Simulation of Sloshing in LNG Tanks

The aim of this paper is to demonstrate the prediction of internal loads on liquified natural gas (LNG) tanker ships and on offshore platforms. We use the moving grid approach and a finite volume solution method designed to allow for arbitrary ship motion. The motion of liquid is computed using an interface-capturing scheme which allows overturning and breaking waves. By performing a coupled simulation of the flow and vessel motion, it is possible to obtain a realistic response of the liquid in a tank to external excitation, e.g. by sea waves. Results are first presented for an LNG tanks whose motion is prescribed in accordance with planned laboratory experiments. Both two-dimensional (2D) and three-dimensional (3D) simulations are performed. The aim is to demonstrate that 1) realistic loads can be predicted using grids of moderate fineness, 2) the numerical method is able to accurately resolve the free surface even when severe fragmentation occurs, and 3) long-term simulations over many oscillation periods are possible without numerical mixing of liquid and gas. The plausibility of a coupled simulation of both vessel motion and the flow inside tanks and outside the vessel is then demonstrated for a full-size ship with partially-filled tanks exposed to head waves. In this simulation the forces and moments exerted by the sea cause the vessel to move, exciting the sloshing of liquid in tanks. For the computation of vessel motion, both sea-induced forces and forces due to sloshing in tanks are taken into account when determining the resultant forces and moments. While there are no experimental data for comparison at this time, the results look plausible and encourage further validation and application studies.