Seismic vulnerability of offshore wind turbines to pulse and non-pulse records

The increasing number of wind turbines in active tectonic regions has attracted scientific interest to evaluate the seismic vulnerability of offshore wind turbines (OWTs). This study aims at assessing the deformation and collapse susceptibility of 2MW and 5MW OWTs subjected to shallow-crustal pulse-like ground motions, which has not been particularly addressed to date. A cloud-based fragility assessment is performed to quantify the seismic response for a given intensity measure and to assess the failure probabilities for pulse-like and non-pulse-like ground motions. The first-mode spectral accelerationS(a)(T-1) is found to be an efficient response predictor for OWTs, exhibiting prominent higher-mode behavior, at the serviceability and ultimate conditions. Regardless of earthquake type, it is shown that records with strong vertical components may induce nonlinearity in the supporting tower, leading to potential failure by buckling in three different patterns: (i) at tower base near platform level, (ii) close to tower top, and (iii) between the upper half of the main tower and its top. Type and extent of the damage are related to the coupled excitation of vertical and lateral higher modes, for which tower top acceleration response spectraS(a,i(Top))is an effective identifier. It is also observed that tower's slenderness ratio (l/d), the diameter-to-thickness ratio (d/t), and the rotor-nacelle-assembly mass (m(RNA)) are precursors for evaluating the damage mode and vulnerability of OWTs under both pulse-like and non-pulse-like ground motion records.