Seismic fragility analysis of jacket type offshore platforms considering soil-pile-structure interaction

There is an ever-increasing demand to assess integrity of the old platforms in which seismic fragility curves need to be developed for the integrity analysis in case of earthquake. Therefore, in this paper, an analytical approach is presented to develop seismic fragility curves of an existing jacket type offshore platform in the Persian Gulf. The method is based on nonlinear dynamic analyses results of the platform subjected to possible ground motions expected at the site. The effects of both aleatory and epistemic uncertainty are considered using the Comprehensive Interaction Incremental Dynamic Analysis (Comprehensive Interaction IDA) method, simultaneously. The influence of aleatory uncertainty is addressed using the Incremental Dynamic Analysis (IDA) method, separately. Then, the results of Comprehensive Interaction IDA are compared with the results of IDA in seismic fragility curves. Even though the probability of failure at a given Sa(g) of Comprehensive Interaction IDA is less than that of IDA, the influence of aleatory uncertainty has the most significant proportion. The seismic fragility curves are defined in terms of cumulative lognormal distribution functions and the combination of a structural damage index with a ground motion index. These are used to define two damage states: an extensive damage state and a collapse state in the case study.