For high speed vessels and offshore structures, wave impact, a main source of environmental loads, causes high local stresses and structural failure. However, the prediction of wave impact loads presents numerous challenges due to the complex nature of the instant structure-fluid interaction. The purpose of the present study is to develop an effective wave impact model to investigate the dynamic behaviors of specific shaped elements as they impact waves. To achieve this objective, a wave impact model with a body swinging on a pendulum system is developed. The body on the pendulum goes through a wave free surface driven by gravity at the pendulum's natural frequency. The system's motion and impact force during the entire oscillation time beginning from the instant of impact are of interest. The impact force is calculated by applying von Karman's method, which is based on momentum considerations. The usual wave forces are presented in the Morison's equation and incorporated into dynamic systems with other wave forces. For each body shape, the dynamic system is described by a strongly nonlinear ordinary differential equation and then solved by a Runge-Kutta differential equation solver. The dynamic response behavior and the impact force time history are obtained numerically and the numerical results show support the selection of a pendulum model as an efficient approach to study slamming loads. The numerical prediction of this model is compared to previous experiments and classification society codes. Moreover, a basic design of wave impact experiments using this pendulum model is proposed to provide a more accurate comparison between numerical results and experimental data for this model. This design will also serve as a first look at the experimental application of the pendulum model for the purpose of forecasting slamming force.

Local and global loadings, which may cause the local damage and/or global failure and collapse of offshore structures and ships, are experimentally investigated in this study. The big research question is how the aeration of water and the elasticity of the structural section affect loading during severe environmental conditions. A further question is how the scattered waves from ships and offshore structures, the mooring line force and the structural response, which are known to affect local load and contribute to global load, will be affected by wave-structure interaction of a ship or offshore structure under non-breaking wave conditions. Three different experiments were undertaken in this study to try to answer these questions: (i) slamming impacts of a square flat rigid/elastic plate, which represents a plate section of the bottom or bow of ship structure, onto pure and aerated water surface with zero degree deadrise angle; (ii) wave impacts on a truncated vertical rigid/elastic wall in pure and aerated water, where the wall represents a plate section of a hull; and (iii) wave-structure interactions of different FPSO-shaped models, where the models were fixed or taut moored. The experiments were carried out at Plymouth University’s COAST Laboratory. Spatial impact pressure distributions on the square plate have been characterised under different impact velocities. It was found that the impact pressures and force in pure water were proportional to the square of impact velocity. There was a significant reduction in both the maximum impact pressure and force for slamming in aerated water compared to that in pure water. An exponential relationship of the maximum force and the void fraction is proposed and its coefficients are found from drop test in this study. There was also a significant reduction in the first phase of the pressure and force impulse for slamming into aerated water compared with pure water. On the truncated wall, aeration also significantly reduced peak wave loads (both pressure and force) but impulses were not reduced by very much. For the case considered here, elasticity of the impact plate has a significant effect on the impact loads, though only at high impact velocities; here the impact loads were considerably reduced with increasing elasticity. Wave loading on the truncated wall was found to reduce with increasing elasticity of the wall for all investigated breaking wave types: high aeration, flip-through and slightly breaking wave impacts. In particular, impact pressure decreases with increasing elasticity of the wall under flip-through wave impact. As elasticity increases, the impulse of the first positive phase of pressure and force decreases significantly. This significant effect of hydroelasticity is also found for the total force impulse on the vertical wall under wave impacts. Scattered waves were generated from the interaction of focused wave groups with an FPSO model. The results show that close to the bow of the FPSO model, the highest amplitude scattered waves are observed with the most compact model, and the third- and fourth-harmonics are significantly larger than the incident bound harmonic components. At the locations close to the stern, the linear harmonic was found to increase as the model length was decreased, although the nonlinear harmonics were similar for all three tested lengths, and the second- and third-harmonics were strongest with the medium length model. The nonlinear scattered waves increased with increasing wave steepness and a second pulse was evident in the higher-order scattered wave fields for the fixed and free floating models. In addition, the higher harmonics of the mooring line force, and the heave and pitch motions all increased with increasing wave steepness. Incident wave angles of 0 (head-on), 10 and 20 degrees were experimentally investigated in this study. As the incident wave angle between the waves and the long axis of the vessel was increased from 0 to 20 degrees, the third- and fourth-harmonic scattered waves reduced on the upstream side. These third- and fourth-harmonic diffracted waves are important in assessing wave run-up and loading for offshore structure design and ringing-type structural response in fixed and taut moored structures. The second-, third- and fourth-harmonics of the mooring line force, and the heave and pitch motions decreased as the incident wave angle increased from 0 to 20 degrees.

