Modeling Nonlinear Irregular Waves in Reliability Studies for Offshore Wind Turbines

Abstract

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. This involves simulation of the stochastic inflow wind field on the rotor plane, of 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 most 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. We compute loads on the monopile support structure of a 5MW offshore wind turbine model for several representative environmental states where we focus on differences in estimates of the extreme tower bending moment at the mudline due to linear and nonlinear waves. Finally, we compare long-term load predictions using inverse reliability procedures with both the linear and nonlinear wave models. We present convergence criteria that may be used to establish accurate 20-year loads and discuss comparative influences of wind versus waves in long-term load prediction.