Experimental and numerical investigation of a jacket structure subject to steep and breaking regular waves

Jacket structures installed in shallow water less than 50 m for offshore wind turbines are frequently exposed to steep and even breaking waves, which may result in severe damage of the structure. However, the models for calculating wave loads were primarily developed for monopile structures, and may not be suitable for jacket structures. The aim of this paper is to improve the understanding of extreme wave loads on jacket structures exposed to steep and breaking waves, based on a combination of numerical modeling and large scale experiments. The experiments with a 1:8 scaled jacket structure exposed to controlled regular waves were conducted in the WaveSlam project. In this study, the measured wave surface elevations and water particle velocities from the experiments are reproduced using a fully nonlinear potential flow solver with a fairly good agreement. Furthermore, the validity of the Morison equation applied to jacket structures is investigated. A reasonably good agreement is achieved for the steep waves without occurrence of breaking. In terms of the breaking waves, the calculated hydrodynamic forcing is examined by subdividing it into a quasi-static and an impulsive slamming force component. The quasi-static force is well reproduced by the Morison equation given the fully nonlinear wave kinematics. Furthermore, eleven degrees of parameterization of the slamming force history are statistically analyzed for a better understanding of the slamming force characteristics. Significant variabilities are presented in these parameters, as a result of the inherent uncertainties in the wave breaking process. In the end, practical implications of the proposed slamming force description to load assessment of a realistic offshore wind jacket structure are discussed.