Nonlinear dynamic response of offshore structures under seismic loading is a challenging and interesting problem to structural engineers. Fixed offshore platform under intensive ground shaking may undergo deformations well into the inelastic range. The first part of this paper discusses the seismic analysis methodology we developed considering the following effects (1) Soil radiation damping; (2) Soil-pile gapping; (3) Multi-level excitation and (4) direction of seismic loading. The second part of the paper uses case studies to demonstrate the methodology. These case studies involve a representative fixed offshore platform, Platform A. Platform A was analyzed against 10,000-year earthquake events. These 10,000-year events are based on actual earthquakes. The ground motion records were scaled up so that the resulting earthquakes would have a return period of 10,000 years. Time history simulations were carried out using the nonlinear dynamic structural analysis program USFOS. Results show that (a) the effect of soil radiation damping is not negligible; (b) Soil-pile gapping and soil radiation damping may cause opposite effects on the platform response; (c) Direction of seismic loading affects soil-structure interaction as well. The goal of the paper is to discuss and illustrate the methodology for advanced nonlinear dynamic analysis of fixed offshore platforms during 10,000-year earthquake events.

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