In the study, the thickness of wave moved-sediment layers was measured under regular wave conditions as well as the initial slope of sea bed with grain size of medium diameter 0.237mm and 0.128mm. The initial bottom slope ( tan£\) is 1/45, and nineteen wave conditions were studied. For each case, we analyzed the results after about 28800 waves were made. Sands are similar density and grain size but different colors. After wave action, the clear boundary between the two layers (two different colors) of sands will be mixed into gray color by wave-induced vortex.According to unmoved-layer(white) and gray-layer to get the total wave-moved sediment quantity. Finally, we get the relationship between the wave-moved sediment quantity per wave( q) and two parameters( £`b and £Kb ) which were established by Liao (2005, doctorate draft). The grain size does not affect £`b , and we get the q= 6.486*10^-5*£`b . The relationships between and ¡G 0.110mm gets q=5.103*10^-6£Kb , 0.128mm gets q=1.139*10^-5*£Kb, and 0.237mm gets q=1.933*10^-5*£Kb .

Over the past decade, offshore wind has positioned itself as one of the most promising renewable energy markets. While this field is currently dominated by fixed-bottom wind turbines located within a limited depth range, floating turbines are showing promise as a way to capture the more developed wind profiles available in deeper waters. Currently, the main challenge with floating offshore wind is that the systems experience larger ultimate loads compared to fixed bottom turbines. These larger loads are caused by the increased motion inherent with floating structures. This study looks to analyze the effects that traditional and inerter based structural control methods can have on vibration reduction of floating offshore wind turbines. Models are developed adding tuned mass dampers (TMD) and tuned inerter dampers (TID) into the three main columns of a semisubmersible platform. Results showed that for free decay tests, heave and pitch root mean square (RMS) values were reduced significantly by the addition of passive structural control. The inerter based structural control consistently outperformed traditional TMD and also allowed for similar performance with significantly reduced physical mass values. For regular wave excitation both control methods resulted in significant reductions to the heave and pitch RMS values compared to the baseline, with the TID outperforming the TMD . And for an irregular wave analysis, it was found that both control configurations were still able to provide meaningful reductions to the baseline model. / Master of Science / Over the past decade, offshore wind has positioned itself as one of the most promising renewable energy markets. While this field is currently dominated by fixed-bottom wind turbines located within a limited depth range, floating turbines are showing promise as a way to capture the more developed wind profiles available in deeper waters. Currently, the main challenge with floating offshore wind is that the systems experience larger ultimate loads compared to fixed bottom turbines. These larger loads are caused by the increased motion inherent with floating structures. This study looks to analyze the effects that traditional and enhanced motion reduction technology can have on floating offshore wind turbines. Models are developed adding the traditional and enhanced motion reduction technology into the three main columns of a semisubmersible platform. Results showed that for several different tests, the motion reduction technology has a positive effect on the turbines. For test dropping the system from a set height, the motion reduction technology allowed the system to come to rest much faster. Moreover, the enhanced technology performed better than the traditional technology. The same results were found when the system was excited by simulated waves.

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.