depending on such factors as location and population density. It is desirable to allow the
owner flexibility in achieving the required level of public safety. B.3.3 The SSE level of seismic loading is to be used for a limit state check on the specified
components. The specified SSE is the minimum level of ground motion that must be used for
the analysis. The actual level must be specified by the owner, and when used in conjunction
with other considerations, such as location, siting, type of impounding system, hazard
control, local climatic conditions, and physical features, it must be sufficient to ensure
adequate public safety to the satisfaction of the regulatory authorities. A risk analysis study is
recommended. At the SSE level of seismic loading, primary components of the LNG
container are permitted to reach the stress limits specified in 7.2.2.7. An LNG container
subjected to this level of loading must be capable of continuing to contain a full volume of
LNG.
B.3.4 The impounding system must, as a minimum, be designed to withstand the SSE level
of loading while empty and the OBE level of loading while holding the volume, V, as
specified in 5.2.2.1. The rationale is that should the LNG container fail following an SSE, the
impounding system must remain intact and be able to contain the contents of the LNG
container when subjected to an aftershock. It is assumed that the strength of the aftershock
can be reasonably represented by an OBE. B.3.5 Systems or components, the failure of which could affect the integrity of the LNG
container, the impounding system, or the system components required to isolate the LNG
container and maintain it in a safe shutdown condition, must be designed to withstand an
SSE. B.3.6 The operator is required to install instrumentation capable of measuring ground
motion at the plant site. Following an earthquake that produces ground motion equal to or
greater than the design OBE ground motion, it is advisable that the operator of the facility
either take the LNG container out of service and have it inspected or prove that the LNG
container components have not been subjected to loading in excess of the container's OBE
stress level and design criteria. For instance, if the LNG container was partially full during
the seismic event, calculations can prove that the container OBE stress levels were not
exceeded.
B.4 Design Response Spectra.
Using the OBE and SSE ground motions as defined in Section B.2 and B.3.1 respectively, vertical and horizontal design response spectra must be constructed that cover the entire
range of anticipated damping ratios and natural periods of vibration, including the
fundamental period and damping ratio for the sloshing (convective) mode of vibration of the
contained LNG.
B.5 Other Seismic Loads.
B.5.1 Small LNG plants consisting of shopbuilt LNG containers and limiting processing
equipment should be designed for seismic loading using the ground motion specified by
ASCE 7, Minimum Design Loads for Buildings and Other Structures. Either a structural
response analysis should be performed or an amplification factor of 0.60 should be applied to