An offshore Gravity Based Structure (GBS) and the underlying seabed soil layers were analyzed to evaluate the seismic retrofit strategy to extend the platform’s useful life and increase its production capacity. The GBS is a steel structure consisting of a box shape caisson substructure and a Box Girder Deck (BGD) which supports the topsides. The BGD and topsides are seismically isolated from the substructure through Lead Rubber Bearings (LRB). The caisson is stabilized on the seabed with a sandcore consisting of 30m of compacted sand layers inside the caisson.

An integrated and nonlinear model was developed and analyzed in time domain where the soil continuum, sandcore, caisson, conductors, BGD, and topsides were modeled in detail and included in a single Finite Element (FE) model. The model scale spans the extremes: from a 0.5km by 0.5km soil domain down to explicit modeling of stiffeners within the steel structure. The nonlinear and pressure dependent response of soil layers were modeled via a plasticity constitutive law. The latter allowed for modeling the significant variation in seismic response of granular seabed soil layers under pressure from the 30m tall sandcore versus the seabed soils located outside the structure’s footprint. Nonlinear elements connecting the BGD to the caisson were calibrated to match the laboratory measured LRB responses. A plasticity model with a damage evolution function and failure criterion was used to model the cyclic response of the BGD’s stiffened bulkheads. Via explicit time integration, the model is capable of capturing cyclic degradation, post-buckling and softening response, and failure. Furthermore, the analysis incorporated the actual construction sequence of topsides placement and LRB retrofits to capture their effect on bearing reactions and stresses in the stiffened bulkhead panels. Also, the BGD and caisson were coupled via contact algorithms to allow modeling of potential pounding after LRB failure. The resulting FE model had over 4.5 Million degrees of freedom.

Before performing the global analyses the response of the key components of the integrated FE model was extensively verified. Among others, the seismic wave propagation, soil material response, and the nonlinear buckling of BGD’s stiffened bulkheads were verified. Site response, single element, and detailed local analyses were utilized for verifications respectively. Once verified, the FE model was used for time-history analyses at multiple hazard levels to support performance-based retrofit design. Finally, safety margin analyses with amplified ground motions beyond the project’s maximum considered event were implemented to evaluate the system’s reserve capacity. High-performance computing was utilized to perform over 20 time history analyses.

The results of the verification studies, a sample of global analyses results, and the safety margin analysis result are presented. The detailed analyses resulted in significant retrofit cost savings through more realistic demand calculation and confidence gained through safety margin evaluations. They also reduced risk through identifying 3D demand amplifications on the topsides. In addition, the detailed analyses provided important insights into the global performance of the retrofit strategy in mitigating the seismic risk.

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