Towards a method to quantify plastic loss of a floating barrier due to wave overtopping

Abstract

Plastic pollution in the marine environment is an increasing problem with severe impacts on ecosystems and economies around the globe. The Ocean Cleanup (TOC) Foundation develops a floating barrier able to intercept, concentrate and extract plastic from the marine environment. TOC has conducted several experiments and numerical studies to determine the plastic capturing efficiency of its system. One of the phenomena leading to degradation of capturing efficiency is wave overtopping. During a wave overtopping event, the system is not able to properly follow the waves. This allows water to wash over the system and possibly lead to plastic, which was retained by the system, escape into the open ocean. This issue is amplified by the use of a stiffer barrier than the original concept that was developed by TOC.
To model and quantify plastic loss due to wave overtopping, the ideal approach would be to use a nonlinear 3D CFD method including hydro-elasticity of the barrier structure. Given the size of the problem and the number of conditions that need to be simulated to characterize the design space of the system, the use of such a method is computationally expensive and therefore unrealistic. Therefore, the objective of this work is to propose an alternative method. A method is presented which aims to quantify overtopping volumes by coupling a hydrodynamic solver to a 2D CFD solver. This thesis work will present the method in three parts.
Part I develops a hydrodynamic model to predict the first order (linear) motion response to wave excitation of a 2D cylinder with a spring attached. It is proposed that this model approximates the motions of a 2D cross-section of a long flexible cylinder subject to random wave excitation. The model’s output is a time trace of the motion response, which allows for coupling to a 2D CFD solver. It is found that the spring constant is dependent on the wave characteristics of the ambient wave field. Also, the ability of the 2D cylinder to follow waves is sensitive to the spring stiffness introduced in the model.
Derivation of the spring constant is handled in part II. Here, A model to approximate bending effects into a linear spring is set up. The ambient 3D wave field surrounding the capturing system is modelled and wave statistics are derived. Based on these wave statistics, spring constants are derived for given physical and environmental configurations. From the results it is found that for the wave cases 1 to 4, values of the equivalent spring stiffness coefficient are significant enough to influence the motion response of the floater. For case 5 and 6, values for the equivalent spring stiffness are relatively low and its influence on the response is expected to be negligible.
Part III handles coupling of the hydrodynamic model to the CFD solver ComFLOW. In the cases with low wave steepness, derivation of overtopping statistics has been achieved and results show that overtopping performance can be assessed by performing the steps taken in this research project. In these cases, the wave field is generally behaving linear and the motion response obtained from the hydrodynamic model can be coupled to the CFD solver. Results show that wave height and the applied spring stiffness are governing parameters for overtopping performance. It was also found that in some cases, non-linearities are introduced in the wave field. In the cases where non-linearities occur, the motion response derived by the hydrodynamic model deviates from the motion response that one would expect.