Numerical modelling currently cannot accurately reproduce the onshore-offshore transport asymmetry observed on gravel beaches. The role of the impulsive pressure response generated by plunging waves has been hypothesised to aid mobilisation of sediment, and thus may contribute to transport asymmetry. This process is not currently included in models. Laboratory tests were conducted across a range of wave conditions to investigate the role of plunging wave-breaker impacts on the mobilisation of sediment of gravel beaches. Pressure records were obtained at positions close to the plunging impact locations, to monitor the localised pressures that lead to sediment mobilisation. The correction of the recorded pressure to the bed surface, for further analysis, was achieved through a two stage approach. Adoption of a new technique for separating the pressure records into two components, each determined by different processes is presented. Each component is then corrected to the bed surface with the application of a pragmatic prediction of the experienced attenuation. Data covering a wide range of Iribarren values was assessed, and the impact pressure was parameterised against the wave-breaker type. This procedure identified a potential peak in the impact pressure-Iribarren space in the plunging breaker region, consistent with the previous hypothesis. Comparison of cross-shore profile records provides further limited evidence that morphological prediction fails to reproduce specific characteristics associated with profiles generated under plunging breaker action. Finally, a brief discussion is provided on how the role of the additional pressure generated under plunging impacts can be incorporated into future numerical models.

This thesis is concerned with the use of the open source computational fluid dynamics (CFD) software package, OpenFOAM® for predicting and analysing the behaviour of a near-shore oscillating wave surge converter (OWSC), when subject to various types of ocean wave conditions in a numerical wave tank (NWT). OpenFOAM® which utilises a Finite Volume Method (FVM) is used to solve the incompressible, Reynolds Averaged Navier-Stokes (RANS) equations for a two-phase fluid, based on a Volume of Fluid (VOF) phase-fraction approach to capture the interface between the air and water phases. Preliminary studies on classic wave-structure interaction benchmark cases, involving a fixed and a vertically oscillating semi-immersed horizontal cylinder are carried out. The gradual transition of the linear to non-linear behaviour of the horizontal and vertical forces induced on a fixed cylinder when subject to various regular waves, and the amplitude ratios of the surface waves elevations generated by the prescribed oscillatory motion of the cylinder, are shown to provide good overall agreement within the limitations of the relevant theory and the experimental data in the literature. The OWSC is modelled with the inclusion of a Power Take-Off (PTO) system, using a linear damping restraint, and simulated in two-dimensional (2D) and three-dimensional (3D) setups. The 2D and 3D numerical results, such as the surface wave elevations, flap angular velocity, PTO torque and flap angular displacement, compare well with one another and with the experimental data for operational regular head-on and oblique wave conditions. Small discrepancies between numerical results and experimental data are likely to be caused by non-linear behaviour of the PTO system. Pressure distributions on the flap surfaces and forces induced on the flap and hinge of the OWSC for various wave conditions are also presented. The effects between 2D and 3D wave-structure interactions become more significant when subject to large waves that break during impact. Comparison between the full scale and 1:24 scale numerical results of the OWSC shows no significant evidence of viscous and scaling effects. The validated 2D OWSC model is also subject to embedded focused waves, to predict the worse possible scenario of wave loading in extreme wave conditions. The delay of the focus event breaking is shown to affect the slamming behaviour for the larger focus event wave heights. Incorporation of a focused wave at different phase positions within a background of regular waves reveals that the focus event wave height has little effect on the peak tangential force on the flap during the slamming event, when a PTO cut-off mechanism is implemented to prevent excessive torque surges. In contrast, the peak radial force on the flap and the maximum resultant force on the hinge appear to respond more sensitively to the focus event wave height. It has been demonstrated that OpenFOAM® is able to provide a comprehensive understanding of the complex hydrodynamic analysis and prediction of highly non-linear wave-structure interactions for an OWSC, which give useful guidance and confidence to WEC developers on the design considerations relevant to the OWSC systems.

