Extreme waves endanger offshore structures under severe environmental conditions. These large and steep waves are highly nonlinear, which can cause high-intensity and short-duration impact forces. It is vital to understand the impact created by such extreme events over any structure. The objective is to investigate wave impact forces on a vertical surface piercing cylinder subjected to breaking waves. The numerical simulation is carried out with a hybrid coupled solver named foamStar. It couples a High-Order Spectral (HOS) based nonlinear potential model and an OpenFOAM based CFD model.The HOS model describes accurately in a fully nonlinear potential flow framework the focused breaking wave without the structure. The CFD model is based on the incompressible Reynolds-averaged Navier–Stokes equations and the volume of fluid for the free surface. The coupled model is a CFD based numerical wave tank. First, the focused wave onset of breaking is validated in HOS-NWT and foamStar, and then its interaction with the cylinder is addressed. The experimental results used in this paper correspond to the experiments performed in Ludwig-Franzius-Institute, Germany[1]. The experimentally measured main wave crest of the breaking focused wave group with its total forces and pressure over a cylinder is fairly well captured in the numerical simulation. Further, the evolution of focused breaking waves along the tank and their characteristics were examined. An overall good degree of agreement is reported, which denotes that the model can be a helpful tool to evaluate breaking wave forces on structures.

In ocean environments, floating wind turbines are subject to both breaking waves and directional waves. While the response is often modelled through radiation-diffraction theory, the effects of higher-order wave loads are hereby limited to second order, and the damping often needs calibration against model tests. Better accuracy and full coverage of nonlinearity can be achieved through CFD. However, there is a limited robust Open Source CFD based six degrees of freedom (6-DOF) rigid body solvers, that can both generate different types of waves and also accurately simulate floating foundation motions. FloatStepper, a recent OpenFOAM-based rigid body motion algorithm, effectively addresses these differences, making it a good alternative to conventional body motion solvers. Its reliability and accuracy have been demonstrated in several benchmark cases, including single-phase and two-phase flows. The present study conducts a comprehensive examination the motion responses and mooring line tensions to unidirectional and multi-directional waves for a design variant of Stiesdal Offshore’s TetraSub floating foundation, using the FloatStepper algorithm. In particular, three categories of waves are investigated: 1) regular waves 2) unidirectional irregular and 3) multi-directional (3D) focusing waves. The numerical results are validated through comparison with experimental data from the 2023 test campaign of the Danish FloatLab project. The study contributes to the efficient use and validation of this robust CFD solver. It further extends the understanding of the multi-directional waves and wave-structure interaction for floating wind turbines, identifying potential limitations and areas for refinement in the modelling of this complex dynamics in CFD.