Dynamic response analysis of a real-world operating offshore wind turbine under earthquake excitations

This paper describes the dynamic monitoring campaign conducted on a monopile offshore wind turbine (OWT). An unexpected harvest is that the dynamic responses of the OWT under earthquake excitations were captured. Therefore, particular emphasis is placed on any abnormal behavior of the OWT. Operational modal analysis with the stochastic subspace identification method is used to reveal any variations in the modal parameters before, during and after the earthquake. The influence of the time-varying yaw angle on modal identification is derived. The results indicate that the earthquake had a colossal impact on the OWT as compared to the wind loading. The earthquake allows the identification of some of the high-frequency bending and twisting modes. Another critical finding is that the first frequency of the OWT increased after the earthquake, possibly implying some quick consolidation of the seabed adjacent to the OWT. Although earthquakes are often considered harmful to the safety of OWTs, it is possible for some special cases to improve the bearing capacity and stability of the monopile foundations. This paper could be the first study on the dynamic response of a real-world OWT under earthquake excitations, in which some beneficial effects of an earthquake are observed.