Fatigue loads for wind turbines operating in wakes

Abstract Loads for a wind turbine operating in the wake of an upstream wind turbine was investigated with the objective of identifying the load generating changes in the wind field parameters compared to a free flow situation. The investigation was based on the measurements from the offshore wind farm Vindeby in Denmark. The following wind field parameters were included in the wind field analysis: Wind speed deficit, turbulence intensity, horizontal shear and turbulence spectrum length scale. The identified wind field changes were implemented in the aeroelastic code HawC, which was used to predict the wind turbine loads. For both free flow and wake cases, good agreement with measured loads was obtained. The most important load generating parameters for the wind farm in Vindeby were the increased turbulence intensity and the reduced turbulence length scale. The increased fatigue loading in the wind farm compared to free flow was found to be between 5% and 15%, depending on the wind farm layout. The load increase caused by wake effects was found to be the same for an offshore and a land site.