KHK level 2

The Basic Plan for Disaster Prevention” by the Central Disaster Prevention Council in Japan requires the introduction of “Two Steps earthquake assessments” for the critical facilities including that of HPG (High Pressure Gas). Seismic design for those facilities shall be both Level 1 earthquake (operating basis earthquake) and Level 2 earthquake (safety shutdown earthquake, probable strongest earthquake even though with low probabilities).

The aim of the seismic design is to keep functionality or safety depending on the earthquake intensity level as follows:

(1) Level 1 Required Seismic Performance (L1-RSP)

The facilities shall not keep any fatal residual deformation and any leakage of the high pressure gas during and after the Level 1 design base earthquake.

(2) Level 2 Required Seismic Performance (L2-RSP)

The facilities of seismic importance Ia and I shall not leak of any high pressure gas during and after the Level 2 earthquake caused by the inertia force/relative displacement and the ground failure.

For Level-2T earthquake, the detrimental ground distortion by possible liquefaction and land sliding shall be taken into account.

The L2-RSP for High Pressure Gas piping systems shall allow non-elastic deformations. The design of HPG piping systems for the evaluation of L2-RSP requires non-elastic analysis instead of elastic analysis for L1-RSP. The simplified seismic design method for the evaluation of L2-RSP is proposed. The evaluation requires investigations of effects of both ground acceleration, and ground displacement (vertical and/or lateral movement) due to liquefaction during and after L2-Earthquake.

Seismic Design Concept of piping for Ground Liquefaction

Ground displacement make foundations of the Seismic Structures (equipment and storage) sink vertically, move laterally or incline and consequently piping systems suffer from relative displacements between supporting points, which may exceed several tens of centimeters. Design Concepts of piping system for the L2-RSP are proposed to absorb this kind of large relative displacements due to ground displacement as follows.

(1) Utilizing plastic deformation capability of elbow to secure the flexibility of piping system. The plastic deformation capability is fairly large considering L2-RSP.

(2) Utilizing deformation capabilities of expansion joint to keep piping from detrimental deformation. The deformation capability is extremely larger than the specification for usual operation.

(3) Letting some supports lose their restraining functions with the progress of relative displacements.

These design concepts was verified by tests using real scale piping models.

On this requirement, the HPG seismic design code (HPGSL) had been amended to introduce the two steps earthquake assessments in the MITI Notification, March 1997.

The seismic design flow diagram is shown in Figure 1. Towers, vessels, tanks, piping and their supporting structures and foundations of the HPG facilities shall be designed against earthquake. All of these seismic design structures are classified into seismic importance (Ia, I, II, III). For the first step seismic assessment, maximum ground acceleration of Level 1 design base earthquake are specified in accordance to the seismic importance. The performance of the seismic design structures during and after the Level 1 earthquake shall be evaluated that the facilities shall not lose their operational functions seriously.

After that the critical piping (Importance Ia and I) shall be evaluated against Level 2 earthquake for the assuring the safety between and after the specified earthquake level. The procedure of bend angles using equivalent linear method for Level 2 seismic performance is shown in Figure 2.

Equivalent Linear Method

The plastic flexibility factor of 90° bend shall be calculated as follows:

(a) Closing in-plane approximation

(b) Opening in-plane approximation

(c) Out-of-plane approximation

(d) Average of closing and opening in-plane and out-of-plane approximation

where, h is flexibility factor of bend, and θ is rotational distortion (degree), Sy is the yield strength of material at temperature, Syo is the reference yield strength (215 N/mm2), and ke (=1.65/h) is the “elastic” flexibility factor defined by ASME B31.3 . Other than 90° bend, the linear interpolation shall be made.

For inertia response analysis, the average of closing and opening in-plane and out-of-plane shall be used because the complexity of response included closing, opening and out-of-plane behavior.

However, as the load due to the ground movement can identify the applied direction to piping configuration, the appropriate plastic flexibility factor shall be used. Or, the severest plastic flexibility factor of opening in-plane (Eq. 2) shall be used in convenience of design.