Modelling the Effect of Sloshing on Ship Motions

Ships with partially filled liquid tanks, such as LNG carriers or FPSOs, are sensitive to sloshing in case they are exposed to waves. The effect of sloshing can have a pronounced effect on the ship motions, in particular roll in oblique seas. In this paper several methods are described which can be used to quantify this effect. The first is a linear diffraction method in which the effect of the liquid in the tank is modeled as a solid inertia of the fluid mass, an added mass and damping of the sloshing fluid and a hydrostatic free-surface correction to the GM. The response is easily computed in the frequency domain. The second is a time domain method in which the sloshing liquid in the tank is modeled with the CFD code ComFLOW. The forces exerted by the liquid on the tank walls are included in a time-domain simulation of the ship motions, based on linear potential flow for the outer domain (ship hull and ocean). The computed ship motions are again input for the motions of the liquid tank, generating a 2-way coupling between the dynamics of the tank and the ship. Both methods are applied to sloshing model tests, published by Molin (2008). In these tests, the response of a barge was measured with a completely filled and partially filled tank in beam seas. The results of the linear diffraction method agree reasonably well, but some differences in the roll response near the first sloshing mode are observed. The coupled time-domain method gives very good results for both low and high sea states.