Prediction of extreme wave loads in focused wave groups

摘要

The Volume of Fluid (VoF) method is a promising tool to predict extreme wave loads on fixed and floating offshore structures. The VoF method described in this paper has been validated step by step by means of model tests like dam break flow, sloshing in LNG tanks and loads on fixed structures in extreme regular waves. Until recently, there was no means to generate a realistic extreme irregular wave in the VoF method. Traditionally, these are generated in time-domain simulations by picking extreme events from long-duration simulations that apply a random phase model to generate waves. Due to the fact that the computational times in the VoF method are large such an approach is not feasible. Instead, an approach based on wave focusing can be applied. The focused wave is designed by choosing the phases of the linear harmonic components such that they are identical at a certain target location. By means of linear dispersion, the wave time trace at the wave generator is predicted and subsequently the required motions of the wave generator. This approach was applied in model tests designed to validate the VoF model in extreme irregular waves. A simplified box-shaped structure was placed in a shallow water basin in the path of the focused wave group and the wave loads were measured. This paper focuses on the ability of the VoF method to reproduce these focused wave groups and the subsequent wave loads on a typical fixed structure, using the motions of the wave generator. The VoF method contains too much numerical dissipation to properly compute wave propagation over long distances (from the wave generator to the target location). Therefore, a non-linear potential flow method is used to simulate the wave propagation from the wave generator to the boundary of the computational domain of the VoF method (close to the target location). At the boundary of the VoF domain the wave kinematics from the potential flow method are used as boundary conditions for the VoF method. The VoF method is then used to determine the focused wave around the target location and the impact on a structure which is located there.