TANKER SAFETY
DEFINITIONS
For the purpose of this Guide the following interpretations apply; other definitions of more limited
application are given in Appendix 3.
Absolute vapour density The mass of a unit volume of gas* under stated conditions of temperature and pressure.
[*For the purpose of this Guide the term 'gas' is regarded as being
synonymous with 'vapour'.]
Adiabatic
Without transfer of heat. Adiabatic expansion is volume change in a liquid or gas with no heat loss or
gain involved (see Appendix 3).
Administration (Flag State) The Government of the country in which the ship is registered. This is the authority that issues all
certificates related to the operation of a ship, and is responsible for inspections to ensure compliance
with appropriate standards.
Administration (Port State) The Government of the country in which a port is situated.
Air lock
A separation area used to maintain adjacent areas at a pressure differential, e.g. an electric motor room
air-lock is used to maintain pressure segregation between a gas-dangerous zone on the open weather
deck and the pressurised gas-safe motor room.
Approved equipment Equipment of a design that has been tested, approved and certified by an appropriate authority, such
as an Administration or Classification Society, as safe for use in a specified hazardous atmosphere.
Anti-freeze
An agent which lowers the freezing point of water, e.g, alcohol, ethanol, methanol.
Asphyxia
The condition arising when the blood is deprived of an adequate supply of oxygen so that loss of
consciousness may follow.
Asphyxiant
A gas or vapour, which may or may not have toxic properties, which when present in sufficient
concentrations, excludes oxygen and leads to asphyxia.
Auto-ignition temperature The lowest temperature to which a solid, liquid or gas requires to be raised to
(Autogenous ignition cause self-sustaining combustion without initiation by a spark or flame or
temperature)
other source of ignition.
B.L.E.V.E.
Boiling Liquid Expanding Vapour Explosion. Associated with the rupture under fire conditions of a
pressure vessel containing liquefied gas.
Boil-off
Vapour produced above a cargo liquid surface due to evaporation, caused by heat ingress or a drop in
pressure.
Boiling point
The temperature at which the vapour pressure of a liquid equals that of the atmosphere above its
surface; this temperature varies with pressure (see data sheets).
Bonding (electrical)
The connecting together of electricity conducting metallic objects to ensure electrical continuity,
Brittle fracture
Fracture of a material caused by lack of ductility in the crystal structure resulting from low
temperature.
Bulk
The term 'in bulk' refers to carriage of cargo in tanks or pressure vessels which are constructed as part
of the ship, the contents being loaded and discharged by the ship's installed handling system.
xiv Cargo area That part of the ship which contains the whole cargo system, cargo pump rooms and compressor rooms,
and includes the full beam deck area over the length of the ship above the cargo containment system.
Where fitted, the cofferdams, ballast or void spaces at the after end of the aftermost cargo space or the
forward end of the forwardrnost cargo space are excluded from the cargo area.
Cargo containment system The arrangement for containment of cargo including, where fitted, a primary and secondary barrier,
associated insulation and any intervening spaces, and adjacent structure, if necessary for the support of
these elements. If the secondary barrier is part of the hull structure it may be a boundary of the hold
space.
Cargo operations Any operations on board a gas carrier involving the handling of cargo liquid or vapour, e.g. cargo
transfer, reliquefaction, venting etc.
Cargo tank The liquid-tight shell designed to be the primary container of the cargo, and other liquid-tight containers
whether or not associated with insulation or secondary barriers or both.
Cargo transfer The transfer of cargo liquid and/or vapour to or from the ship.
Cavitation Uneven flow caused by vapour pockets within a liquid. In a pump impeller casing this can
occur even if the pump suction is flooded.
Certificate of fitness A certificate issued by the flag administration confirming that the structure, equipment, fittings,
arrangements and materials used in the construction of a gas carrier are in compliance with the relevant
IMO Gas Codes. Such certification may be issued on behalf of the Administration by approved
Classification Societies.
Certified gas-free A term signifying that a tank, compartment or container has been tested by an
(see also Gas-free) authorised person (usually a chemist from shore) using an approved testing instrument, and found to be
in a suitable condition - i.e. not deficient in oxygen and sufficiently free from toxic and chemical gases -
for a specified activity, such as hot work, and that a certificate to this effect has been issued.
Certified safe electrical (See Approved Equipment)
equipment
Chemical absorption An instrument used for the detection of vapours which works on the
detector principle of a reaction between the gas and a chemical agent in the apparatus; the gas discolours the
agent or the agent dissolves some of the gas (see Appendix 6).
Closed gauging system A system in which the contents of a tank can be measured by means of a
(closed ullaging) device which penetrates the tank, but which is part of a closed system preventing the release of tank
contents. Examples are float-type systems, electronic probe, magnetic probe and bubbler tubes (see
Appendix 6).
Cofferdam The isolating space between two adjacent steel bulkheads or decks; it may be a void or ballast
space.
Combustible gas detector An instrument for detecting a flammable gas/air mixture and usually
('Explosimeter') measuring the concentration of gas in terms of its Lower Flammable Limit (LFL). No single instrument
is reliable for all combustible vapours (see Appendix 6).
Coefficient of cubical The fractional increase in volume for a I'C rise in temperature. The increase is
expansion X of this value for a 1 'F rise.
Critical pressure The pressure of a saturated vapour at its critical temperature.
xv
ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) xvICS TANKER SAFETY GUIDE (LIQUEFIED GAS) xvi
Critical temperature The temperature above which a gas cannot be liquefied by pressure alone (see Appendix 3).
Dew point
The temperature at which the water vapour present in a gas saturates the gas and begins to condense.
Endothermic
A process which is accompanied by absorption of heat.
Exothermic
A process which is accompanied by evolution of heat.
Explosion proof/flame Equipment or apparatus which will withstand, without damage and in
proof equipment
accordance with its prescribed rating (including recognised overloads), any explosion of a prescribed
flammable gas to which it may be subjected under practical operating conditions and which will
prevent the transmission of flame to the surrounding atmosphere (see Appendix 7).
'Explosimeter'
(See Combustible Gas Detector)
Filling limit (or ratio) That volume of a tank, expressed as a percentage of the total volume, which can be safely filled,
having regard to the possible expansion (and change in density) of the liquid.
Flame arrester
A device used in gas vent lines to arrest the passage of flame into enclosed spaces.
Flame proof equipment (See Explosion Proof Equipment)
Flame screen (gauze screen) A portable or fitted device incorporating one or more corrosion resistant wire woven fabrics of very
small mesh used for preventing sparks from entering a tank or vent opening, or for A SHORT
PERIOD OF TIME preventing the passage of flame, yet permitting the passage of gas (not to be
confused with Flame Arrester).
Flammable
Capable of being ignited and burning in air.
Flammable gas
A vapour/air mixture within the flammable range.
Flammable limits
The minimum and maximum concentrations of vapour in air which form flammable (explosive)
mixtures are known as the lower flammable limit (LFL) and upper flammable limit (UFL)
respectively. (These terms are synonymous with lower explosive limit (LEL) and upper explosive
limit (UEL) respectively.)
Flammable range
The range of flammable vapour concentrations in air between the lower and upper flammable limits.
Mixtures within this range are capable of being ignited and of burning.
Flash point
The lowest temperature at which a liquid gives off sufficient vapour to form a flammable mixture with
air near the surface of the liquid or within the apparatus used. This temperature is determined by
laboratory testing in a prescribed apparatus.
Gas absorption detector (See Chemical Absorption Detector)
Gas-dangerous space or A space or zone within the cargo area which is designated as likely to contain
zone
flammable vapours and which is not equipped with approved arrangements to ensure that its
atmosphere is maintained in a safe condition at all times.
Gas detector
An instrument which alerts someone to the presence of gas, especially in spaces where gas is not
normally expected.
Gas-free
Gas-free means that a tank, compartment or container has been tested using approved gas detection
equipment and found to be sufficiently free, at the time of the test, from toxic, flammable or inert
gases for a specific purpose.
xvi ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) xvii
Gas-freeing The process of displacing toxic or flammable vapours, or inert gas from a tank, compartment or container,
followed by the introduction of fresh air into the tank, compartment or container (for correct procedures, see
Chapter 4).
Gas-safe space A space not designated as a gas-dangerous space.
'Cascope' A trade name for an instrument used to detect and indicate the presence of cargo vapour. (See Appendix 6, section 7)
Gauze screen (See Flame Screen)
Hold space The space enclosed by the ship's structure in which a cargo containment system is situated (see Cargo Containment
System).
Hot work Work involving flames, incendive sparks or temperatures likely to be sufficiently high to cause ignition of
flammable
gas. The term includes any work involving the use of welding, burning or soldering equipment, blow torches,
some
power~driven tools, portable electrical equipment which is not intrinsically safe or contained in an explosion-proof
housing, and equipment with internal combustion engines.
Hot work permit A document issued by an authorised person permitting specific work to be done for a specified time in a defined
area employing tools and equipment which could cause ignition of flammable gas (see 'Hot work').
Hydrates The compounds formed at certain pressures and temperatures by the interaction between water and hydrocarbons.
IMO The International Maritime Organization; this is the United Nations specialised agency dealing with maritime
affairs.
IMO codes The IMO Codes for the Design, Construction and Equipment of Ships carrying Liquefied Gases in Bulk. They are
described in Appendix 2, section 5.
Incendive spark A spark of sufficient temperature and energy to ignite flammable gas.
Inert gas A gas (e.g. nitrogen) or mixture of gases, containing insufficient oxygen to support combustion.
Inerting The introduction of inert gas into a space to reduce and maintain the oxygen content at a level at which combustion
cannot be supported.
Inflammable (See Flammable)
Inhibited cargo A cargo which contains an inhibitor.
Inhibitor A substance used to prevent or retard cargo deterioration
or a potentially hazardous chemical self-reaction, e.g. polymerisation.
Insulating flange An insulating device placed between metallic flanges, bolts and washers, to prevent electrical continuity between
pipelines, sections of pipelines, hose strings and loading arms, or equipment/apparatus.
Interbarrier space The space between a primary and a secondary barrier, whether or not completely or partially occupied by
insulation or other material.
Intrinsically safe Intrinsically safe equipment, instruments, or wiring are incapable of releasing sufficient electrical or thermal
energy under normal or abnormal conditions to cause ignition of a specific hazardous atmospheric mixture in its
most easily
ignited concentration Appendix 7, section 2),
Liquefied gas A liquid which has an absolute vapour pressure exceeding 2.8 bar at 37.8'C, and certain other substances of
similar characteristics specified in the IMO Codes.
xvii ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) xviii
Lower flammable limit (See Flammable limits)
(LFL)
LNG Liquefied Natural Cas; the principal constituent of LNG is methane.
LPG Liquefied Petroleum Cases - these are mainly propane and butane, shipped either separately or in
mixtures. They may be refinery by-product gases or may be produced in conjunction with crude oil or
natural gas.
MARVS The Maximum Allowable Relief Valve Setting of a cargo tank.
MARPOL The International Convention for the Prevention of Pollution from Ships, 1973, as modified by its
Protocol of 1978.
Mole The a mount of a substance, in any convenient system of weight measurement, which corresponds to
the numerical value of the molecular weight of the substance (e.g. for propane, molecular weight of
44.1, a gram-mole weighs 44.1 grams; a pound-mole weighs 44.1 pounds).
Mole fraction The number of moles of any component in a mixture divided by the total number of moles of each
component.
Mole percentage The mole fraction multiplied by 100.
Oxygen analyser An instrument used to measure oxygen concentrations, expressed as a percentage by volume.
Peroxide A compound formed by the chemical combination of cargo liquid or vapour with atmospheric oxygen,
or oxygen from another source. These compounds may in some cases be highly reactive or unstable
and constitute a potential hazard.
Polymerisation The phenomenon by which the molecules of a particular compound link together into a larger unit
containing anything from two to thousands of molecules, the new unit being called a polymer.
A compound may thereby change from a free-flowing liquid into a viscous one or even a solid. A
great deal of heat may be evolved when this occurs.
Polymerisation may occur spontaneously with no outside influence, or it may occur if the compound
is heated, or if a catalyst or impurity is added. Polymerisation may, under some circumstances, be
dangerous, but may be delayed or controlled by the addition of inhibitors.
Pressure Force per unit area,
Primary barrier The inner element designed to contain the cargo when the cargo containment system includes two
boundaries.
Purging The introduction of a suitable cargo vapour to displace an existing tank atmosphere.
Relative vapour density The mass of the vapour compared with the mass of an equal volume of air, both at standard conditions
of temperature and pressure,
Thus vapour density of 2.9 means that the vapour is 2.9 times heavier than an equal volume of air
under the same physical conditions.
Reliquefaction Converting cargo boil-off vapour back into a liquid by refrigeration (see Appendix 3).
Responsible officer The Master or any officer to whom the Master may delegate responsibility for any operation or duty.
xviii ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) xix
Responsible terminal
The shore supervisor in charge of all operators and operations at the terminal
representative associated with the handling of products, or his responsible delegate.
Restricted gauging system A system employing a device which penetrates the tank and which, when in
(also known as restricted rise, permit., a ,mall quantity of cargo vapour or liquid to be released to the
ullage system) atmosphere. When not in use the device is completely closed (see Appendix 6).
Secondary barrier The liquid-resisting outer element of a cargo containment system designed to afford temporary
containment of any envisaged leakage of liquid cargo through the primary barrier and to prevent the
lowering of the temperature of the ship's structure to an unsafe level. Types of secondary barriers are
more fully defined in the IMO Codes.
Self-reaction The tendency of a chemical to react with itself, usually resulting in polymerisation or decomposition.
Sloshing Wave formations which may arise at the liquid surface in a cargo tank from the effects of ship
motions.
SOLAS The International Convention for the Safety of Life at Sea.
Span gas A vapour sample of known composition and concentration used to calibrate (or span) a ship's gas
detection equipment.
Specific gravity The ratio of the weight of a volume of a substance at a given temperature to the weight of an equal
volume of fresh water at the same temperature or at a different given temperature.
(Since temperature affects volume, the temperature at which a specific gravity comparison is made
needs to be known and is stated after the ratio.)
Static electricity The electrical charge produced on dissimilar materials through physical contact and separation.
Tank access hatch The access hatch for tank entry, fitted to the tank dome.
Tank cover The structure intended to protect the cargo containment system against damage where it protrudes
through the weather deck, or to ensure the continuity and integrity of the deck structure, or both.
Tank dome '1'he upward extension of a portion of the cargo tank. For below-deck cargo containment systems the
tank dome protrudes through the weather deck, or through a tank skirt.
Tank skirt The vertical cylindrical structure attached to the ship's foundation deck, supporting a spherical cargo
tank at its equator.
Thermal conductivity meter A fixed or portable instrument used to detect the presence of gas
concentrations from 0 to 100%. It must be calibrated for a particular gas. (See Appendix 6)
Threshold limit value The 'time-weighted average' (TWA) concentration of a substance to which it is
(TLV) believed workers may be repeatedly exposed, for a normal 8-hour working day and 40-hour working
week, day after day, without adverse effect. It may be supplemented by a 'short-term exposure limit'
(STEL) TLV.
Upper flammable limit (See Flammable Limits)
Vapour Density See Absolute Vapour Density and Relative Vapour Density
Vapour pressure The pressure exerted by the vapour above the liquid at a given temperature (see Appendix 3).
xix ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) xx
Ventilation
The process of maintaining in a space an atmosphere suitable for human access, by natural or mechanical means
using a fixed or portable system. (Reference should be made to the relevant IMO Code chapters for specific
requirements.)
Venting The release of cargo vapour or inert gas from cargo tanks and associated systems.
Void space The enclosed space in the cargo area external to a cargo containment system, not being a hold space, ballast
space, fuel oil tank, cargo pump or compressor room, or any space in normal use by personnel.
Water fog Very fine droplets of water generally delivered at a high pressure through a fog nozzle.
Water-spray system A system of sufficient capacity to provide a blanket of water droplets to cover the cargo manifolds, tank domes,
deck storage tanks, and boundaries of superstructure and deckhouses.
THE PROPERTIES
AND HAZARDS OF CHAPTER 1
LIQUEFIED GASES
1.1 INTRODUCTION
This chapter deals with the properties common to all or most bulk liquefied gas cargoes. These cargoes are normally carried
as boiling liquids and, as a consequence, readily give off vapour.
The common potential hazards and precautions are highlighted in the following sections.
1.2 FLAMMABILITY
Almost all cargo vapours are flammable. When ignition occurs, it is not the liquid which burns but the evolved vapour.
Different cargoes evolve different quantities of vapour, depending on their composition and temperature (see Section 1.6).
Flammable vapour can be ignited and will burn when mixed with air in certain proportions. If the ratio of vapour to air is
either below or above specific limits the mixture will not burn. The limits are known as the lower and upper flammable
limits, and are different for each cargo.
Combustion of vapour/air mixture results in a very considerable expansion of gases which, if constricted in an enclosed
space, can raise pressure rapidly to the point of explosive rupture.
Details of the precautions necessary to avoid fire are given in Chapter 3.
1.3 HEALTH HAZARDS
1.3.1
Toxicity
Some cargoes are toxic and can cause a temporary or permanent health hazard, such as irritation, tissue damage or
impairment of faculties. Such hazards may result from skin or open-wound contact, inhalation or ingestion (see relevant
data sheets at Appendix 1).
Contact with cargo liquid or vapour should be avoided. Protective clothing should be worn as necessary (see Section 9.2)
and breathing apparatus should be worn if there is a danger of inhaling toxic vapour (see Sections 9.4 and 9.5). The toxic
gas detection equipment provided should be used as necessary and should be properly maintained (see paragraph 5.3.6).
1.3.2
Asphyxia
Asphyxia occurs when the blood cannot take a sufficient supply of oxygen to the brain. A person affected may experience
headache, dizziness and inability to concentrate, followed by loss of consciousness. In sufficient concentrations any vapour
may cause asphyxiation, whether toxic or not.
Asphyxiation can be avoided by the use of vapour and oxygen detection equipment and breathing apparatus as necessary
(see Appendix 6, section 7).
1.3.3
Anaesthesia
Inhaling certain vapours (e.g ethylene oxide) may cause loss of consciousness due to effects upon the nervous system. The
unconscious person may react to sensory stimuli, but can only be roused with great difficulty. ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 2
Anaesthetic vapour hazards can be avoided by the use of cargo vapour detection equipment and breathing apparatus as
necessary (see Appendix 6, section 7).
1.3.4
Frostbite
Many cargoes are either shipped at low temperatures or are at low temperatures during some stage of cargo operations.
Direct contact with cold liquid or vapour or uninsulated pipes and equipment can cause cold burns or frostbite. Inhalation
of cold vapour can permanently damage certain organs (e.g. lungs).
Ice or frost may build up on uninsulated equipment under certain ambient conditions and this may act as insulation. Under
some conditions, however, little or no frost will form and in such cases contact can be particularly injurious.
Appropriate protective clothing should be worn to avoid frostbite, taking special care with drip trays on deck which may
contain cargo liquid (see paragraph 1.7.2). For treatment of frostbite see Section 9.9.
1.4 REACTIVITY
A liquefied gas cargo may react in a number of ways: with water to form hydrates, with itself, with air, with another cargo
or with other materials.
1.4.1
Reaction with Water - Hydrate Formation
Some hydrocarbon cargoes will combine with water under certain conditions to produce a substance known as a hydrate
resembling crushed ice or slush. The water for hydrate formation can come from purge vapours with an incorrect dew
point, water in the cargo system or water dissolved in the cargo. Care should be taken to ensure that the dew point of any
purge vapour or inert gas used is suitable for the cargo concerned, and that water is excluded from the cargo system.
Hydrates can cause pumps to seize and equipment to malfunction. Care should therefore be taken to prevent hydrate
formation.
Certain cargoes, notably LPGS, may contain traces of water when loaded. It may be permissible in such cases to prevent
hydrate formation by adding small quantities of a suitable anti-freeze (e.g. methanol, ethanol) at strategic points in the
system. It is emphasised that nothing whatsoever should be added to any cargo without the shipper's permission. For LPG
mixtures a small dose of anti-freeze may be permissible, but for chemical cargoes such as ethylene the addition of even one
litre per two hundred tons could make the cargo commercially valueless. In the case of inhibited cargoes the anti-freeze
could adversely affect the inhibitor.
If the use of anti-freeze is permitted it should be introduced at places where expansion occurs because the resultant lowering
of temperature and pressure promotes hydrate formation (see Appendix 3, paragraph 7.7)
Anti-freeze additives are often flammable and toxic, and care should be taken in their storage and use.
1.4.2
Self-reaction
Some cargoes may react with themselves. The most common form of self-reaction is polymerisation which may be initiated
by the presence of small quantities of other cargoes or by certain metals. Polymerisation normally produces heat which may
accelerate the reaction.
The IMO Codes require cargoes which may self-react either to be carried under an inert gas blanket, or to be inhibited
before shipment. In the later case a certificate must be given to the ship, stating:
•
the quantity and name of the inhibitor added;
•
the date it was added and how long it is expected to remain effective;
•
the action to be taken should the voyage exceed the effective lifetime of the inhibitor;
•
any temperature limitations affecting the inhibitor.
An example of an inhibitor certificate is given in Appendix 11.
2 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 3
Normally there should be no need to add any inhibitor to the cargo during the voyage. If it should become necessary,
however, any such additions should be made in accordance with the shipper's instructions.
The inhibitor may not boil off with the cargo and it is possible for reliquefaction systems to contain uninhibited cargo. The
system should therefore be drained or purged with inhibited cargo when shut down.
Many inhibitors are much more soluble in water than in the cargo, so to avoid a reduction in inhibitor concentration, care
should be taken to exclude water from the system. Similarly the inhibitor may be very soluble in anti-freeze additives if
these form a separate phase and the shipper's instructions on the use of anti-freeze should be observed. If the ship is
anchored in still conditions the cargo should be circulated daily to ensure a uniform concentration of inhibitor.
Certain cargoes which can self-react (e.g ethylene oxide, propylene oxide), but which cannot be inhibited, have to be carried
under inert gas. Care should be taken to ensure that a positive pressure is maintained in the inerted atmosphere at all times
and that the oxygen concentration never exceeds 0.2% by volume.
(Note: For provisions concerning the avoidance of uninhibited stagnant liquid pockets refer to the IMO IGC Code,
paragraph 17.4.2.)
1.4.3
Reaction with Air
Some cargoes can react with air to form unstable oxygen compounds which could cause all explosion. The IMO Codes
require these cargoes to be either inhibited or carried under nitrogen or other inert gas. The general precautions in
paragraph 1.4.2 apply and care should be taken t() observe the shipper's instructions.
1.4.4
Reaction with Other Cargoes
Certain cargoes can react dangerously with one another. They should be prevented from mixing by using separate piping
and vent systems and separate refrigeration equipment for each cargo. Care should be taken to ensure that this positive
segregation is maintained.
To establish whether or not two cargoes will react dangerously, the data sheet for each cargo should be consulted. This
issue is also covered in various national regulations, which should be observed.
