A dual response surface-based efficient fragility analysis approach of offshore structures under random wave load

Abstract

This paper presents an efficient fragility analysis approach for offshore structures subjected to random wave load considering parameter uncertainty. Fragility analysis of structures under random dynamic load is generally accomplished by the direct Monte Carlo simulation (MCS), which requires extensive computational time. Hence, in the present study, a new cumulative distribution function matching-based fragility analysis approach is proposed in the framework of dual response surface method to reduce this computational burden. The proposed approach does not require MCS during failure probability computations once the two response surfaces of mean and standard deviation of response are obtained. Thereby, a substantial amount of computational time is saved. The proposed approach is generic in nature and can be applied to any offshore structure, with different probability distributions of system parameters and higher uncertainty levels, as well. The record-to-record variation of random wave force time-histories (that occurs even for the same set up of hazard parameters) can be easily considered by the proposed approach. The accuracy of the proposed approach is maintained by applying the moving least-squares method, in place of the conventional least-squares method, which is often reported to be a source of error. Two illustrative examples (one simple benchmark problem and another practical steel offshore platform structure) are presented to demonstrate the efficiency of the proposed fragility analysis approach. The results indicate that the present moving least-squares method yields more accurate solutions than the conventional least-squares method. Also, the computational efficiency by the proposed approach over the usual MCS-based approach can be clearly envisaged from the numerical studies.