Mid-fidelity Model with CFD Damping Correction for Floating Offshore Wind Turbine Productivity Evaluation

Renewable Energy Sources play a central role in the fight against climate change, as they represent one of the most important paradigms on which a renovated and sustainable society should be built. Among them, offshore wind energy is growing in popularity, as it represents a great opportunity to efficiently harness the vast potential of wind energy at sea. In particular, floating offshore wind turbines (FOWT) are the only viable solution at higher bathymetry and thus they could open the possibility for a much wider installation worldwide. FOWTs are stabilized thanks to the structure on which they are built, moored to the seabed. Usually, they are divided in three categories: mass stabilized structures, buoyancy stabilized structures, and tension leg platforms. Each solution has different advantages and disadvantages, mostly linked to their performance, construction methods and cost. As researchers and the industry aim at installing bigger and more efficient turbines, there is a strong commitment to the development of innovative solutions to support floating offshore wind turbines. However, experts have to deal with multiple interconnected problems in the design of these solutions, such as the prediction of their interaction with wave and wind loads or the design of an effective control system. The focus of this thesis is the accurate evaluation of the energy produced by a floating offshore wind turbine. In particular, this work is centered around the UMaine VolturnUS-S Reference Platform, which is a semi-submersible floating platform specifically designed to work with the IEA 15MW reference turbine. This turbine is currently the largest available reference device and it is an excellent example of the trend to go towards bigger and more powerful turbines. In order to estimate the productivity of the turbine, it is necessary to correctly model the interaction between the structure and the environment in which it should be installed. As it is a body with a complex geometry, an accurate description of its dynamic properties could lead to an improvement in the prediction of its performance. Therefore, a CFD simulation of the semi-submersible structure is performed within Star-CCM+, posing particular attention on the free decay motion for the pitch and heave motions. Post-processing techniques are then used in order to extrapolate first and second order damping coefficients. These results, obtained from a high accuracy model, are then used in lower-accuracy models to check their effect on the prediction of the productivity of a turbine under a set of reference conditions. In this work, an accurate analysis of the CFD model is performed, in order to optimize the simulation in terms of computational time and accuracy, while testing the effect of different mesh parameters, simulation strategies and numerical models. From the result of this process, a set of accurate descriptors for the body motion is obtained. The main goal is to assess the benefits that a preliminary CFD analysis can bring to a lower-accuracy simulation of the turbine performance, in order to improve the overall quality of future works on the subject.