Simulation techniques and a Volterra functional polynomial are applied as two alternative methods of calculating ship roll response to irregular waves. The roll motion is modeled by a single degree of freedom differential equation, with two alternative nonlinear damping functions. Estimation techniques are developed to obtain the coefficients of the damping functions from decay tests and from forced rolling tests. A linear plus quadratic form of damping function is found to be slightly preferable to a linear plus cubic form. The roll response process is found to be non-Gaussian, and characterised by negative values of the coefficient of kurtosis. Simulation results agree well with results obtained from the functional polynomial for low response levels, but show increasing disagreement as the response level increases, due to divergence of the functional polynomial representation. Analyses of results from model tests in irregular waves and from sea trials confirm the non-Gaussian nature of the roll response. A "constrained" form of the generalised gamma distribution function is found to provide an improved fit to the roll maxima and tothe roll minima, as compared to the Rayleigh distribution. The model tests also show some asymmetry in the roll response, which is not predicted by the theoretical model. It is suggested that this asymmetry may primarily be due to the combined effect of horizontal drift forces and the restraining system used to keep the model on station.

Most designs for wave energy converters include a hydraulic (or pneumatic) interface between the wave device and the generator to smooth electricity production, but a direct drive power take-off system is a possible way of increasing the power transfer efficiency and the reliability, which was first adopted by Archimedes Wave Swing. Direct drive wave energy systems normally include a low speed linear generator directly coupled with the wave device. With no mechanical interface, the mechanical energy loss and maintenance requirements can, in theory, be significantly reduced. To maximize the energy capture, the motion of the wave energy converter must be controlled to achieve mechanical resonance so that the velocity is in phase with the incoming waves. So far, a number of control methods have been proposed, but few of them have been tested experimentally. For direct drive linear generators in real sea conditions, reaction force control is shown to be an effective way to achieve control where knowledge of future wave could not be required. Different reaction force control methodologies are suggested where the force is provided directly from the linear generator. Among these methodologies, complex conjugate control is regarded as the optimal control and can be used to achieve mechanical resonance. When resonance occurs, some system parameters such as the system excursion and required power take-off force become extremely large, and may exceed the design parameters. In this thesis, the system is modelled under reaction force control taking into account practical considerations which are based on design parameters. A novel control scheme for a direct drive linear generator to achieve such reaction force control in irregular waves is proposed, where a voltage-source rectifier is employed as the bridge between the linear generator and the dc bus. The application of linear generator in real wave conditions not only has inherent advantages, but also present a big challenge for controller design in order to obtain maximum power production. For a linear generator in real sea states, reaction force control idea can be implemented to adjust the velocity of motion, hence to maximize the power production, where the required currents in the generator coils to provide the desired force are constantly varying in frequency and amplitude. The control strategy of the active rectifier is developed based on the derived three-phase currents and the dynamic response of the system to determine varying modulation indices. The unknown situations and some unmeasurable parameters in the system degrade the performance of the control system, hence the current feedback and PI controller are both adopted to reject the effect of the disturbance. Simulation verifications are included for the proposed control idea.

The waves traveling on the surface of the world’s oceans carry atremendous amount of energy. The ability to convert this energy forhuman use has the potential to help solves the worlds energy problem. Adirect-drive linear generator point absorber is a wave energy converter(WEC) that aims to reduce the complexity of the overall system andshelter the most vulnerable parts of the system by placing them on theseabed. This concept builds around the buoy moving up and down indifferent sea-states which leads to a correlating vertical movement of thestroke in the generator resulting in the conversion of mechanical energyto power. This report aims to explore the possibility to use the open codeWEC-Sim to model the Uppsala University direct-drive linear generatorWEC in extreme sea states and to identify the resulting extreme loads. Theconstructed WEC-Sim model constrained the buoys motion in heave andsurge and limited its range of motion by modeling the generators upperend-stop spring. Simulations were run for different sea-states and theresulting forces on the system were analyzed. The peak line force for thedifferent sea states was calculated and compared to previous studies. Theresults validated the model as they showed a good correlation for mostsea-states. It was only for larger significant wave heights that there was adivergence compared to the results in previous studies.

Nos travaux ont porté sur l’étude des structures hydrodynamiques spatio-temporelles présentes dans le sillage d’un mât d’éolienne en mer, assimilé ici à un cylindre, pour une application future au parc éolien de Courseulles-sur-Mer. L’analyse hydrodynamique s’est focalisée sur le phénomène de synchronisation entre des vagues régulières et le sillage généré par le courant derrière deux cylindres verticaux. Un diagramme d’état résumant les différents régimes de synchronisation par rapport aux fréquences des vagues et amplitudes étudiées, a été établi. Pour compléter les résultats obtenus avec les vagues régulières, la synchronisation a aussi été étudiée en présence de vagues irrégulières. Dans le cas de vagues irrégulières, le coefficient de diffusion de la phase, K a été utilisé pour expliquer la synchronisation des fréquences. Après l’étude du phénomène de synchronisation, des essais ont été consacrés au transport sédimentaire (affouillement et rides) autour et dans un environnement proche d’un cylindre vertical sous l’effet de plusieurs types d’écoulement. Parallèlement aux expériences en laboratoire, une étude numérique a été amorcée. Dans un premier temps, une simulation DNS (Direct Numerical Simulation) d’un écoulement d’un courant seul autour d’un cylindre vertical a été effectuée. Un cas d’écoulement plus compliqué (i.e. Comprenant des vagues) et un domaine numérique simulant l’impact hydrodynamique d’un réseau de cylindres (modélisant un réseau de mâts d’éoliennes) soumis à un courant seul ont également été mis en oeuvre. Ce réseau comprend 4 cylindres disposés sur 2 rangées. Après l’étude numérique d’un courant seul, le solveur olaFoam a été utilisé pour imposer un écoulement composé d’un courant et de vagues dans un domaine relativement simple : sans obstacle et avec un fond lisse. Les résultats numériques se sont révélés être en assez bon accord avec la littérature au voisinage de la surface libre, montrant la décélération du courant de surface lorsque les vagues et le courant en entrée se propagent dans le même sens. / The various experiments carried out have laid emphasis on the study of hydrodynamic spatio-temporal structures found in an offshore wind turbine’s mast, coupled to a cylinder, which will later on be used at the Courseulles-sur-Mer wind farm. The hydrodynamic analysis focused on the synchronization phenomenon between regular waves and the Von Karman Street generated by a current, behind two different vertical cylinders. A diagram showing the different synchronization regimes with respect to the frequencies of the waves and the amplitudes studied, has been drawn. In addition to the results obtained with regular waves, the synchronization phenomenon has also been analysed using irregular waves. In the case of irregular waves, the phase diffusion coefficient, K, has been used to explain the different frequency synchronization observed. After having studied the synchronization phenomenon, tests were carried out on the study of sediment transport (scour and ripples) under the effect of different types of flows in an environment close to a vertical cylinder. In parallel to the laboratory experiments, a numerical study has also been conducted by studying the DNS (Direct Numerical Simulation) of a steady flow of current around a vertical cylinder. In addition to that a more complicated flow of a steady current around a network of cylinders comprising of a series of 4 cylinders arranged in 2 rows (representing a network of wind turbine masts) have also been observed and analysed. After having studied the flow made up by a steady current, the olafoam solver was used to apply a flow made up of both currents and waves in an obstacle free environment with a smooth floor. The numerical results obtained close to the free surface have been found to fairly agree with the findings of other related studies conducted. We observed a reduction in the intensity of the surface current when the waves and the current propagate in the same direction.