IMPACT OF NUMBER OF SELECTED RECORDS ON PROBABILISTIC SEISMIC PERFORMANCE OF OFFSHORE JACKET PLATFORMS

Intensive seismic excitations might be considered as one of the most important loading cases imposed on offshore jacket platforms, especially for the ones located in regions of high seismic hazard. For evaluating probabilistic seismic performance of such intricate structures, Incremental Dynamic Analysis (IDA) could be employed as an efficient andapplicable procedure. In order to conduct IDA, appropriate suites of ground motions are required that can be selected by different scenarios and methodologies. Furthermore, due to 3-dimensional numerical models of jacket platforms with a lot of nonlinear structural elements time history analyses of these infrastructures are truly time consuming. Hence, number of selected records can substantially change the computational costs of such assessments. This paper aims to study the effects of sample size of ground motions on probabilistic seismic performance of offshore jacket platforms. In this process, IDA has been performed considering a suite of 40 far field records (selected as explained in (Abyani et al., 2017; Baker et al. 2011)) to model the aleatoric variability of the records. To this end, a numerical model of the investigated jacket platform is created in OpenSEES software. The considered sample platform is located in the Persian Gulf in water depth of about 75 m. The platform comprises a 4-Ieg jacket supporting a wellhead deck, which was designed and analyzed in accordance with the requirements of the American Petroleum Institute. The jacket is approximately 84 m high from mud line up to top of the jacket. The lateral resisting systems of the jacket are the V-bracing and inverted V-bracing systems, which constitute a multistory X-bracing system at the second and fourth horizontal plan. The jacket is fixed to the ground by four through leg piles and the gap between pile and leg is filled with grout to provide a composite section named grouted section. All top side and jacket masses including structural and hydrodynamic masses have been considered as concentrated masses at the element joints. Accordingly, all top side and jacket loads are applied at joints as equivalent point loads.

Keywords:

Seismic fragility , Offshore , Genetic algorithm , Seismic performance , Jacket platform