Evaluation of Optimal IM-EDP pairs for Typical South Pars Fixed Pile-Founded Offshore Platforms

Probabilistic seismic demand models (PSDMs) for typical South Pars fixed pile-founded offshore platforms, utilizing probabilistic seismic demand analysis (PSDA) have been presented in this study. It expresses the probability that a system experiences a certain level of engineering demand parameter (EDP) for a given intensity measure (IM) level. Utilizing Bin approach, 80 ground motion records have been selected. A three dimensional (3D) nonlinear model has been generated considering the effects of soil-pile-structure interaction (SPSI) and analyzed for each ground motion. The process involves a modal analysis to determine natural frequency as well as a static pushover analysis to establish yield values, and mode shape information, and finally 80 dynamic time-history analyses to determine demand, given IMs. With the probabilistic models being traditionally conditioned on a single seismic IM and single EDP, the degree of uncertainty in the models is dependent on the IM and EDP used. The present study evaluated optimal PSDMs build from 16 IMs against a wide range of EDPs in levels of local, intermediate and global. From a large combination of IM-EDP pairs, a selection of the optimal pairs has been made owing to the criteria of practicality, effectiveness, efficiency, and sufficiency. Results indicate the absolute superiority of velocity-related IMs compared to acceleration, displacement and time-related ones for most of EDP types. In particular, Housner Intensity-Global Drift and Specific Energy Density-Global Ductility (in global level), Housner Intensity-Jacket Drift (in intermediate level) and Housner Intensity- TopDeck Differential Settlement (in local level) result in optimal pairs. Conversely, Sa(T1, 5%), the widely used IM in probabilistic assessment of fixed pile-founded offshore platforms, demonstrates relatively poor performance in predicting the demand parameters.