Free-Surface Hydrodynamic Analysis of Floating Offshore Structures (FOS) Using Multiphase Flow Modeling

Floating Offshore Structures like platforms or buoys subjected to cyclic loads caused by wind, waves and currents. The motion of FOS and the resultant loads can be predicted with increased accuracy using new CFD techniques. Although studies of the deformed free surface near the waterline have been already performed, CFD studies of FOS separate the aerodynamic and hydrodynamic phenomena. While the separation of aero- and hydrodynamics saves time for modeling and simulation, it does not take into account the strong interaction between the forces associated with each. Moreover the role of viscous effects on the free surface elevation is often neglected, and this prevents such things as water flow separation from being properly captured. The aim of this study is to couple CFD analysis of the aerodynamic flow around the masts, together with the hydrodynamic flow around the hull and mooring line of a cylindrical buoy, including viscous effects and surface wave generation at the waterline. The volume of fluid (VOF) model in FLUENT software was used for this purpose. This multiphase model is designed for immiscible fluids (air and water in this case), and tracks the interfaces between them. The environmental conditions were adopted based on the met-ocean data of the Persian Gulf. The results of particular interest from the analysis include the free surface elevation with the related wave drag, and the aerodynamic and hydrodynamic lift and drag acting on the masts, hull, and mooring line. The lift and drag results can be used to derive the thrust and lateral forces on the buoy, which can then be used to predict the buoy velocity corresponding to a fixed configuration in terms of heave, drift and heel angle