Multi-hazard stochastic response analysis and reliability evaluation of a 5 MW monopile offshore wind turbine based on extended platform FAST-S

This study investigates the random dynamic response of a 5 MW monopile offshore wind turbine (MOWT) subjected to wind, wave and earthquake loads. Based on the aero-hydro-servo-elastic code FAST, a multi -field coupled simulation module considering earthquake, FAST -S, is developed for the fully coupled multi -hazard analysis of OWTs. Compared to the original FAST code, a seismic analysis module (SeismoDyn) and a soilstructure interaction (SSI) module (SoilDyn) have been supplemented in FAST -S. SeismoDyn is developed by modifying the Kane's kinetic equation in the FAST, and the SoilDyn is developed through Winkler foundation model in order to consider the nonlinear SSI. Moreover, FAST -S has been validated by other recognized codes. Based on FAST -S, a fully coupled numerical analysis model of the NREL 5 MW MOWT is established, the random responses and reliability evaluation of the offshore wind turbine under wind, wave and earthquake loads are analyzed by probability density evolution method. The results show that the aerodynamic and hydrodynamic actions increase the damping of the wind turbine system, which has a certain effect on inhibiting the tower's vibration caused by the earthquake. Therefore, compared with the coupled wind -wave -earthquake case, the sole earthquake case may be more unfavorable, and should be paid more attention to in high risk earthquake regions. The results show that the probability density functions of offshore wind turbine response under strong earthquakes obviously deviates from the typical regular distribution models. Based on the theory of equivalent extreme value event, the dynamic reliability of the offshore wind turbine is calculated. Seismic actions maybe the major factor in the design of the support structure. Nevertheless, owing to the aerodynamic and hydrodynamic damping effects of wind and wave loadings, the structural responses under wind -wave -earthquake are more moderate than ones subjected to solely earthquake. The strong seismic loads would cause significant complex variations of time -dependent probability density function of the OWT response.