The seismic analysis of an offshore platform may be accomplished using classical finite element techniques, modified to take into account the fluid-structure and the soil-pile interaction effects. However, a major uncertainty exists as to what constitutes an appropriate seismic forcing function. Current practice employs seismic data recorded from onshore sites. Recent research, on the other hand, suggests that the seafloor does not respond in the same manner as onshore sites during an earthquake. The presence of the water column above the seafloor, soft-saturated seafloor sediments, and the dissimilarity between the impedance at the soil-water interface can interact to cause marked differences in the resulting ground motions. In this dissertation, a series of free-field strong ground-motion records obtained from actual seafloor sites, located offshore southern California in the Santa Barbara Channel, are analyzed and compared to onshore earthquake records to further identify and substantiate the dissimilarity between the two conditions. In addition, an offshore steel-jacket structure, typical of the type employed by the oil and gas industry, is analyzed using compatible earthquake time-series from both onshore and offshore sites to investigate differences in platform response. As a result of this investigation, recommendations are put forth for the type of seismic forcing functions to be used in the design of new or in the requalification of existing offshore structures, as well as for future research required to further advance the state-of-the-practice.
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