Sustainable and renewable energies are essential in achieving climate targets set by the Paris Agreement and Sustainable Development Goals (SDGs). Floating offshore wind turbines (FOWTs) represent an innovative technology with considerable potential to contribute to these goals. However, being a relatively new technology, FOWTs pose challenges that must be addressed to enhance their development. To speed up FOWT installation and optimize their design, accurate numerical simulation tools are essential, particularly mid-fidelity models, which use less computational time but require precise hydrodynamic characteristics to be tuned. These tuning parameters are obtained with experimental tests or with Computational Fluid Dynamics (CFD) simulations.
This thesis aims to analyze the added mass and drag coefficients of the MaRINET2 semi-submersible wind turbine floater when subjected to forced oscillatory motion, utilizing a validated CFD model. To achieve the objective, a nonlinear Navier Stokes numerical model was developed within the open-source CFD software OpenFOAM. The interFoam solver was employed for the simulations. A mesh convergence study identified an optimal mesh configuration, balancing computational efficiency with result accuracy. Two types of simulations were conducted: free decay simulations for model validation against University of Strathclyde (UoS) experimental data and forced oscillatory simulations to compute added mass and drag coefficients under varying oscillation parameters.
It was found that heave added mass is sensitive to both oscillation amplitude and period, while heave drag coefficients displayed minimal influence at shorter periods. For high-amplitude oscillations, surge drag coefficients remained stable across oscillation periods, yet longer periods increased coefficients for
mid and low amplitudes. Pitch added mass coefficients remained unaffected by oscillation amplitude but increased with longer periods. Comparing these hydrodynamic parameters with two reference papers revealed both similarities and discrepancies in trends. It is challenging to attribute these discrepancies
only to floater geometry due to the lack of specific experimental data for this floater.

The advancement of floating wind turbine technology in recent years necessitates accurate predictions of their behaviour using various simulation tools. Utilizing multi-fidelity tools like OpenFAST allows for comprehensive calculations of the entire floating wind turbine structure. However, the precision of these calculations is challenged by the uncertainties associated with different system parameters.

The main goal of this research is to conduct a comprehensive exploration of various parameters, with a particular emphasis on hydrodynamic factors, and their impact on the structural and aerodynamic performance of semi-submersible floating wind turbines. To achieve this, an extensive sensitivity analysis approach is employed, utilizing the multi-fidelity model OpenFAST. The simulations are conducted on a semi-submersible floating wind turbine, specifically the one used in the OC4 experiment, featuring the 5MW NREL wind turbine. The Elementary Effects (EE) method is employed in this study to assess the significance of various factors. These factors include the hydrodynamic drag coefficients, wave height, peak wave period, wind speed, wind direction, current speed, current direction, and turbulence intensity. Their influence on the Damage Equivalent Load (DEL) of different loads acting on the floating offshore wind turbine (FOWT) is thoroughly examined.

The analysis conducted in this study has unveiled several key findings. The most influential parameters identified are the current speed, primarily impacting mooring line tension, and the significant wave height, which exhibits a balanced effect across various outputs, including hydrodynamic loads, tower loads, and rotor dynamics. Additionally, turbulence intensity emerges as a significant factor, particularly concerning rotor thrust and torque.

When employing the Morison equation, the order of parameter significance remains consistent with the hybrid theory (Potential + Drag). However, it is noteworthy that the added mass coefficients carry greater significance compared to the drag coefficient. Furthermore, simulations conducted using probability distributions for different locations confirm that wave height consistently ranks as the most significant parameter. However, the overall significance may vary and decrease in more severe environmental conditions.

The comparison with the OC3 spar platform revealed a similar pattern when examining the influence of current speed and significant wave height. Nonetheless, the simple hydrodynamic drag coefficient used for the whole platform also displayed significance, emphasizing the sensitivity of the spar platform loads to this modelling parameter. This significance is much higher than the corresponding one of the semisubmersible platform.

The research underscores it is imperative to mitigate the uncertainty associated with various parameters. This can be achieved through experimental verification and establishing correlations between modelling parameters and the specific conditions being simulated. Additionally, given that the most influential parameters are related to external conditions, it becomes crucial to simulate conditions that closely resemble the anticipated deployment location for the floating offshore wind turbine (FOWT). By doing so, the accuracy and reliability of the simulations will be enhanced to better inform FOWT design and deployment strategies.