Wind/Wave Misalignment in the Loads Analysis of a Floating Offshore Wind Turbine

Wind resources far from the shore and in deeper seas have encouraged the offshore wind industry to look into floating platforms. As a result, the International Electrotechnical Commission is developing a new technical specification for the design of floating offshore wind turbines that extends existing design standards for land-based and fixed-bottom offshore wind turbines. The work summarized in this paper supports the development of best practices and simulation requirements in the loads analysis of floating offshore wind turbines by examining the impact of wind/wave misalignment on the system loads under normal operation. We conducted simulations of a spar-type floating offshore wind turbine system under a wide range of wind speeds, significant wave heights, peak-spectral periods, and wind/wave misalignments using the aero-servo-hydro-elastic tool FAST. The extreme and fatigue loads were calculated for all of the simulations. The extreme and fatigue loading as a function of wind/wave misalignment are represented as load roses and a directional binning sensitivity study is performed. This study focused on identifying the number and type of wind/wave misalignment simulations needed to accurately capture the extreme and fatigue loads of the system in all possible meteorological and ocean conditions considered, and for a down-selected set of conditions identified as the generic U.S. East Coast site. For this axisymmetric platform (except for the mooring lines), perpendicular wind and waves play an important role in the loading of the support structure. Therefore, including these conditions in the design loads analysis can improve the estimation of extreme and fatigue loads. However, most support-structure locations experience their highest extreme and fatigue loads when the wind and waves are aligned. These findings are specific to the spar-type platform, but we expect that the results presented here will be similar to other floating platforms.