Extreme waves at the sea surface can have severe impacts on marine structures. One of the theoretical mechanisms leading to extreme waves is the instability of deep-water wave trains subject to initially small perturbations, which then grow exponentially. The present study focuses on the two-dimensional Benjamin–Feir (or modulational) instability and the three-dimensional crescent (or horseshoe) waves, also known as Class I and Class II instabilities, respectively. Numerical studies on Class I and Class II wave instabilities to date have been limited to models founded on potential flow theory, thus they could only properly investigate the process from initial growth of the perturbations to the initial breaking point. The present study conducts numerical simulations to investigate the generation and development of wave instabilities involving the wave breaking process. A CFD model solving Reynolds-averaged Navier-Stokes (RANS) equations coupled with turbulence closure in terms of the anisotropic Reynolds stress model is applied. Wave form evolutions, Fourier amplitudes, and the turbulence beneath the broken waves are investigated.

Gravity-based offshore foundations generally consist of a bottom slab and one or more cylindrical shafts on top of it. The geometry of the structure can strongly affect the flow pattern, dynamic wave pressure and further soil response and the liquefaction risk in the vicinity of the foundation. In this work, gravity-based foundations with bottom slabs of cylindrical shape and hexagonal prismatic shape are investigated. An integrated wave-structure-seabed interaction model applied in this work is developed in Open-FOAM, incorporating a nonlinear wave solver, a linear elastic structure solver and an anisotropic Biot’s poroelastic soil solver consisting of consolidation and liquefaction modules. Soil consolidation behavior in the presence of the foundations is investigated. It is found that the corners of the hexagonal foundation cause stress concentration in the soil. Therefore the initial effective stress around the hexagon corners is relatively high. Then, fully nonlinear waves modelled by fifth-order stream functions are simulated. Wave-induced pressure distributions and momentary liquefaction depths around the foundations are predicted.

In order to prevent the future risk of soil and structural failures, it is essential to evaluate the dynamic seabed soil behaviors in the vicinity of the offshore foundations under dynamic wave loadings. Three-dimensional (3D) numerical analysis is conducted on the interaction between waves, seabed soil and a gravity-based wind turbine foundation. An OpenFOAM based numerical code developed by Tang [1]for wave-structure-seabed interaction is applied. The nonlinear waves are modeled by solving the Navier-Stokes equations for incompressible flow. The dynamic structural response of the foundation is computed using a linear elasticity solver. The transient responses of the seabed are solved by an anisotropic poro-elastic soil solver. The dynamic interaction between different physical domains is implemented by boundary condition coupling and updating in the integrated FVM based framework. The dynamic wave pressure on the structure and the seabed, the elastic responses of the structure and the changes of the pore pressure, shear stress and seepage flow structure in the seabed are investigated. Highest wave-induced shear stress along the foundation is predicted by solving the deformable structure model. For the seabed soil in the vicinity of the foundation, it is found that the presence of the foundation affects the soil responses by amplifying the wave induced shearing effect on the underlying seabed. Vertical distributions of the pore pressure in the seabed beneath the foundation are investigated with different angles relative to the wave propagation direction. A parametric study of isotropic and anisotropic soil permeability is performed and demonstrates that for the simulated soil in this work, the consideration of the anisotropic permeability is suggested.

SeaWEED (Sea Wave Energy Extraction Device) is a multi-body floating wave energy converter (WEC) with hinged joints developed by Grey Island Energy Inc. (GIE) in Canada. Initial conceptual studies have been carried out to evaluate the performance of the first generation device by testing a 1:16 scale model in a wave basin. The experimental results were compared with the numerical solutions. Based on the experimental studies, improvements were made and a second generation model with a new geometry of the hull and a new connection structure was developed. This paper is mainly focused on the numerical analysis and optimization of the second generation SeaWEED model. In the numerical studies, the hydraulic power take-off (PTO) system was simulated by a linear spring damper system coupled with the motion of the hinged bodies. The vertical hinge motion was computed at a series of wave periods using WAMIT. Optimization was focused on the PTO damping and the geometrical parameters in terms of the draft and the length of the truss structure between hinged bodies by using the response surface method. An optimal combination of length, draft and PTO damping was recommended for an intended operation location.