The Effect of Boundary Condition on Predicting the Thrust Loss of Azimuth Thruster in Ship-Type Offshore Structures

For various offshore structures including ship types such as drillship, Wind Turbine Installation Vessel (WTIV), Floating Production Storage and Off-loading (FPSO), and Floating Liquefied Natural Gas (FLNG), the dynamic positioning (DP) is a fundamental function. For effective DP operation, the special propulsor, for example, azimuth thruster was chosen in general. It is well known that several azimuth thrusters are installed in offshore structures, the thrust loss might be induced by various interaction phenomena. In this study, the influence of boundary conditions is mainly examined for predicting thrust loss on numerical simulation. WTIV and FPSO were chosen as a target vessel, respectively. Two different types of boundaries were defined. The first consideration is that the boundary condition was assigned with the direction of the azimuth angle of the thruster whereas it is fixed regardless azimuth angle of the thruster. The predicted thrust loss according to these boundary conditions showed a difference and this is originated from the current load of the vessel. The current load on the ship depends on the ship type, draft, and inflow direction, the magnitude of the current load is affected by those conditions. Since the current load might be induced unintentionally in numerical simulation, the boundary condition is important to obtain a realistic thrust loss.