Dynamics of offshore wind turbine and its seabed foundation under combined wind-wave loading

Abstract

Over the past two decades, a great number of offshore wind turbines (OWTs) have been built in offshore areas worldwide to harvest clean wind energy resources. The dynamic response and stability of OWTs under environmental loads, e.g., wind and wave, is a key technical problem that engineers are most concerned about. In this study, a wave-structure-foundation integrated numerical model OlaFlow–ABAQUS, which was independently developed through the secondary development, is used to study the dynamics characteristics of a thin-walled steel monopile offshore wind turbine and its seabed foundation under extreme wave and wind loads that would occur in typhoon weather. The wave-induced load imposed on the cylindrical tower of the OWT is determined by solving the VARANS (Volume Average Reynolds Average Navier-Stokes) hydrodynamic equation, the wind load on the turbine blades is estimated by applying the pulsating wind theory, and the mechanical behavior of seabed foundation soil is described by the ideal elastoplastic Mohr-Coulomb model. The computational results show that the OWT and its seabed foundation have a strong dynamic response under the combined action of extreme wind and wave, and there is intensive interaction between them. There is a transient liquefaction zone in the seabed foundation, and the maximum liquefaction depth, in this case, is approximately 2 m. Residual horizontal displacement with a magnitude of approximately 0.4 m occurs at the top of the cylindrical tower of the OWT, resulting in the overall tilting of the OWT. The maximum inclination angle of the cylindrical tower is approximately 0.51°. This angle of inclination may cause significant bending moment and stress concentration in some mechanical components such as the drive shafts and gearboxes that connect to the turbine blades. It reduces the service life of the OWT significantly and also is detrimental to the long-term service performance of the OWT.