1.4.5
Reaction with Other Materials
The data sheets list materials which should not be allowed to come into contact with the cargo. The materials used in the
cargo systems must be compatible with the cargoes to be carried and care should be taken to ensure that no incompatible
materials are used or introduced during maintenance (e.g. gaskets).
Reaction can occur between cargo and purge vapours of poor quality: for instance, inert gas with high CO, content can
cause carbamate formation with ammonia (see paragraph 4.6.1). Reaction can also occur between compressor lubricating
oils and some cargoes, resulting in blockage and damage.
1.5 CORROSIVITY
Some cargoes and inhibitors may be corrosive. The IMO Codes require materials used in the cargo system to be resistant to
corrosion by the cargo. Care should therefore be taken to ensure that unsuitable materials are not introduced into the cargo
system. All precautions specific to the cargo should be strictly observed (refer to data sheets at Appendix 1).
Corrosive liquids can also attack human tissue and care should be taken to avoid contact: reference should be made to the
appropriate data sheets. Instructions about the use of protective clothing should be observed (see Section 9.2).
1.6 VAPOUR CHARACTERISTICS
One characteristic of liquefied gases is the large quantity of vapour readily produced by a small volume of liquid (I m' of
LNG will produce 60Om' of vapour at ambient temperature). The venting of cargo vapour should therefore be avoided.
However, if the venting of cargo vapour is unavoidable, it should be done with care and in full knowledge of the potential
hazards. In most port areas the venting of flammable or toxic vapours is forbidden, and applicable local regulations should
be observed (See Sections 2.9 and 4.16).
1.7 LOW TEMPERATURE EFFECTS
As liquefied gas cargoes are often shipped at low temperatures it is important that temperature sensing equipment is well
maintained and accurately calibrated (see paragraph 5.3.6 and Appendix 6, section 5).
Hazards associated with low temperatures include:
1.7.1
Brittle Fracture
Most metals and alloys become stronger but less ductile at low temperatures (i.e. the tensile and yield strengths increase but
the material becomes brittle and the impact resistance decreases) because the reduction in temperature changes the
material's crystal structure.
Normal shipbuilding steels rapidly lose their ductility and impact-strength below O'C. For this reason, care should be taken
to prevent cold cargo from coming into contact with such steels, as the resultant rapid cooling would make the metal brittle
and would cause stress due to contraction. In this condition the metal would be liable to crack. The phenomenon occurs
suddenly and is called 'brittle fracture'.
However, the ductility and impact resistance of materials such as aluminium, austenitic and special alloy steels and nickel
improve at low temperatures and these metals are used where direct contact with cargoes at temperatures below -55'C is
involved.
1.7.2
Spillage
Care should be taken to prevent spillage of low temperature cargo because of the hazard to personnel (see Section '1.3) and
the danger of brittle fracture (see paragraph 1.7.1). If spillage does occur, the source should first be isolated and the. spilt
Liquid then dispersed (see paragraph 7.3.3). (The presence of vapour may necessitate the use of breathing apparatus.) If
there is a danger of brittle fracture, a water hose l-nay be used both to vaporise the liquid and to keep the steel warm. If the
spillage is contained in a drip tray the content., should be covered or protected to prevent -accidental contact and allowed to
evaporate. I.iquefied gases quickly reach equilibrium and visible boiling ceases; this quiescent liquid could be mistaken for
water and carelessness could be dangerous.
Suitable drip trays are arranged beneath manifold connections to control any spillage when transferring cargo or draining
lines and connections. Care should be taken to ensure that unused manifold connections are isolated and that if blanks are
to be fitted the flange surface is clean and free from frost. Accidents have occurred because cargo escaped past incorrectly
fitted blanks.
Liquefied gas spilt onto the sea will generate large quantities of vapour by the heating effect of the water. This vapour may
create a fire or health hazard, or both. Great care Should be taken to avoid such spillage, especially when disconnecting
cargo hoses.
1.7.3
Cooldown
Cargo systems are designed to withstand a certain service temperature; if this is below ambient temperature the system has
to be cooled down to the temperature of the cargo before cargo transfer. For LNG and ethylene the stress and thermal shock
caused by an over-rapid cooldown of the system could cause brittle fracture. Cooldown operations should be carried out
carefully in accordance with instructions (see paragraph 4.7.2). ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 5
1.7.4
Ice Formation
Low cargo temperatures can freeze water in the system leading to blockage of, and damage to, pumps, valves, sensor lines,
spray lines etc. Ice can be formed from moisture in the system, purge vapour with incorrect dewpoint, or water in the cargo.
The general precautions given in paragraph 1.4.1 apply. The effects of ice formation are similar to those of hydrates, and
anti-freeze can be used to prevent them.
1.7.5
Rollover
Rollover is a spontaneous rapid mixing process which occurs in large tanks as a result of a density inversion. Stratification
develops when the liquid layer adjacent to a liquid surface becomes more dense than the layers beneath, due to boil-off of
lighter fractions from the cargo. This obviously unstable situation relieves itself with a sudden mixing, which the name
'rollover' aptly describes.
Liquid hydrocarbons are most prone to rollover, especially cryogenic liquids. LNG is the most likely by virtue of the
impurities it contains, and the extreme conditions of temperature under which it is stored, close to the saturation
temperatures at storage pressures.
If the cargo is stored for any length of time and the boil-off is removed, evaporation can cause a slight increase in density
and a reduction of temperature near the surface. The liquid at the top of the tank is therefore marginally heavier than the
liquid in the lower levels. Once stratification has developed rollover can occur.
No external intervention such as vibration, stirring or introducing new liquid is required to initiate rollover. The response to
a small temperature difference within the liquid (which will inevitably occur in the shipboard environment) is sufficient to
provide the kinetic energy to start rollover, and release the gravitational driving forces which will invert the tank contents.
The inversion will be accompanied by violent evolution of large quantities of vapour and a very real risk of tank over
pressure.
Rollover has been experienced ashore, and may happen on a ship that has been anchored for some time. If such
circumstances are foreseen the tank contents should be circulated daily by the cargo pumps to prevent rollover occurring.
Rollover can occur if similar or compatible cargoes of different densities are put in the same tank. For example, if tank
pressure is maintained by boil-off reliquefaction, the condensate return may be of slightly different temperature (and hence
density) from the bulk liquid, and likewise if condensate from two or more cargoes is returned to one tank. In such
circumstances, rollover may be prevented by returning condensate that is less dense than the bulk liquid to the top of the
tank, and condensate that is denser to the bottom of the tank.
Rollover may also occur when two part cargoes are loaded into the same tank (e.g. propane and butane). In this case there
will be a large boil-off (up to 3% of the total liquid volume) due to the temperature difference between the two. For this
reason, the practice is considered unsafe unless a thorough thermodynamic analysis of the process is undertaken, and the
loading takes place under strictly controlled conditions.
Rollover in a ship on passage is most unlikely. Essentially, stratification and the subsequent rollover process is confined to
shore LNG storage. However, if the use of LNG carriers for floating storage were to be introduced, personnel manning
such vessels would need to be as aware of the problem and as vigilant to avoid rollover as their counterparts managing shore
based storage.
1.8 PRESSURE
Liquefied gases are normally carried as boiling liquids at either:
•
ambient temperature (fully pressurised ships), or
•
atmospheric pressure (fully refrigerated ships), or
•
intermediate temperatures and pressures (semi-pressurised ships, often referred to as semi-refrigerated).
5 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 6
Particularly hazardous cargoes such as ethylene oxide and propylene oxide may be carried below their boiling points to
reduce boil-off and increase safety. In such cases the tank pressure is maintained above atmospheric with nitrogen padding.
Any heat input to the cargo will vaporise some of the liquid and gradually increase the tank pressure. Pressure vessels are
designed to accommodate this increase, but on fully or semi-refrigerated ships the boil-off is condensed by the
reliquefaction system and returned to the cargo tanks as a boiling liquid. On LNG vessels cargo tank pressure is almost
always controlled by burning the boil-off in the main propulsion system or in rare cases (e.g. emergency) by venting it to
atmosphere.
If the pressure above a boiling liquid is increased, vaporisation from the surface is reduced, and vice versa.
1.8.1
High acid Low Pressure Effects
Pressures above or below the design range can damage a system, and operating personnel should be fully aware of any
pressure limitation for each part of the cargo system; pressures should always be kept between the specified maximum and
minimum.
1.8.2
Pressure Surge
High surge pressures (shock pressures or 'liquid hammers') can be created if valves are opened or shut too quickly, and the
pressure may be sufficient to cause hose or pipeline failure (see paragraph 4.5.2 and Appendix 8).
1.8.3
Pressurised Systems
In pressurised systems, with the cargo at ambient temperature, there is normally no external frosting to indicate the presence
of liquid or vapour anywhere in the system. Cheeks should be made for the presence of high pressure vapour or liquid by
gauges and test cocks before opening valves etc.
It is possible for vapour trapped in a system to condense in cold weather, causing a slight reduction in pressure. If the cargo
is inhibited, this condensed liquid will be uninhibited and the precautions given in paragraphs 1.4.2, 1.4.3 and 1.8.4 should
be observed.
1.8.4
Reciprocating Compressors
If vapour trapped in a reciprocating compressor condenses, it can dilute the lubricating oil in the crankcase which could
cause bearing failure, overheating or possibly an explosion. The crankcase heating equipment, if fitted, should be used to
reduce the possibility of cargo condensing and should be operated before the compressor is started. Liquid condensed in the
compressor may also cause mechanical damage.
1.8.5
Cargo Tank Pressures
Cargo tank pressure should normally be maintained above atmospheric pressure to prevent the ingress of air and the
possible formation of flammable mixtures. Positive pressures should be maintained if the tank contains any cargo vapour or
inert gas.
However, many pressure vessels are designed to withstand vacuum and it is possible to reduce tank pressure below
atmospheric without drawing in air, for example during inerting and gas freeing (but see paragraph 4.6.4).
Cargo operations such as cooldown, warm-up, loading and discharge may affect pressures ill hold or interbarrier spaces.
Pressures can also be affected by climatic changes and the variation in temperature between day and night.
Pressure in cargo tanks and hold or interbarrier spaces should be closely monitored, especially during cargo operations, and
the equipment provided should be used to make the necessary adjustments. Particular care is necessary with membrane or
semi-membrane systems which are vulnerable to damage from vacuum or incorrect differential pressures because of the thin
barrier material. ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 7
•
•
•
•
Pressures in cargo tanks may be maintained above atmospheric by:
equalising pressures between tanks which contain the same cargo, or
circulating cargo liquid or vapour, or both, between tanks containing the same cargo, or
circulating cargo within a tank by use of the cargo pumps, or
allowing the cargo to warm up.
1.8.6
Liquid Gas Samples
Liquid gas samples should not be placed in containers which cannot withstand the pressure created by the sample at the
highest ambient temperature expected. Sufficient ullage should be left in the container to ensure that it does not become
liquid full at the highest temperature anticipated (see paragraph 4.18.1). Liquid gas samples should be stored within the
cargo area.
1.8.7
Sloshing
Within a range of tank filling levels, the pitching and rolling of the ship and the liquid free-surface can create high impact
pressure on the tank surface. This effect is called 'sloshing' and can cause structural damage. Filling levels within this
range must therefore be avoided.
However, some cargoes may be carried safely within the range specified for a particular system if the sloshing forces are
permissible; guidance should be sought from the shipowner, the designer and the Classification Society.
1.8.8
Pressure Relief Valves
Pressure relief valves depend on accurate setting of opening and closing pressures for effective operation (see paragraph
5.3.8 and Appendix 5, Section 9).
1.8.9
Cargo Heat Exchanges
Heat exchangers should be pressure tested prior to use. This is especially important after a long period of idleness and
before a ship is delivered on time charter. In addition to testing the tubes for tightness, the seawater low temperature cut-out
must be tested to ensure that the cargo inlet valve to the heater closes, thereby avoiding damage to the tubes from freezing
should the outlet temperature of the seawater fall below 5'C.
In use, seawater flow through the heater must be established before product flow commences.
7 EMPTY PAGE
ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 8ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 9
GENERAL
PRECAUTIONS CHAPTER
2
2.1 INTRODUCTION
This chapter covers general precautions which should be observed irrespective of the cargo carried. Additional precautions
for specific cargoes are dealt with in other chapters. The existence of local regulations is mentioned in this chapter; it is the
master's responsibility to see that any applicable local regulations are understood and observed.
2.2 CARGO INFORMATION
The IMO Codes require the following information to be available to every ship and for each cargo:
•
•
•
•
•
•
•
•
•
•
a full description of the physical and chemical properties necessary for the safe containment of the cargo
action to be taken in the event of spills or leaks
counter-measures against accidental personal contact
fire-fighting procedures and fire-extinguishing agents
procedures for cargo transfer, gas freeing, ballasting, tank cleaning and changing cargoes
special equipment needed for the safe handling of the particular cargo
minimum inner hull steel temperatures
emergency procedures
compatibility
details of the maximum filling limits allowed for each cargo that may be carried at each loading temperature, the maximum
reference temperature and the set pressure for each relief valve.
The master should request the correct technical name of the cargo as soon as possible and before loading. The master must
only load a cargo which is listed on his certificate of fitness. Data sheets for these cargoes should be on board.
The master and all those concerned should use the data sheet and any other relevant information to acquaint themselves with
the characteristics of each cargo to be loaded. If the cargo to be loaded is a mixture (e.g LPG), information on the
composition of the mixture should be sought; the temperature and pressure readings in the shore tank can be used to verify
this information.
Special notes should be made of any contaminants that may be present in the cargo, e.g, water.
2.3 MOORINGS
The consequences of a gas carrier ranging along or breaking out of a berth could be disastrous, particularly during cargo
transfer when damage could be caused to loading arms or hoses. Correct mooring is therefore of the utmost importance.
Mooring requirements are usually determined by the terminal, supplemented by advice from the pilot. For general guidance
on moorings, see the OCIMF publication 'Effective Mooring'.
Once the vessel has been secured, moorings should be regularly checked and tended to ensure that they remain effective.
9 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 10
2.4 EMERGENCY TOWING-OFF WIRES (FIRE WIRES)
The ship should provide towing-off wires, ready for immediate use without adjustment, in case the ship needs to be moved
in the event of fire or other emergency.
Wires should be positioned fore and aft on the offshore side of the ship, be in good condition, of adequate strength, and
properly secured to the bitts such that full towing loads can be applied. The eyes should be maintained at or about the
waterline in a position that tugs can reach without difficulty. Sufficient slack to enable the tugs to tow effectively should be
retained between the bitts and the fairlead, but prevented from running out by a rope yarn or other easily broken means.
There are various methods currently in use for rigging emergency towing wires, and the arrangement may vary from port to
port. A terminal which requires a particular method to be used should advise the ship accordingly.
2.5 ACCESS TO SHIP
2.5.1
Means of Access (Gangways or Accommodation Ladders)
Personnel should only use the designated means of access between ship and shore.
When a ship is berthed or at anchor, the means of access should be so placed as to be convenient for supervision and if
possible away from the manifold area. Where practicable two means of access should be provided. Gangways or other
means of access should be properly secured and provided with an effective safety net. In addition, suitable life-saving
equipment should be available near the access point to shore.
2.5.2
Lighting
During darkness the means of access and all working areas should be adequately illuminated.
2.5.3
Unauthorised Persons
Persons who have no legitimate business on board, or who do not possess the master's permission to be there, should be
refused access. The terminal, in agreement with the master, should restrict access to the jetty or berth.
A crew list should be given to the terminal security personnel.
2.5.4
Persons Smoking or intoxicated
Personnel on watch on a gas carrier must ensure that no one who is smoking approaches or boards the ship. The company
policy on drugs and alcohol should be strictly enforced.
2.6 NOTICES
2.6.1
Permanent
Permanent notices or internationally accepted signs should be displayed in conspicuous places on board, indicating where
smoking and naked lights are prohibited, and where ventilation is necessary before entry.
2.6.2
Temporary
On arrival at a terminal, a gas carrier should display temporary notices at points of access, in appropriate languages, to
indicate the following:
1 0 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 11
WARNING
NO NAKED LIGHTS
NO SMOKING
NO UNAUTHORISED PERSON
In addition, when the liquefied gases being handled present a health hazard, further notices in appropriate languages should
be prominently displayed stating:
WARNING
HAZARDOUS LIQUEFIED GAS
Local regulations may require additional notices and such requirements should be observed.
2.7 CRAFT ALONGSIDE
Unauthorised craft should be prohibited from securing alongside or approaching close to the ship.
No tugs or other self-propelled vessels should be allowed alongside during operations which involve the venting of cargo
vapours.
Regulations against smoking and naked lights should be strictly enforced on any craft permitted alongside and on shore if
applicable. Operations should be stopped if these rules are violated and should not be restarted until the situation has been
made safe.
2.8 WEATHER PRECAUTIONS
2.8.1
Wind Conditions
In conditions of little or no wind, vapour resulting from an accidental release or from purging or gas-freeing operations may
persist on deck. A strong wind may create low pressure on the lee side of a deckhouse or structure and thereby cause
vapour to be carried towards it.
In any such conditions it should be assumed that local high concentrations of vapour may exist, and all cargo operations
should cease.
2.8.2
Electrical Storms
Cargo operations or the venting of flammable cargo vapours should be stopped during electrical storms in the immediate
vicinity of the ship. See Section 8.3 for action if a vent mast is struck by lightning.
2.8.3
Cold Weather
Particular attention should be paid to pneumatic valves and control systems which can freeze in cold weather if the control
air supply is damp, and to relief valves and cooling water systems. If fitted, heating systems should be used. Any water
collected on the discharge side of relief valves should be drained off. Cooling water systems should either be dosed with
anti-freeze or drained. If a system is drained, the fact should be logged and the system refilled before subsequent use. Water
in fire main or spray systems should be circulated continuously or drained where there is a risk of freezing. Attention
should be paid to emergency showers or eye-wash stations to ensure availability of facilities.
Cold weather can also cause cargo vapour trapped in rotating equipment (e.g. in a cargo compressor) to condense, enter the
crankcase and dilute the lubricating oil, and cause damage. Crankcase heaters should be used if fitted, and started in ample
time before running up cargo compressors (see paragraph 1.8.4). ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 12
2.9 DISPERSAL OF VENTED CARGO VAPOURS
Cargo vapour, whether toxic or flammable, should be vented to atmosphere with extreme caution, taking account of
regulations and weather conditions (see Section 2.8).
If the temperature of the vented vapour is below atmospheric dewpoint, clouds of condensed water vapour will form.
Condensed water vapour (fog) is heavier than air whereas the cargo vapour may or may not be heavier than air, depending
on temperature. The cargo vapour cloud is likely to be oxygen deficient, and should only be entered by personnel wearing
breathing apparatus. Furthermore, it should never be assumed that the cargo vapour is contained entirely within the
boundaries of the visible water vapour cloud.
If the cargo vapour is heavier than air it may accumulate on deck and enter accommodation spaces. The precautions in
Section 2.10 should therefore be observed. In some cases it may be possible to heat vapour before venting to reduce its
density and assist dispersion. If such facilities are provided they should be used.
2.10 OPENINGS IN DECKHOUSES AND SUPERSTRUCTURES
Regulations require that superstructures are designed with certain portholes fixed shut and openings positioned to minimise
the possibility of vapour entry. These design features should not be modified in any way.
All doors, portholes and other openings to gas-safe spaces should be kept closed during cargo operations. Doors should be
clearly marked if they have to be kept permanently closed in port, but in no circumstances should they be locked.
Mechanical ventilation should be stopped and air conditioning units operated on closed cycle or stopped if there is any
possibility of vapour being drawn into the accommodation.
2.11 ENGINE AND BOILER ROOM PRECAUTIONS
2.11.1
Combustion Equipment
Boiler tubes, uptakes, exhaust manifolds and combustion equipment should be maintained in good condition as a precaution
against funnel fires and sparks. In the event of a funnel fire, or if sparks are emitted from the funnel, cargo operations
should be stopped and, at sea, the course should be altered as soon as possible to prevent sparks falling onto the tank deck.
2.11.2
Blowing Boiler Tubes
Funnel uptakes and boiler tubes should not be blown in port.
At sea they should only be blown in conditions where soot will be blown clear of the tank deck.
2.11.3
Cargo Vapour
Care should be taken to ensure that cargo vapour does not enter the engine or boiler room from any source. Special
attention should be paid to engine room equipment connected to the cargo plant e.g. the inert gas plant, with its cooling
water system. Particular care is necessary if LNG cargo vapour is used as fuel (see paragraph 4.9.3).
If malfunction of equipment, explosion, collision or grounding damage should give rise to a situation where cargo vapour is
likely to enter the machinery space, immediate consideration should be given to its possible effect on the operation of
equipment. Any necessary action should be taken; e.g. isolating the source, closing access doors, hatches and skylights,
shutting down mechanical ventilation system, auxiliary and main machinery, or evacuation.
Apart from the obvious hazards, diesel engines are liable to overspeed and destroy themselves if flammable vapour is
present in the air supply, even at concentrations well below the lower
12 flammable limit (LFL). It is recommended that diesel engines are fitted with a valve on the air intake to stop the engine in
these circumstances.
2.12 CARGO MACHINERY ROOM PRECAUTIONS
Cargo vapour may be present in cargo pump or compressor rooms, and gas detection systems are installed to warn of its
presence. In ships carrying cargoes whose vapours are lighter than air (e.g. ammonia) and heavier than air (e.g. LPG) gas
detector points are fitted at high and low levels and the relevant detector points should be used for the cargo carried.
Ventilation systems are provided to disperse any vapour that may collect in the pump or compressor room. The space
should be ventilated for at least ten minutes before cargo operations begin and throughout their duration, and also if liquid
or vapour leakage is suspected. Ventilation systems should be maintained carefully; if the fans fitted are of non-sparking
design their design features should not be modified in any way.
The precautions given in Section 6.3 should be observed before personnel enter cargo machinery rooms.
Lighting systems in cargo machinery rooms must be certified flame proof. It is essential to ensure that such systems are
properly maintained. Additional lighting, if required, should be of a suitably safe type (see paragraph 3.5.2).
Gas-tight bulkhead gland seals and air lock doors to cargo machinery electric motor rooms should be carefully checked and
maintained to ensure that cargo vapour does not enter.
2.13 SHIP’S READINESS TO MOVE
At all times during discharge, loading and ballasting operations the ship should have adequate stability and suitable trim to
allow for departure at short notice in the event of an emergency. While berthed at a terminal the ship's boilers, main
engines, steering machinery, mooring equipment and other essential equipment should be kept ready to permit the ship to
move from the berth at short notice, and in accordance with the terminal regulations.
Repairs and other work which may immobilise the ship should not be undertaken at a berth without the prior written
agreement of the terminal. It may also be necessary to obtain permission from the local Port Authority before carrying out
such work.
2.14 NAVIGATION
The normal high standards of navigation should be maintained and any navigational restrictions (routeing, reporting
requirements etc) should be observed. If the ship is permitted to burn LNG vapour in the main machinery at sea, it may be
necessary to change over to oil fuel when manoeuvring or when entering restricted or territorial waters.
