Investigations of Offshore breaking Wave Impacts on a large offshore Structure

Abstract

This paper describes numerical and laboratory investigations that have been carried out to gain a better understanding of the physical processes involved in offshore breaking wave impacts on a large offshore structure. The findings are relevant to offshore and coastal structures and to identifying the extreme loads, peak pressures and maximum run-up needed for their design. A truncated wall in a wave flume is used to represent a vertical section of an FPSO (Floating Production Storage and Offloading) hull, which is a typical large offshore structure. Four types of wave impact were identified in the tests, and are referred to as slightly-breaking, flip-through, large air pocket and broken wave impacts. Physical modelling was undertaken in Plymouth University's COAST Laboratory and the open source Computational Fluid Dynamics (CFD) package-Open Field Operation and Manipulation (OpenFOAM) was adopted to study focused wave generation and wave impact on the hull. The method solves incompressible Unsteady Reynolds-averaged Navier–Stokes Equations (URANSE) using a finite volume method with two phase flows. A Volume of Fluid (VoF) interface capturing approach is used to model the free surface. A NewWave boundary condition is used to generate focused wave groups based on the first plus second-order (hereafter second-order) Stokes wave theory in the Numerical Wave Tank (NWT). By changing the focus location with respect to the wall, the wave impact type was altered in both the numerical and laboratory investigations. The results show that for the four wave impact types tested good agreement was achieved between numerical predictions and experimental measurements of surface elevation, run up and impact force. The peak pressures predicted by the simulation are lower than the experimentally measured results due to time step constraints, although the shape of the pressure time history is very similar. Four distinct wave impact types are identified for the vertical hull section and are found to be similar in character to those observed for a full depth vertical wall. The predicted force on the hull is found to be greatest for the large air pocket impact, and the highest run-up for the slightly-breaking wave impact. The pressure records show a high degree of spatial and temporal variation though the highest pressure recorded at any location was due to flip-through. This research has shown that different characteristic wave impact types are responsible for maximum load and greatest wave run-up and so need to be considered separately for design purposes.