Impact of hydrodynamic drag coefficient uncertainty on 15 MW floating offshore wind turbine power and speed control

Abstract

The main objective of this study is to analyse the impact of the drag coefficient uncertainty on large floating offshore wind turbines with respect to the power performance and generator speed control. In order to do so, the recently published IEA 15 MW reference wind turbine mounted on two different floating platform types (semi-submersible UMaine VolturnUS-S and spar WindCrete) has been selected. The platforms have been divided by components (columns, heave plates and pontoons) and the drag coefficient uncertainty has been quantified for each component, using a range of values from the literature that is representative for normal sea state conditions. Dynamic simulations have been performed for both floating platforms operating in a variety of environmental conditions (wind and waves), with a range of drag coefficients to account for the variability. Then, operating variables (generated power, generator speed, blade pitch angle.), platform motions and hydrodynamic forces have been analysed, to determine how large the impact due to the variations of the drag coefficient is. Results have shown that the considered range of drag coefficient uncertainty, although being quite large, has negligible impact on the performance of the wind turbine control response, irrespective of the floating platform type.