Numerical Assessment and Validation of Floating Offshore Wind Turbines in One Fully Coupled CFD Simulation

Abstract In this work we present a coupled computational fluid dynamics and fluid-structure interaction approach for simulating the dynamics of floating offshore wind turbines (FOWTs). The multiphase flow over the platform is solved using a cartesian cut-cell approach coupled with an environmental flow generation module which can model a variety of wind-wave conditions. This is in turn coupled with a 6-degree-of-freedom fluid-structure interaction model to simulate the platform motion, while the mooring lines are represented using a dynamic lumped element model. Finally, an actuator line model (ALM) is used to model the wind turbine rotor. We have applied this methodology to study both the aerodynamic loading on the rotor and the platform dynamics of a FOWT design consisting of a 3.6-MW horizontal-axis wind turbine attached to a Stiesdal TetraSpar platform in a variety of conditions such as free decay, regular and irregular waves. The FOWT geometry and cases presented here are based on those specified by Phase IV of the OC6 (Offshore Code Comparison Collaboration, Continued with Correlation and unCertainty) project under the International Energy Agency Wind Technology Collaboration Programme (IEA Wind) Task 30. We have selected a set of cases to study the response of the FOWT to various types of loading. The first set of tests are to determine the exact equilibrium location of the FOWT for numerical simulation. In these tests we will also investigate the effect of grid resolution on the results. Next, we simulate the free decay response of the FOWT in the surge, heave, and pitch degrees of freedom. After that we investigate the turbine thrust for both floating and fixed platform configurations. Finally, we present a set of cases that illustrate the platform response to both regular and irregular waves. The simulation results match very well with the experimental data for all the cases that we have studied. Thus, this approach can be seen as a viable method to carry out predictive simulations for studying FOWT platform motions, hydrodynamic loading, and aerodynamic performance.