Mitigation of offshore wind turbine responses under wind and wave loading: Considering soil effects and damage

The present paper studies the mitigation of monopile offshore wind turbines subjected to wind and wave loading. Soil effects (SE) and damage are considered. A semiactive tuned mass damper (STMD) capable of retuning its natural frequency and damping property in real time is utilized to mitigate the nacelle/tower top dynamic response. Based on the Euler–Lagrangian equation, an analytical model of the wind turbine coupled with an STMD is established wherein the interaction between the blades and the tower is modeled. Wind turbulence is generated via mapping a three-dimensional wind field profile onto the rotating blades. Aerodynamic loading is computed using the blade element momentum method where the Prandtl's tip loss factor and the Glauert correction are considered. Wave loading is computed using Morison's equation together with the strip theory. The National Renewable Energy Laboratory monopile 5-MW baseline wind turbine model is employed to examine the performance of the STMD. It is found that the SE and damage presence in the foundation or/and the tower can change the dominant frequency, thereby rendering the conventional TMD detuned and ineffective. In comparison, the STMD retuned in real time by the proposed algorithm can mitigate the nacelle/tower and foundation response more effectively with a smaller stroke. Results indicate that the STMD has significant effectiveness improvement over the TMD when the SE and/or damage are considered.