While addressing different load cases according to the IEC guidelines for offshore wind turbines, designers are required to estimate long-term extreme and fatigue loads; this is usually done by carrying out time-domain stochastic turbine response simulations. The simulation of offshore wind turbine response involves simulations of the stochastic inflow wind field on the rotor plane, of the irregular (random) waves on the support structure, and of the turbine response. Obtaining realistic response of the turbine depends, among other factors, on appropriate modeling of the incident wind and waves. The current practice for modeling waves on offshore wind turbines is limited to the representation of linear irregular waves. While such models are appropriate for deep waters, they are not accurate representations of waves in shallow waters where offshore wind turbines are commonly sited. In shallow waters, waves are generally nonlinear in nature. It is, therefore, of interest to assess the influence of alternative wave models on the behavior of wind turbines (e.g., on the tower response) as well as on extrapolated long-term turbine loads. The expectation is that nonlinear (second-order) irregular waves can better describe waves in shallow waters. In this study, we investigate differences in turbine response statistics and in long-term load predictions that arise from the use of alternative wave models.
Among several nonlinear irregular wave models available in the literature, the one that has been recommended by offshore design guidelines and that has been increasingly applied to a variety of problems in recent years is the second-order nonlinear irregular wave model developed by Sharma and Dean. We discuss how this model may be adapted for use with turbine response simulation software. We compute loads on the monopile support structure of a NREL 5MW offshore wind turbine model for several representative environmental states where we focus on differences in extreme tower bending moment at the mudline due to linear and nonlinear waves. Finally, we compare long-term load predictions using inverse FORM with both linear and nonlinear wave models. We discuss convergence criteria that may be used to predict accurate 20-year loads and discuss wind versus wave dominance in the load prediction.
Agarwal, P. and Manuel, L., “Modeling Nonlinear Irregular Waves in Reliability Studies for Offshore Wind Turbines,” 28th International Conference on Ocean, Offshore and Arctic Engineering, Honolulu, Hawaii, 31 May – 5 June, 2009.