Fully Nonlinear Numerical Simulation And Analysis Of Sloshing And Coupling Ship Motion

With the increasing requirement of natural gas all over the world,liquefied natural gas tanker(LNG),which performs as the main transportation tool of natural gas,has been playing a more and more important role in new shipbuilding market.Meanwhile,study on LNG tankers has become a hot point in academic area.Different from ordinary ships,LNG ship always has liquid tanks with it.Under the action of sea waves,the ship motion will cause sloshing motion inside the liquid tank,and the sloshing-induced loads will affect the ship motion responses in return,which always exert adverse effect on sea-keeping performance of ship.Therefore,study on the interaction mechanism of liquid-tank sloshing and the ship motion itself has significant meaning in engineering practice.Based on fully nonlinear potential theory,the 2-D boundary element method is used to study the free motion of floating body under wave actions,and tank-sloshing problem under forced excitation in this paper.Then the coupling numerical model of sloshing and ship motion in waves is established in time domain.The instantaneous free surface is tracked by Semi-Mixed Euler-Lagrange method(Semi-MEL)and the numerical integrations about free surface boundary conditions and the rigid motion equation are conducted by using the four-order Runge-Kutta method;and the time derivative of the velocity potential is solved by auxiliary function method.Based on the above numerical methods,the coupling interactions between wave actions,sloshing inside tank and ship motion are solved.Compared with the published numerical results and the experimental results,the accuracy of the established model is well verified.In order to investigate the nonlinear coupling effects of sloshing flow on the ship responses,the corresponding response amplitude operators(RAOs)of ship motion for various wave frequencies,amplitudes and filling conditions are obtained.It is found that the amplitudes of wave-excitation and sloshing-induced force and phase difference between them result in the value of RAO.The most important coupling effects on roll motion are the split of peak,and the second peak is caused by sloshing motion.In addition,due to the nonlinearity of sloshing flow,the roll motion amplitude is not linearly proportional to wave amplitude and the nonlinear variation of sloshing-induced moment results in the nonlinear roll motion of ship.