2.15 POLLUTION PREVENTION
It is the responsibility of the master or those in charge of transfer operations involving cargo or bunkers to know the
applicable pollution prevention regulations and to ensure that they are not violated. Exercises should be held to train
personnel in accordance with the Shipboard Oil Pollution Emergency Response Plan, and recorded.
There is a danger of violating pollution prevention regulations if ballast taken on in polluted waters is discharged in another
port. If ballast has to be taken on in polluted areas, it may be necessary to exchange it for clean ballast when in deep water
on passage. Some terminals have specific requirements in this respect, and the master should ensure that they are observed.
13
ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 132.16 FIRE-FIGHTING AND FIRE PROTECTION EQUIPMENT
Fire-fighting appliances should always be kept in good order, tested regularly, and available for immediate use at all times
(see Section 3.8).
2.17 HELICOPTERS
Gas carriers are recommended not to undertake routine helicopter operations unless a purpose-built helicopter platform is
provided. Whenever helicopter services are used the safety measures recommended in the ICS 'Guide to Helicopter/Ship
Operations' should be taken into account.
14
ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 14FIRE HAZARDS
AND PRECAUTIONS CHAPTER
3
3.1 INTRODUCTION
This chapter addresses the hazards presented by flammable liquefied gases and vapour emissions, and recommends
practices to prevent the risk of fire. Information is also provided on precautions against the dangers of inhaling vapour and
of fire hazards from sources other than the cargo.
The avoidance of cargo fires depends upon preventing flammable cargo vapour, oxygen and sources of ignition coming
together.
Cargo vapours in flammable concentrations are likely to be present in areas such as cargo tanks, cargo machinery spaces
and at times on deck. It is essential that all possible sources of ignition are eliminated from these areas, both by design and
operation.
Sources of ignition are inevitably present in spaces such as the accommodation, galleys and engine rooms, and it is essential
to prevent cargo vapour entering these spaces.
Personnel should be continuously on their guard, not only against the more obvious dangers, but also against unforeseen
circumstances which could lead to flammable vapours and sources of ignition coming together.
3.2 FLAMMABILITY OF LIQUEFIED GASES
It is the vapour given off by a liquid and not the liquid itself which burns. A mixture of vapour and air cannot be ignited
unless the proportions of vapour and air lie between two concentrations known as the Lower Flammable Limit (LFL) and
the Upper Flammable Limit (UFL). The limits vary according to the cargo (see data sheets). Concentrations below the
lower limit (too lean) or above the upper limit (too rich) cannot burn. However, it is important to remember that
concentrations above the upper limit can be made to burn by diluting then] with air until the mixture is within the flammable
range, and that pockets of air may exist ill ally system, leading to the creation of a flammable mixture.
A liquid has to be at a temperature above its flash point before it evolves sufficient vapour to form a flammable mixture.
Many liquefied gas cargoes are flammable, and since they are shipped at temperatures above their flash points flammable
mixtures can be formed.
The source of flammable material may be vapour from the cargo, or from anything else that will burn. Oxygen normally
derives from the atmosphere, which contains approximately, 2117, oxygen by volume. Ignition can be caused by anything
capable of providing the necessary energy,, such a., a naked flame, an electrical or electrostatic spark, or a hot metal
surface.
Fire is prevented by ensuring that at least one of these three elements is excluded.
In the presence of a flammable substance, sources of ignition or oxygen should be excluded. Oxygen can be restricted to a
safe level within the cargo system by keeping the pressure above atmospheric pressure with cargo vapour or inert gas.
Many sources of ignition are eliminated during the design stage and care should be taken to ensure. that design features are
not impaired in any way. Other sources of ignition need to be excluded by correct operational practices.
Where sources of ignition and oxygen are likely to be present, such as in accommodation, engine and boiler rooms, galley,
motor rooms etc., it is vital to exclude flammable vapour. Particular care is necessary if there is a direct connection between
the engine room the cargo system (e.g. when cargo vapour is burnt asfuel, see paragraph 4.9.3), or if the inert gas plant is
located in the engine room.
1 5
ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 15ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 16
3.3 GENERATOR AND DISPOSAL
Liquefied gas cargoes are usually carried either fully refrigerated or pressurised in order to avoid loss of cargo. Cargo
vapour is evolved and is normally treated in the following ways:
•
•
•
During loading, vapour is displaced by cargo liquid; this vapour is either reliquefied and returned to the tanks as a boiling
liquid or returned to shore through a vapour return line.
During carriage, the cargo will boil off because of heat transfer through the insulation. In this case the vapour is either
reliquefied or (in the case of LNG only) burnt in the main engines. If the cargo system is fully pressurised any vapour will
be retained within the cargo tank.
During gas-freeing at sea, the vapour is normally a mixture of cargo vapour and inert gas or inert gas and air. It cannot be
used as fuel or reliquefied, and is vented to atmosphere. During gas-freeing in port, the vapour is returned through a
shoreline.
Whatever methods are provided for handling vapour, it is essential to ensure that they function properly and are operated
correctly. Failure to do so may create a hazard to the ship, the ship's crew or the environment.
3.4 ATMOSPHERE CONTROL
3.4.1
General
When carrying a flammable cargo the cargo system contains liquid and vapour. The atmosphere around the cargo tanks is
normally inerted to prevent the formation of flammable mixtures. The IMO Codes use the term 'environmental control' to
describe this process. The precautions necessary to ensure safety are dealt with in the following paragraphs.
3.4.2
Hold and lnterbarrier Spaces
These spaces may have to be filled with inert gas if the cargo is flammable. Different cargo containment systems require
different procedures, as follows:
Containment System
Hold or Interbarrier Space Atmosphere
Full secondary
Dry inert gas or nitrogen;
barrier
maintained with make-up gas provided by the shipboard inert gas generation system,
or by shipboard storage which should be sufficient for at least 30 days at normal
rates of consumption.
Partial secondary
Dry inert gas or nitrogen;
barrier
maintained with make-up gas provided by the shipboard inert gas generation system,
or by shipboard storage which should be sufficient for at least 30 days at normal rates
of consumption. Alternatively, subject to certain conditions, the space may be filled
with dry air (see Regulation 9.2.2.2 of the IGC Code).
No secondary
Dry air or dry inert gas depending on the cargo;
barrier
maintained either with dry air provided by suitable air drying equipment, or with
make-up inert gas provided by the shipboard inert gas generation system or
shipboard storage.
3.4.3
Cargo Tanks and Piping Systems
The formation of a flammable vapour mixture in the cargo system should be prevented by replacing the air in the system
with inert gas before loading, and by removing cargo vapour by inert gas after discharge, prior to changing cargoes or gas
freeing. Suitable pipe connections should be provided for this purpose. Inerting should be continued until the concentration
of oxygen or cargo vapour in the space is reduced to the required level. The tank atmosphere
16 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 17
•
•
•
•
•
should be monitored at different levels to ensure there are no pockets of excessive concentrations of oxygen or cargo
vapour, particularly in tanks with complex internal structures or bulkheads.
Some cargoes require the oxygen content in the vapour space to be kept extremely low (in some cases less than 0.2%) to
prevent a chemical reaction occurring. For instance, ethylene oxide/propylene oxide mixtures can decompose
spontaneously unless special precautions are taken to control the atmosphere; and butadiene can react with oxygen to form
unstable peroxide compounds. The oxygen content in the tanks must be reduced as necessary before loading begins. While
such cargoes remain on board, oxygen is excluded either by keeping the ullage space full of inert gas at a positive pressure
or, in the case of butadiene, by keeping the cargo vapour above atmospheric pressure. In every case, shippers' requirements
should be strictly observed.
3.4.4
Inert Gas Quality
Inert gas used for atmosphere control should be suitable for the intended purpose, regardless of source. In particular it
should:
be chemically compatible with the cargo and the materials of construction throughout the full range of operating
temperatures and pressures;
have a sufficiently low dewpoint to prevent condensation, freezing, corrosion, damage to insulation etc. at the minimum
operating temperature;
have an oxygen concentration not exceeding 5%, but as low as 0.2% if the cargo can react to form peroxides;
have a low concentration of C02 to prevent it freezing out at the anticipated service temperature (see paragraph 4.6.1);
have minimal capacity for accumulating a static electrical charge.
3.4.5
Inert Gas Hazards and Precautions
The main hazard associated with inert gas is asphyxiation of personnel due to lack of oxygen. Asphyxiation can occur in
those parts of the cargo system which have been inerted, or in other enclosed spaces into which inert gas has leaked.
Nobody should enter spaces which are not in common use until it has been established that the atmosphere will support life
(see Chapter 6).
As the inert gas plant is often situated in the engine room, great care should be taken to ensure that cargo vapour does not
flow back along inert gas supply lines; non-return valves should be tested for effectiveness, at regular intervals. Any
temporary connection between the inert gas plant and the cargo systems should be disconnected and tightly blanked after
use.
If a liquid nitrogen system is used, care should be taken to avoid contact with skin and eyes, or severe cold burns will be
caused. Any metal structure or component likely to come into contact with liquid nitrogen could suffer brittle fracture
unless it has been designed for a service temperature of -196'C. Great care should be taken to ensure that vaporisers are
used correctly.
3.5 PRECAUTIONS AGAINST SOURCES OF IGNITION
3.5.1
Smoking
Company policy and local regulations should be strictly observed.
Smoking can only be permitted under controlled conditions. The designated smoking places on a gas carrier must be known
to the crew, and when in port should be agreed in writing between the master and the terminal representative before cargo
operations start. The master is responsible for ensuring that all persons on board the tanker are informed of the places in
which smoking is permitted, and for posting suitable notices,
The agreed smoking places should be confined to locations abaft all cargo tanks, and should not have doors or portholes
which open directly to open decks.
The use of matches and cigarette lighters outside designated smoking spaces should be
17 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 18
•
•
•
•
•
prohibited. The risks involved in carrying matches and, more particularly, cigarette lighters should be impressed on all
personnel. The use of lighters should be discouraged. Matches used on board should be of the safety type.
3.5.2
Portable Electrical Equipment
Portable electrical equipment (self-contained or on extension cables) should not be used inside cargo tanks, cargo
pumprooms, compressor rooms, or adjacent spaces unless:
the equipment circuit is intrinsically safe;
the equipment is contained within an approved explosion-proof housing;
flexible cables are of a type approved for extra hard use, have an earth conductor, and are permanently attached to the
explosion-proof housing in an approved manner;
the compartments around and within which the equipment and/or cable are to be used are free from flammable vapour
throughout the period during which the equipment is in use; and
adjacent compartments are free from flammable vapour or have been made safe by inerting or completely filling with water,
and all connections with other compartments that are not free from flammable vapour are firmly closed and will remain so.
If the equipment is only to be used on the tank deck, explosion-proof and other types of certified safe equipment can be
used (see Appendix 7).
Air-driven lamps of an approved type may be used in non gas-free atmospheres, although to avoid the accumulation of
static electricity on the lamp it should either be earthed or the hose should have a resistance low enough to allow static
dissipation.
Only approved safety torches or hand lamps should be used.
Small battery powered personal items such as watches and hearing aids are not significant ignition sources when correctly
used. However portable domestic radios, electronic calculators, tape recorders, cameras and other non-approved battery
powered equipment should not be used on the tank deck or wherever flammable vapour may be encountered.
When in port, reference should be made to local regulations which may totally prohibit the use of any electrical equipment.
All portable electrical equipment should be carefully examined for possible defects before use, Special care should be taken
to ensure that insulation is undamaged, that cables are securely attached and remain so while the equipment is in use, and
that mechanical damage to cables is prevented.
3.5.3
Communication Equipment in Port
Main radio transmitters should not be used and the main aerials should be earthed during cargo operations because energy
may be induced into conducting objects in the radio wave field. This energy can be sufficient to create a spark if
discontinuity occurs. Heavy sparking can also occur at the insulators, particularly in humid weather. Permanently and
correctly installed VHF equipment is not affected.
If it is necessary to operate the ship's radio in port for maintenance etc., the agreement of the terminal and port authorities
should be sought. The issue of a work permit may be necessary,
and to ensure safety the terminal may require operation at low power, use of a dummy aerial load, or transmission only
when no cargo operations are in progress.
It is advisable to consult the terminal before radar scanners or satellite communication equipment are used, because they
may include non-approved equipment such as drive motors. The radiation itself is considered not to present an ignition
hazard.
Loud hailers, searchlights, signal lamps, etc. should not be used in port.
3.5.4 Use of Tools
Although grit blasting and the use of mechanically powered tools are not normally considered to fall within the definition of
hot work, both these operations should only be permitted under controlled conditions. Section 1 of the Hot Work Permit is
suitable (see Appendix 12). ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 19
The work area should not be subject to vapour release or a concentration of combustible vapours, and should be free from
combustible material. The area should be gas-free, and tests with a combustible gas indicator should give a reading of not
more than 1 % LFL. The ship must not be alongside at a terminal. There must be no cargo, bunkering, ballasting, tank
cleaning or gasfreeing operations in progress.
The hopper and hose nozzle of a gritblasting machine should be electrically earthed to the deck or fitting being blasted.
There is a risk of perforation of pipelines when grit blasting or chipping, and great care must be taken over planning such
work. Cargo and inert gas pipelines should not be blasted or mechanically chipped unless the entire ship is gas-free.
Adequate fire fighting equipment must be laid out and ready for immediate use.
The use of hand tools such as chipping hammers and scrapers for steel preparation and maintenance may be permitted
without a hot work permit. Their use must be restricted to deck areas and fittings not connected to the cargo system. The
work area should not be subject to vapour release or a concentration of combustible vapours. The area should be gas-free
and clear of combustible materials. There must be no cargo, bunkering, ballasting, tank cleaning or gasfreeing operations in
progress. Work on cargo pipelines and inert gas pipelines should be subject to the same precautions as applies to powered
tools.
3.5.5
Aluminium Equipment and Paint
Aluminium equipment should not be dragged or rubbed across steel since it may leave a smear. If a heavy smear of
aluminium on rusty steel is struck it is possible to cause an incendive spark.
Extensive experience indicates that the normal use of aluminium paint creates no special hazard.
3.5.6
Ship/Shore Insulating, Earthing and Bonding
In order to provide protection against static electrical discharge at the manifold when connecting and disconnecting cargo
hose strings and metal arms, the terminal operator should ensure that they are fitted with an insulating flange or a single
length of non-conducting hose, to create electrical discontinuity between the ship and shore. All metal on the seaward side
of the insulating section should be electrically continuous to the ship, and that on the landward side should be electrically
continuous to the jetty earthing system.
The insulating flange or single length of non-conducting hose must not be short-circuited by contact with external metal; for
example, an exposed metallic flange on the seaward side of the insulating flange or hose length should not make contact
with the jetty structure either directly or through hose handling equipment.
Simply switching off a cathodic protection system is not a substitute for the installation of an insulating flange or a length of
non-conducting hose.
Cargo hoses with internal bonding between the end flanges should be checked for electrical continuity before they are taken
into service and periodically thereafter.
A ship/shore bonding cable is not effective as a safety device and may even be dangerous. A ship/shore bonding cable
should therefore not be used.
Note: Although the potential dangers of using a ship/shore bonding cable are widely recognised, attention is
drawn to the fact that some national and local regulations may still require a bonding cable to be connected. If a
bonding cable is demanded, it should first be visually inspected to see that it is mechanically sound. The connection
point for the cable should be well clear of the manifold area. There should always be a switch on the jetty in series
with the bonding cable and of a type suitable for use in a hazardous area. It is important to ensure that the switch is
always in the 'off' position before connecting or disconnecting the cable. Only when the cable is properly fixed and
in good contact with the ship should the switch be closed. The cable should be attached before the cargo hoses are
connected and removed only after the hoses have been disconnected.
19 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 20
3.5.7
Auto-Ignition
The vapours from flammable liquids (including fuel and lubricating oil) may ignite, even in the absence of external flame or
sparks, if the liquid comes into contact with a surface heated above its auto-ignition temperature (e.g. steam lines,
overheated equipment). This is called 'auto-ignition'. In any case, evaporation of the liquid will create an additional fire
hazard.
Immediate steps should be taken to remedy any leakage. Care should also be taken to avoid rags or other materials soaked
in oil or chemicals from coming in contact with hot surfaces. Lagging should not be allowed to become saturated with oil.
3.5.8
Spontaneous Combustion
Wet, oily or chemically impregnated fibrous materials are liable to ignite as a result of a gradual build-up of heat due to
oxidation. The hazard is increased if the material is kept warm, for example by proximity to a hot pipe. Furthermore,
contact with other liquids, such as strong acids, may cause such materials to ignite or to be destroyed by chemical attack.
Cotton waste, canvas, or similar absorbent materials should therefore not be left lying on decks, on equipment, or on or
around pipelines, and should not be stowed near oil, paint etc. If such materials become damp or contaminated they should
either be cleaned and dried before storing, or destroyed.
3.6 HOT WORK
It is anticipated that owners and operators of liquefied gas tankers will issue clear guidance to masters and crews on
the control of hot work outside repair yards. The following is intended to assist safety by indicating principal areas
that should receive attention.
3.6.1
General
Hot work means any work requiring the use of electric arc or gas welding equipment, cutting burner equipment or other
forms of naked flame, as well as spark generating tools. It covers all such work, regardless of where it is carried out aboard
a ship, including open decks, machinery rooms and the engine room.
Repair work outside the engine room which necessitates hot work should only be undertaken when it is essential for the
safety or immediate operation of the ship, and no alternative repair procedure is possible.
Hot work outside the engine room (and in the engine room when associated with fuel, lubrication or cargo systems) must be
prohibited until the requirements of national legislation and other applicable regulations have been met, safety
considerations taken into account, and a hot work permit has been issued. This may involve the master, owners'
superintendent, shore contractor, terminal representative and port authority as appropriate.
Hot work in port at a gas terminal is normally prohibited. If such work becomes essential for safety or urgent operational
needs, then port regulations must be complied with. Full liaison must be arranged with port and terminal authorities before
any work is started.
3.6.2 Assessment of Hot Work
The master shall decide whether the hot work is justifiable, and whether it can be conducted safely. Hot work in areas
outside the engine room should not be proceeded with until the master has informed the ship's operators of details of the
work proposed, and a procedure has been discussed and agreed.
Before hot work is started a safety meeting under the chairmanship of the master must be held, at which the planned work
and the safety precautions are carefully reviewed. The meeting should be attended at least by all those who will have
responsibilities in connection with the work. An agreed written plan for the work and the related safety precautions should
be made. The plan must clearly and unambiguously designate one officer who is responsible for the supervision of the
work, and another officer who is responsible for safety precautions and communications between all parties involved.
20 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 21
A flow chart to assist is shown overleaf.
All personnel involved in the preparations and in the hot work operation, must be briefed and instructed on their own role.
They must clearly understand which officer is responsible for work supervision and which for safety precautions. A written
hot work permit should be issued for each intended task. The permit should specify the duration of validity, which should
not exceed a working day. An example of a hot work permit is given in Appendix 12.
3.6.3
Preparations for Hot Work
No hot work must be undertaken inside a compartment until it has been cleaned and ventilated, and tests of the atmosphere
in the compartment indicate 21 % oxygen content by volume, not more than 1 % LFL and it is free from toxic gases. It is
important to continue ventilation during hot work (see Chapter 6).
No hot work should be undertaken on the open deck unless the area is free from flammable vapour and all compartments,
including deck tanks, within a radius of at least 30 metres around the working area have been washed and freed of
flammable vapour and/or inerted.
All sludge, cargo-impregnated scale, sediment or other material likely to give off flammable or toxic vapour, especially
when heated, should be removed from an area of at least 10 metres around all hot work. All combustible material such as
insulation should either be removed or protected from heat.
Adjacent compartments should either be cleaned and gas freed to hot work standard, freed of cargo vapour to not more than
1 % by volume and kept inerted, or completely filled with water. No hot work should be undertaken in a compartment
beneath a deck tank in use.
Care should be taken to ensure that no release of flammable vapour or liquid can occur from non-adjacent compartments
that are not gas-free.
No hot work should be carried out on bulkheads of bunker tanks in use. An adjacent fuel oil bunker tank may be considered
safe if tests using a combustible gas indicator give a reading of not more than 1 % LFL in the urage space of the bunker
tank, and no heat transfer through the bulkhead of the bunker tank will be caused by the hot work.
All pipelines interconnecting with cargo spaces should be flushed, drained, vented and isolated from the compartment or
deck area where hot work will take place.
Hot work on pipelines and valves should only be permitted when the item needing repair has been detached from the system
by cold work, and the remaining system blanked off. The item to be worked on should be cleaned and gas freed to a safe
for-hot work standard, regardless of whether or not it is removed from the hazardous cargo area.
All other operations utilising the cargo or ballast system should be stopped before hot work is undertaken, and throughout
the duration of the hot work. If hot work is interrupted to permit pumping of ballast or other operations using the cargo
system, hot work should not be resumed until all precautions have been re-checked, and a new hot work permit has been
issued.
3.6.4
Checks by Officer Responsible for Safety During Hot Work
Immediately before hot work is started the officer responsible for safety precautions should examine the area where it is to
be undertaken, and ensure that tests with a combustible gas indicator show not more than 1 % LFL, and that the oxygen
content is 21 % by volume.
Adequate fire-fighting equipment must be laid out and ready for immediate use. Fire watch procedures must be established
for the area of hot work and in adjacent, non-inerted spaces where the transfer of heat may create a hazard. Effective means
of containing and extinguishing welding sparks and molten slag must be established.
The work area must be adequately and continuously ventilated. Flammable solvents must not be present, even for cleaning
tools.
21 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 22
This flowchart assumes the work is considered essential for safety or the immediate operational capability of the ship, and that it
cannot be deferred until the next planned visit to a repair yard.
3.6.2 HOT WORK
Can the task be achieved
without using hot work?
NO
YES
Is the part of the ship requiring work a
pipeline or other fitting or is it
permanent structure?
FITTINGS
Can the fitting be disconnected
and removed from hazardous
cargo area before hot work?
PERMANENT
STRUCTURE
Description of work necessary and
proposed procedures to be sent to
ship’s operators for prior consent or
alternative plans to be considered
YES
NO
Fitting to be isolated
from all pipelines and
blanks attached
PLAN WORK
ACCORDINGLY
Operators
concurrence
received?
YES
NO
NO HOT
WORK
PERMITTED!
Master to hold safety meeting on board attended
by all having responsibilities during work
Is master satisfied that work can be
completed safely?
NO
YES
Written statement of work to be drawn up showing
separate responsibilities for work supervision and safety
Hot work permit to be issued showing
task and time
Complete all preparations for hot work
Stop all other work in hazardous cargo
area
PERFORM
TASK
REPORT COMPLETION
TO OPERATORSICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 23
•
•
•
•
The frequency of atmosphere monitoring must be established. Atmospheres should be re-tested at regular intervals and
after each break in work periods. Checks should be made for flammable vapours or liquids, toxic gases or inert gas from
non-gas free spaces.
