Reliability and fragility assessment of offshore floating wind turbine subjected to tropical cyclone hazard

In the present study, an overall simulation-based procedure to assess the fragility and reliability of offshore wind turbines (WTs) subjected to tropical cyclone (TC) hazard is presented. The procedure relies on establishing the synthetic databases of TC track, TC wind and wave fields, and structural response. The construction of the databases and their use for reliability analysis are described. The proposed procedure can take into account the dependent TC induced wind and wave loads to estimate failure probability by considering multiple performance limits of various components. The overall procedure is illustrated for the reliability evaluation of an offshore semi-submersible WT placed at four potential locations near the coastline of mainland China. The results indicate that the failure probability of the considered WT is greater than the optimal value suggested in the literature, showing that the TC risk of such a WT could be very high if it is installed at offshore locations near the coastline of mainland China. The component failure analysis also shows that the failure probability is dominated by the tower buckling, followed by the platform overturning. Sensitivity analysis results indicate that the annual failure probability of the WT for the considered offshore locations is greater than 10^-3 if the site-specific 50-year return period values of the annual maximum wind speed and significant wave height are used for the design and without additional load factor. An increased design environmental loads need to be considered to achieve a further reduction in the failure probability.