Offshore wind turbine wakes measured by sodar

A ship-mounted sodar was used to measure wind turbine wakes in an offshore wind farm in Denmark. The wake magnitude and vertical extent were determined by measuring the wind speed profile behind an operating turbine, then shutting down the turbine and measuring the freestream wind profile. These measurements were compared with meteorological measurements on two offshore and one coastal mast at the same site. The main purposes of the experiment were to evaluate the utility of sodar for determining wind speed profiles offshore and to provide the first offshore wake measurements with varying distance from a wind turbine. Over the course of a week, 36 experiments were conducted in total. After quality control of the data (mainly to exclude rain periods), 13 turbine-on, turbine-off pairs were analyzed to provide the velocity deficit at hub height as a function of the distance from the turbine. The results are presented in the context of wake measurements at other coastal locations. The velocity deficit is predicted with an empirical model derived from onshore measurements based on transport time dependent on surface roughness. The measurements are closer to those predicted using an onshore rather than an offshore roughness despite the relatively low turbulence experienced during the experiments.