The coupling between vessel motion and inner-tank sloshing is investigated by a potential-CFD (Computational Fluid Dynamics) hybrid method in time domain. Potential-theory-based 3D diffraction/radiation panel program is used to simulate vessel motions in time domain. The liquid sloshing in tanks is simulated with smaller time steps by using the improved MPS (Moving Particle Simulation) method. The ship motion, which is influenced by sloshing-induced tank forces, is in turn inputted to the MPS system as forced motions. For the verification of the coupling, a barge-type FPSO hull with two partially filled inner tanks is selected and the numerically simulated results correlate well against the measurements done by MARIN for various fill ratios. It is seen both in prediction and experiment that the roll RAOs (Response Amplitude Operators) are sensitive to the amount of liquid cargo and can be increased or decreased by a factor of 2 or 3 in some wave frequency range compared to the bare-hull case. It is also shown that the nonlinear sloshing effects can alter the vessel-motion characteristics in relatively high waves. The simulation is extended to dual vessels in side-by-side arrangement to simulate LNG offloading operation.
In conventional ship-motion analyses, the effects of inner free surface have been usually simplified/neglected due to the unavailability of well-developed vessel-motion/liquid-sloshing coupled dynamic analysis computer programs. However, recently built complex vessels, such as LNGCs (Liquefied Natural Gas Carriers), LNG-FPSOs (Floating Production Storage Offloadings), and FSRUs (Floating Storage Re-gasification Units), are equipped with large inner tanks, so the inner-sloshing effects can no longer be ignored in many applications. Especially, in the case of side-by-side arrangement i.e. two floating units are operated in close proximity to each other, the hydrodynamic interactions may further increase the liquid sloshing motion and its effects on ship responses.
