Hydrodynamic loading of breaking waves on offshore inspection platforms

Abstract

The most violent breaking waves cause the largest platform forces at sea, however it is unknown which wave in a sea state leads to these forces if run-up is included. Offshore inspection platforms give access to wind turbine monopiles. They are positioned above mean sea level. Wave interaction with monopiles leading to runup causes vertical impact forces on inspection platforms. Predicting the vertical impact is challenging. The objective of this article is to find the wave in a sea state that causes the largest vertical impact force on a horizontal platform. It is hypothesized that breaking waves will cause the largest vertical forces. Detailed numerical OpenFOAM Volume-of-Fluid simulations are performed to investigate wave breaking interaction with structures. long-crested wave propagation was considered in 2D with a schematized inspection platform, composed of a horizontal deck and a vertical wall above mean sea level – the minimum requirements to still cause wave run-up and a vertical impact. This simplified numerical setup results in high resolution particle velocities that we require for further investigation with the kinematic breaking criterion. A potential flow solver, OceanWave3D, was used to generate a three-hour sea state representative of North Sea conditions. From the sea state, 17 waves were selected based on the kinematic breaking criterion and their surface elevation. The selected waves were fed into an OpenFOAM domain with and without structure to determine their breaking index, particle velocities and forces on the structure. This is the first study to use the breaking index as a screening parameter for critical wave selection. The three waves causing the largest forces on the structure were simulated again with the structure present at various horizontal positions relative to the point of wave overturning. The wave with the highest surface elevation and the highest breaking index overturning one third of the peak wave length before the inspection platform caused the largest vertical forces. This is due to the strong relation between impacts loads and the free surface particle velocity