High-Fidelity Modelling of Floating Offshore Wind Turbine Platforms

There are already several examples of commercially operating floating offshore wind turbines (FOWTs) delivering electricity to the grid. These devices operate usually in harsh environments due to high waves, strong winds and currents. Guaranteeing the survivability of these complex and expensive constructions is crucial but not straightforward, and this may be partially controlled by the associated hydrodynamic damping. FOWT structures are still designed and analysed using potential-flow and empirical tools. However, nowadays, almost all fields of maritime problems are seeing an increased use of higher-fidelity viscous-flow tools (CFD, or Computational Fluid Dynamics), due to its increased modelling accuracy. Therefore, in this work, a FOWT is analysed in detail using CFD with emphasis on the hydrodynamic damping. This damping arises due to wave radiation , skin friction, eddy making and lift, and drag of the mooring lines. The CFD associated turbulence model influences largely the eddy making and skin friction components of the damping. Moorings are also a critical issue when assessing the damping of a floater. Catenary moorings are towed along with the motion of the floater and induce loads (drag and inertia) on the lines, effects which cannot be represented by static or quasi-static approaches. And numerical errors, often neglected or not quantified , can influence all the previous aspects and the final accurate prediction of the structure damping. In this paper, all these aspects are investigated by performing CFD decay tests, and their contribution quantified in terms of linear and quadratic damping components. The following issues were found to have a major contribution to the hydrodynamic damping and consequently on the motions of the platform: numerical discretisation (temporal and spatial), Reynolds number, free surface, turbulence and accurate mooring modelling.