Offshore Platform Seismic Analysis

SC Solutions was engaged to conduct seismic analysis and design check of an offshore gas platform, which is comprised of a four-legged steel gravity substructure (SGS) and Topsides.  The platform is supported on square foundations located directly beneath each of the four vertical columns of the SGS at a water depth of approximately 70 meters.  Due to the unique design of the platform, rock blanket, and foundation arrangement, the following analyses were performed:

• Independent verification of input ground motions
• Site response analysis to develop the strain compatible soil properties
• Development of foundation dynamic soil spring stiffness and damping (impedance) for structural analysis and code compliance check
• Development of a SASSI structural model from an ANSYS detailed SGS model and an SACS detailed Topsides model
• Seismic structural evaluation analyses for the following: Extreme Level Earthquake (ELE, 500-year return period), Abnormal Level Earthquake (ALE, 3,000-year return period) and Safety Case (10,000-year return period)

To properly account for the effect of the weight on the stiffness of the soil, a unique iterative procedure to obtain strain-compatible properties from soil block analyses for an ALE event was developed and implemented by using the System Analysis for Soil Structure Interaction (SASSI) program.  The soil block model consists of the detailed Topsides and simplified SGS models and a foundation soil block composed of 3D soil elements with nonlinear properties. The final strain-compatible properties were extracted once the soil block model results converged. Average strain-compatible properties were calculated from results at each soil layer and used as input soil properties for the SSI analysis of the detailed models.
Using the strain-compatible soil properties developed by SC Solutions, SSI analyses were performed for the ELE and ALE events to generate (1) maximum foundation loads to evaluate the foundation design, (2) in-structure acceleration response spectra at various locations in the Topsides for design of pipe and equipment support, and (3) dynamic foundation impedances for ELE and ALE dynamic analyses in other software for design evaluations of SGS and Topsides structural components.