Wave disturbance rejection for monopile offshore wind turbines

The dimensions of offshore wind turbine (OWT) support structures are governed by fatigue considerations. For 6‐ to 10‐MW OWTs, wave loads are often dominating in terms of fatigue utilization. The present work proposes a control scheme to reduce the wave‐induced fatigue loads in OWT support structures. The control scheme applies collective pitch control to increase both the damping and stiffness of the fore‐aft vibration modes. With conventional active tower damping, efficient wave disturbance rejection is restricted to a narrow frequency range around the first fore‐aft modal frequency. The proposed control scheme achieves efficient wave disturbance rejection across a broader frequency range. Here, tower feedback control is implemented via an auxiliary control loop. Based on a low‐fidelity model, the effect of the tower feedback loop on the stability margins of the basic controller is analysed. The results show that, within certain boundaries, the stability margins are improved by the stiffness term in the tower feedback loop. Consequently, the need to reduce the bandwidth of the basic controller to accommodate tower feedback control is relaxed. Based on time‐domain simulations carried out in an aero‐hydro‐servo‐elastic simulation tool, the lifetime effects of the proposed control scheme are analysed. Compared with conventional active tower damping, a more favourable trade‐off between adverse side effects and the support structure's fatigue damage is achieved with the proposed control scheme.