The Effects of Tank Sloshing on Lng Vessel and Floating Terminal Responses

In the conventional ship-motion analysis, the effects of inner free surface have been usually ignored. Recent experimental and numerical studies have shown that the coupling effect between liquid cargo sloshing and LNG(Liquefied Natural Gas) ship motion can be significant at certain frequency range of partially filled tanks. This is of great concern to the LNG FPSO/FSRU operation in the production site and offloading operation of LNG carriers close to LNG terminal. The coupling effects are expected to become more important as the size of LNG carriers significantly increases with rapidly growing demand. The coupling between ship motion and sloshing has been studied by Molin et al(2002), Malenica(2003), and Newman(2005) based on linear potential theory in the frequency domain. In time domain, Kim et al.(2003,2007) studied the sloshing effect on ship motion with 2-D and 3-D viscous FDM sloshing codes. Lee et al.(2007) also investigated the sloshing effect of multiple tanks on ship’s roll motion with 3-D FDM calculation, which is further extended in this paper to include two floating-body interactions. In the present study, a potential-viscous hybrid method for multiple-vessel responses with multiple tanks is developed. The ship motion is solved in time domain by using linear potential theory and three-dimensional panel method, while the liquid sloshing in the inner tanks is solved by 3D-FDM Navier-Stokes solver including free-surface nonlinearity through SURF scheme. However, for simplicity, the single-valued surface profile is assumed in the sloshing calculation i.e. very violent free-surface motions such as overturning and splash are not considered. For comparison, an equivalent linear potential program in frequency domain is independently developed to solve the same interaction problem assuming small liquid motions. When the inner-fluid motion is mild, both approaches should produce similar coupling effects unless viscous effects are not negligible. In the present study, the ship and liquid-cargo motions are coupled in time domain by the kinematic and dynamic relations in that the vessel motions excite the tank sloshing, while the sloshing-induced loads in turn influence vessel motions. The calculated ship motions with or without considering liquid sloshing are compared with the model test results. The model test was conducted by MARIN as a part of SALT JIP (Gaillarde, 2004). The numerical results generally compare well with the measured data. After verifying the single-body case through comparison with experimental data, the ship and liquid motion interactions are further extended to two floating body problems in side-by-side offloading arrangement. In this case, the LNGC motions can be increased by the two-body interaction effects and the increased motion can further excite the inner tanks and vise versa. In addition, the roll motion of LNGC can be excited by two-body interactions even in the head-sea condition, which is otherwise zero due to the geometric symmetry. Therefore, it is expected that the ship-motion and tanksloshing interaction effects are even more pronounced during the side-by-side offloading operation.