Welding and other equipment to be used should be carefully inspected before each occasion of use to ensure that it is in
good condition. Where required it must be correctly earthed. Special attention must be paid to electric-arc equipment to
ensure that:
electrical supply connections are made in a gas-free space;
existing supply wiring is adequate to carry the electrical current demanded without overloading and consequent heating;
flexible electric cables laid across the deck have sound insulation; and
the cable route to the worksite is the safest possible, only passing over gas-free or inerted spaces.
3.7 STATIC ELECTRICITY
Static electricity can cause sparks capable of igniting a flammable gas. Some routine operations can cause electrostatic
charging, and precautions to minimise the hazard are given below.
3.7.1
Electrostatic Generation
The extent to which materials, whether solid, liquid or vapour, can generate and retain a static charge depends on their
electrical resistance. If the resistance is high, a charge can be built up. It is also possible for a charge to build up on
materials in a system with low resistance (e.g. metals) that are electrically insulated from each other The cargo system of a
gas carrier is electrically bonded to the ship's hull to prevent charge build-up. It is important that such bonding connections
are maintained in an efficient condition.
Hoses are normally bonded to their flanges by the metal reinforcement, and thus provide a continuous path to earth through
the ship's manifold and the hull. If an insulating flange is inserted at the shore manifold, the intermediate flanges and metal
reinforcement will still provide that continuous path.
A significant static electrical charge can be caused by high fluid velocities, change from liquid to vapour/liquid droplet
flow, small particles carried in a vapour stream and by impingement.
In an unbonded system static electricity could be generated by:
•
•
•
flow of liquid through pipes
flow of liquid/vapour mixtures through spray nozzles
flow of a vapour containing particles (e.g. rust) through piping.
The risk of causing ignition by static electricity is reduced if the system is correctly bonded or if flammable mixtures are
avoided.
3.7.2
Steam
High velocity water droplets in a jet of steam may become charged in passing through a nozzle and could produce a charged
mist. For this reason steam should not be injected into a tank,
compartment or piping system which contains a flammable mixture. Steam may sometimes be used to provide external heat
to defrost or dry a bonded system containing flammable liquid or vapour, but only if the surrounding atmosphere is non
flammable.
3.7.3
Carbon Dioxide
When liquid carbon dioxide under pressure is released at high velocity, rapid evaporation causes cooling and particles of
solid carbon dioxide may form. The solid particles in the cloud of CO, may become electrostatically charged. For this
reason carbon dioxide should not be released into spaces containing a flammable mixture.
3.8 FIRE-FIGHTING AND FIRE PROTECTION EQUIPMENT
3.8.1
Fire-fighting Equipment
Fire-fighting appliances should always be kept in good order, tested regularly, and available for immediate use at all times.
When the ship is berthed, the responsible officer should familiarise himself with the availability of the shore fire-fighting
services and with the means of communicating with the appropriate authorities. This information should be obtained while
completing the ship/shore safety checklist prior to cargo operations.
Immediately prior to commencing cargo transfer, the ship's fire-fighting system should be made ready. If practicable, a
pump should maintain pressure on the fire water main, but in any case it should be on standby. Fire hoses should be
uncoiled and connected to the main: at least two should be placed near the manifold, one forward and one aft of it.
The water spray system to protect the manifold area should be tested. Fixed monitors should be made ready and, if
remotely activated, adjusted to protect the manifold before operations begin. A portable dry powder extinguisher should be
placed conveniently for use near the manifold or a hose from a fixed dry powder monitor uncoiled and placed upwind of the
manifold (see also Chapter 8).
3.8.2
Flame Arresters and Gauze (Flame) Screens
Flame arresters and screens, when required to be fitted, should be maintained in good condition and replaced if they become
defective. Passage of gas may be dangerously restricted if these devices become blocked; the passage of cold vapour
through a damp screen can cause freezing and blockage. Flame screens should never be painted.
3.8.3
Inert Gas
If inert gas is used in the cargo system (e.g. tanks, hold or interbarrier space) the gases in each space should be checked
regularly to ensure that the oxygen concentration is below the required level and that the pressure is above atmospheric. All
instruments and equipment used in the system should be maintained in good condition.
It should be remembered that an inert gas/cargo vapour mixture may become flammable if it should escape to the
atmosphere.
24 CARGO
OPERATIONS CHAPTER
4
4.1 INTRODUCTION
This chapter outlines the range of cargo operations normally encountered on liquefied gas carriers and the general safety
precautions to be observed in connection with these operations. The procedures outlined should be considered as general
guidance only; there is considerable variation in the design of cargo containment and cargo handling systems, and specific
instructions should be prepared for inclusion in the cargo operations manual for individual ships. These instructions should
be carefully studied by all personnel involved in cargo handling operations.
Although the cargo containment and handling systems have been carefully designed, and have been constructed under strict
supervision, the required levels of safety in cargo operations can only be achieved if all parts of systems and equipment are
maintained in good working order. Similarly, the personnel involved in cargo operations must be fully aware of their duties
and thoroughly trained in the correct procedures and handling of the equipment.
Training in emergency procedures is particularly important (see Chapter 7).
4.2 RESPONSIBILITY
It is the responsibility of the master to ensure that the officers and crew are properly and correctly informed of their duties,
and understand how to fulfil them.
The master or an officer appointed by him is responsible for the safety of the ship and all cargo operations. The responsible
officer should be present at all times and be satisfied that all equipment under his care is in good working condition.
The master should ensure that there is proper liaison between the responsible officer on the ship and his counterpart at the
shore installation (see Ship/Shore Safety Checklist in Appendix 9). These personnel should establish the programme for all
cargo operations and the procedures to be adopted in the event of an emergency. Details of emergency contact names,
positions, telephone numbers etc. should be distributed before cargo operations begin. Any special safety requirements of
the shore installation should be brought to the attention of those concerned.
4.3 COMMISSIONING THE CARGO SYSTEM
Before a new ship is commissioned to carry liquefied gas it is essential that all parts of the cargo system are clean and dry to
prevent faults or damage and that safety equipment has been checked and tested.
After drydocking or repairs, cargo tanks should be cleaned and inspected at all levels to ensure that accumulations of rust,
water and possible loose objects have been removed. Internal fittings should be checked for tightness and security of nuts,
bolts etc., which preferably should have been spot welded. Manholes should be checked for the correct type of gaskets and
possible damage; covers should be properly tightened down.
It is essential that pipelines, valves and pumps are carefully dried out. Piping systems should be thoroughly blown through
with adequate quantities of compressed air followed by nitrogen, making full use of the drains in the system in proper
sequence. Special attention should be paid to the body cavities of valves and to convolutions of expansion bellows.
25
ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 25The emergency shutdown system and the operation of all actuator valves, compressor and pump cut-outs etc. should be
checked.
Final adjustment and testing of some cargo plant control equipment can only be carried out with cargo on board.
Arrangements should be made in advance with the shore installation to allow this work to be carried out by competent
personnel during early stages of first loading. Pipe supports should be checked, especially where expansion bellows are
fitted.
4.4 GENERAL CYCLE OF CARGO OPERATIONS
Every liquefied gas carrier must have a Procedures and Arrangements Manual which gives specific operating instructions.
The following sequence outlines a general cycle of operations, and supplementary comments are made where relevant:
Preparation for
cargo transfer
ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 26
Gas Inerting and
freeing purging
Changing Cooldown
cargoes
Discharge Loading
Cargo Conditioning
4.5 PREPARATION FOR CARGO TRANSFER
4.5.1
General
Before cargo transfer starts, the responsible officer should be satisfied that the precautions set out in Chapters 2 and 3 are
being observed. The use of safety check lists, appropriately adapted for the specific ship, is strongly recommended.
The following pre-arrival checks should be made by the ship:
•
•
•
•
•
•
•
•
•
•
•
•
deck lighting is working to provide for safe working conditions;
ventilation systems are in operation as necessary (see section 6.4);
fixed gas detection systems are calibrated for the cargo concerned and are in operation;
fire protection equipment has been tested and is ready for immediate use, including water spray (see Section 3.8);
personnel protection equipment has been checked and breathing apparatus air bottles are fully charged;
protective clothing and breathing apparatus are being worn or are immediately available as necessary;
no unauthorised work is being done in the cargo area and non-essential personnel are being kept away from the cargo area;
restricted liquid level gauges are secured in the closed position if their use is not permitted with the cargo concerned;
adjustable relief valves are correctly set: if multi set point relief valves are fitted the correct setting device should be used;
flame screens or similar devices in the vent system are clear, will not restrict gas flow and are suitable for the product;
the oxygen content is below the specified maximum level in spaces required to be kept inerted, and a supply of inert gas is
available to maintain a slight positive pressure in these spaces during all cargo transfer operations;
spill pans or trays are in place beneath manifold connections or portable extension pipes, the relevant drain valves are closed
and a hose for draining spillage is connected if required;
26 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 27
•
•
•
•
•
•
•
•
•
all cargo pipelines, filters, plant, instrumentation and controls have been checked and are in good working order;
those involved in operations know who is responsible for instructions to open or close valves and to stop or start equipment;
the ship's pipeline system is set for the relevant operation, and the valves have been checked; the stem cargo line, if fitted, is
isolated if it is not to be used; any removable pipe sections or hoses connecting the cargo system to the ship's inert gas plant
have been removed and blind flanges properly fitted.
The following ship-shore checks should be made:
local regulations have been ascertained and are being observed;
agreement has been reached with the responsible terminal representative about:
- the signals to indicate 'Stand by', 'Start operations', 'Slow down', 'Stop operations';
- pumping rates;
- pumping or loading sequence;
- action to be taken in the event of fire or other emergency;
- emergency shutdown procedures: the ship's system should have been tested and the shore warned of the shutdown period of
the ship's isolating valves;
- if the ship and shore systems have been linked, their functioning should have been tested;
- access to the ship, and smoking restrictions;
a ship-shore bonding connection, if used, is made before hoses are connected; if an insulating flange is used, its insulation
has not been impaired;
cargo hoses, loading arms and gaskets are suitable for the cargo and are in good condition;
flexible hoses are suspended from suitable equipment, are not subjected to excessive bending and are not putting excessive
strain on the manifold (especially when the manifold is extended by unsupported reducing pieces); care is being taken not to
damage mechanical loading arms; and the ship-shore flange connection is specially checked by the responsible cargo
officer.
Full specimen Ship/Shore Safety Checklists are given in Appendix 9.
4.5.2 Pressure Surge
The potential hazards of pressure surges (shock pressures or 'liquid hammers') resulting from rapid operation of valves
should be emphasised to all personnel engaged in cargo transfer. Pressure surges can be created when the flow in a liquid
line is stopped too quickly. The hazard is greatest when cargo is being transferred over long distances and at high velocity.
If a valve is shut too quickly under these conditions the deceleration of the large column of liquid in the line sets up shock
waves which can travel up and down the line causing extremely high surge pressures. The cargo hose is most vulnerable to
failure in these circumstances.
Pressure surges may be caused by automatic shut-off valves operated by level sensors. It is important that emergency
shutdown valve systems are well maintained and accurately adjusted. Such valves often have different torque
characteristics at service and ambient temperatures. If possible, they should be connected to the terminal system so that the
shore and ship systems operate together. The operation should be adjusted so that upstream valves close first to safeguard
the cargo hose or loading arm. Where such co-ordinated systems do not exist, those responsible for cargo operations should
be aware of the potential hazard of the ship shutting down against the shore pumps, or vice versa.
The following precautions should be taken to avoid pressure surge during cargo transfer:
During loading, when flow is diverted from one tank to another, the valves on the tank about
to receive cargo should be fully opened before the valves on the tank being isolated are shut. On completion of loading the
flow should be stopped by the terminal using the shore valves to prevent overstressing of the cargo hose.
During discharge, cargo flow should be controlled by the pump discharge valves or the valve on the tank dome, if possible,
to minimise pressure effects and restrict them to a short length of pipe mostly within the cargo tank. Likewise pump
discharge valves should be shut before ship and shore manifold valves are closed.
27 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 28
•
While cargo is being transferred, valves in the liquid system should not be opened or shut suddenly
Valves should be set in the position required before transfer begin, and only those needed for throttling or control duty
should be used during operations. Manual valves not required for normal operations may be lashed, but should never be
locked m case they have to be used m an emergency
•
pressure surge can be generated when a valve maintaining a pressure difference in the liquid line is opened. If the pressure
difference is high and the valve is opened too quickly a high flow velocity will result, giving rise to a high surge pressure
when the flow is stopped. This could occur, for example, when liquid is trapped between valves in a deck line and becomes
warm: in such cases the valve should be opened very carefully to equalise the pressures slowly. Liquid lines should be
drained after use to prevent this problem.
A more detailed description of the pressure surge phenomenon is given in Appendix 8.
4.6 INERTING AND PURGING
4.6.1
General
The term 'inerting' generally refers to the replacement of air or cargo vapour by inert gas before loading or gas-freeing
respectively, to prevent the formation of flammable mixtures.
The term 'purging' generally refers to the introduction of a suitable cargo vapour to displace an existing tank atmosphere.
The extent of purging and the vapours used will normally be laid down by the IMO Codes or shippers' requirements.
Shippers should always be consulted about the atmosphere required on arrival at the loading port.
Inerting or purging operations may take place at sea if the ship is suitably equipped, or in harbour. In either case, due
consideration should be given to the safety of venting cargo vapour to atmosphere, and any local regulations should be
observed. Venting is not normally permitted in port. If venting is unavoidable, operations should be carefully controlled to
prevent dangerous vapour concentrations in the vicinity of the ship. Facilities may however be available for venting vapour
to shore flare facilities.
During inerting or purging the relevant gas concentrations should be monitored regularly at different tank levels to ensure
safe concentrations. This is particularly important in tanks with internal structures, wash bulkheads etc.
Tanks can be inerted or purged separately, in parallel or in series according to the arrangement of the system. If tanks are
inerted or purged in parallel, back pressure effects in the line may cause an uneven vapour supply to each tank unless the
vapour supply to each tank can be accurately measured and controlled.
Ship generated inert gas produced by combustion will contain up to 15% carbon dioxide and is unsuitable for use in certain
circumstances. Such inert gas should not be used before loading a cargo with a temperature below -55C because at such
temperatures the CO2 will freeze out and may contaminate the cargo. The CO2 can also react with ammonia to produce
carbamates which will be deposited on the tank walls and may block pipelines etc. To prevent reaction, for instance when
preparing to load ammonia after carrying LPG, the inerted tank should be ventilated with air before the ammonia is loaded;
and when preparing to load LPG after carrying ammonia, the ammonia concentration should be reduced to 10Oppm by gas
freeing with air before inerting,
4.6.2 Inerting
The purpose of inerting is to prevent the formation of explosive vapour/air mixtures. The production of inert gas is outlined
in paragraphs 5.3.12 and 5.3.13 and the gas used for inerting should be suitable for the purpose (see paragraph 3.4.4).
When inerting a tank which has been ventilated with air, the oxygen content should be checked regularly. The oxygen
content after inerting should never exceed 5% by volume, and should normally be in the order of 2% to allow for uneven
distribution. Much lower levels may be required for oxygen reactive cargoes (e.g. butadiene). The dew point temperature
for the particular cargo to be loaded must be achieved during this operation.
28 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 29
When inerting a tank which has contained cargo vapour, the process should be continued until the cargo vapour
concentration is sufficiently low to prevent formation of flammable mixtures during subsequent ventilation with air. For
further information on the oxygen and cargo vapour concentration for each cargo see data sheets in Appendix 1.
To avoid possible back-flow of cargo vapour, the cargo lines should be opened to the vent system before the inert gas
system is connected. It is advisable to use hot gas or some other suitable method to warm up tanks that have contained low
temperature cargoes before inerting, so that the steel temperature is above the dew point temperature. Failure to do so
means that much larger quantities of inert gas will be required and moisture or CO2 will freeze out. Similarly, if cold
nitrogen vapour is used for inerting, atmospheric moisture is likely to be deposited in tanks.
4.6.3
Purging
The purpose of purging is to prepare tanks for receiving cargo. Normal shipboard inert gas may have to be displaced with
pure nitrogen for cargo requirements (e.g. to remove C02or to obtain a low dew point). Neither inert gas nor nitrogen can
be condensed by the ship's reliquefaction plant. Purging with cargo vapour is therefore necessary before loading so that the
ship's reliquefaction plant can operate continuously (see paragraph 4.9.2).
Cargo vapour for purging can be taken from a shipboard storage vessel via a vaporiser and the cargo vapour/inert gas
mixture vented to atmosphere. When purging with cargo vapour in port, the cargo vapour/inert gas or nitrogen mixture
should be led to a proper vent or flare stack ashore for safe disposal. Purging is completed when analyses of vapour
samples from the tank are satisfactory, or when the inert gas concentration is sufficiently low for the reliquefaction plant to
operate continuously, or both.
4.6.4
Methods of Inerting and Purging
One of three methods may be adopted:
•
Stratification (Displacement)
This method makes use of the difference in vapour densities between the gas in the tank and the inerting or purging gas.
A fairly distinct layer (or stratum) is formed between the two gases because of the density difference. Most hydrocarbons
are heavier than inert gas, whereas ammonia is lighter than air. Nitrogen and inert gas have almost equal densities though
nitrogen is denser at low temperatures. It is advisable and economical to use warm nitrogen.
The lighter gas is passed into or vented from the top of the tank and the heavier gas is passed into or vented from the
bottom. Considerably less inerting or purging vapour is used than with other methods, but the vapour has to be introduced
at a controlled rate, otherwise the turbulence will cause mixing and prevent distinct layers from forming. This method can
be used for all types of tanks but is most efficient for those with a simple internal structure.
•
Turbulence (Dilution or Mixing)
Large volumes of inerting or purging gas are blown into the tanks and mixed with the existing vapour. The inerting or
purging gas should be blown in vigorously to reduce the possibility of isolated pockets of vapour remaining undisturbed.
•
Vacuum/Pressure
Warning: This is now an uncommon procedure which can result in a total low-pressure shutdown, making
renewed start-up a very complex process. It should only be employed with the utmost care.
The method is used with pressure or semi-pressure type cargo tanks. A vacuum is created in the tanks (within design
limitations) using the ship's compressors. Inerting or purging gas is then admitted until a positive pressure is achieved. The
process is repeated until the required inert or purge gas concentration is reached. In order to assist the process, the original
gas in the tank should be reduced to a minimum and the inerting or purging gas should be introduced rapidly at several
points to give maximum mixing.
29 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 30
4.7 PREPARATION FOR LOADING, INCLUDING COOLDOWN
4.7.1
General
After a tank has been inerted or purged as necessary, it has to be correctly prepared for loading in order to prevent
uncontrolled pressure rises and unsafe temperature gradients developing during the initial stages of loading.
Most cargoes have sub-zero boiling points at atmospheric pressure (butane -5C, ammonia -33C, propane -42C, ethylene -
104'C, methane -163C). These boiling points are correspondingly higher at higher pressures (see data sheets). Cargo liquid
entering cargo tanks and piping which are at ambient temperature and pressure will therefore immediately begin to boil, and
its temperature will correspond to the boiling point at atmospheric pressure. Boiling and evaporation will continue to result
from heat transfer into the liquid until the structure reaches the liquid temperature.
This initial boiling will cause a rapid pressure rise in the cargo system. The pressure attained will depend on the quantity of
liquid, the heat available for evaporation and the capacity of the vapour return line or vent stack. The ship's reliquefaction
plant cannot condense vapour until very nearly all the incondensible inert gas has been removed. If the rate of vapour
generation exceeds the vapour return capacity, the pressure will increase quickly and may exceed the relief valve setting,
causing venting through the vent mast. Care should therefore be taken to introduce cargo liquid into warm cargo tanks
sufficiently slowly to avoid an alarming pressure rise and uncontrolled venting.
The initial boiling will also cause local cooling of the tank structure, resulting in thermal stresses. Great care should
therefore be taken during cargo operations to avoid undue thermal stress or shock, particularly in the case of low boiling
point cargoes.
4.7.2
Cooldown
Cooldown of tanks and pipelines is undertaken to control thermal stresses, and loading rates should be restricted during
cooldown. If tanks are fitted with spray equipment it should be used, and the liquid distributed around the inside of the tank
as evenly as possible to avoid thermal stresses. Spray cooling is essential for very cold cargoes such as ethylene or LNG.
Certain restrictions on the rate of cooldown may also apply to LPG carriers. Pressure build-up in the tanks will restrict the
rate at which liquid is introduced. The use of a vapour return line is recommended to avoid cooldown and loading rates
being dictated by the capacity of the reliquefaction plant.
Cargo pipework and equipment should be cooled down by circulating liquid at a controlled rate. The system should reach
liquid temperature sufficiently slowly to prevent undue thermal stresses in materials or expansion/contraction fittings. The
liquid used can come from the shore, shipboard storage vessels or cargo tanks. The temperature sensors will indicate when
liquid is present on the tank bottom, but the liquid should be introduced slowly until the bottom is completely covered.
The cooldown of tanks may cause a pressure reduction in sealed hold or interbarrier spaces, and dry air, inert gas or dry
nitrogen should be introduced in order to maintain a positive pressure. This is usually done by automatic equipment.
Pressure gauges should be observed regularly during cooldown to ensure that acceptable pressures are maintained.
4.7.3
Ice or Hydrate Formation
Ice or hydrates may form during cooldown if moisture is present in the tank atmosphere.
During cooldown, valves should be operated frequently to ensure that they are free; where practicable, pump shafts should
be turned manually at intervals.
LPG from pressure storage at above O'C may contain water. If used for cooldown before loading LPG, ice or hydrates can
form where expansion occurs. Water can be prevented from freezing by dosing with anti-freeze (see paragraph 1.4.1). Anti
freeze should only be used with the approval of the shipper.
30 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 31
4.7.4
Minimum Cargo Tank Temperature
Some cargo tanks, such as fully pressurised tanks and some sen-d-pressurised tanks, have a minimum allowed cargo tank
temperature, which is higher than the boiling point for one or more of the products which the ship is certified to carry.
In order to avoid cooling such tanks below their limit during loading, they must be pressurised until the corresponding
liquid temperature is above the minimum permissible tank temperature.
On completion of discharge any remaining liquid must be thoroughly stripped before the tank vapours are evacuated, in
order to prevent the temperature of any remaining liquid from dropping below the minimum permissible tank temperature.
4.8 CARGO LOADING
Before cargo is loaded, a Pre-arrival and Ship/Shore Checklist should be completed (see paragraph 4.5.1), and the
responsible officer must be satisfied that the cargo system is in all respects ready. Loading must not commence until
information on the cargo has been obtained (see Section 2.2).
During loading, cargo is transferred from shore through the appropriate midships or stern manifolds, and led into the cargo
tanks via the filling lines, which usually terminate close to the tank bottoms. If the tank has not been cooled down it is
normal to by-pass some of the incoming liquid through the tank sprays, if fitted, to reduce the temperature gradient from
tank top to bottom, and to even out the rate of boil-off. The loading rate is determined by the rate of change of the tank
pressure.
As the liquid level in the tank rises, the tank pressure is increased by.
•
vapour pressure of the 'warm' cargo;
•
vapour displaced by the incoming liquid;
•
vapour generated by heat transfer through the tank walls to the liquid; and
•
vapour generated by heat transfer from the ship and shore pipelines and the shore pumps.
