Surge Decay Simulations of a Semi-Submersible Floating Offshore Wind Turbine

The state of the art to simulate the dynamic behaviour of floating wind turbines is to use Blade Element Momentum Theory (BEMT) for the aerodynamics and potential flow based tools for the hydrodynamics, see Cordle and Jonkman (2011) and Robertson et al. (2017). These methods are often used in the analysis and design of FOWTs. The OC5 (Offshore Code Comparison, Collaboration, Continued, with Correlation) project was focused on the validation of numerical simulations of the DeepCwind semi-submersible floating wind turbine with experimental data obtained from physical tests performed at the facilities of MARIN. The results will be presented shortly in Robertson et al. (2017). In this paper the focus lies on the prediction of the hydrodynamic damping for surge motion. It is assumed that the hydrodynamic damping consists of wave radiation damping, viscous damping and drag of the mooring lines. The viscous damping cannot be predicted by potential flow based methods. On these grounds, the viscous flow solver ReFRESCO (http://www.refresco.org/) is used to simulate the surge decay test of the OC5 semi-submersible floating wind turbine. There are many mooring models available with lots of different properties, see e.g. Davidson and Ringwood (2017). Dynamic mooring models are needed to account for the drag of the mooring lines moving in water. For this work the moorings are considered with a linear stiffness matrix in the equations of motion, i.e. no drag of the mooring lines is considered in the current study. Nevertheless, several surge decay simulations are performed to understand the effects of scaling and wave radiation on the hydrodynamic damping. Numerical results are compared with the above mentioned experimental data.