Revealing inflow and wake conditions of a 6MW floating turbine

Abstract. We investigate the characteristics of the inflow and the wake of a 6MW floating wind turbine from the Hywind Scotland offshore wind farm, the world's first floating wind farm. We use two commercial nacelle-mounted lidars to measure the up- and downwind conditions, with a fixed and a scanning measuring geometry, respectively. In the analysis, the effect of the surge and sway motion of the nacelle on the lidar measuring location is taken into account. The upwind conditions are parameterised in terms of the mean horizontal wind vector at hub height, the shear and veer of the wind profile along the upper part of the rotor and the induction of the wind turbine rotor. The wake characteristics are studied in two narrow wind speed intervals 8.5–9.5 ms^-1 and 12.5–13.5 ms^-1, corresponding to below and above rotor rated speeds, respectively, and for turbulence intensity values between 3.3 %–6.4 %. The wake flow is measured by a wind lidar scanning in a horizontal plan position indicator mode, which reaches ten rotor diameters downwind. This study focuses on the downstream area between 3 and 8 rotor diameters. In this region, our observations show that the transverse profile of the wake can be adequately described by a self-similar wind speed deficit, that follows a Gaussian distribution. We find that even small variations (∼1 %–2 %) of the ambient turbulence intensity can result in an up to 10 % faster wake recovery. Furthermore, we do not observe any additional spread of the wake due to the motion of the floating wind turbine.