The initial separation of mixed oil/gas and water from risers is done by wash tanks inside the FPSO/FLNG hull. As the vessel size increases, larger-sized separators/wash tanks and storage tanks are considered. The performance of separators/wash tanks is in general affected by the vessel motions, and the vessel motions themselves are influenced by multi-layer-liquid sloshing motions inside the wash tanks. The MPS (Moving Particle Simulation) method has shown that it is adequate in predicting violent sloshing patterns and the corresponding impact loading on tank walls in single-phase-liquid problems. However, the application of the MPS method to the multi-layer-liquid system has been very rare in the open literature. In the multi-liquid system, a proper buoyancy model including surface tension has to be incorporated to more accurately simulate the behavior among different-density particle members. Another important factor of the multi-liquid problem is the reasonable treatment for tracing multiple interfaces and imposing proper kinematic and dynamic boundary conditions at the interfaces. The newly developed MPS method for multiple-liquid layers is validated by comparison against linear potential theory (in the case when interfacial amplitudes are small) and by comparison against some available experimental results. The multi-liquid MPS sloshing program is also coupled in time domain with a ship-motion program to assess their interactions in a typical operational sea environment. The generation of interfacial sloshing waves depending on the excitation wave period is clearly demonstrated, and the internal waves are in several cases much greater than the free-surface waves. Since various interfacial sloshing motions of different frequencies can be generated at the respective interfaces, the influence of large separators on vessel motion can be more complicated than the single-liquid tank.

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