The Impact of Peak Spectral Period in the Design of Offshore Wind Turbines for the Extreme Sea State

Most offshore wind turbines (OWTs) are designed according to the international standard International Electrotechnical Commission (IEC) 61400-3 which requires consideration of several design load cases under extreme sea state conditions during which the wind turbine is in survival mode (i.e. the rotor is parked and blades are feathered). Each of these load cases depends on combinations of two random variables, the mean wind speed and the significant wave height, both with a mean return period of 50 years. The response of an offshore wind turbine under wave loading is known to be sensitive to both the significant wave height and a frequency measure of the sea state such as the peak spectral period. The IEC standard states that design calculations for the extreme sea state should be based on values of peak spectral period which result in the highest loads acting on the structure, but does not provide additional guidance. The Standard does provide a deterministic range for the period of the extreme wave conditioned on the significant wave height, and this can be converted to a range of peak spectral period using published empirical relationships. This paper considers an offshore location off the coast of Georgia, where the National Oceanic and Atmospheric Administration (NOAA) buoy 41008 is located, and shows that a deterministic range of peak spectral period converted from the range provided in the IEC Standard may not accurately represent measured data. Moreover, the paper shows that the response of a hypothetical offshore wind turbine, installed at this location and supported by a monopile foundation, is sensitive to variation in the peak spectral period, emphasizing the importance of modeling the turbine for an appropriate and possibly site-specific range of peak spectral periods. A probabilistic approach is proposed to find an appropriate and site-specific range of the peak spectral period for the design of offshore wind turbines under the extreme sea state.