A framework for rapid seismic fragility analysis of offshore wind turbines considering wind, wave and scour

The rapid growth of the offshore wind turbine (OWT) industry has led to the development of wind farms in earthquake-prone regions. OWT structures are dynamically affected by environmental forces such as wind, waves, and currents. Seismic analysis of these structures is challenging due to the nonlinear behavior of soil under various dynamic loads and the scour effects resulting from waves and currents. This study aims to evaluate the seismic performance of a 5-Megawatt (MW) monopile-supported OWT subjected to simultaneous wind, wave, and earthquake excitations under scour conditions. For this purpose, the Endurance Time (ET) method, a runtime-efficient approach, is employed to conduct probabilistic seismic assessments and develop fragility functions under various scour depths and soil types. Load excitations on the OWT are introduced through endurance time excitation functions (ETEFs) and dynamic wind and wave loads. Validation with incremental dynamic analysis reveals that the ET method accurately estimates the seismic behavior of MOWTs with low computational cost. Overall, the study concludes that increasing scour depth or decreasing soil resistance leads to higher fragility values and exceedance probabilities over a 50-year period, along with a reduction in the reliability indices for all damage indicators associated with the serviceability and ultimate limit states. Scour can also cause the location of maximum demand to shift along the height of the OWT. Additionally, under severe scour depths (greater than 1.5 times the pile diameter), the OWT undergoes significant damage due to operational environmental loads, with the increase in vulnerability being particularly more pronounced for the serviceability limit state compared to the ultimate limit state.