Analysis of a Wave Energy Converter with Particular Focus on the Effects of Power Take-Off Forces on the Structural Responses

Wave energy is regarded as a major and promising renewable energy resource. The most critical factor to the success of deploying a wave energy converter in an ocean environment is the cost. The key factors affecting the costs include the performance, capital costs, operation and maintenance costs and risk. Changes to the design can affect both the performance and the costs simultaneously.

The objective of this thesis is to develop design tools and methods for the design process of wave energy converters in order to make them more competitive. Wave to wire models are numerical models used to evaluate the electrical power generated by a given wave energy device from a given wave condition. The first part of this work focuses on the development of such a numerical model. An important task is to quantify the wave-induced load effects to ensure that the input is correct and a safe and robust design of the converter can be carried out. One can achieve this task by running experiments, using simplified models from the classification societies or carrying out numerical calculations.

In this thesis, the dynamical properties of a semi-submerged hemisphere oscillating around a pivot point where the vertical height of this point is above the mean water level are investigated. The numericalmodel includes the calculation of the nonlinear hydrostatic restoring moment by a cubic polynomial function fit to laboratory test results. Moreover, moments due to viscous drag are evaluated on the oscillating hemisphere considering the horizontal and vertical drag force components. The influence on the motions of this nonlinear effect is investigated by a simplified formulation proportional to the quadratic velocity. Results from experiments are shown in order to validate the numerical calculations. All the experimental results are in good agreement with the linear potential theory as long as the waves are sufficiently mild. For steep waves however, the relative velocities between the body and the waves increase thus requiring inclusion of the nonlinear hydrostatic restoring moment to effectively predict the dynamics of the wave energy converter. For operation of the device with a passively damping power take-off the moment due to viscous drag is found to be negligible.

In the second part of the thesis the focus lies on the structural modeling of a wave energy converter. The challenge here is to incorporate different control strategies in the analysis and investigate their effect on the mechanical stresses. A fatigue model is set up which can be used as an independent and generic toolbox to calculate the fatigue damage at an early stage of the project. The stress responses due to the stochastic wave loads are computed by a Finite Element model in ANSYS using the frequency-domain approach. The fatigue damage is calculated based on the spectral-based fatigue analysis in which the fatigue is described by the given spectral moments of the stress history. The question will be answered, which control case is more favorable regarding the trade off between fatigue damage reduction and increased power production.