Effects of Operational Conditions on TMD Control Efficiency of Offshore Wind Turbines Subjected to Wind–Wave Seismic Multi-Hazard Loads

To elucidate the influence of operational conditions on the seismic responses of offshore wind turbines (OWTs) and the vibration mitigation efficacy of tuned mass dampers (TMDs) under multi-hazard scenarios, time-domain dynamic analyses are performed for OWT systems subjected to combined wind, wave and seismic excitations. Five typical operational conditions are considered, including cut-in operation, rated-power operation, cut-out shutdown, 1-year return-period extreme shutdown, and 50-year return-period extreme shutdown. The nacelle acceleration and tower-top displacement responses of the uncontrolled structure are comparatively characterized, the peak and root-mean-square (RMS) vibration reduction ratios of the TMD for fore-aft vibrations are quantitatively assessed, and the intrinsic mechanism governing the response discrepancies across operational conditions is elucidated. Numerical results demonstrate that seismic excitation dominates the extreme structural responses of the OWT system. Under the rated-power condition, the peak acceleration and displacement under coupled seismic loading reach 6.90 and 2.19 times the corresponding values under wind–wave loads alone, respectively. Substantial discrepancies in structural responses are observed across operational conditions, with aerodynamic damping magnitude and the spectral properties of hub rotational loads identified as the key influencing factors. The TMD exhibits reliable vibration control performance overall: the optimal control efficacy is achieved under the 1-year return-period shutdown condition, with a peak acceleration reduction ratio of 34.8%—by contrast, its mitigation performance degrades significantly under the 50-year return-period extreme-turbulence condition, with the peak acceleration reduction ratio dropping to merely 15.8%.