Seismic Design of Subsea Pipeline Systems: Evaluation Study

Abstract

Abstract 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. Pipeline systems are typically analyzed for seismic loadings by the modal response spectrum method, or time history analysis. The modal response spectrum method is a linear method and is not adequate when non-linearity should be considered (e.g. ALE). It is desirable that the designer should make every effort to ensure that the ALE is a confirmatory check, which should not lead to major adjustments in the design. To achieve this vision, the designer should use ductile material and the design should have sufficient flexibility; meaning that the system should be able to undergo large deformations without loss of containment. In essence, a time history method should be adopted. However, it is not often economical or a trivial task to perform such analysis in the early phases of the project definition (i.e. concept selection or pre-FEED) or even during the FEED phase. Taking a case study, the application of different seismic analysis approaches to a pipeline system, including pipelines, tie-in spools, and subsea structures, is presented and evaluated. This paper also recommends the appropriate level of seismic analysis for different phases of project definition.