Active Load Mitigation to Counter the Fatigue Damage Contributions From Unavailability in Offshore Wind Turbines

The offshore wind industry continues to grow, but there is still a need for more economical designs. As unavailability conditions can be critical for the fatigue damage in support structures, design standards use conservative values for availability. This leads to most turbines having an unused fatigue capacity at the end of the lifetime. This paper investigates the potential for reducing this unused capacity in order to reduce the capital expenses. The proposed strategy is to design the turbines for a higher availability, closer to the expected value. For individual turbines that experience lower availability than the design value, active load mitigation is imposed to reduce the fatigue damage. The potential of this methods is explored, together with its limitations. It is found that the effect of faults occurring early in the turbines lifetime can be reduced. This is not the case for faults occurring towards the end of the lifetime.