Time-dependent reliability analysis of fixed offshore structures under stochastic loadings

Abstract

Fixed offshore structures under stochastic loadings usually have time-dependent reliability due to time-varying statistics of nonstationary load effects. Since closed-form solutions for such time-varying statistics are difficult to obtain, traditional methods cannot analytically evaluate reliability of structures. In this paper: (1) the explicit closed-form solutions for variances of nonstationary load effects at arbitrary time of linear structures are derived by a pole-residue method; and (2) the explicit solution of a time-dependent reliability index of the structure is then established. For computing displacement variances of offshore structures to waves using the pole-residue method in step (1), an innovative approach of writing the wave force power spectral density into its pole-residue form is developed. To verify the efficiency of the proposed method, a numerical example studies the time-dependent reliability index of the monopile foundation of offshore wind turbines under random waves. Furthermore, a hybrid numerical–experimental study of a jacket platform model to ground motions is conducted. The results indicate that reliability of structures at the nonstationary part is significantly smaller than that at the stationary part. This study provides a reference and advocates attention to structural failures generated by nonstationary responses to extreme loads, e.g., earthquakes.