Seismic demand of jacket and monopile offshore wind turbine support structures under combined loading: Effect of ground motion significant duration

Offshore wind turbines in seismically active regions face wind, waves, currents, and earthquake loading simultaneously, yet how ground motion significant duration shapes cumulative structural damage under combined loading remains poorly understood. This study examines how D 5-95 affects the seismic demand of jacket and monopile foundations supporting a 10 MW offshore wind turbine. Twenty ground motion records spanning D 5-95 from 2.25 s to 73.4 s were applied to nonlinear finite element models in ABAQUS across two soil profiles, with all load components acting concurrently, ductile material damage, pile-soil contact, and explicit dynamic integration. The monopile consistently produced higher demand than the jacket. Tower-top accelerations were 2.5 to 4.0 times larger and the plastic energy ratio reached 34.2. Stiff soil drives higher accelerations while soft soil drives larger displacements, a reversal with direct implications for foundation selection. The jacket remained largely elastic while the monopile accumulated up to 718 kJ of plastic energy. Regressing D 5-95 against bending moment yielded R 2 of 0.57 to 0.59, while the duration trend in plastic energy spanned three orders of magnitude. Significant duration predicts cumulative damage far more reliably than peak force demand, and excluding it from seismic assessment of offshore wind foundations is a meaningful omission.