Hydroelastic wave deformation of Very Flexible Floating Structures

Abstract

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.