On fully or semi-pressurised ships the vapour pressure increase during loading can be reduced by spray loading, provided
the cargo temperature will give a saturation pressure safely below the relief valve set pressure. With fully or semi
pressurised tanks, the boil-off and displaced vapour is either returned to shore or condensed by the ship's reliquefaction
plant. Venting during loading must be avoided. In the case of LNG the boil-off cannot normally be condensed and the ship
will be dependent on full vapour return to shore.
The responsible officer should ensure that the following precautions are observed, in addition to those set out in Section 4.5:
•
In the event of an emergency, the emergency shutdown procedures should be implemented.
•
All fixed gas detection equipment should be operated throughout all loading operations.
•
During the early stages of loading, the incoming liquid may be relatively warm and generate quantities of vapour in excess
of capacity of the reliquefaction plant or vapour return line. The tank pressure should be regularly observed during loading
and the loading rate reduced in good time before approaching safety valve set pressures.
If reducing the loading rate does not reduce the pressure rise, loading should cease immediately, and the terminal should be
notified to enable proper steps to be taken in the event of hazard to the adjacent shore areas.
If venting occurs it will cause self-refrigeration, thus reducing the cargo temperature and pressure.
•
Filling of the cargo tanks may cause a significant loss of pressure in the hold or interbarrier spaces, depending upon the
cargo system design. This should be continuously monitored and pressure maintained by the addition of supplementary
inert gas, dry air or dry nitrogen.
31 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 32
•
Filling limit regulations should be observed. The maximum liquid level in each tank should be calculated before or during
the early stages of loading.
•
If large quantities of vapour are being generated (i.e. the cargo liquid is boiling rapidly) the bubbles created will increase the
liquid volume. To measure tank contents accurately under such circumstances it is recommended that vapour removal
should be reduced temporarily to allow the liquid level to stabilise.
•
In all tanks, whether or not being loaded, the liquid level and pressure readings should be monitored throughout loading. A
reading which does not change as expected may indicate a fault which should be investigated.
•
When liquid flow is diverted from one tank to another the valves on the tank about to receive cargo should be fully opened
before those on the tank being isolated are shut.
•
On completion of loading all ship's lines should be drained into the cargo tanks using the facilities provided. Liquid in
hoses or loading arms should also be drained to the cargo tanks, if possible, or blown to shore and pressed past shore valves
by vapour pressure. If possible the ship-shore connection should be purged before being disconnected. Ship-shore
connections should not be broken until it has been ascertained that all liquid has been removed and the lines are
depressurised. Adjacent isolating valves on ship and shore and any other relevant valves should then be closed before
connections are broken.
•
Bonding wires, if fitted, should not be disconnected until after the hoses have been disconnected.
•
The relief valves of some ships have dual or multiple settings, either for operational purposes or to meet differences in
national regulations. Changes to the relief valve setting should be carried out in accordance with the procedures specified
and under the supervision of the master. Changes should be recorded in the ship's log and a sign posted at the relief valve
and in the cargo control room, if provided, stating the set pressure.
• It is normally necessary to deballast during loading and the precautions set out in Section 4.12 should be observed.
4.9 CARGO CONDITIONING
4.9.1
General
The term 'cargo conditioning' refers to the care and attention given to the cargo on passage to ensure that:
•
there are no undue losses in cargo quantity;
•
cargo tank pressures are kept within design limits; and
•
cargo temperature is maintained or adjusted as required.
These aims are achieved either by reliquefaction or, on most LNG ships, by using boil-off as propulsion fuel. Cargo
conditioning may not be necessary on ships with pressure vessel tanks.
If reliquefaction plant is fitted the responsible personnel should have a thorough understanding of its operational principles.
When running, the plant should be monitored so that anything which might adversely affect its safety or efficiency is
quickly recognised and corrective action taken. Plant is normally fitted with shutdown devices to sense high liquid level,
temperature or pressure.
4.9.2
Reliquefaction and Boil-off Control
General guidance on safe procedures for reliquefaction and boil-off control is given below. The detailed instructions for
any ship depend upon the system fitted, and manufacturers' operating instructions should be closely followed. Procedures
and precautions for individual components in the system are given in Appendices 5 and 6.
There are several different types of reliquefaction system and these are discussed in Appendix 3. The most common
involves compressing the cargo vapour and condensing it in a seawatercooled condenser. Alternatively the condenser may
be cooled by a refrigerant from a secondary refrigerating unit (cascade-type refrigeration). Another type of reliquefaction is
achieved by circulation of the refrigerant through coils inside the tank or through a separate heat exchanger outside the tank
(indirect cooling). Cargo-incompatible refrigerants should not be used, nor refrigerants which are known to have a high
ozone depleting potential.
32 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 33
The vapour of certain cargoes (e.g. ethylene oxide, propylene oxide) cannot be compressed. Such cargoes can only be
refrigerated by indirect cooling and cargo compressors usually have to be isolated or blanked off.
LPG is normally reliquefied by direct compression and condensation in one or two stages, with condensation against water,
in what is called a direct rehquefaction system (see Appendix 3 Section 6.2 and 6.3). Colder cargoes such as ethylene,
although still requiring direct compression, require a cascade system with the cargo condensing against a secondary
refrigerant, which is condensed using water as the coolant (see Appendix 3 Section 6.4).
A reliquefaction plant is not normally fitted to LNG carriers. Instead the boil-off is used as fuel for main propulsion
machinery. During ballast passages the tanks are kept cold using cargo deliberately retained on board: this cargo is known
as a 'heel'. Boil-off from the heel is also used for propulsion during the ballast voyage. The retention of a heel requires
consideration of sloshing loads (see paragraph 1.8.7): care has to be taken to ensure that the retained liquid is properly
distributed. A heel is often also retained on board fully refrigerated or semirefrigerated LPG carriers to enable the tanks to
be kept cold on the ballast voyage. As LPG boiloff is heavier than air, regulations do not permit it to be used as propulsion
fuel and it is therefore reliquefied and returned to the tanks. Return should be by the spray line, if fitted, for best cooling
efficiency.
The specific operating instructions for the system fitted should be observed in addition to the following precautions:
•
The purpose of the rehquefaction system is to prevent loss of cargo and ensure that the cargo liquid is either kept at the
loading temperature or is at the temperature required for discharge on arrival. In the latter case it may be necessary either to
cool or to warm up the bulk liquid on passage. If the system is used only to keep cargo tank pressure just below the relief
valve set point, the cargo will warm up to a new temperature and it may be too hot for discharge at the terminal. If it is
necessary to cool down the liquid on passage, the loading temperature and system capacity should be assessed to ensure that
the necessary operations can be completed during the voyage.
•
If two or more cargoes are carried simultaneously, they should be segregated throughout all cargo operations. Particular
care is required with incompatible cargoes (see Section 4.13).
•
Gas detection equipment in spaces containing reliquefaction plant, instrumentation and controls should always be activated.
Upper and lower sample points (if fitted) should be selected according to the relative vapour density of the cargo (see data
sheets).
•
Ventilation equipment for the reliquefaction plant space should be started well in advance of activating the plant.
•
Filters on the suction side of compressors should be checked and carefully cleaned if necessary. If they are blocked the
efficiency of the plant may be reduced drastically.
•
The lubricants used for all machines should be compatible with the cargo and suitable for the temperatures and pressures
experienced both in operation and when stopped. Oil levels should be checked and crankcase heaters started if necessary
before the plant is activated.
•
All plant, instrumentation, control and shutdown equipment should be tested on a regular basis.
•
The precautions on ice or hydrate formation, reactivity and cargo contamination should be observed (see Sections 1.4 and
4.13).
•
All pipelines and valves should be double-checked to ensure that they are correctly set before starting the plant.
•
To prevent overheating, the cooling water supply to condensers should be established and the refrigerant system (where
fitted) started before cargo compressors are run.
•
Cargo compressors should never be operated with discharge valves shut.
•
Sub-atmospheric pressures should normally be avoided in any part of the system to prevent the ingress of air. Flammable
vapour/air mixtures should never be passed through cargo compressors.
•
Refrigerant or cargo vapour compressors should be started and suction valves opened very slowly to prevent damage from
liquid carry-over.
•
If the capacity of cargo or refrigerant compressors is controlled manually, plant should be started on the minimum setting
and the capacity increased gradually as necessary.
33 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 34
•
Operation of the reliquefaction plant will be affected by any incondensible gases in the vapour drawn from the cargo tanks.
These incondensibles may originate from the cargo itself (e.g. ethane, methane) or may be inert gas remaining from
previous purging. Incondensibles will cause abnormally high condenser pressure and will reduce condensation of the cargo
vapour. To re-establish full condensation the incondensibles have to be vented regularly. Problems with incondensibles
mainly arise during the early stages of reliquefaction. Reliquefaction plant liquid levels should be checked regularly during
operation to prevent overfilling of receivers or condensers, which may be caused by sticking control valves or expansion
valves. It is desirable to keep comprehensive records so that any unexpected changes can be quickly noticed and remedial
action taken.
•
Care should be taken to prevent liquid cargo from entering compressors, particularly if liquid separation equipment is not
fitted. In heavy weather this could be a significant problem which may require shutdown of compressors. Under certain
conditions liquid entrainment may also occur during spray cooling of the tanks. Liquid entrainment in the vapour may
cause severe mechanical damage to compressors.
•
If condensate is returned to more than one tank simultaneously, or if vapour is taken from several tanks and is returned to a
single tank, the liquid levels should be checked regularly and remedial action taken to avoid possible overfilling.
4.9.3
Use of Cargo as Fuel
Boil-off from LNG cargo may be burnt as fuel in the main propulsion system. Two factors influence the sanctioning of this
practice:
•
LNG vapour, being mainly methane, is lighter than air at ambient temperatures. It is therefore safe to be used because if it
were to leak into the machinery space it would escape through exhaust vents and not accumulate within the machinery
space. Consequently LNG is the only cargo vapour allowed to be used as fuel.
•
Reliquefaction of LNG would require a complex refrigeration cycle requiring considerable power, and such equipment is
rarely fitted.
It is possible to burn LNG vapour in boilers, diesel engines or gas turbines. In each case cargo vapour is introduced into a
space from which it is normally excluded, and the design of the cargo vapour-to-fuel system is therefore subject to strict
requirements. It is vital to ensure that the integrity of the system is not impaired in any way.
LNG boil-off may be either vented or burnt (or both) to keep tank pressures at the required level. The decision whether to
vent or burn the boil-off depends on many factors, some economic, some the result of regulations. Regulations may, for
instance, either prohibit venting or the use of cargo as fuel in certain places. Such regulations should always be observed.
[Note: Attention should also be paid to Chapter 16 of the IGC Code, Regulation II-2/15.1 of the SOLAS Convention, IMO
recommendations concerning the use of low flashpoint cargoes as fuel e.g. IMO Resolution A565(14), and to classification
requirements.]
On the high seas, cargo vapour may provide the main fuel, though oil pilot burners are also required. In the case of steam
plants, cargo vapour may also be burnt when propulsion machinery is not in operation provided that means for steam
dumping are installed.
Boil-off is normally heated and pressurised before delivery to the machinery space; it is sometimes odourised as well. The
pressure of the vapour is boosted to promote stable and efficient combustion. It is heated so that conventional steel can be
used in the system, fuel economy is increased and any vapour which may leak from the system will more readily rise and
escape.
The following precautions should be observed:
•
Personnel should fully understand the system, its limitations, maintenance requirements and the danger of cargo leakage.
The system should be kept clean and efficient and machinery performance logged so that changes can be identified.
•
Ventilation fans for the machinery space and the fuel supply line trunking should be operated before and during gas burning
operations. Attention should be paid to the ventilation of any areas near untrunked gas piping.
34 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 35
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Gas detection equipment for the system should be working throughout burning operations
Supply lines should be purged with inert gas immediately before and after burning operations.
All operating instructions for the system should be observed. Safety equipment (such as interlocks) should not be
overridden.
If the gas flame goes out, the reason should be established before it is relit. If both oil and gas flames are lost all
combustion spaces should be ventilated of flammable vapour before the flame is relit, otherwise an explosion could occur.
Attention should be paid to flame failure sensors; low sensitivity will result in failure to shut down and high sensitivity will
cause unnecessary shutdowns.
Cargo tank pressures should be monitored during all burning operations: if boil-off is removed too fast, the pressure could
be reduced below atmospheric and air drawn into the tank, creating a flammable mixture. Cargo tank pressures should be
maintained above atmospheric at all times.
Care should be taken to prevent liquid cargo from entering compressors, especially if liquid separators are not fitted. Rapid
changes in supply pressures should be avoided, otherwise the flame will not be stable.
The gas supply lines should be checked regularly for leaks. If a leak does occur, the fuel supply should be isolated
immediately and not reconnected until the leak has been repaired.
No modification whatsoever should be made to the system without the permission of a responsible authority.
Reference should be made to Appendices 5 and 6 for safety precautions specific to instrumentation or items of plant.
All joints in the supply line should be pressure tested after maintenance before the system is recommissioned.
Water should be drained from carbon steel fuel lines to prevent corrosion.
Flame screens may be fitted in the supply line or within each burner: they have very small holes which are easily blocked,
and should be cleaned regularly.
The gas heaters should be regularly checked to ensure that no leakage occurs between the gas and steam systems. Steam
condensate has to be returned to the feed water system via a ventilated drain tank: the water level in these tanks should be
maintained and vents checked periodically for blockage which could cause gas to enter the feed system.
Gas booster compressors should be carefully maintained and attention paid to the condition of shaft seals.
All incidents, however trivial they may seem, should be recorded and brought to the attention of the responsible officer.
4.10 CARGO DISCHARGE
The general precautions in Section 4.5 should be observed during cargo discharge, and particular attention should be paid to
cargo equipment (e.g. pumps, compressors, vaporisers). If the cargo is to be discharged by pumping and the shore cannot
accept the full pump capacity, flow should be reduced by shutting down pumps or recirculating rather than by throttling, as
throttling will tend to heat the cargo.
Pressure-vessel ships may discharge cargo by displacement (i.e. by increasing the pressure above the liquid with the
compressors) instead of or in addition to the use of pumps.
The following precautions should be observed in addition to those in Section 4.5:
•
•
•
•
Fixed gas detection equipment should be working throughout cargo discharge.
If a submerged electrical pump is fitted, the insulation reading should be checked before starting up.
Cargo pumps are normally started with the discharge valve shut or fractionally open to reduce both the starting load and
pressure surge. It may also be necessary to recirculate to adjust pressures and cool deck lines. Pump manufacturers'
instructions and ship's operating manuals should be consulted. Booster pumps should be circulated with cargo from the
main cargo pumps and should not be started until there is sufficient liquid supply to prevent cavitation.
Pressures in the liquid lines will be considerably higher during discharge than during loading. Joints and glands are
therefore more likely to leak during this operation and should be inspected frequently.
35 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 36
•
•
•
•
•
•
•
The tank pressure will tend to fall as cargo is removed. If the discharge rate is high there may be insufficient boil-off to
maintain positive pressure in the tank, and vapour should be added to prevent a vacuum. The vapour may come from the
shore, or can be generated aboard by diverting some cargo liquid to a vaporiser. Tank pressures should be monitored
throughout the discharge.
Discharge can cause pressure changes in the hold or interbarrier spaces, the rate of change depending upon the cargo system
design. Pressures in such spaces should be watched during discharge and any necessary action taken.
All cargo tank level readings should be watched, whether or not their cargo is being discharged. Any reading that does not
change as expected may indicate a fault that needs to be investigated. Care should be taken to ensure that cargo pumps do
not cavitate: it is normal to close the discharge valve gradually when a tank is nearly empty both to prevent cavitation and to
assist in pumping out the maximum quantity of cargo. It is dangerous to run pumps dry as the cargo liquid provides the
required lubrication and cooling for bearings, seals, glands, etc.
In the event of any emergency, the emergency shutdown procedures should be implemented.
On completion of discharge, liquid lines and cargo hoses or loading arms should be drained, purged and depressurised using
the facilities provided. The isolating valves should then be closed and the ship-shore connections can then be broken.
Bonding wires, if fitted, should not be disconnected until after the hoses have been disconnected.
See Section 4.13 for precautions when two or more cargoes are transferred simultaneously and Section 4.18 for sampling
procedures.
The ship's stability should be carefully checked at all stages during the discharge and necessary precautions taken (see
Section 4.12).
4.11 CARGO TRANSFER BETWEEN VESSELS
If cargo is to be transferred from one ship to another, or to a barge, the precautions above should be observed (see Sections
4.5, 4.8 and 4. 1 0). In addition, the relevant precautions in the ICS/ OCIMF publication 'Ship to Ship Transfer Guide
(Liquefied Cases)' should be closely observed.
Before starting transfer operations the two masters involved should agree on every aspect of the transfer procedure and
appoint a person in overall charge. Transfer operations between gas carriers should be carried out in accordance with the
requirements of the receiving vessel.
In all cases, however, each master remains fully responsible for the safety of his own ship, its crew and cargo, and must not
permit safety to be prejudiced by the actions of the other master concerned.
Transfer operations should only be carried out in favourable weather conditions and should not begin until the master or
responsible officer of each vessel is satisfied that the situation is safe. A safety checklist should be used prior to
commencing operations and, in the event of subsequent stoppages, a further check should be made before resuming
operations.
During operations the maximum transfer rate must be consistent with the receiving vessel's reliquefaction capacity.
Alternatively, a vapour return hose connection should be made to the discharging vessel.
In the case of ship to barge transfers the following additional precautions should be taken:
•
•
•
•
•
before transfer begins the person in overall charge should be satisfied that the barge personnel are fully conversant with the
nature of the hazards presented by the cargo being transferred and with the necessary safety precautions;
moorings should be of such a nature that the barge can be quickly released in an emergency;
the rate of transfer should be controlled according to the nature and size of the barge;
operations should be stopped immediately if the barge fails to comply with the safety requirements in any respect;
the barge should be requested to move from alongside as soon as possible after completion of loading or discharge.
36 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 37
4.12 BALLASTING/DE-BALLASTING
Depending upon ship design, it may be necessary to undertake loading/de-ballasting or discharging/ballasting operations
simultaneously. If this is the case, consideration should be given to the stability of the ship, especially to free surface effect
in tanks, correct use of cargo tank centreline bulkhead valves, and cargo and ballast distribution to ensure adequate stability.
Care should also be taken to ensure that the weight distribution does not lead to excessive trim, list or stress in transverse
and longitudinal directions.
Concern about the introduction of alien organisms into environmentally sensitive waters and adjacent areas has prompted
some national administrations to establish controls on the discharge of ballast water from ships. If it is necessary to change
ballast at sea, the same care and attention must be paid to trim, stress and stability.
4.13 SEGREGATION OF CARGO
When common pipeline systems are provided for various cargo-related operations, contamination will occur when different
grades of cargo are carried simultaneously. If segregation is needed to avoid cargo contamination, shippers' instructions and
regulatory requirements must be observed. If a common piping system has to be used for different cargoes, great care
should be taken to ensure complete drainage and drying of the piping system before purging with new cargo.
Wherever possible separate reliquefaction systems should be used for each cargo. However, if there is a danger of chemical
reaction, it is necessary to use completely segregated systems, known as positive segregation, at all times, utilising
removable spool pieces or pipe sections. This restriction should apply equally to liquid, vapour and vent lines as
appropriate. Whilst positive segregation may be acceptable for most cargoes, some substances may require totally
independent piping systems. Special treatment of certain cargoes is specified in the relevant IMO Gas Carrier Code.
If there is any doubt about the reactivity or compatibility of two cargoes, the data sheets for each cargo and a cargo
compatibility chart should be checked and advice sought from shippers or other authority. If this advice seems
inconclusive, the cargoes should be treated as incompatible and positive segregation provided.
The following precautions should be observed:
•
•
•
•
•
Where codes and regulations call for segregation, the position of the valves, blanks, portable bends and spool pieces
associated with such segregation should be carefully arranged and clearly identified. These arrangements for segregation
must be followed as part of the approved system.
If the cargoes to be carried are not compatible, the responsible officer should ensure that the pipeline systems for each cargo
are completely isolated from each other This entails checking that all necessary blanks are fitted or that pipe spool pieces
have been removed. A cargo log book entry should be made of the action taken.
In cases where two cargoes are compatible and an apparent negligible mix is permitted, the adjacent systems carrying the
different cargoes should be isolated by at least two valves at each connection, or by one positive visible blank.
Common pipelines and associated equipment should be drained, dried, ventilated and monitored before being used for
another cargo.
All temporary pipework should be gas-freed, monitored, disconnected and properly stored when not in use.
4.14 CHANGING CARGOES
4.14.1
Cargo Stripping
Before changing cargoes or gas-freeing it is most important to remove all cargo liquid from tanks, piping, reliquefaction
plant and any other part of the cargo system. Any remaining cargo liquid will continue to give off vapour and will frustrate
subsequent purging or gas-freeing.
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If previous and subsequent cargoes are similar in chemical properties - e.g. propane and butane purging may not be required
(subject to shippers' considerations), but even in such cases it is normally required that no previous cargo liquid remains.
Shippers' instructions regarding purging requirements should always be sought.
To achieve maximum drainage of liquid during discharge, the following advice should be followed:
Careful trimming or listing of the ship can, depending on the design of a tank, assist drainage of liquid.
If pumps are used for discharge, the pump discharge valve should be throttled towards completion of discharge to maintain
suction to minimum liquid level. Manufacturers' instructions should be consulted as to the liquid level at which throttling
should be started and the pump pressure that has to be maintained during later stages of pumping to obtain maximum
stripping. Each pump should be kept under continuous control during stripping to obtain the best results without pumps
running dry.
Even with good operation of cargo pumps, some liquid will remain in the tanks at termination of pumping. In the case of
ships whose cargo tanks can accept overpressure, further stripping of liquid may be achieved by increasing tank pressure
sufficiently to press out the liquid through the piping system ashore. Alternatively, all strippings may be collected in one of
the tanks for subsequent discharge ashore. The use of cargo compressors, taking suction from other tanks, will ensure that
all tanks and associated piping systems are left liquid-free. Proper stripping of tanks should be checked by the bottom
sampling line or temperature sensors.
In the case of ships with cargo tanks designed for pressures only slightly above atmospheric (fully refrigerated ships),
stripping by pressure alone is not possible. On such ships (and on ships with pressure tanks, if pressure stripping is not
successful) the remaining liquid should be boiled off by introducing hot vapour from the cargo compressors to the bottom of
the tanks, through puddle heat coils (if fitted). During such operations the tank pressure must be closely observed, to avoid
exceeding the relief valve set pressure. When pressure has increased to a safe level below the relief valve pressure, the
cycle is reversed by starting compressor suction from the tank, reliquefying the vapour in the condenser, and discharging the
condensate to shore or retaining on board in a deck pressure vessel. Alternatively, if the ship is at sea the vapour may be
vented instead of being reliquefied.
