Deepwater subsea developments require installation of relatively large, heavy structures in poor soil conditions. In many cases, the foundations of choice are large diameter suction piles. The industry has excellent experience in design of these foundations for operational conditions, but offshore codes provide little guidance regarding performance during earthquakes. This paper presents an example of a recent project where suction piles were used to support large manifold structures in a seismically active region. Preliminary verifications using a conventional pseudostatic approach showed that the planned foundations would have been unsatisfactory for seismic loadings. More detailed performance based design was employed to avoid over-dimensioning the piles. Dynamic finite element analysis was used to evaluate structure displacements during and after the design earthquake. Advanced soil models were required to capture the nonlinear behavior of the soft soils at the site. The manifold displacements were compared to operational requirements to assess foundation acceptability. The approach provided considerable savings. This paper outlines the main aspects of the two approaches, and shows the advantages of performance based design for these structures. Our conclusion is that careful analysis is required for the soil conditions typically found at deepwater sites; standard simplified approaches may not suffice.

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