Dans le cadre du stockage de Gaz Naturel Liquéfié (GNL) dans des réservoirs flottants, tels que les méthaniers, les contraintes imposées à la cuve par le ballotement de la cargaison doivent être quantifiées. La plupart des études expérimentales ou numériques actuelles ne prennent pas en compte la possibilité de changement de phase entre le GNL et sa vapeur lors d'un impact du liquide sur la paroi. L'objectif de cette thèse est l'ajout de ce phénomène physique dans un code de mécanique des fluides numérique pour la simulation de l'impact d'une vague déferlante sur une paroi.Dans ce but, un état de l'art des différentes modélisations possibles du changement de phase en mécanique des fluides est présenté. Il a été choisi de modéliser le changement de phase entre le liquide et le gaz à une interface franche sans hypothèse d'équilibre thermodynamique à l'interface. Un système hyperbolique de lois de conservation incluant le changement de phase interfacial hors-équilibre est présenté.Deux approches sont utilisées pour la résolution numérique de ce système. La première utilise un modèle de mélange pour décrire les mailles contenant l'interface liquide-vapeur. Dans la seconde méthode, l'interface est reconstruite et évolue de manière lagrangienne. Les deux approches sont basées sur un schéma volume fini de type Roe.L'enjeu de la simulation numérique du changement de phase interfacial est la capacité du code à gérer un rapport de densité loin de 1 et une chaleur latente élevée, qui entrainent respectivement de fortes variations de pression et de température à l'interface. L'aspect thermique est le phénomène limitant dans le cadre de la simulation d'impacts de vagues avec changement de phase. Seule une fine couche limite thermique autour de l'interface tend à revenir à l'équilibre thermodynamique liquide vapeur, ce qui limite l'effet quantitatif du changement de phase. / In the context of Liquefied Natural Gas (LNG) transportation in floating tanks, such as in LNG carriers, the constraints imposed by the sloshing of the liquid cargo on the tank have to be estimated. Most experimental and numerical studies until now do not take into account the possibility of phase change between the LNG and its vapor during the impact of liquid on the wall. The goal of this thesis is to include this physical phenomenon into a CFD code for the simulation of a breaking wave impact on a wall.A state of the art of the different modelisations of phase change in fluid mechanics is thus presented. This work focus on the modeling of phase change between the liquid and the gas at a sharp interface, without any equilibrium hypothesis. An hyperbolic system of balance laws including non-equilibrium interfacial phase change is presented.Two approaches are used to solve numerically this system. The first one relies on a mixture model for the description of the finite volume cells containing the interface, whereas in the second approach the interface is reconstructed and evolves in a lagrangian way. Both methods are based on a Roe-type finite volume scheme.The challenge of the numerical simulation of interfacial phase change is the capacity of the code to deal with density ratio far from 1 and high latent heat, as the lead to high temperature and pressure variations at the interface. The thermal aspect is the limiting phenomenon in the frame of wave impact simulation with phase change. Only a thin boundary layer around the interface tends to return to thermodynamical equilibrium, thus limiting the quantitative effect of phase change.

[Truncated abstract] The hydrodynamics of wave impact on offshore structures is not well understood. Wave impacts often involve large deformations of water free-surface. Therefore, a wave impact problem is usually combined with a free-surface problem. The complexity is expanded when the body exposed to a wave impact is allowed to move. The nonlinear interactions between a moving body and fluid is a complicated process that has been a dilemma in the engineering design of offshore and coastal structures for a long time. This thesis used experimental and numerical means to develop further understanding of the wave impact problems as well as to create a numerical tool suitable for simulation of such problems. The study included the consideration of moving boundaries in order to include the coupled interactions of the body and fluid. The thesis is organized into two experimental and numerical parts. There is a lack of benchmarking experimental data for studying fluid-structure interactions with moving boundaries. In the experimental part of this research, novel experiments were, therefore, designed and performed that were useful for validation of the numerical developments. By considering a dynamical system with only one degree of freedom, the complexity of the experiments performed was minimal. The setup included a plate that was attached to the bottom of a flume via a hinge and tethered by two springs from the top one at each side. The experiments modelled fluid-structure interactions in three subsets. The first subset studied a highly nonlinear decay test, which resembled a harsh wave impact (or slam) incident. The second subset included waves overtopping on the vertically restrained plate. In the third subset, the plate was free to oscillate and was excited by the same waves. The wave overtopping the plate resembled the physics of the green water on fixed and moving structures. An analytical solution based on linear potential theory was provided for comparison with experimental results. ... In simulation of the nonlinear decay test, the SPH results captured the frequency variation in plate oscillations, which indicated that the radiation forces (added mass and damping forces) were calculated satisfactorily. In simulation of the nonlinear waves, the waves progressed in the flume similar to the physical experiments and the total energy of the system was conserved with an error of 0.025% of the total initial energy. The wave-plate interactions were successfully modelled by SPH. The simulations included wave run-up and shipping of water for fixed and oscillating plate cases. The effects of the plate oscillations on the flow regime are also discussed in detail. The combination of experimental and numerical investigation provided further understanding of wave impact problems. The novel design of the experiments extended the study to moving boundaries in small scale. The use of SPH eliminated the difficulties of dealing with free-surface problems so that the focus of study could be placed on the impact forces on fixed and moving bodies.

