Hurricane-Induced Loads on Offshore Wind Turbines with Considerations for Nacelle Yaw and Blade Pitch Control

Abstract

During extreme tropical storm systems such as hurricanes, offshore wind turbines are required to have adequate structural integrity in parked condition and with blades pitched to feather. Such turbine states are preferred in order to mitigate loads on the turbine blades; simultaneously, yaw control is required so as to track the changing wind direction in this configuration. During a hurricane, however, it is possible that a turbine's yaw control system might operate abnormally due to damage of the control and protection system or due to loss of the electric grid and/or insufficient backup power. In earlier studies, the authors have shown that feathered blades can lead to higher tower bending moments in the side-to-side (lateral) direction rather than in the fore-aft (longitudinal) direction. In the present study, we carry out an in-depth investigation of the effect of several alternative parked configurations on an offshore turbine's response using numerical simulations with coupled wind-wave fields during a hurricane. We use these output wind-wave fields obtained from the University of Miami Coupled Model (UMCM), a fully coupled atmosphere-wave-ocean model that is used to simulate the storm and associated environmental fields and is able to represent relevant physical processes at the air-sea interface. In this study, we evaluate: (1) the effect of different nacelle yaw angles relative to the wind direction (i.e., different amounts of yaw error or misalignment); (2) the effect of different blade pitch angles; (3) the effect of different turbine parking strategies—e.g., parked and at a standstill or in an idling state; and (4) load maxima at different blade azimuthal configurations while the turbine is in a standstill state. The effects of the different turbine parked configurations on turbine response are evaluated under only wind loads first; later, wave loads are included to reflect possible joint metocean conditions likely to be encountered during a hurricane.