Seismic fragility analysis of monopile offshore wind turbines considering ground motion directionality

The necessity of performing a detailed seismic fragility analysis of offshore wind turbines (OWTs) is well established in the wind power industry. This paper presents a seismic fragility analysis for large-scale monopile offshore wind turbines (MOWTs) considering ground motion directionality. Based on recommendations by the Eurocode EN 1993-1-6, DNVGL-ST-0126 and literature study, the critical displacement, rotation, local buckling strength for structural components were proposed. Fragility curves under different angles of ground motion incidence were obtained by using the truncated incremental dynamic analysis (TIDA) and compared between the first scenario of the wind turbine parked in a calm sea and the second scenario of the wind turbine in normal operation at the rated wind speed of the turbine. The results show that ground motion directionality has obvious influence on the seismic fragility of the MOWT. Wind and wave loads increase the probability of the MOWT exceeding limit states and amplify the ground motion directionality effects on the structural fragility. Neglecting aerodynamic damping leads to an overestimation of the structural fragility. The excessive displacement at the tower top, local buckling at the bottom of the tower and in the monopile at the mudline section, and tension fracture in the grouted connection are the critical failure modes of the MOWT being subjected to earthquakes.