At the moment, the world is at the verge of an energy transition. One of the most promising green resources is solar energy, which is a rapidly growing market. However, to fully use its potential of economy of scale, the application of offshore floating solar should be explored. A promising option is the use of a flexible type of Very Large Floating Structures (VLFSs), which are called Very Flexible Floating Structures (VFFSs). They are characterised by their large length to height ratio compared to rigid bodies and depending on their material properties have a hydroelastic response to the incident wave. In the late 1990s, a lot of research has been done on VLFSs by Tsubogo and Okada (1998) who derived an analytical dispersion relation assuming a zero­draught structure. However, only recently, Schreier and Jacobi (2020b) did experimental research on VFFSs in a towing tank at the Delft University of Technology, as little is still known about flexible structures. This report focuses on a numerical alter­ native that covers both VLFSs as well as VFFSs using a Finite Element Method (FEM) Fluid Structure Interaction (FSI) model which has been built based on potential flow to model the fluid and a dynamic Euler­Bernoulli beam that represents the floating structure using the Julia package Gridap. One of the main advantages is that the zero­draught assumption is not necessary and, therefore, structures with larger draughts can also be modelled. Next to this, the numerical model is able to cope with irregular shapes, for which no analytical method yet exists. The model is built such that it can handle 2D as well as 3D domains. A 2D analysis has been made to understand the influence of hydroelastic wave deformation of the incident wave, in terms of wavelength dispersion as well as amplitude dispersion on floating structures. To verify the model, the numerical results were compared to the analytical solution and experimental research in a towing tank, which showed accurate results. Test runs were set up that mimicked the towing tank set­up and a full­scale solar park. Furthermore, a sensitivity study was executed that shows the limits of the flexible domain and to see in which cases significant (>1%) hydroelastic wave deformation would occur using governing mean and extreme ocean waves, as well as a typical lake wave. Finally, the influence of the draught of the structure was examined. This report provides a good overview of when wave deformation should be accounted for in terms of bending rigidity and density. Confirming existing theory, it was found that the stiffness of the VFFS causes wave stretching and the draught of the structure influences the extent of wave shortening. It was also found that significant wave deformation will not occur for ocean waves as the required stiff­ ness is beyond existing materials. For extreme ocean waves, there is even no dispersion at all. As the wave frequency increases, the hydroelastic interaction gets stronger. The typical lake wave showed to be well within the flexible regime and also showed significant dispersion with realistic material parame­ ters. The numerical model is able to cope with large draught scenarios which lead to wave shortening, which in its turn leads to wave focusing. Ultimately, the numerical model showed to be a good alternative to existing methods to investigate the behaviour of VLFSs outside the floating solar domain, where one could think of ice floes, floating islands or floating airports.

Large-scale cultivation of seaweed presents opportunities for multiple global challenges currently at play. Cultivated seaweed can provide a sustainable source of protein for humans and cattle without competing for land, freshwater supply or the use of fertilisers. Kelp forests are known to be a solid basis for an elaborate biome that supports biodiversity in areas that have been damaged by over-fishing or rising sea temperatures. Additionally, kelp forests can lock-in large amounts of Blue Carbon, expanding the oceans’ buffering capacity to mitigate anthropogenic emissions. Furthermore, with their densely seeded lines, offshore kelp farms are found to attenuate wave amplitude, thus providing coastal protection and benefits like increased workability for offshore operations. Both academic publications and industry reviews underline the potential of this sector and significant growth in cultivation is expected in the near future. Methods currently used for quantification of the damping effects of large-scale offshore kelp farms are diverse and entail varying degrees of accuracy and computational cost. Experimental observations that support the outcomes of these methods are limited to scaled experiments in wave flumes, with various methods used to mimic vegetation. No convergence is found in the most suitable methods for application to large-scale offshore kelp farms. This research presents a novel modelling framework based upon the Finite Element method, implemented using Julia Programming Language. The effects of the vegetation on the wave climate are represented with a Darcy-Forchheimer term borrowed from porous medium flow theory, including a linear and a quadratic resistance term. The framework comprises a numerical wave tank, using the incompressible Navier Stokes equations. The single-phase model captures the free surface using the coupling of dynamic pressure with a virtual elevation variable through a linearized transpiration boundary condition. Wave energy dissipation is shown to increase significantly by moving the farm structure close to the water surface. Similarly, a decrease in relative water depth - compared to the vegetated height - increases damping potential. Wave period is found to be of strong influence on dissipation, where short waves are attenuated more. Scaling vegetation length with wave length, however, diminishes the reduction in damping of longer waves. Conversely, wave amplitude is shown to be of less influence on the transmission of amplitude through a vegetated patch. The framework presents a method that is easily scalable, flexible in application on a wide range of flows and vegetation characteristics, and at reasonable computational cost. Introduction of both the linear and quadratic terms extends applicability compared to traditional methods. The approach is verified using convergence studies, application of the model is validated by comparison to existing experimental data. It is shown that experimental set-ups can be reproduced effectively, and simulation results coincide with experimental findings. Validation of outcomes on scales larger than common wave tanks was found unfeasible due to a lack of measurement data. A theoretical case study was performed to predict wave damping of a full-scale kelp farm, demonstrating promising potential with up to 40% wave energy reduction at the local peak wave period. Further research into the establishment of the Darcy- and Forchheimer-coefficients is recommended. A preliminary range of values has been found, based upon calibration on existing experiments that represent realistic ranges of vegetation characteristics. Furthermore, the main conditions of the flow and vegetation that dictate damping potential are identified. On this basis, research into a physics-based determination of the coefficients is recommended. Additionally, full-scale measurements are advised to validate application on future kelp farm designs. Through this novel approach, the range of application is increased compared to existing methods, while straightforward setup and usage are governed, and limited computational costs allow for simulation without the need for a dedicated computer setup. The framework is shown to be robust by generating consistent simulation results. In summary, the established framework shows to be a good alternative to existing approaches to investigate the wave damping potential of large-scale offshore kelp farms.