Numerical Simulation of Large Amplitude Sloshing in FPSO Tanks Under Irregular Excitations Based on Youngs VOF Method

Recently, with the increasing demand for oil and natural gas in the sea promoted, large FPSOs (Floating Production, Storage and Offloading System) win people’s popularity. Large FPSOs with liquid tanks of large volume may suffer from random waves at real seas frequently, and this results in 6-DOF motions, such as roll and sway. Due to the excitations from irregular 6-DOF motions, liquid sloshing in tanks shows complicated behaviors. In severe sea states, violent sloshing yields great impact load to tank structure, even cause damage of tank structure. Since it is important to evaluate safety performance on liquid sloshing and its effects for large FPSOs, effective simulation of large amplitude liquid sloshing, including turning, merging, breaking of free surface et al, attracts researchers’ attention. Among all capturing methods for free surface, VOF is widely used for its simplicity as well as effectiveness. For the method to solve the VOF equation, Youngs method approximates free surface by oblique line in a cell, and possesses advantage of higher accuracy and higher sharpness for free surface capturing. Based on the establishments of basic control equations and boundary conditions, the authors establish numerical model for liquid motion using finite difference method. To solve the VOF equation, Youngs method [1] is adopted. And the velocity boundary conditions in free surface are specially analyzed. Finally a computer code for 2-D sloshing in time-stepping format is developed taking the above conditions into account. Considering characteristic FPSOs’ motions, roll, pitch and heave affect tank sloshing dominantly than others. In this paper, the authors predict such kinds of ship motions based on the strip theory, and take them due to irregular long-crest waves as compulsive external excitation, and numerically simulate large amplitude sloshing phenomenon in rectangular tanks of different filling level. Furthermore, the sloshing load to structure is also computed and some simulation results are compared with published experimental data.