Nonlinear Time History Analysis for Assessment of An Existing Offshore Jacket Platform

Abstract eng:
It is often necessary to reassess the seismic performance of existing platforms due to changes to platform deck loading, increase in number of well conductors, or because modern characterizations of earthquakes result in more severe ground motions than were considered at the time of the design of existing platforms (20-30 years ago). The primary goal of seismic reassessment of an existing platform is to assure that the platform system will have sufficient strength and ductility to withstand the Ductility Level Earthquake (DLE) without collapsing, although structural damage may occur. For this study an existing jacket platform off the coast of California was selected. The seismic analyses were based on new design response spectra and three sets of conforming ground motions for the 1000-year DEL. They were developed using the New Generation of Attenuation Relationships considering the influence of near-source and rupture directivity effects. Ground motions were developed for non-liquefied and liquefied conditions. The seismic assessment was carried out in three phases. The first phase of the study focused on the assessment of the foundation of the platform for the new DLE ground motions. Sensitivity analyses were undertaken to determine the effect on the platform’s response to variations in key parameters such as liquefaction and, structural and soil damping. The second phase assessed the platform with an additional 18 conductors. In the final phase of the study, analyses were conducted to determine the Reserve Strength Ratio (RSR) by assessing the sensitivity of the structure and its foundation to the intensity of the motions and to soil capacity. The results of the analyses are presented in this paper.

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