Wave-structure interaction of offshore wave energy converters


William Finnegan – Doctoral Dissertation, National University of Ireland, Galway – November 2013

Abstract

With the continuing rise in oil prices and greater concern for the damage to the atmosphere, the world is continually looking for a cleaner and more sustainable form of energy. Ocean wave energy as a renewable source of energy, which as of yet is relatively unexploited, offers a possible solution to this energy crisis. The concept of harnessing ocean wave energy is by no means a new idea. However, the topic only gained international interest in the 1970s with the publication of Stephen Salter’s ground-breaking paper on his Wave Energy Duck. The current research study aims to aid the exploitation of this resource by developing robust and reliable analytical and numerical models. These numerical models will provide a platform for designers to optimise their marine renewable energy devices, in particular wave energy converters, before venturing into large scale physical testing, which is a very costly procedure. Therefore, the main objective associated with the current research is to develop numerical models which can accurately perform the interaction between an ocean wave and a structure to assist in the design of wave energy converters. However, in order to achieve this, two subtasks must be completed, which are: (1) to derive an analytical approximation in order to determine the wave excitation forces on a floating truncated cylinder in water of infinite depth and (2) to develop a computational fluid dynamics numerical model for a wave tank that can accurately simulate interaction between an irregular water wave and a floating structure. In the derivation of the analytical approximation, the method of separation of variables was employed in solving the appropriate boundary value problem to derive the velocity potentials. Graphical representations of the analytical approximation for the truncated vertical cylinder and the cylinder of infinite depth are presented. The presented analytical approximation was found to be in good agreement when compared with the results from computational fluid dynamics analysis, using a commercial boundary element package, and with independent experimental data. The novel contribution of the presented analytical approximation is that it provides a solution which is far easier to use and implement than already available analytical solutions. A methodology for developing a numerical model for a wave tank, commonly known as a numerical wave tank (NWT), that can accurately simulate linear regular waves and perform linear wave-structure interaction was then derived. In the current study, the finite volume commercial software ANSYS CFX, which uses a solver based on the Reynolds-averaged Navier-Stokes equations, was used to perform the numerical analysis. This methodology was validated by comparing the outputs to physical experimental studies performed using the in-house wave flume and good agreement between the two were found. The state-of-the-art contribution is the methodology for the development of an optimum numerical model of a wave tank, in terms of the desired wave period generated. The numerical model was then advanced in order to generate linear irregular water waves. The waves generated are simulated measured real sea waves, which were recorded at the Atlantic marine energy test site (AMETS) off the west coast of Ireland. Finally, a breakwater type floating structure was introduced into the model to explore the interaction between an irregular ocean wave and a structure. The results of this study were found to be in good agreement with the prediction from a hydrodynamic analysis of the structure. The ability of the model to accurately model measured ocean waves and their interaction with a floating structure is the novel aspect here. Finally, numerical CFD models were developed to aid in the design of offshore wave energy converters (WECs). One application is in the development of a methodology to optimise the dynamic heave response of the floating oscillating part of the WEC through form finding of the geometric configuration of its structure. The state-of-the-art aspect lies within the methodology itself. It offers a designer a method of optimising the performance of a WEC, in terms of its geometric configuration, at a given location using a single wave energy spectrum as the input. In this study, the wave energy spectrum was derived from three years of data recorded at the Atlantic marine energy test site.

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