Field observational study of offshore wind turbine dynamic response to a marine earthquake event (Ms 5.0) in the South Yellow Sea, China, on November 17, 2021

Abstract

A Ms 5.0 earthquake occurred in the South Yellow Sea in the area offshore of Dafeng (China) on November 17, 2021. The authors were monitoring wind turbine vibrations at regional offshore wind farms during the earthquake period, collecting valuable field data. Records from several surrounding onshore seismic stations were also examined. Analysis of the self-vibration status of offshore wind turbines revealed that the signal spectrum of the working status of the turbines was broadly the same before and after the earthquake, indicating that the earthquake had no impact on the self-vibration frequency or the structural system of the turbines. National seismic monitoring network data revealed that Yancheng station was the only land-based seismic station recording seismic acceleration of 0.04 m/s², which was much lower than that at the bottom of the wind turbine tower, i.e., 0.1–0.2 m/s². The seismic action, input from the tower bottom, had a frequency higher than the first-order self-oscillation frequency (0.25 Hz) and lower than the second-order self-oscillation frequency (2 Hz) of the turbines. The turbines experienced both first-order and second-order vibration under seismic action, with lower parts vibrating at a higher frequency and upper parts close to the first-order self-oscillation frequency (0.27 Hz) measured during the earthquake. As height increased, the wind turbine acceleration response changed from high to low frequency. Due to these factors, the top of the wind turbine exhibited low-frequency vibration, differing from the main seismic frequency, resulting in a weak seismic response. Notably, the seismic response of several turbines in the Dafeng wind farm was largest at 0.66 times the hub height, about twice the magnitude of the response at the tower base. This research contributes field observational data that can serve as a valuable reference for dynamic model testing and numerical simulations, thereby enhancing the efficacy of practical engineering design.