Provided the temperature of the liquid remaining in the tanks is above the saturation temperature corresponding to
atmospheric pressure, the liquid may also be boiled off by using the compressor to draw off gases from the tank
(maintaining the tank pressure at atmospheric pressure), instead of using hot gas. This is known as the vacuum method.
The quantity of liquid that can be removed by this method is limited, however, as the boiling will soon sub-cool the liquid
and no further evaporation will take place. The presence of subcooled liquid still remaining may be difficult to establish, as
there will be insufficient tank pressure to detect it using the bottom sampling line. It will take some time before the liquid
picks up sufficient heat from the surrounding tank structure to start boiling again and raise the pressure in the tank.
Evaporation of the remaining liquid by means of hot gas is therefore recommended rather than the vacuum method.
(Note: the sub-cooled temperature should not fall below the temperature assigned to the tank (see paragraph 4.7.4). To
ensure that this does not occur tank bottom temperatures should be closely observed.)
Some ships are fitted with heating coils in the tank bottom to evaporate liquid residues. The heating medium is hot cargo
vapour for internal coils or may be thermal heating oil for coils fitted externally to the tank. Vapour circulating coils should
be purged either with inert gas, or with vapour from the subsequent cargo if it is compatible with the previous cargo.
Similar precautions should be taken with cargo compressors.
Liquid is removed from the piping system and equipment by blowing through with vapour (see above). Hot gas from the
compressors passed through the liquid lines will provide heat to evaporate liquid not removed by pressure displacement. In
cold weather and in insulated pipelines, liquid butane, butadiene etc. may evaporate very slowly even at atmospheric
pressure.
It may be necessary to change compressor lubricating oils when changing cargoes (thecompressor manufacturer's
instructions should be observed).
38 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 39
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4.14.2
Change of Cargo
When all remaining liquid from the previous cargo has been removed from the system, a slight overpressure should be
maintained to prevent the entry of air until the preparation necessary for the next cargo is established.
When preparing for the next cargo the possibility of a chemical reaction with the previous cargo should be checked by
referring to the data sheets. In addition care must be taken to avoid contaminating the new cargo. Many products are
subject to very strict specifications for commercial reasons, and vapour from the previous cargo, even if compatible with the
new product, may be sufficient to cause unacceptable contamination. Shippers' instructions should be obtained as far in
advance of loading as possible.
In some rare cases the tanks have to be inspected internally before loading, requiring gas-freeing and ventilation with air
before arrival. The precautions in Section 6.3 should be observed before tank entry. Before loading, tanks should be
inerted and purged again as necessary (see Section 4.6).
4.14.3
Displacing Atmosphere with Inert Gas (Inerting)
Vapours from the last cargo in the system are displaced by inert gas from the ship's inert gas generator, or by pure nitrogen
from shore. If the ship's inert gas is used, the cargo piping system from the tank should be opened to the vent before the
inert gas supply is connected as an additional precaution against the possible backflow of flammable vapour to the
generator.
Regulations regarding venting of cargo vapour in port should be observed. Such regulations may require that vented cargo
vapours should be led to a flare or vent stack ashore.
Inerting is continued until the required dew point or concentration of cargo vapour or oxygen level has been reached.
4.14.4
Displacing with Vapour of the Next Cargo (Purging)
If the two cargoes are compatible, in terms of both chemical reaction and shippers' requirements, vapour from the previous
cargo may be replaced (purged) directly by vapour from the next cargo, from shipboard storage vessels or from shore (see
paragraph 4.6.3).
If purging is carried out in port, local regulations may require the expelled vapours to be led ashore for safe disposal, either
to a vent or flare stack or for use in the shore plant.
In all cases advance notification should be given to the port authorities and permission obtained before starting the
operation. Purging should be carried out in the most suitable way (see paragraph 4.6.4) until acceptable conditions for the
next cargo are reached.
4.14.5
Water Washing after Ammonia Cargoes
Ammonia vapour is normally removed from the tanks at sea by introducing large amounts of air and ventilating to
atmosphere. However, the removal of all traces of ammonia by ventilation alone is a lengthy process.
If desirable, remaining traces of ammonia may be removed by water washing or water sweeping. Ammonia is extremely
soluble (one volume of water dissolves up to 1000 volumes of ammonia vapour), and the introduction of water into tanks
containing high concentrations of ammonia may immediately cause dangerous vacuum conditions unless unrestricted access
of air is provided. Ship's inert gas containing C02 should never be used for purging after ammonia cargoes as carbamates will
be formed which may block the cargo pipe lines (see paragraph 4.6.1).
If water washing, the following precautions should be taken.
Personnel should wear breathing apparatus and protective clothing as necessary.
Low ammonia concentrations must be achieved before water washing. All tank manhole covers should be opened up to
provide unrestricted access of air and prevent dangerous vacuum conditions which may cause the tank to collapse.
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Fresh water should be used, because sea water will leave deposits which are difficult to remove and will considerably
increase rust formation on steel surfaces.
Tanks should not be washed if they contain submersible pumps unsuitable for water immersion. Even if water is not
harmful to the cargo pumps, separate portable pumps should preferably be used for removal of washing water from the
tanks. If the use of cargo pumps is proposed, the possibility of over-loading pump motors when pumping water (SC = 1)
should be considered; pump manufacturers' instructions should be consulted.
After pumping out as much water as possible, any remaining water on the tank bottom should be wiped up and the tanks
dried out by ventilation before being closed. For water washing to be successful it is essential to dry the tank and cargo
lines with dry air from the inert gas system. In conditions of high relative humidity ventilation with warm air may be
necessary. If cargo pumps have been used for pumping washing water, they should be carefully dried out by ventilation and
treated with anti-freeze. Excess anti-freeze collecting at the tank bottom should be wiped up, as it may not be acceptable for
the next cargo.
The discharge overboard of ammonia washings may be prohibited in certain areas, and care should therefore be taken. The
requirements for the control of pollution in Annex 11 of the MARPOL 73/78 Convention should be observed. If discharged
overboard, ammonia contaminated washings should not be allowed to enter the ship's seawater intakes because ammonia is
corrosive to copper-based alloys in the seawater system.
4.15 GAS-FREEING
To gas free a cargo tank the following procedures should be followed:
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Remove any remaining cargo liquid (see paragraph 4.14.1). Pressure should be released with caution.
If the tank temperature is near cargo saturation temperature at atmospheric pressure, the tank atmosphere should be warmed
up by circulating hot gas. This is a very time-consuming operation and it is essential that all the steel in the tank has
reached a temperature above dew point. It will assist in evaporating any remaining liquid (including condensation on tank
structures) and will reduce the quantity of inert gas required (see paragraph 4.6.2).
Remove the cargo vapour in the system with inert gas (see Section 4.6). This stage may be omitted when gas-freeing after
the carriage of ammonia (see paragraph 4.14.5).
Alter inerting the system to a safe cargo vapour concentration (see data sheets), it may be necessary to ventilate the system
with air to provide safe access for inspection or repairs. Venting with air should be continued until an oxygen content of
21% by volume is obtained. Samples should be taken at various levels, and sampling repeated some time after the first
acceptable readings are obtained, to allow possible pockets of inert gas to mix with the air, and the consequent reduction in
oxygen content to be detected before tanks are entered.
When a tank and associated pipelines have been certified gas-free maintenance work may take place. The precautions set
out in Sections 3.5 and 6.3 and in Appendices 5 and 6 should be observed.
4.16 VENTING AT SEA
In favourable ambient temperature and wind conditions venting may take place at sea. Rapid dispersion and dilution of toxic
or flammable cargo vapour vented to the atmosphere is essential to safety. The critical factor is the ability to disperse
potentially high concentrations of vented vapour. Such vapour needs to be diluted many times to bring its concentration
below the Lower Flammable Limit, and even greater dilution may be necessary if inhalation of vapour could cause danger to
personnel. Furthermore, the density of the escaping vapour mixture may be much greater than that of air and consequently
the vapour mixture will tend to sink and form a layer across the deck.
Wind speed plays an important part in the dispersion of vapour It may be necessary to suspend operations in still air
conditions (see paragraph 2.8.1). Wind direction relative to the ship's course is also important. Eddy currents may cause
pockets of high gas concentrations to form in the lee of superstructures. Alterations to the ship's course and speed should be
made to assist dispersal of cargo vapour.
40 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 41
Cold cargo vapour will freeze out moisture in the air in the venting system and in the vicinity of the vent outlets.
On ships designed for carrying soluble toxic gases such as ammonia a scrubber is sometimes fitted to reduce the quantity of
vapour emitted. If such a system is fitted it should be used.
4.17 DECK STORAGE TANKS
Some ships have dedicated storage tanks on deck for cargo liquid which can be used to purge cargo tanks at sea. Normally
these tanks are pressure tanks designed for containinent of the liquid gas at ambient temperature.
The following precautions should be observed for pressure storage tanks on deck:
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Filling limits should be heeded, particularly when handling cold liquid which will expand as its temperature increases
towards ambient.
If storage tanks contain gas different from that carried in the cargo tanks, the storage tanks should be completely segregated
from the cargo system by the removal of relevant spool pieces to prevent cargo contamination.
For gas-freeing and access, the precautions set out in Sections 4.15 and 6.3 should be observed. When not in use, storage
tanks should be gas-freed and preferably filled with clean, dry inert gas to prevent corrosion. They should be completely
segregated from the cargo system by removal of relevant spool pieces and fitting of blank flanges.
4.18 SAMPLING
Cargo is normally sampled by shippers' or receivers' personnel, or by authorised petroleum inspectors. The responsible
officer should make proper records of the samples taken as they may subsequently be of considerable value. A good rule is
to request that samples be taken from the liquid shore connections at the start of loading to safeguard against possible
contamination from shore transfer lines.
The following precautions should be taken when sampling cargo liquid or vapour.
4.18.1 Liquid Samples
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The responsible officer should be present when any cargo sampling is carried out. He should be fully conversant with all
aspects of the ship's sampling system including the operational characteristics of all valves. He should clearly recognise
that the responsibility rests entirely with him for ensuring that sampling operations are conducted in a safe and efficient
manner which will preclude any escape of cargo liquid or vapours to the atmosphere beyond that required by the sampling
process, whoever is performing the actual sampling operation.
The responsible officer should satisfy himself that the sampling equipment is compatible with the ship's sampling points
before starting any sampling operation. If the two are incompatible for any reason he should ensure that any action taken to
rectify the situation does not impair the gas tight integrity of any part of the ship's system or endanger life or property.
Sample containers should be completely clean and compatible with the cargo to be sampled. They should be of a
recognised standard and able to withstand the extremes of temperature and pressure anticipated.
Sample containers should be purged with nitrogen before use.
If the sample is to be representative its container has to be purged thoroughly with cargo from the sampling connection.
Sufficient cargo should be passed through the container to cool it down to liquid temperature. If cargo is a mixture (which
is often the case) the most volatile components will evaporate more rapidly than the heavier fractions as the container is
cooled down. This will leave the sample with a higher concentration of the heavier fractions than is present in the cargo,
and it will therefore be unrepresentative. To counteract this effect, sample containers should be turned with the vent valve
downwards during cooldown, to drain off any liquid which collects. For the same reason, samples taken from the bottom of
41 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 42
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cargo tanks at or just after the beginning of loading may not be representative. If possible the cargo should be circulated,
using the cargo pump, before taking bottom samples.
It is imperative that sufficient ullage or vapour space is left in the sample container to allow the liquid to expand when the
temperature increases to ambient. To this end a container should be used which is suitably designed for the product being
sampled, with a built-in ullage tube and bursting disc. The safe ullage space is created by holding the sample container
vertically, with the ullage tube end at the top. The container is then filled from the bottom connection and thus cannot be
overfilled above the level set by the ullage tube.
Unless the sample container is free from cargo vapour, it should not be stored in an unventilated space.
Gloves, goggles and necessary protective clothing should be worn when sampling cold cargoes.
If the cargo is toxic, self-contained breathing apparatus must be worn. If sampling in an enclosed space, a respirator is
insufficient because lack of oxygen may lead to asphyxiation.
Any electrical equipment used when taking samples should be of the certified-safe type.
4.18.2
Vapour Samples
The precautions in paragraph 4.18.1 above should be observed when sampling cargo vapour or inert gas. Plastic sample
bags are sometimes used for collecting vapour samples. They should be handled carefully, never used for liquid samples
and always purged after use.
Appendix 4 outlines specific precautions to be observed during drydocking and refit periods.
42 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 43
CARGO
EQUIPMENT CHAPTER
5
5.1 INTRODUCTION
Much of the equipment in the cargo system of a gas carrier is precision made. It depends upon correct assembly and
maintenance of design tolerances for safe operation. Equipment should always be operated in accordance with
manufacturers' and owner's instructions, and with due regard for the properties of the cargo. Equipment should never be
operated outside its specified limits. General operational and maintenance precautions are outlined in this chapter;
precautions for individual types of equipment and instrumentation are given in Appendices 5, 6 and 7.
5.2 OPERATIONAL PRECAUTIONS
5.2.1
Maintenance
Any defect can impair operation and present a hazard to personnel, equipment, the ship or the environment. Equipment
should be carefully maintained and the following precautions should be observed:
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All maintenance should be undertaken carefully by personnel who are familiar with the equipment. The manufacturer's
instructions should be complied with.
If equipment has been exposed internally to cargo liquid or vapour, all toxic or flammable vapours should be drained and
purged thoroughly before the equipment is dismantled. If the internal volume is significant, any inert gas should be
removed by ventilation with air to prevent a hazard to personnel from asphyxia.
No dismantling should begin until the relevant equipment has been depressurised and isolated, and a full working
knowledge obtained of the internal construction and assembly details. Associated controls should be rendered inoperative,
and the situation adequately logged and reported to subsequent watches.
All sensing and control piping should be leakproof, especially if the system operates under a vacuum.
All spares used should be at least equivalent to the manufacturer's specification and compatible with the cargoes to which
they may be exposed, and should be suitable for the design temperatures and pressures of the system.
During reassembly of an internal component of a system all nuts, bolts and other fastenings should be checked and suitably
locked in position.
All instruments used to calibrate equipment should be accurate; the composition and concentration of any gas samples used
for calibration should be accurately known. Calibration should be recorded on or near the equipment.
All maintenance work should be recorded.
If maintenance involves hot work, the possible necessity for subsequent stress relief should be considered before the work is
undertaken, especially for cargo equipment. The precautions for hot work given in Section 3.6 should be observed.
Suspected leaks from piping and equipment should be investigated using safe means such as portable gas detectors or soapy
water. A naked light should never be used.
(N.B. Soapy water can freeze, thereby sealing the leakage.)
5.2.2 Action in the Event of a Defect
The following action should be taken if a defect is discovered:
The responsible officer and all personnel concerned should be informed of the nature of the defect.
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A suitable entry should be made in the cargo log.
The defect should be rectified as soon as possible.
An alternative back-up or duplicated system should be activated: this may entail a manual operation. If equipment is
temporarily decommissioned it is essential that it is isolated from the cargo systems. All associated controls should be
rendered inoperative. The facts of the situation should be adequately logged and conveyed to subsequent watches.
All sensing and control piping to the defective equipment should be isolated to prevent leakage or malfunction of other
equipment.
If any alarm is activated, immediate investigation is necessary and the appropriate action should be taken.
5.3 PLANT AND EQUIPMENT PRECAUTIONS
Equipment should be operated in accordance with the instructions for the particular ship, manufacturers' instructions and the
cargo properties outlined in Chapter 1. Logs of equipment readings should be kept. General precautions are given below
and further precautions for individual types of equipment are given in Appendices 5, 6 and 7.
5.3.1
Pumps
Ships pumps should be used and maintained with care. The following precautions should be observed:
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Before starting, check deepwell pumps manually to establish that the pump is free to turn, dosing with anti-freeze if
necessary (but see paragraph 1.4.1). If the pump is submersible check the electrical resistance.
Start up in accordance with instructions. Pay special attention to pump priming, discharge valve setting and what to do if
the pump does not gain suction immediately.
When pumps are started, valves should be opened slowly, allowing flash gas to dissipate, and avoiding cavitation.
Discharge pressure must be maintained above manifold pressure. If the manifold pressure exceeds the pump capacity, then
a booster pump will have to be used. The pump capacity curve will give the design pressure at which the pump should be
operated.
During normal discharge the flow should be controlled using a by-pass (recirculation) rather than by throttling. Prolonged
throttling will heat the cargo and the pumps, and where ball valves are installed the seal will be damaged.
Towards the end of pumping, discharge valves should be throttled to maintain suction and improve drainage.
Manufacturers' instructions should be observed.
During maintenance particular attention should be paid to keeping filters clean and to the condition of seals, bearings and
pressurising circuits.
5.3.2
Compressors
Compressors are used in cargo systems to compress vapour for pressurisation, reliquefaction or, in the case of LNG,
delivery to the engine room. Compressors are easily damaged by liquid that has condensed in the cylinders, crankcases or
separators. The following precautions should be observed:
Before starting, check that no liquid has condensed in the machine, that heating systems, if fitted, are operating as required,
that filters are dean and that cut-outs are correctly set.
When running, open suction valves slowly to prevent liquid carry-over. Keep lubricating oil clean and separators working
efficiently. Check for signs of leakage, especially on the discharge side, and watch pressures. Pressures higher than
expected could be due to incondensible gases or blockage downstream, e.g. a level gauge stuck, or an expansion valve iced
up.
After shutting down and when changing cargoes it may be necessary to change lubricating oil (e.g. after ammonia,
butadiene or vinyl chloride cargoes). Oil coolers, filters, separators, crankcases and any traps where remaining lubricating
oil can accumulate should be checked for cleanliness after carriage of butadiene.
During maintenance particular attention should be paid to cut-outs, bulkhead glands and crankcase seals, suction filters
(both to prevent damage and because blockage reduces efficiency), and all pipe flange joints.
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5.3.3
Heat Exchangers
Heat exchangers may be used for a number of purposes, including condensing and reliquefying cargo vapour, vaporising
cargo liquid, puddle heating, inter-cooling and drying. The specific instructions for the operation concerned should be
observed, especially with regard to the sequence for introducing the 'hot' and 'cold' phases, and the relative pressures of
each. The equipment should be kept free from fouling (see also paragraph 1.8.9).
Sea water cargo heaters should be operated with care, especially if the cargo is at low temperature. The sea water supply
should be established first and the cargo liquid supply carefully regulated to prevent the water freezing, which could block
the equipment and cause damage. When the sea water temperature is below 5C such equipment cannot be used.
5.3.4
Electrical Equipment
The design and installation of fixed electrical equipment in the cargo area is subject to strict regulations to prevent fire or
explosion. It is essential that this design safety is maintained. Portable electrical equipment which is not certified safe
should not be taken anywhere where cargo vapour might be present (see paragraph 3.5.2).
The following maintenance precautions should be observed:
Equipment should be de-energised and isolated before maintenance is undertaken.
High voltage test equipment should only be used on circuits which can withstand it, and never on intrinsically safe circuits.
Low voltage circuits may be permanently damaged and sparks may be created.
If there is internal condensation in any equipment, that equipment should be isolated, opened up and dried out before being
resealed.
On reassembly, cable penetrations should be fitted with proper seals (not stuffing tubes or putty) and all bolts etc. should be
replaced.
5.3.5 Control and Alarm Systems
Gas carrier cargo systems are often complex, with many remote control and automatic cut-out systems. It is important that
these control systems are kept in good working order.
All control systems should be checked regularly, especially shut-down systems, and adjustments made if necessary;
equipment should be at service temperature when this is done. Checks or tests should not be carried out during cargo
operations or cargo transfer as they may create unforeseen situations for shore installations or other vessels alongside.
Control fluids and control air should be kept clean, dry and uncontaminated, and should be replenished as necessary. Filters
in the system should be kept clean.
Control fluids should only be drained when strictly necessary. If a different fluid is to be used, it must be compatible with
the gasket and sleeve materials.
5.3.6
Instrumentation
Instruments should be treated with great care. The lives of personnel and the safety of the ship often depend on decisions
based on readings from delicate and sensitive instruments. In particular the following precautions should be observed:
Manufacturers' instructions should be studied carefully before use or calibration and recommendations on factory
reconditioning should be observed as closely as possible.
Instruments should only be used for their intended purpose and records of readings should be kept.
With chemical absorption or reaction equipment, the tubes or fluids have a limited life, which should not be exceeded.
45 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 46
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Equipment should be calibrated as often as necessary (N.B. gas detection equipment readings may 'drift', even during cargo
operations). Calibration equipment should be accurate and of good quality. Supplies of calibration gas samples (span gas)
should be available at all times and should be replenished frequently. Zero points should be checked and adjusted if
necessary, and the equipment should then be calibrated throughout its operating range. Calibration or overhaul should be
logged on or near the instrument.
5.3.7
Valves
The valves used on gas carriers differ in detail from those used on other tankers because of the temperatures and pressure
ranges they are subjected to. It is important to keep valves leak-tight and functioning properly.
The following precautions should be observed:
Valves should be set correctly before cargo operations begin. They should be operated during cooldown to check that they
are free.
During operations, spindles should be kept free from ice. Manual valves should be operated slowly to prevent pressure
surge. Protective gloves should be worn when handling cold spindles, wheels etc.
When changing tanks during loading, the valve to the tank about to be filled should be opened before closing the valve to
the tank which is almost full.
During maintenance, valve packings should be checked carefully.
Care should be taken when draining lines to avoid trapping liquid between adjacent valves. In such circumstances excessive
pressure may build up if the trapped liquid heats up.
5.3.8
Cargo Vent Systems
Vent systems are provided for the disposal of cargo vapour from tanks and liquid retained in pipelines. It is essential that
cargo vent systems are kept clear otherwise they will become overpressurised and may be damaged.
The following precautions should be observed:
If relief valves have multiple settings, then any changes in the settings should be made in accordance with instructions, and
should be logged.
Vent systems should be kept clear; water should be drained off regularly from stacks. Drains should be kept closed when
not in use, otherwise liquid on the discharge side of relief valves could be ejected onto the deck when a valve operates.
Flame screens, if fitted, should be clean and in good condition but not painted. Snuffer flaps, if fitted, should be kept free to
move.
5.3.9
Expansion/Contraction Fittings
The cargo systems of all gas carriers are subject to considerable temperature variation and some means of accommodating
expansion and contraction has to be provided. It is important that the procedures work properly, otherwise parts could be
over-stressed and damaged.
The following precautions should be observed:
Sliding feet should be kept free to move.
Fittings should be protected from freezing.
A check should be made that constraints which should be in place have been fitted and that all others have been removed.
The condition and alignment of bellows should be checked. Bellows should be protected against mechanical damage.
5.3.10
Cargo Pipelines
Liquid and vapour lines can be complicated and difficult to trace, so it is important that the settings of valves etc. are
carefully checked before any line is used. Pipelines are particularly vulnerable to pressure surges and to the risk of freezing
of trapped water.
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The following precautions should be taken:
Manifold flanges should be cleaned before blanks or hoses are connected, paying particular attention to the removal of ice.
Protective gloves should be worn by personnel handling cold lines or blanks etc.
Filters should be cleaned and baskets replaced carefully.
All missing supports, chocks etc. should be replaced.
