Offshore wind turbine tower fore-aft control: Trade-off fatigue load reduction and pitch activity

The tower contributes a significant share to the overall cost of an offshore wind turbine. To allow a more cost-effective design and prevent the occurrence of unscheduled maintenance, it is highly desirable to mitigate severe fatigue loading conditions and constantly monitor the dynamic behavior that the tower exhibits during operation. This paper sought to understand whether state-of-the-art control techniques for tower load reduction might be coupled with an online time-frequency analysis of tower vibrations, in order to extend the ongoing research effort in the area of situation control. Therefore, we performed hydro-aero-elastic simulation for a commercial 5MW offshore wind turbine, for two different sets of wind-sea conditions; simulations were iterated for three alternative control scenarios: baseline control, conventional tower control and advanced tower control. Results showed how the proposed method was able to reduce fatigue loads, without severely impairing pitch actuators, as is the case with classic state-of-the-art tower control methods. Furthermore, connecting these already established, but apparently independent fields of research may result in a sounder technical solution, which is better tailored to the technology requirements that offshore wind energy demands.