Semi-active control of monopile offshore wind turbines under multi-hazards

•A fully-coupled three dimensional analytical model of the monopile offshore wind turbine with a semi-active tuned mass damper (STMD) has been established.•Dynamic response under the combined wind, wave and seismic loading is evaluated considering soil effects and damage.•The proposed control algorithm can track the response variation in real-time.•The natural frequency and damping property of the STMD are retuned by the control algorithm.•The STMD outperforms the passive TMD in mitigating the structural and foundation dynamic responses. The present paper studies the control of monopile offshore wind turbines subjected to multi-hazards consisting of wind, wave and earthquake. A Semi-active tuned mass damper (STMD) with tunable natural frequency and damping ratio is introduced to control the dynamic response. A new fully coupled analytical model of the monopile offshore wind turbine with an STMD is established. The aerodynamic, hydrodynamic and seismic loading models are derived. Soil effects and damage are considered. The National Renewable Energy Lab monopile 5MW baseline wind turbine model is employed to examine the performance of the STMD. A passive tuned mass damper (TMD) is utilized for comparison. Through numerical simulation, it is found that before damage occurs, the wind and wave induced response is more dominant than the earthquake induced response. With damage presence in the tower and the foundation, the nacelle and the tower response is increased dramatically and the natural frequency is decreased considerably. As a result, the passive TMD with fixed parameters becomes off-tuned and loses its effectiveness. In comparison, the STMD retuned in real-time demonstrates consistent effectiveness in controlling the dynamic response of the monopile offshore wind turbines under multi-hazards and damage with a smaller stroke.