5.3.11
Ships' Cargo Hoses
The liquid and vapour hoses are the most vulnerable part of the cargo transfer system and should be treated with great care,
both when in use and during storage. During cargo loading or discharging all non-essential personnel should keep away
from the manifold area.
If cargo hoses are carried on the ship the following precautions should be observed:
Hoses should be checked to ensure that they are suitable, in terms of chemical compatibility, temperature and pressure
ratings etc. for the cargo to be carried. The hose details should be checked (see Appendix 13) and the condition of the hose
inspected. Hoses should be tested at least every six months and the test results recorded: the tests should be at ambient
temperature and up to the working pressure. The ICC Code, Section 5.7 should be referred to.
Caskets should be checked for suitability.
The hose should be supported correctly at all times, especially during use, when tidal and draught variations should be taken
into account.
Pipeline flanges should be cleaned before connecting.
Nuts and bolts should be of the correct size and material, and damaged bolts should not be used. A bolt should be fitted in
every hole and tightened correctly.
Hose bonding or insulation should be checked (see paragraph 3.5.6).
Hoses should be drained, purged and depressurised before disconnection. Hoses not purged of cargo vapour should not be
stored in enclosed spaces.
Hose ends should be blanked before storage.
5.3.12 Inert Gas Systems
Inert gas has an important role in maintaining safety aboard a gas carrier and the inert gas system should be kept in good
working order. Regardless of frequency of use it should be tested regularly to prevent deterioration and enable any faults to
be detected and rectified.
The following precautions should be observed:
The whole system should be visually checked before starting up, in particular the deck nonreturn valves.
The piping system to the vent outlet should be opened to release any pressure and prevent back-flow, and the temporary
connections to the cargo system fitted.
The scrubber water supply should be started before beginning combustion.
The gas produced should be vented to atmosphere until it is of sufficiently good quality for use.
The air supply should be adjusted to produce the best quality inert gas possible: oxygen, carbon dioxide, carbon monoxide
and soot levels should be controlled (see paragraph 4.6.2). If the air supply is reduced in order to lower the oxygen
concentration the gas produced may often have a high soot content which can clog driers, non-return valves etc.
The gas quality should be continually monitored while the plant is in use.
After use, the temporary connections to the cirgo system must be disconnected and the flanges blanked securely.
5.3.13
Nitrogen System
If liquid nitrogen is used as an inert gas, it should not be allowed to come into contact with any metal (other than the
dedicated nitrogen storage and piping system) with a service temperature above -196'C. If it does, brittle fracture will
occur. If the storage bottles and transfer line are vacuum insulated, the vacuum should be carefully maintained to prevent
excessive boil-off. It is rarely possible to replace lost nitrogen except in port. (See also Appendix 5, section 13.)
47 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 48
Ships may have the means to produce nitrogen gas on board by physical separation from the atmosphere, using the pressure
swing absorption method or the membrane method. It is important to appreciate that the exhaust from the plant will be
oxygen-rich compared to normal atmosphere.
5.3.14
Ventilation Equipment
All ventilation motors and fans should be well maintained. Poor electrical contacts, blocked air intakes and interference
between moving parts should be avoided. In some installations the fan impellers are made from special materials that
cannot create sparks. When maintenance of such equipment is undertaken, care should be taken to ensure that the design
safety features are not impaired in any way.
All components in ventilation systems, such as non-return flaps, flame arresters and dampers should be kept in good
working order.
48 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 49
ENCLOSED
SPACES CHAPTER
6
6.1 INTRODUCTION
When it is intended that personnel should enter or work in an enclosed space, great care should be taken to create and
maintain safe working conditions, even if the duration of the work is to be short. The hazards and precautions associated
with entering or working in enclosed spaces are outlined in this chapter.
6.2 ATMOSPHERE IN ENCLOSED SPACES
When an enclosed space is left closed and unventilated for any length of time, the internal atmosphere may become unsafe
either because it contains less than 21 % oxygen, or because it contains contaminants, or both. The oxygen content may be
reduced by the presence of inert gas or by the process of rusting, which absorbs oxygen from the air. Cargo vapour is the
most common contaminant, although fumes from other sources (e.g. stores) may present a hazard.
Cargo vapour or inert gas should always be anticipated in cargo tanks and hold or interbarrier spaces. Leakage should be
suspected in the case of enclosed spaces separated by a single gastight bulkhead from cargo tanks and hold or interbarrier
spaces. Similarly, leakage should be suspected in the case of any space containing cargo or inert gas equipment (e.g.
compressor rooms and control rooms with direct connections to the cargo systems).
The concentration of oxygen in fresh air is approximately 21 %. An atmosphere with a lower oxygen concentration can be
breathed for some minutes before the effects become apparent. If the oxygen supply to the brain is depleted the victim will
feel dizzy and have a headache before losing consciousness. This is particularly dangerous because he may not recognise
that he is in danger or be capable of finding his way out of the space, He therefore becomes a risk to himself and others.
There is a danger of permanent brain damage after only four minutes in a very oxygen-deficient space. A successful rescue
depends upon the victim being resuscitated in the shortest possible time.
If the space contains hydrocarbon vapour the victim may act as though drunk, and behave aggressively. His judgement may
be impaired and he may not recognise the danger before losing consciousness.
It is therefore vital that nobody ever enters an enclosed space without breathing apparatus until it has been confirmed that
the atmosphere is safe and will remain so. As a general rule, enclosed spaces should not be entered unless it is absolutely
necessary. If entry is essential the precautions set out below should be followed.
Suitable notices should be prominently displayed to inform personnel. Instructions should be given detailing the
precautions to be taken when entering tanks or other enclosed spaces, and listing any restrictions placed upon the permitted
work.
6.3 ENTRY INTO ENCLOSED SPACES
6.3.1
General
No one should enter an enclosed space which is known or suspected to contain cargo vapour, or in which the atmosphere
may be deficient in oxygen, unless it is essential. The master or responsible officer should ensure that the space is
sufficiently ventilated and that company procedures covering entry permit requirements or check lists are correctly
observed. In particular it is critical to ensure that:
49 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 50
•
•
•
•
•
•
•
•
•
•
the levels of oxygen and contaminants are continuously checked, and are within safe limits; or
suitable breathing apparatus and life-saving equipment is worn, including a life-line if practicable.
In either case it should be confirmed that:
the space is ventilated while personnel remain inside;
a rescue plan has been drawn up;
an experienced crew member is standing by at the entrance;
a reliable system of communication has been established and is understood both by those entering and by the crew
member standing by at the entrance;
a rescue team is readily available with escape equipment, breathing apparatus and resuscitation equipment placed near the
entrance.
The emergency plan should clearly set out how to raise the alarm and summon assistance. Access to the space concerned,
deployment of reserve equipment and communication between the emergency party and command
centre should also be arranged (see Chapter 7).
6.3.2
Testing Before Entry
Before the space is entered it should be thoroughly ventilated. The time necessary to ensure thorough ventilation depends
upon the size of the space, the capacity of the system used, the level of contamination and the efficiency of the ventilation
system.
Once the space has been ventilated, the atmosphere should be checked as follows:
the oxygen content should be sampled with a suitable and reliable detector: 21% oxygen is required for entry;
if a flammable cargo vapour may be present, a combustible gas indicator should also be used: a content of not more than 1
% LFL is required for entry;
if a toxic gas may be present, the appropriate toxic gas detector should be used.
It is vital that the correct instruments are used: a combustible gas indicator will not measure an oxygen deficiency, the
presence of toxic gas or the presence of flammable vapour in inert gas. It is also essential to ensure that readings are taken
at several levels, bearing in mind that vapours which are heavier than air will be found at the bottom of any space; the air at
this point should be sampled if the suspected vapour has a relative vapour density greater than that of air (see data sheets).
Similarly, the top of a space will have to be sampled if the suspected vapour has a relative density less than that of air.
Vapour will also tend to remain where the ventilating airflow is least effective. Ventilation should be stopped about 10
minutes before tests are made and not restarted until the tests are completed. Sampling the atmosphere may require the use
of breathing apparatus. A number of samples may have to be taken before the air in the whole space can be judged safe.
It is essential that all gas testing equipment used is of an approved type. It must be correctly maintained and, where
appropriate, frequently check-tested against standard samples. Gas testing should be done by personnel familiar with the
use of the equipment and sufficiently knowledgeable to understand the results obtained.
Chemical absorption indicators with the appropriate detector tube can be used for measuring oxygen content but the reading
may be less accurate if other chemical vapours are present. An indicator which may be reliable for measuring oxygen
content in a space after thorough ventilation may not be suitable for checking the oxygen content in a cargo vapour/air/inert
gas mixture.
If the atmosphere in the space has been found safe for entry, the precautions in paragraph 6.3.1 should also be rigorously
observed.
Even after a space has been made gas-free and found to contain a respirable atmosphere, local pockets of gas should always
be suspected. Hence a person moving around to different areas of a tank or compartment, or descending to the lower part
after work in the upper part, should remain alert to the possible need for further tests to be made. Generation of vapour
should always be considered possible even after loose scale has been removed.
50 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 51
Ventilation should be continued and the atmosphere monitored at regular intervals while personnel are inside the space. If
they begin to feel dizzy or unwell they should leave the space at once. Care should be taken when pipes or equipment in the
space are opened up; if liquid or vapour escapes, the space should be evacuated and not re-entered until the atmosphere in
the entire space has again been found to be safe. Frequent gas tests should be made, appropriate to the work in hand or to
any change in conditions. In particular, tests should be made before each daily resumption of work or after any interruption
or break in the work. Tests should be so arranged that readings representative of the condition of the entire space are
obtained.
6.3.3
Breathing Apparatus
Unless all the above precautions can be followed, spaces should only be entered by personnel wearing breathing apparatus,
and, if practicable, a life-line.
Personnel using breathing apparatus, as well as their support teams, should be thoroughly familiar with the equipment and
with action to be taken in the event of an accident whilst using it.
6.3.4
Rescue from Enclosed Spaces
It is imperative that regular drills and exercises in rescue from enclosed spaces are carried out and that every member of a
rescue team knows what is expected of him.
When personnel are in need of rescue from an enclosed space, the first action must be to raise the alarm. Rescue and
resuscitation equipment should already be positioned at the entrance. Although speed is often vital in the interest of saving
life, rescue operations should not be attempted until the necessary assistance has been obtained. There are many examples
of lives having been lost through hasty, ifi-prepared rescue attempts.
Whenever it is suspected that an unsafe atmosphere has been a contributory factor to an accident, breathing apparatus and,
where practicable, life-lines must be used by persons entering the space. A code of signals should be agreed in advance.
The officer in charge of the rescue should remain outside the space, where he can exercise the most effective control.
6.4 VENTILATION OF SPACES
Ventilation equipment should be carefully maintained (see paragraph 5.3.14). irhe ventilation precautions for various parts
of the ship, which are additional to those in Section 6.3, are discussed in the following paragraphs.
6.4.1
Cargo System
Cargo tanks, cargo piping and cargo equipment will contain cargo vapour unless they have been gas-freed, or inert gas
unless they have been ventilated.
Other parts of the cargo system may contain inert gas. Care should be taken to ensure that oxygen levels in the space are
safe before personnel enter without breathing apparatus. The other precautions listed in Section 6.3 should also be
observed.
Cargo equipment may contain cargo vapour or inert gas. It is important to ensure that such equipment has been adequately
ventilated before it is opened up for maintenance.
When ventilating membrane cargo systems, care should be taken to ensure that manufacturers' instructions on differential
pressures are observed, otherwise damage may occur.
6.4.2
Enclosed Spaces Separate from the Cargo System
No cofferdam, ballast tank, peak tank, fuel or lubricating oil tank, fresh water tank, duct keel, void space, access trunk, or
any other enclosed space should be entered unless the precautions listed in Section 6.3 are strictly observed.
51 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 52
The principal danger in such spaces is that rusting has depleted the oxygen content of the atmosphere to the point where it
cannot support life. However, it is also possible for cargo vapour or inert gas to leak into them, and the atmosphere should
therefore be checked for both oxygen content and cargo vapour before entry.
6.4.3
Cargo Control Rooms
Any cargo control or instrument room which is not classified as gas-fiee should be ventilated thoroughly before entry (see
paragraph 6.3.1), but access doors or hatches should never be left open. Ventilation and gas detection equipment should be
operated and checked throughout the period that the room is in use. If fixed equipment is not fitted or is not working,
portable equipment should be used.
In ships designed to carry cargoes whose vapours are either lighter or heavier than air,
alternative upper and lower ventilation points and gas sampling heads are normally provided. The changeover devices
should be set according to the relative vapour density of the cargo (see data sheets).
6.4.4
Cargo Pump or Compressor Rooms, Motor Rooms and Air Locks
The following precautions are additional to those in paragraph 6.4.3.
Ventilation fans should be run continuously for at least 10 minutes before cargo operations begin, and throughout their
duration. Fans should also be run continuously when leakage of vapour or liquid into the space is suspected.
Safety interlocks are provided to ensure that no machinery can be started until the ventilation system has been operating for
at least 10 minutes, long enough to have dispersed any toxic or flammable vapour that may have collected in cargo
pumprooms or compressor rooms, and to build up sufficient pressure in motor rooms and air locks. Loss of ventilation
pressure can cause shutdown of equipment.
Regular inspections should be undertaken of inlet and outlet grilles to ensure that they have not become obstructed.
6.4.5
Engine or Boiler Rooms
On LNG ships using cargo boil-off as fuel, the ventilation equipment in the gas supply system should be running before gas
is allowed to pass to the machinery space. All detection equipment in the supply system and machinery space should be
working before gas supply begins.
If a gas leak is detected, the gas supply should be stopped until the leak has been repaired.
52 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 53
EMERGENCY
PROCEDURES CHAPTER
7
7.1 INTRODUCTION
It is impossible to predict the nature of every potential emergency, but standard emergency procedures should be
developed for each ship and it is essential that personnel are properly trained for these procedures. The
overriding consideration of those responsible for operations should be the continued safety of all personnel on
board and anyone in the immediate vicinity. General guidance on procedures for the most readily foreseeable
emergencies is given in this chapter.
In addition the extensive advice given in the ICS/OCIMF/SIGTTO Contingency Planning Guides should be
consulted.
7.2 PRE-PLANNING
Emergency procedures have to be pre-planned and ready for immediate ftnplementation in the event of an emergency. The
procedures must anticipate and cover such foreseeable types of emergencies which might be encountered at sea or in port as
grounding, fire, collision and cargo spill. In each situation, the first stages of a plan should be:
•
raising the alarm,
•
locating and assessing the incident, the possible dangers, and action to be taken,
•
organising manpower and equipment.
The detailed circumstances of an actual emergency will differ in many cases from those envisaged during pre-planning;
however, the standard procedures should ensure that basic action can be taken quickly and that decisions on how to tackle
any additional problems can be made in an orderly and adequate manner.
Company regulations will be tailored to individual ships, and will cover organisation, preliminary action and procedures to
be followed. This guide gives general advice on aspects relevant to the carriage of liquefied gas.
7.3 EMERGENCIES
7.3.1
Water Leakage into Hold or Interbarrier Space
If water leaks into a hold or interbarrier space, it may damage the insulation and, in the case of a membrane tank system,
result in tank wall corrosion. These spaces are normally equipped with a water detection alarm system which will indicate
if leakages occur; the water should be pumped out and the leakage remedied if possible.
7.3.2
Hose Burst, Pipework Fracture or Cargo Spillage
This is likely to result from the effects of pressure surge, excessive ship movement, defective hoses, leaking flange packing
or overfilling of tanks.
The following actions should be taken immediately:
•
•
the alarm should be raised and the terminal informed immediately;
all cargo operations should be stopped and all valves in the liquid line closed both on the ship and ashore as necessary;
53 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 54
•
•
•
•
•
•
•
•
•
•
all accommodation access doors should be shut and all ventilation (except closed-circuit systems) shut down;
smoking and naked lights should be prohibited everywhere on the ship, and electrical switches used as little as possible;
if liquid spillage occurs, fire hoses and water sprays (which should always be ready for immediate action) should be used to
disperse the liquid overboard and to maintain steel temperatures so that brittle fracture is avoided: water sprays from hoses
can also be used to deflect a gas cloud: for this reason water spray equipment should be available in way of the manifold
during cargo operations and transfer (see paragraph 7.3.3); and
appropriate fire-fighting equipment and breathing apparatus should be assembled for
immediate use: the emergency parties should wear breathing apparatus and protective clothing.
7.3.3
Dispersion of Liquid Spill and Vapour Emissions by Water Spray
The best design and operational technique is to prevent liquid spillage and vapour emission incidents altogether. However,
if such incidents occur existing fire-fighting water monitors and hand-held water spray nozzles can often provide a rapid
and flexible means of effecting dispersion:
by controlling the direction of the dispersion;
by diluting the gas with air entrained in the water spray;
by heating the relatively cold gas cloud to increase its buoyancy;
by absorbing some toxic gases which are soluble in water e.g. ammonia and chlorine.
The size of a spillage or vapour emission which can be controlled or dispersed by water spray will depend upon available
fire main water pressure and the number of jet/spray nozzles which can be employed. Leakages which can generally be
dealt with will be those from loading arm swivel joints, pipeline flange connections and cargo pump shaft seals. Vapour
vented from a ship's mast riser due to the operation of a cargo tank pressure relief valve may also be dispersed in this way.
However, large flammable gas leakages require extreme caution in the use of water sprays as the spray may not dilute the
gas to below the LFL but simply increase the volume of the prefixed cloud, increasing its buoyancy and thus enhancing the
likelihood of access to an ignition source. Similarly the use of water sprays may not prevent ignition: indeed, the turbulence
and mixing caused by water spray may increase the flame speed on ignition. On the other hand, water spray will assist the
protection of people, structures and equipment from radiation heat damage should ignition occur.
If a flammable gas cloud is to be controlled by water spray, for example to prevent it reaching a potential source of ignition,
the maximum quantity of water spray available should be brought to bear as quickly as possible to redirect the gas cloud
away from the ignition source. Small pipeline liquid leakages may be vapourised with water from one hose and dispersed to
below LFL with water spray from another hose. It is normal practice to protect the steel deck structure of a ship by sluicing
liquid spills over the side, and additional water jets can assist spill dispersal down and away from the ship's side.
7.3.4
Tank Leakage
Cargo tank leakage to the hold space or interbarrier spaces is detected by the gas detection equipment, and constant
monitoring will give continuous information on the change of vapour concentration. The stability or rate of change of
equipment readings will indicate the magnitude of the leakage and, together with constant monitoring of the hold or
interbarrier space pressure and temperature, will enable the operator to establish the leak rate. All leakages from cargo
tanks should be regarded as serious and reported immediately.
Any specific instructions for the ship should be observed, and the following courses of action should be considered:
pumping liquid in the hold or interbarrier spaces into an undamaged tank with compatible cargo and sufficient ullage
available,
using the reliquefaction plant or other means in order to reduce the tank pressure, and therefore the static head on the leak.
Care should be taken to avoid drawing air into the tank, thereby creating a flammable mixture.
54 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 55
•
•
•
•
•
•
7.3.5
Emergency Discharge of Cargo at Sea
If any tank develops a serious defect at sea, cargo should be transferred to any other tank containing compatible cargo and
with sufficient ullage available. Remaining cargo which cannot be transferred will need to be discharged overboard, taking
into account trim, stability and stress considerations, local circumstances and the amount of cargo to be discharged.
If cargo is to be discharged, the stern line should be used. If no stern line is fitted, an extension pipe for the midship
crossover should be provided, sufficiently long to extend over the side and properly supported to prevent overstressing the
manifold. The extension piece should be of material suitable for the cargo, angled downwards and fitted at the end with a
suitable reducer to increase the discharge velocity and to prevent liquid from coming into direct contact with the hull and
causing brittle fractures.
If emergency cargo discharge has to be undertaken, the precautions listed in paragraph 7.3.2 should be observed. In
addition, the following points should be considered:
informing or consulting the operator;
advising local authorities such as the Coast Guard;
broadcasting a radio warning to all other ships in the vicinity;
heading the ship so that the direction of discharge is down the relative wind, if possible, while ensuring that the vessel is
free from gas clouds;
controlling the pumping rate in such a way as to get the cargo as far as possible from the ship, and
taking any other precautions specified for the particular ship.
7.3.6
Accidents Involving Personnel
If personnel come into contact with the cargo, the emergency action specified in the data sheet for that cargo should be
taken (see Chapter 9 and Appendix 1).
If personnel are overcome or affected by the cargo, the alarm should be raised and the rescue team mobilised. The agreed
rescue plan should be implemented and the responsible officer informed.
55 Empty page 56
ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 56ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 57
FIRE-FIGHTING CHAPTER
8
‘The first few
moments
are vital!’
8.1 INTRODUCTION
If a fire occurs, the action taken in the first few moments is vital. The man on the spot should raise the alarm and assess the
situation. The emergency plan should be implemented (see Chapter 7).
With liquefied gas fires it is essential to isolate the fuel source before extinguishing the flames to minimise the danger of a
potentially flammable gas cloud forming.
8.2 FIRE-FIGHTING ORGANISATION
The requirements for fire-fighting equipment are laid down by national and international regulations and are not covered in
this guide. General fire-fighting theory is included in the International Safety Guide for Oil Tankers and Terminals
(ISCOTT).
Company regulations will be tailored to individual ships, and win cover organisation and training of personnel and
maintenance of fire-fighting equipment. Fire-fighting cannot be successful unless all equipment is operational and all
personnel are well trained in the use of the equipment and in emergency procedures.
8.3 SPECIAL CONSIDERATIONS FOR FIGHTING LIQUEFIED GAS FIRES
8.3.1
Isolating the Source
The main considerations in fighting a liquefied gas fire are the large quantity of vapour given off by the liquid and the
considerable heat generated by the flames. In the event of fire every effort should be made to isolate the fuel source: dry
powder or water sprays should be used on local fires which prevent access to valves. The flames should not be extinguished
before the source of fuel has been shut off, to prevent a potentially flammable gas cloud forming and being re-ignited
downwind or by surfaces heated in the original fire. If the fuel source cannot be isolated it is safer to let the fire burn while
continuing to cool the area.
8.3.2
Use of Dry Powder
It is not beneficial to use low expansion foam or water for liquefied gas fires because their application increases the rate of
vapourisation. Dry powder is used instead (see Section 8.4), although it provides a negligible cooling effect. Cooling is
required to prevent re-ignition until all liquid has dispersed and the area is free from flammable vapour. It is best achieved
by water from fitted spray systems or hand hoses. Sprays from hand hoses are excellent in protecting fire-fighters from the
radiant heat of a liquefied gas fire.
57 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 58
•
•
•
•
•
•
•
•
Care should be taken to ensure that the limited capacity of dry powder extinguishing systems is used effectively, especially
when being used with other fire-fighting media. It is possible to waste a large proportion unless there has been careful
planning of large scale fire-fighting operations.
A fire in stores or machinery spaces may affect the cargo and increase boil-off. This can be reduced by cooling the area
with water sprays, use of reliquefaction plant (provided the power supply is still available) or both. However, a water jet
should never be used on a liquid fire.
