Wave Models for Offshore Wind Turbines

Abstract

In the design of wind turbines—onshore or offshore—the prediction of extreme loads associated with a target (long) return period requires statistical extrapolation from available loads data. The data required for such extrapolation are obtained by stochastic timedomain simulation of the inflow turbulence and of the turbine response. Prediction of accurate loads depends on assumptions made in the simulation models employed, both for the turbine and for the input wind/wave conditions. While for the wind, inflow turbulence models are fairly well established due to the relative maturity of the industry for onshore wind energy development, for wave input, the current practice is to model irregular (random) waves using a linear wave theory. These wave modeling assumptions do not adequately represent waves in shallow waters where most offshore wind turbines are being sited. As an alternative to these less realistic (albeit simple) representations of ocean waves, the present study investigates the use of irregular nonlinear (second-order) waves for estimating loads on the support structure (monopile) of an offshore wind turbine. We use a 5MW utility-scale wind turbine model for the simulations. Using, first, the simpler linear irregular wave modeling assumptions, we establish long-term loads and identify controlling environmental conditions (i.e., the wind speed and wave height) that are associated with the 20-year return period load derived using the inverse first-order reliability method. We then present the theory for an improved nonlinear second-order irregular wave modeling approach which we use to compute hydrodynamic loads on a stand-alone monopile analyzed quasi-statically for the controlling long-term design environmental conditions identified from the linear model. We demonstrate that computed hydrodynamic loads are generally larger with the nonlinear wave modeling assumptions; this establishes the importance of using such refined nonlinear wave models in stochastic simulation of the response of offshore wind turbines. This motivates the need for continuing work directed towards incorporation of nonlinear irregular wave models in turbine response simulation studies and loads extrapolation for design.