Lors du tsunami de Tohoku en 2011, des relevés de terrain sur les côtes japonaises ont montré la fragilité des structures côtières, où le plus grand brise-lames du monde (brise-lames de Kamaishi) a été fortement endommagé dû à cet événement. Dans ce doctorat, l'objectif est d'étudier l'interaction entre les brise-lames , les structures côtières les plus communes protégeant les zones urbaines et les entrées des ports, et les vagues, en particulier les tsunamis.Dans la première partie de ce travail, la transformation du tsunami en bore ondulaire dans les zones côtières est étudiée numériquement avec le code de calcul BOSZ (modèle Boussinesq). Les résultats montrent que la deuxième vague générée par le tsunami de Tohoku s'est transformée en un bore ondulaire. En revanche, la première vague n'était pas assez cambrée pour permettre une telle transformation. Les forces et les moments dus aux vagues ainsi que la contrainte normale appliquée par la base arrière du caisson sur le sol de fondation sont calculés à l'aide de deux modèles numériques différents: BOSZ et THETIS (modèle Navier-Stokes). Les résultats de BOSZ sont comparés avec THETIS pour l'interaction tsunami-structure. L'étude d'impact est réalisée à relativement grande échelle dans le but d'obtenir une première estimation des efforts d'un tsunamiPar la suite, une expérience numérique utilisant le modèle THETIS a été réalisée pour étudier les impacts du type flip-through sur des brise-lames. Ces impacts de vagues sans air emprisonné sont considérés comme le type d'impact le plus extrême dans la littérature (e.g. Cooker & Peregrine (1992), Hofland et al. (2011)). L'influence de l'inclinaison de l'interface sur la dynamique d'impact et les pressions générées sont analysées dans une configuration de brise-lames réelle. Le modèle d'onde solitaire est utilisé pour générer trois impacts caractéristiques du type flip-through: peu cambré, moyen et très cambré. Le champ de vitesses et la pression à l'intérieur de la fondation sont également étudiés dans cette partie. Les forces horizontales et verticales appliquées sur le caisson sont estimées en intégrant les distributions de pression données par THETIS.La dernière partie de ces travaux montre la stabilité des caissons de brise-lames soumis à des impacts du type flip-through, qui sont ici assimilés à un jet triangulaire (e.g. Cumberbatch (1960), Kihara et al. (2015)). Cette approche simple permet de formuler un modèle semi-analytique pour prédire le mouvement des caissons dû à ce type d'impacts. Après validation avec des simulations numériques, la méthode du jet triangulaire permet d'obtenir des informations sur les forces, la durée du mouvement et le déplacement total en fonction des caractéristiques de la vague et des dimensions du caisson du brise-lames impacté. / During the Tohoku tsunami in 2011, field surveys of the east coast of Japan showed the weakness of coastal defences, as even the world largest tsunami breakwater (Kamaishi) almost completely collapsed due to this event. In this PhD, the aim is to investigate the interaction between breakwaters, the most common offshore coastal structures protecting urban areas and harbour entries, and waves and especially tsunami waves.In the first part of the work, the generation of undular bores in the near-shore area of Sendai during the Tohoku event is numerically investigated with the numerical model BOSZ (Boussinesq-type model). It is shown that the second wave, which stroke the coast during this event, transformed into an undular bore, whereas the first wave did not due to steepness differences. Tsunami loads, moments and bearing stress applied on the offshore breakwater of the Soma Port are calculated using two models: BOSZ and THETIS (Navier-Stokes VOF model). BOSZ results are compared to THETIS for the tsunami wave-breakwater interaction. The impact study is carried out at a relatively large scale aiming to have a first estimation of tsunami efforts. Then, a numerical experiment using THETIS is carried out to investigate flip-through impacts on vertical breakwaters. This non-aerated wave impact is considered as the most severe type of impact in the literature (e.g. Cooker & Peregrine (1992), Hofland et al. (2011)) in terms of maximum pressure generated. The influence of the front interface on the impact dynamics and the pressure induced is analysed in a realistic breakwater configuration. Solitary waves are used to obtain three characteristic flip-through impacts involving least steep, medium steep and steepest wave front. The flow field and pressure inside the porous rubble mound are then investigated as well as horizontal and uplift forces applied on the breakwater caisson. The last part of this study is devoted to the stability of breakwater caissons submitted to flip-through impacts. The latter are here assimilated to water wedges (e.g. Cumberbatch (1960), Kihara et al. (2015)). This simple approach allows to formulate a semi-analytical model to predict caisson motion due to this type of impacts. After validation with numerical results, the water wedge method gives rich informations about forces, motion duration and sliding distance depending on the wave impact characteristics and breakwater caisson dimensions.