NUMERICAL MODELLING OF THE AERODYNAMIC CHARACTERISTICS OF A FLOATING OFFSHORE WIND TURBINE UNDER YAWED ROTOR CONDITIONS

The aerodynamic characteristics of floating wind turbines in yaw are more complex than those of turbines with fixed foundations as a result of the floating platform dynamics under wave action. This paper applies numerical simulation tools to investigate the time varying rotor thrust and shaft power characteristics of offshore floating wind turbines (OFWTs) under different rotor yaw angles and regular sea wave conditions. The study is based on the NREL1 5 MW baseline OFWT installed on the MIT2 tension-leg platform (TLP). Both the wind speed and rotor speed are maintained constant throughout the analysis, though different sea wave heights and periods are considered. The predictions from three different aerodynamic models are compared. These include the Blade Element-Momentum (BEM) and the General Dynamic Wake (GDW) methods and a higher fidelity Free-Wake Vortex model (FWVM) that is capable of modelling the unsteady skewed helical wake development of the yawed rotor. Initially the motions of the OFWT under both axial and yawed rotor conditions are estimated in a time domain using FAST, an open source software developed by NREL. These motions are then prescribed to WInDS, a FWVM developed by the University of Massachusetts Amherst, to determine the aerodynamic rotor thrust and power as a function of time. The three models have consistently shown that the TLP motion under the modelled wave states exhibits a negligible impact on the time-averaged rotor shaft thrust and power of the yawed rotor. On the other hand, the cyclic component of rotor thrust and power are found to be significantly influenced by the wave state at all yaw angles. Furthermore, significant discrepancies between the predictions for this cyclic component from the three models observed.