Andrew Stephen Zurkinden

Comparison Between Numerical Modeling and Experimental Testing of a Point Absorber WEC Using Linear Power Take-Off System

Currently, a number of wave energy converters are being analyzed by means of numerical models in order to predict the electrical power generation under given wave conditions. A common characteristic of this procedure is to integrate the loadings from the hydrodynamics, power take-off and mooring systems into a central wave to wire model. The power production then depends on the control strategy which is applied to the device. The objective of this paper is to develop numerical methods for motion analysis of marine structures with a special emphasis on wave energy converters. Two different time domain models are applied to a point absorber system working in pitch mode only. The device is similar to the well-known Wavestar prototype located in the Danish North Sea. A laboratory model has been set up in order to validate the numerical simulations of the dynamics. Wave Excitation force and the response of the device for regular and irregular waves were measured. Good correspondence is found between the numerical and the physical model for relatively mild wave conditions. For higher waves the numerical model seems to underestimate the response of the device due to its linear fluid-structure interaction assumption and linearized equation of motion. The region over which the numerical model is valid will be presented in terms of non-dimensional parameters describing the different wave states.Copyright

Fatigue Analysis of a Wave Energy Converter Taking into Account Different Control Strategies

Point absorber wave energy converters (WEC) are subjected to random wave loads. In addition, the power production of a WEC and the motions are considerably influenced by the applie d control mechanism. For small waves, with a wave period close to the natural period of the oscillating system, the power ou tput may be controlled passively by means of a constant dampin g coefficient. The energy is extracted proportionally to the s quare of the body 0 s velocity. If the wave period is away from the natural period, reactive power may be applied in order to enlarg e the resonance bandwidth. Recent studies on a point absorber have shown that the stresses at a particular section of the st ructure depend on the control parameters. The power is increase d by choosing a more advanced control mechanism. The consequences are that the stress amplitudes are higher than for th e more conservative control case. In this study, the focus is g on the fatigue damage calculation of a structural detail by t aking

STRUCTURAL MODELING AND ANALYSIS OF A WAVE ENERGY CONVERTER APPLYING DYNAMICAL SUBSTRUCTURING METHOD

This paper deals with structural modeling and analysis of a wave energy converter. The device, called Wavestar, is a bottom fixed structure, located in a shallow water environment at the Danish Northwest coast. The analysis is concentrated on a single float and its structural arm which connects the WEC to a jackup structure. The wave energy converter is characterized by having an operational and survival mode. The survival mode drastically reduces the exposure to waves and therfore to the wave loads. Structural response analysis of the Wavestar arm is carried out in this study. Due to the relative stiff behavior of the arm the calculation can be reduced to a quasi-static analysis. The hydrodynamic and the structural analyses are thus performed separately. In order to reduce the computational time of the finite element calculation the main structure is modeled as a superelement. The structural detail, where the stress analysis is carried out, is connected with the superstructure by interface nodes. The analysis is conducted for two different control situations. Numerical results will be presented which can be further used to carry out fatigue analysis in which a more refined FE model is required to obtain the stress concentration factors.Copyright

Nonlinear Motion Analysis of the Wavestar Wave Energy Converter With a Focus on the Structural Responses

The scope of this paper is to connect a nonlinear WEC numerical model with a structural response model. The numerical WEC model takes into account the nonlinear hydrostatic restoring moment of the Wavestar float. A parameterized structural model of the Wavestar arm is developed in ANSYS APDL. Based on the assumption that the structural displacements remain small, linear first order theory is used to calculate the structural response. The section moments and forces are thus superimposed according to the superposition law. The effect of the nonlinear hydrostatic restoring moment on the structural response is investigated. Moreover, an analysis is carried out which shows that reactive control, applied as a closed loop control, increases the section moments and shear forces.Copyright