Numerical Modeling of Wave Basin Experiments of a Floating Wind Turbine with Active Thrust Emulation: A Discussion of Important Aspects

This article addresses some important issues that may arise when experimental data from seakeeping tests are used to verify the numerical model of a floating wind turbine. The discussions are based on an extensive set of model tests performed with a floater whose shape follows that of the UMaine VolturnUS-S 15MW, although with different dimensions. The model was moored with three catenary lines and anchors on the basin’s floor. Active ballast compensation was not considered. In this case, the rotor was not modeled in the reduced scale and its action was emulated by means of an active loading system that imposed the expected aerodynamic thrust on the top of the tower (thus one force component only) in a software-in-the-loop approach. The numerical models were built in OpenFAST and comprised both the full-scale FOWT and a simplified version that aimed at reproducing the experimental conditions more closely. From the hydrodynamic modeling point of view, three subjects are discussed: the importance of considering directional drag coefficients for the pontoons, the relevance of second-order resonant motions in both the horizontal and vertical planes, and the influence of floater tilt on the wave-induced dynamics. Regarding the aerodynamic modeling, on the other hand, the main concerns involved the simplifications imposed on the rotor loads. For the simulation of the experimental tests, modifications were made in the numerical model of the IEA 15MW turbine in order to ensure that the comparisons would be consistent, in a “model the model” approach. All the aerodynamic rotor forces/moments other than thrust were disregarded as well as the effects of blade elasticity, since these effects were not captured by the active loading system. Finally, a set of comparisons with the full-scale numerical model are presented in order to assess the impact that these simplifications might have on the predictions of the real FOWT dynamics.