Good morning Alex,
First to clarify, I am not mathematically trained.
I know the Launch Process. I am not the numbers guy.
As a supplement to this lack of formal education, MIL STD 1625D allows me to use my IDNA department to figure the Launch calcs.
Below is a collaboration of IDNA (Internal Design and Naval Architecture) and me.
Amount of allowable movement of the ship.
We define Creep as ship movement downhill and Settling is the ships weight pressing down on its supports.
I don’t know of a number that is specified as “Allowable ship movement”
Ship creep down the ways is monitored throughout the launching sequence on average 1 week prior to Launch and ship settling is monitored throughout the erection cycle.
In the graving dock, the ship is erected on transverse I-beam spreaders supported by feet on the outboard ends. The ship comes down on the order of ¼ - ½” so that there are three points of contact: the two outboard and the centerline. I have attached 2 Keel graphs. These are the measurements we take throughout the erection cycle. 1 graph show a typical Keel Reading of a ship in our graving dock (MLP1) with a 1 to 2” settling. The other graph shows a ship on the ways, the T-AKE 5.
This ship had one of our worst out of level settlings, with a 7” deviation from initial keel setting to Launch. The actual settling was 4 ½” because the Keel line was no longer at zero when the blocks were erected. Although a 4” settling is not unheard of, a better job needed to be done by the alignment team.
When the ship is being built, do you note any significant movement when, for example, adding large parts?
At the start of a new contract, IDNA will send me a blocking plan designed to support the completed ship.
I will make recommendations for temporary blocking or stands to support erection breaks or heavier areas.
IDNA will review these recommendations for impact to hull structure and slab the set limits on how long these temporary support can remain in contact with the hull.
When large parts of the ship are added, there can be significant movement in the ship. For this reason sometimes the ship need to be fully welded out before adding heavy items above like LNG tanks, engines or generators. If this ship isn’t welded out and still supported by temporary screw jacks, all the load goes though a single point into the ground, which can cause buckling in the ship’s structure.
Is there any effort to diminish this movement or any associated calculations?
There are temporary screw jack supports (a screw jack is an adjustable topped stand that can support up to 200t) that support the ship before it is fully welded out. They can be adjusted up and down to place the block in the correct position.
Is there some allowable movement in event of an earthquake?
We don’t have an “allowable movement limit” but we do have an allowable pressure (800psi, the limit on douglas fir). There are seismic blocks that support the ship in the event of an earthquake. These blocks also must support a minimum of 10% displacement located off centerline.
Is there any calculations associated with that?
Yes these are in compliance with Military Sealift Command STD001-085-6229179.
Does anything of the above come into consideration when designing the ground supports?
In an earthquake, the ship will dent before the blocks give out, causing damage to the ship but making sure there is no catastrophic failure that could result in loss of life.
The softcap used in launch isn’t added until the week prior to launch because the amount of settling over the erection cycle can vary on the order of inches.
Are any thermal effects taken into consideration?
The temperature of the structure must fluctuate during construction, expanding and contracting.
Is there a procedure to work around that, or calculations associated with that?
Larry will know more about structure fluctuation, but from a launch standpoint. Higher temperatures lower the grease’s friction coefficient and result in the ship reaching a higher speed during launch. Thermal effect can reek havoc on the alignment process. Especially at this time of year when we still can have sunny days and the nights get colder. Full sun on one side of a completed ship can cause the bow to an inch or more depending on the surface area being heated by the sun. That is why we make all readings on critical blocks in the early morning when the steel is cold and in a condensed state.
Do you experience any creep?
Over a period of time the weight of the ship creeping into the supports.
Is there any record of this or calculations associated with that?
The ship does settle into the cribbing over the erection cycle, and that is monitored. There is enough surface area on the blocks that the settling is not in risk of catastrophic block failure. However, in the 10 hours or so prior to launch, the cribbing blocks are removed and the ship’s weight is transferred to the launch cradle. The surface area on this wood is much less, and significant crushing occurs. If the ship sits too long only supported by the cradle, the blocks will eventually fail and the ship will launch itself.
What safety factors are used in construction and support.
I am particularly interested in safety factors pertaining to any chains used, and any safety factors associated with the supports or anything else above.
Significant safety factors are used in all rigging operations. I can’t comment on the exact numbers, but typically, the lower the confidence in the weight estimate of a block, the larger the safety factor used. In some instances where there is a low confidence in a weight estimate, safety factors for padeyes can be as high as 7.
Email back if you have any more questions.
Hope this helps,
Larry