In this paper, two methods are studied for determination of the sloshing load in tanks of LNG and FPSO vessels. The first method is based on potential theory; and the second one is a hybrid method with linear potential model solution of ship motion and nonlinear CFD tank flow simulation. A special treatment with adoption of an artificial damping is proposed in the potential method to improve the accuracy of the cases close to natural frequencies of the tank flow. A simplified approach for quick estimation of tank flow natural frequency is also discussed. Results of both methods are compared with the experimental data, and feasibility and accuracy of the two methods are discussed.
Accurate prediction of the load due to sloshing of the liquid in a partially filled tank is important for the structural analysis in design of LNG and FPSO vessels. Significant sloshing load can be observed near the natural frequencies of the liquid flow in the tank, and it may cause structural damage. In most of design practices by now, this sloshing load is still estimated using the empirical formula based on experimental observation. A CFD model is generally required to simulate the violent strong non-linear flow in the tank. However, the required computation is very costly and not practical, considering that the liquid motion in the tank needs to be solved together with the ship motion, since they are fully coupled with each other. Instead, the linear velocity potential model is usually applied in the analysis of this coupled seakeeping problem. In this paper, we study two methods for determination of the sloshing load. In the first method, linear velocity potential theory is applied for both the inner tank flow problem and the outer ship wave making problem.