Wind energy has seen a transition from onshore to offshore in the last decades. The lack of space on land and up-scaling of turbine sizes led to the need of exploiting further away and harsher environments, where more space is available and there is arguably less of an impact on society. This together with a higher supply of wind resources has led to the development of large numbers of offshore wind farms, of which many are still to be built. Taking Europe as an example, currently the goal is to achieve at least 450[GW] of installed offshore wind power by the year 2050, of which only 25[GW] has been installed up until 2020. This sketches out the enormous challenge that is being tackled by governments and the public market as we speak. The market is a highly competitive one, where small increases in efficiency of one of the many complex steps within the development of offshore wind farms can make or brake companies and projects. The eventual goal of increasing the efficiency of the overall process, is to decrease the levelized cost of energy of a wind farm. One of the main approaches of increasing this efficiency, has been to up-scale wind turbines, in the last decades the size and power output of wind turbines has already more than doubled from an average of 3MW of installed power per wind turbine 15 years ago to an average of 8.2MW in 2020. This trend does not seem to stop any time soon, as wind turbines of 12-15MW are in active development. The industry has already expressed interest into turbines of 20-25MW and even beyond this. The turbines beyond these sizes will be referred to as Ultra Large Wind Turbines(ULWTs). Due to the size and weight of components of ULWTs traditional top-down installation techniques cannot be used anymore due to relative motion related difficulties during the mating of components. Thus, the following research question will be answered during the defence: "What would be feasible novel on-site installation methods for a bottom founded Ultra Large Wind Turbines(ULWT) using floating vessels?" During the research, the base case of, floating installation on a pre-installed jacket type foundation is used as a starting point for the generation of concepts. A total of 15 concepts has been generated and pooled with concepts proposed in the industry and academia. All these concepts were screened to find the most promising ones. These concepts were sequentially tested on their technical feasibility for use with ULWTs, to identify which concepts can possibly be used. The final step was to perform a comparative analysis of the economic feasibility of the technically feasible concepts, to identify the differences in performance and usability of the concepts. From the technically feasible concepts, two main approaches could be identified which are promising for the installation of ULWTs. Further work must be performed to identify the best option from these two and describe their overall economic feasibility. Self-climbers: Devices which climb the tower without assistance of an installation vessel. Especially interesting due to the fact that they can be used with existing mono-hull lifting vessels. Main downsides are the relatively low flexibility w.r.t. wind turbine size, usability and have a long installation time. They are promising, as they decouple the size of the installation vessel and size of the turbine entirely. Large semi-submersible vessels: Either using lifting equipment, so that existing vessels can be used. Or specifically designed vessels which provide large flexibility w.r.t. size of turbine and usability. Main downside are the size and costs of these vessels. They are promising, as they provide a platform on which the needed height can be reached effectively.

Floating offshore wind turbines can only withstand a limited amount of (heave) motions before the equipment fails. In order to reduce the heave motion, the DeepCwind floater for offshore floating wind turbines makes use of heave plates. This semi-submersible floater consists of three cylindrical columns with a heave plate attached to the bottom of each column. Potential flow models are often used in order to assess the response. However, potential flow theory does not take into account the viscosity and the vorticity of the fluid. Therefore, this thesis examines the effect of a heave plate on a cylindrical column's response in heave direction and subjected to wave loads with both a potential flow model and a fully nonlinear numerical wave tank. Specifically, the difference between a potential flow model and a fully nonlinear numerical wave tank is examined. The simulations with the fully nonlinear numerical wave tank have been carried out using the open source computational fluid dynamics (CFD) software package OpenFOAM (version 1606+). An unresolved direct numerical simulation (DNS) approach is used throughout this work. Best practices for the dimensions of the wave tank, the mesh and settings of the solver where obtained from Bruinsma (2016) and Rivera-Arreba (2017). The OpenFOAM waves2Foam toolbox (developed by Jacobsen et al. (2012)) has been used to generate waves in the wave tank. The two phase solver interDyMFoam for moving bodies was coupled to the waveFoam solver from the waves2Foam toolbox in order to simulate a moving body under wave loads. The simulations in OpenFoam were carried out on a 1:50 scale. The potential flow model WAMIT has been used in order to obtain the response amplitude operator (RAO), added mass, damping and wave excitation forces from potential flow theory. A single cylindrical column has been tested in the numerical wave tank both with and without heave plate. Firstly, a heave decay test has been carried out. As a result, the linear damping ratio and the linear and quadratic damping coefficients have been determined. Secondly, the structure was exposed to incoming waves. The response of the structure has been assessed under three different wave periods, which were selected in order to align with Rivera-Arebba (2017). The response of the structure was measured, filtered on the frequency of the incoming wave and compared with the RAO from the potential flow model. Also, the wave excitation forces of the potential flow model have been compared with wave loads from the numerical wave tank, based on simulations where heave motion of the structure was constrained. It was found that both the wave excitation forces and the RAO of the potential flow model are in agreement with the CFD model results. The viscous effects included in the CFD model affect the response of the structure only very lightly. The largest differences between the potential flow and CFD model were found around the heave cancellation wave frequency. At the heave natural period of the structure, the heave plate increases the linear damping coefficient with ca. 50%. The damping at this period was dominated by viscous effects. In general, the potential flow model produces an accurate RAO, due to the fact that the system is lightly damped and the damping therefore plays a minor role in the structure's response. The outcome of this work contributes to the understanding of the effects of heave plates in general and can be used to assess the added value of computational expensive CFD software in the design process of floating wind systems.