When subsea pipelines and tie-in spools are supported directly on the ground, the differential ground movement due to seismic wave propagation may be neglected. This is because pipelines and spools have their centers of mass very close to the ground level. However, when the pipelines/spools are supported on, or connected to different structures (platform, subsea isolation skids, PLETs, etc.), the impact of the differential movements of these supports and structures on the pipeline and spools response should be considered. The industry approach to the seismic design of a subsea pipeline system is that the system is designed for Extreme Level Earthquake (ELE) and is able to withstand Abnormal Level Earthquake (ALE) with some damage, but not loss of containment. The design of a pipeline system is iterative by nature. The pipeline loads on the subsea structure are input to the subsea structure design, while the subsea structure foundation resistance is an input to the pipeline integrity checks. Hence, an integrated model incorporating the pipeline, tie-in spools and subsea structures is the most comprehensive design method.
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