If a fire occurs in a cargo equipment space, such as a compressor room, the source of fuel should be cut off and the fire
attacked in the first instance with dry powder. If necessary all personnel should be evacuated, the compartment closed
down and the fixed fire-fighting system activated. The area should be cooled with water sprays. As soon as the fire has
been extinguished, the space should be ventilated carefully to disperse any vapour.
8.3.3
Vent Mast Fires
Ignition can be caused at the vent mast by a lightning strike or other source of ignition when venting a flammable vapour.
The following actions should be considered:
stopping venting;
injecting inert gas into the vent if possible;
spraying the mast head with water.
Venting may be resumed when the mast head and its surroundings are cool and the electrical storm is over.
8.3.4
Fires Near to the Ship
In the event of a fire in the immediate vicinity of the ship, whether ashore or aboard another vessel, the following actions
should be considered:
making ready the ship's fire-fighting organisation and equipment;
stopping all cargo and bunkering operations;
isolating and disconnecting hoses;
closing all compartment openings;
bringing the main engines to immediate readiness.
8.4 DRY CHEMICAL POWDER AS AN EXTINGUISHING AGENT
Dry chemical powder (also known as 'dry powder') is a flame inhibitor. Discharged from an extinguisher as a free-flowing
cloud it can be effective in dealing initially with a fire resulting from a liquid spill on deck or in a confined space. It is
especially effective on burning liquids such as liquefied gas, or oil escaping from leaking lines and joints, and on vertical
surfaces e.g. diesel equipment fires. It is a non-conductor and thus suitable for use in dealing with electrical fires, although
there is a possibility of some damage to the electrical machinery from its abrasive nature.
Dry powder has a negligible cooling effect and so may not give protection against possible re-ignition from a hot surface.
Certain types of dry powder can cause a breakdown of a foam blanket and only those known to be 'foam compatible' should
be used in conjunction with foam.
58 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 59
PERSONNEL
PROTECTION
AND LIFESAVING
CHAPTER 9
9.1 INTRODUCTION
The minimum requirements for lifesaving equipment on board all ships are laid down by national and international
regulations and are not repeated in this guide. All equipment should be inspected regularly and kept ready for immediate
use in a clearly marked and accessible place. Practical demonstrations, training and drill should be regularly undertaken so
that personnel become experienced in the use of all safety equipment and know the location of each item.
9.2 PROTECTION CLOTHING
Appropriate protective clothing should be worn as necessary to protect those involved in cargo operations from the hazards
associated with the cargo. The suits, gloves, boots, goggles, face shields and other items used should be suitable for the
cargo. Many plastics become brittle and crack when subjected to low temperatures, or can be dissolved by the cargo,
although clothing of PVC or similar material is less susceptible to absorption, and should be worn when exposure to vapour
or liquid emissions is involved.
In particular, gloves should be worn when handling cold equipment, valves or slip tubes, face protection should be worn
when there is a danger of liquid emission (e.g. dismantling cargo equipment, using slip tubes, or sampling) and respiratory
protection should be worn during cargo operations involving toxic or asphyxiating gases (see Sections 9.4 and 9.5).
Cargo vapour may be absorbed into working clothing in sufficient quantities to create a hazard when taken into
accommodation, galley, smoke room etc.
9.3 DECONTAMINATION WATER SPRAYS AND SHOWERS
Full use should be made of changing rooms between deck areas and accommodation, and of showers which may be
provided. Personal hygiene is very important if the cargo is toxic.
9.4 CANISTER OR FILTER TYPE RESPIRATORS
!
Canister or filter type respirators should never be used
in enclosed spaces where the oxygen content of the
atmosphere may be insufficient to sustai
n life. Such equipment filters out toxi
c or poisonous elements but does not
replace oxygen.
Canisters are available for absorption of a variety of different vapours, but the following precautions should be observed:
•
•
•
The correct type of canister should be fitted for the vapour concerned: it may be necessary to change canisters when
changing cargoes;
Canisters should not be opened to the atmosphere until needed for use because they may become gradually saturated and
ineffective;
Canisters have a limited life: they should be discarded and destroyed after use unless it is known with certainty how much
of their life remains.
59 9.5 BREATHING APPARATUS
Breathing apparatus should be used as necessary by personnel engaged in cargo operations involving toxic cargoes, by fire
fighters, and when entering an unsafe space. Tanks or compartments which are not gas-free, which are deficient in oxygen
or which contain smoke should not be entered unless absolutely necessary.
Breathing apparatus should always be used in accordance with manufacturers' instructions. Practical demonstrations and
training in the use of breathing apparatus should be carried out to give personnel experience in its use.
Ships carrying toxic cargoes are provided with small breathing apparatus sets supplying air for approximately 15 minutes.
This equipment is for emergency escape only and should not be used for other purposes.
9.6 CITADEL AREAS
When certain toxic cargoes such as chlorine are carried in gas tankers, the IMO ICC Code requires that a space within the
accommodation area be arranged to provide a safe haven for personnel against the effects of a major cargo release. This is
commonly referred to as the ‘citadel’. The protected area, usually the bridge and the cargo control room, is required to
house the whole of the ship's company and provide uncontaminated air for a period of not less than four hours. Access is to
be easily and quickly available from the open deck and the accommodation area via an airlock which has a decontamination
shower adjacent to it.
9.7 FIRST AID
The first aid procedures for accidents involving cargo are given in the data sheets in Appendix 1.
If cargo liquid should enter the eye, the correct treatment for most cargoes is to flood the eye with clean water and to
continue washing for at least 15 minutes. If cargo liquid comes into contact with the skin, the affected area should be
washed and any contaminated clothing removed. If frostbite has occurred it should be treated by immersing the affected
part in warm water (see Section 9.9).
If any personnel experience the symptoms of vapour exposure they should leave the area and advise other personnel in the
vicinity. The possibility of similar symptoms in others should be constantly borne in mind.
Emergency treatment, correct for most cargoes, is to remove the victim to fresh air and, if breathing is weak or irregular or
has stopped, provide resuscitation.
9.8 RESUSCITATION
Personnel should be instructed in the technique of mouth-to-mouth and mouth-to-nose resuscitation as the most important
and effective means of resuscitating a victim. However, if the victim has inhaled toxic vapour or inert gas the person
providing resuscitation may be overcome by the gas pressed out from the victim's lungs. In this case a resuscitator should
be used, or the person giving resuscitation should wear breathing apparatus and remove the mask only when blowing air
into the victim's lungs.
Responsible personnel should be instructed in the use of resuscitation apparatus. Specially marked cylinders should be used
for training purposes so that, in an emergency, only fully charged cylinders can be selected for use. This apparatus should
NOT normally be kept locked up. The operating instructions for the apparatus should be clearly displayed.
60
ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 60Resuscitation equipment should not be taken into an enclosed space containing flammable cargo vapour unless the
equipment is approved as safe for use. As a general rule it is better to concentrate on removing the victim to a safe place
first than to attempt resuscitation in a hazardous area.
9.9 FROSTBITE
This term applies to cold injury where there is destruction of tissue by freezing, usually localised but possibly covering an
extensive area. The parts most commonly affected are fingers, toes, cheeks, ears and nose.
Ice crystals form in skin and other tissues of the affected part. The patient may not be aware of it until told. Frostbitten
tissue appears white or greyish-yellow. There may be early pain which subsides, or a stinging aching sensation. In severe
cases the area feels numb, hard and solid. As the affected area thaws it becomes red and swollen: gangrene and tissue death
can be the end result.
Treatment should be started as soon as possible. Wet, rapid warming, as explained below, is the preferred method. Once
treatment is started it must be maintained until thawing is complete.
To rewarm the victim, remove cold wet clothing and constricting items such as shoes and socks, and completely immerse
the affected area in warm water at around 42C (but not more than 44'C). Never use dry heat. Thawing may take from 15 to
60 minutes, and should be continued until the pale blue colour of the skin turns to pink or red. Avoid bending of joints or
massaging the flesh. Pain killers (Paracetamol or morphine) and tranquillisers may be required to control pain during
thawing.
After rewarming, gently cleanse the affected area with water and soap, and apply a soft sterile dressing. Ointment or creams
should not be used. Keep the patient warm in bed, with the affected part elevated if possible. Avoid contact with
bedclothes by using an improvised bed cradle.
Guidance from Radio Medical Advice should always be sought.
Bending of joints or massaging of flesh should be avoided.
Alcohol and cigarettes decrease the blood flow to the damaged tissue, and neither should be given to the patient.
61
ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 61ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 62ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 63
CARGO INFORMATION
(DATA SHEETS) APPENDIX
1
A1.1 GENERAL
The safe handling and carriage of liquefied gases demands a knowledge of the physical and chemical properties of the
cargo, of the hazards it presents, and of the action to be taken in the event of an emergency. The data sheets provide
essential information about the liquefied gases and other products listed in Chapter 19 of the IMO International Code for the
Construction and Equipment of Ships Carrying Liquefied Cases in Bulk (the IGC Code). Some data sheets refer to
chemicals which may be carried and used for operational reasons, e.g. to prevent ice formation or as a refrigerant for the
reliquefaction plant.
The data generally are for individual liquefied gases, but some commercial cargoes are mixtures: for example, LPG cargoes
may be mixtures of propane, butane and other gases in varying amounts, for which it is not possible to give complete data.
Information on any mixture should be obtained from the shipper and/or the terminal management before loading is started.
A specimen cargo information form is given in Appendix 10.
Al.1.1 Guidelines on the Use of Cargo Data Sheets
The cargo data sheets and these accompanying guidelines have been compiled to provide background information on the
safe handling of liquefied gases. While every effort has been made to ensure that they are both comprehensive and
accurate, the International Chamber of Shipping cannot accept any liability for their use howsoever arising.
Al.1.2 Cargo Data Sheets
The data sheets have been laid out to convey health and safety information in a convenient and consistent manner, and every
effort has been made to use clear and understandable language. However, certain basic assumptions have been made, and
the purpose of these guidelines is to explain the data sheets so that the maximum benefit can be derived from their use.
A second copy of each cargo data sheet is included, printed on sturdy material, and it is recommended that the appropriate
sheet(s) for the cargo(es) being carried are made available to all personnel by display on a notice board.
The content of the data sheets is in eight sections. The layout ensures that the information most likely to be needed in an
emergency is near the head of the sheet.
An explanation on what each section covers is given below.
A1.2 GENERAL INFORMATION
This section contains:
•
•
•
•
•
•
The name of the product and its synonyms.
The United Nations (UN) number
Appearance of the product: colourless, green gas, amber liquid, etc..
Odour of the product: pungent, suffocating, mild aromatic, fishlike, odourless, etc.
The Main Hazard: flammability, toxicity, corrosivity, reactivity or any combination of the above, or if it is non-hazardous.
The IMO Medical First Aid Guide (MFAC) Table Number.
63 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 64
A1.3 EMERGENCY PROCEDURES
This table is set out in such a way that the action to be taken in an emergency is clearly indicated, in the form: 'If this
happens .... Do this".
First aid information is given for the forms of exposure expected. It is emphasised that this is only first aid; qualified
medical advice should be obtained where so instructed, or if there are any doubts about the effectiveness of treatment.
First aid treatment must be applied quickly and efficiently. Swift action can minimise harmful effects while delay can lead
to unnecessary injury or complications. The correct table in the IMO Medical First Aid Guide should be consulted.
Fire: Fire-fighting procedures will depend greatly on the circumstances and should be under the control of the responsible
senior officer. To assist such personnel, this section gives general guidance on fire-fighting and extinguishing agents. In an
cases where hazardous substances are likely to be released by the fire, or by fire fighting methods, the precautions to be
observed by fire fighters are given. As a general rule fire fighters should wear protective clothing and respiratory protection
equipment. Refer to Chapter 8 on fire-fighting for more detailed guidelines.
Liquid in Eye: This normally gives rise to local eye effects such as irritation or burns, but there are a few materials which
can be absorbed into the body through the eye and which may cause systemic toxic effects.
Liquid on Skin: Two effects may occur:
•
•
•
•
•
Local reactions such as skin irritation arising from direct action by the material at the site of first contact. Direct contact
with low temperature liquefied gases or uninsulated equipment and pipes can cause cold bums (frostbite).
Systemic effects - toxicity caused in tissues remote from the site of first contact due to the material penetrating the skin and
passing into the body
Vapour Inhalation: Three effects may occur:
Lack of oxygen can cause asphyxiation, whether or not the vapour is toxic.
Toxic effects can cause poisoning which may or may not be reversible once the victim is in fresh air.
Inhalation of cold vapours may permanently damage the respiratory system.
Spillage: If spillage occurs prompt consideration must be given to stopping the outflow of liquid. Personnel engaged in
spill control should be safeguarded against contact with cargo liquid or vapour. Potential sources of ignition should be
extinguished.
A1.4 HEALTH DATA
Threshold Limit Values (TLVs): The values shown on the data sheets are normally drawn from the lists of recommended
values published by the American Conference of Governmental Industrial Hygienists, the United Kingdom Health and
Safety Executive and other authorities. Where a product is not included in these lists, the TLV is determined on the basis of
experience, knowledge and chemical industry research. Some differences in published values exist, and the figure quoted is
the lowest. All cargoes where the TLV is very low should be treated with respect.
TLVs take account of factors which include discomfort, irritation and acute toxic effects, as well as chronic toxicity, and the
exposure levels at which these occur. The data are obtained from animal toxicity studies and from previous human
experience. A safety factor allowing for variations in individual susceptibility is added.
TLVs for gases and vapours are expressed as parts per million parts of air.
64 Odour Threshold: Odour threshold is generally expressed in parts per million parts of air. Odour is a very unreliable
check on the presence of material as the nose can readily become accustomed to the vapour.
Nature of Hazard (Effects of Liquid and Vapour): Descriptions of the effects of liquid and vapour by each route of
exposure are arranged in the same order as the first aid information. Where appropriate a statement such as "No hazard by
this route in normal industrial use" will appear indicating that the amount of a substance to which an individual would be
exposed by this route, under conditions of normal industrial use, would be insufficient to lead to toxic effects.
Personal Protection: Wherever possible engineering control procedures, such as proper
ventilation, should be adopted to control hazardous substances at source. Personal protection and emergency response
equipment should be considered as secondary lines of defence against exposures which are unavoidable.
Where personal protective equipment and clothing is used it must be carefully chosen to ensure correct fit and suitability for
its purpose. It must be kept clean and in good repair. Cleaning procedures should be specified to ensure safe removal of
harmful substances.
A1.5 FIRE AND EXPLOSION DATA
Flash Point: Flash point can be determined by a number of test methods which are of either the open cup or closed cup
types; the method may be stated with the value given.
Auto-Ignition Temperatures: In the course of obtaining these values from reference sources, several instances of
conflicting values were found. In such cases the value quoted is the lowest value obtained. These values must not be
regarded as absolute but as indications only since auto-ignition temperature is particularly sensitive to changes in pressure
and humidity.
Flammable Limits: When known, lower and upper explosive limits are quoted in per cent by volume.
Explosion Hazard: To supplement information provided under flash point and auto-ignition temperatures, the explosion
hazards associated with the product are given in general terms.
A1.6 CHEMICAL DATA
Data shown are for 100% concentrations, unless otherwise stated.
Formula: The chemical formula is the representation of the nature and number of the atoms present in a molecule of a
compound by means of letters and numbers (e.g. C2H6 for ethane).
Chemical Family: Alkali, aliphatic, halogen, etc. are compounds all exhibiting similar chemical properties and reactivities.
Chemical and physical properties show a gradual variation from one compound to the next within the family.
A1.7 REACTIVITY DATA
In this section substances are listed which are known to react in a hazardous manner with the substance covered by the data
sheet. It should be emphasised that the type of mixture referred to here is an uncontrolled or accidental mixing.
If hazardous decomposition occurs, this is indicated in the Notes.
It is strongly emphasised that this list is not exhaustive and other references, e.g. the IMDG Code, should be consulted.
PHYSICAL DATA
Typical physical properties are given in this section. They should not be construed as exact specifications for the product
being handled.
Boiling Point: Unless otherwise stated, all boiling points are quoted at a pressure of 760 mmHg, equal to 101.3 kPa or 1
atmosphere.
Vapour Pressure: Wherever possible vapour pressure has been indicated at several temperatures over the normal operating
range. It is normally quoted in S.I. units i.e. kilopascals. To allow conversion the relationships between different systems
of units are as follow:
1 mmhg
0.133 kPa
1 psi
6.89 kPa
1 atm
101.3 kPa
N.B. These are all absolute, not gauge pressures.
Specific Gravity: For the majority of materials specific gravity is quoted at 15.5/15.5C (i.e. relative to water at 15.5C),
unless otherwise stated.
Coefficient of Cubic Expansion: The volumetric coefficient of thermal expansion per 'C rise in temperature. This is used
to determine the maximum volume of cargo with which a tank can be filled, and for cargo quantity calculation purposes.
Vapour Density: Vapour density is expressed relative to the density of air under given physical conditions. It is used in the
calculation of cargo in the vapour phase and will be a governing factor in the dispersal or accumulation of accidental
releases of vapour.
Molecular Weight: The weight of one kmole of the substance. One kmole consists of 6.02309x1021 molecules and
occupies a volume of 22.4 M3 at O'C and 1.01325 bar (standard conditions). The molecular weight is useful in converting
from molecular units to weight units and in calculating the pressure, volume and temperature relationships for gaseous
substances. The molecular weight is expressed in kg/kmole.
Enthalpy: Enthalpy is the sum of the internal energy of a substance plus the product of the substance's volume multiplied
by the pressure exerted on the substance by its surroundings. It is also known as heat content, sensible heat or total heat.
Enthalpy is expressed in kj/kg. For additional guidance refer to Appendix 3.
Latent Heat of Vaporisation: The value is the heat that must be added to a specified weight of a liquid before it can
change to vapour. It varies with temperature. The value given is that of the boiling point at 1 bar: the unit used is kj/ kg.
Freezing Point: The freezing point (or melting point) of a pure substance is the temperature at which its crystals are in
equilibrium with the liquid phase at atmospheric pressure.
Electrostatic Generation: This section refers to the risk exhibited by low conductivity products which are liable to create
electrostatic charges during transfer or handling.
A1.9 CONDITIONS OF CARRIAGE
This section gives an indication of the conditions under which the product is normally carried, with particular reference to
the mode of transportation (pressurised, fully refrigerated), ship type as defined by the IMO Codes, type of gauging and
vapour detection system requirements etc.
The following conditions are indicated:
•
•
•
normal carriage condition
ship type
whether independent tank required
66 ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 67
•
•
•
control of vapour within cargo tanks
vapour detection
gauging
A1.10 MATERIALS OF CONSTRUCTIONS
General guidance is given here on materials which are unsuitable and suitable for containment or transfer operations.
Written primarily from a hazard prevention standpoint, account has been taken of known effects of different product quality.
A1.11 NOTES AND SPECIAL REQUIREMENTS
Any notes expanding upon the information in the other sections are given here. Attention is also drawn to special
requirements which must be complied with when handling and carrying the substance.
A1.12 INTRODUCTION TO DIAGRAMS
Flammability Diagram
When this information is available, the relationship between composition and flammability of mixtures of cargo vapour, air
and inert gas is given in the form of a flammability diagram. The purpose of these diagrams is to enable procedures to be
developed for avoiding flammable mixtures in the cargo system at all times.
When inert gas or nitrogen is added to a mixture of air and flammable vapour the result is to raise the lower flammable limit
concentration and to decrease the upper flammable limit concentration. These effects are illustrated in Figure Al.1, which
should be regarded only as a guide to the principles involved.
Every point on the diagram represents a mixture of air, flammable vapour and inert gas, specified in terms of its flammable
vapour and oxygen content. Air and flammable vapour mixtures without inert gas lie on the line AB, the slope of which
reflects the reduction in oxygen content as
67 the flammable vapour content increases (i.e.. at 50% air and 50% cargo vapour, oxygen is 101/2'% of tank atmosphere).
Points to the left of the line AB represent mixtures in which the oxygen content is further reduced by the addition of inert
gas.
The lower and upper flammability limits for mixtures of flammable vapour and air are represented by the points C and D.
As the inert gas content increases so the flammable limits change, as indicated by the lines CE and DE, which finally
converge at the point E. Only those mixtures represented by points in the shaded area within the loop CED are capable of
burning.
It is evident from Figure Al.la that as inert gas is added to flammable vapour and air mixtures the flammable range
decreases until the oxygen content reaches a level at which no mixture can burn.
On such a diagram, changes in the composition of the tank atmosphere are represented by movements along straight lines.
When adding air the line is directed towards point A, at which only pure air is left in the tank. When adding inert gas the
line is directed towards a point on the x-axis corresponding to the oxygen content of the inert gas, at which only inert gas is
left in the tank (and in the case of nitrogen will be 0%). These lines are shown on Figure Al.la for an inerted mixture with
concentrations corresponding to point F. When such an inserted mixture is diluted by air its composition moves along the
line FA and therefore enters the shaded area of flammable mixtures.
ICS TANKER SAFETY GUIDE (LIQUEFIED GAS) 68
Figure AI.IB shows that a point C can be established from which a line CA will separate all mixtures (above and to the
right, including point F) which will pass through a flammable condition as they are mixed with air during a gas-freeing
operation, from those mixtures which will not become flammable on dilution with air (those below and to the left of line
CA, including point H). The line CA is called a line of critical dilution. Note that it is possible to move from mixtures such
as at point F to one such as at point H by dilution with additional inert gas. Likewise there is a line of critical dilution when
inerting a cargo vapour atmosphere or purging a tank with cargo vapour, and this line is JB; mixtures above and to the right
of the line JB go through a flammable condition, mixtures below and to the left of the line JB do not.
It can be seen that an initial oxygen content of less than J% will ensure that no flammable mixtures are formed when
purging with cargo vapour, and an initial cargo vapour content of less than C% will prevent the formation of flammable
mixtures when gas-freeing with air. In practice a safety factor of 2 is adopted to account for less than perfect mixing,
equipment error etc. Therefore, the cargo vapour concentration in the cargo system after inerting should not exceed
68 (G/2)% before gas-freeing begins and the oxygen concentration should be below (J/2)% after inerting before purging with
cargo vapour. Although a safety factor of 2 is adopted, every effort should be made to ensure that the inerting and purging
operations are carried out properly using correct equipment and procedures, and accurately calibrated gas detection
equipment.
The diagrams attached to relevant data sheets indicate the limits of flammability of the product in mixtures of air and
nitrogen. They also show the lines of critical dilution.
Properties of Liquefied Gases
The properties of liquefied gases that influence their carriage by sea are saturation pressure, temperature, enthalpy of the
boiling liquid and the saturated vapour, heat of vapourisation, and the specific gravity of the boiling liquid and the saturated
vapour.
The required figures for each product can today be obtained from published tables and computer programmes, with more
accuracy than graphs provide. Nevertheless, to indicate how the different properties vary as a function of temperature, and
their interrelationship in a typical cargo, the graph for methane is shown in figure Al.2 overleaf.