Offshore wind turbine loads under the coupled influences of wind, waves, and currents during hurricanes

In the United States, potential offshore wind plant sites have been identified along the Atlantic seaboard and in the Gulf of Mexico. At such sites, it is imperative that we consider load cases for and define external conditions associated with hurricanes and severe winter storms for which wind turbines may need to be designed. We select two hurricanes, Ike (2008) and Sandy (2012), and investigate what effect these tropical storms would have on bottom-supported offshore wind turbines that were hypothetically in or close to their path as they made landfall. For realistic turbine loads assessment, it is important that the coupled influences of the changing wind, wave, and current fields are simulated throughout the evolution of the hurricanes. We employ a coupled model—specifically, the University of Miami Coupled Model (UMCM)—that integrates atmospheric, wave, and ocean components to produce needed wind, wave, and current data. The wind data are used to generate appropriate vertical wind profiles and full wind velocity fields including turbulence; the current field over the water column is obtained by interpolated discrete output current data; and short-crested irregular second-order waves are simulated using output directional wave spectra from the coupled model. Two monopile-supported offshore wind turbines sited in 20 meters of water in the Gulf of Mexico are studied so as to estimate loads during Hurricane Ike; a jacket space-frame platform-supported offshore wind turbine sited in 50 meters of water in the mid-Atlantic region is studied so as to estimate loads during Hurricane Sandy. We discuss in detail how the simulated hurricane wind, wave, and current output data are used in turbine loads studies. We discuss important characteristics of the external conditions including the relative importance of swell versus wind seas, of aerodynamic versus hydrodynamic forces, of current velocity effects, of yaw control options for the turbine, of hydrodynamic drag versus inertia forces, and of soil-structure interaction effects. A detailed framework is presented that explains how coupled inputs can be included in turbine loads studies during a hurricane; this can aid in future efforts aimed at developing offshore wind turbine design criteria and load cases related to hurricanes.