Time-dependent seismic fragility for onshore monopile-supported wind turbines considering wind-structure-soil interaction and long-term operational processes

Monopile-supported wind turbines (MWTs) are increasingly deployed in seismically active regions, where seismic loads can significantly affect their structural safety. Meanwhile, over a service life of 25-30 years, MWTs are continuously subjected to wind loads and 1P/3P loads. These lateral forces lead to the continuous evolution of soil properties, resulting in time-dependent seismic performance of MWTs. This study specifically focuses on onshore MWTs with the water table located below the pile tip, for which the effects of offshore environmental loads such as waves and currents are not considered. On this basis, a framework is proposed for evaluating the seismic response of MWTs after long-term operation, with comprehensive consideration of wind-structure-soil interaction. The framework can effectively characterize the long-term evolution of soil properties and analyze its influence mechanism on the seismic response. On this basis, the incremental dynamic analysis (IDA) method is employed to conduct a time-dependent seismic fragility analysis, and the seismic fragility curves under different operational histories are obtained. The results indicate that, with increasing operational duration, the probability of the wind turbine structure exceeding the serviceability limit state and the ultimate limit state increases significantly. Moreover, the influence mechanisms of factors such as wind characteristics and seismic characteristics on the structural seismic performance change significantly. Overall, the operational history of onshore MWTs has a pronounced degrading effect on their seismic performance.