| Floating Liquefied Natural Gas(FLNG)system,as a new type of platform for the gas exploration with the function of mining,liquefying,storing and unloading of natural gas/oil gas,has demonstrated great advantages over the flexibility,independence and reutilization during the working circle.Research on the hydrodynamic performance of FLNG and key technical concerns has become a leading subject of current offshore engineering field,as well as an important issue for the exploition of oil and gas resources in South China Sea.This paper has investigated the hydrodynamic characters of an FLNG which is designed to be located in Liwan 3-1 gasfield through the methodology of model tests and numerical simulations.The study focuses on the influence of inner-tank sloshing.Decay test and white noise test of FLNG model in various ballast conditions were carried out to obtain the damping curves and motion responses of FLNG in frequency domain.Besides,a 3-D linear potential theory is applied to conduct a coupling calculation of sloshing and FLNG motion.A numerical results under 75% loading condition were compared with the experimental one and a fair agreement has been achieved.On this basis,the contrast numerical calculation were carried out to study the effect of sloshing and the results shows the following conclusions: inner-tank sloshing has a positive effect on the roll motion of FLNG in a high filling condition.Meanwhile it increases the roll period;Surge and sway motion show a response around the first-order resonant period of the LNG-tank;the effect of sloshing on motion response in the modes of pitch,heave and yaw are insignificant;sloshing shows significant influence on added mass and reduces the drift force of FLNG;the impact of inner tank sloshing on roll motion has been demonstrated to be sensitive to sea condition;a larger wave heading leads to a higher LNG bulkhead pressure and its distribution is affected by the wave phase.Numerical model of FLNG has been established through the verification above.Factors affecting the hydrodynamic performance of FLNG in frequency domain were discussed.Though the numerical simulation,following conclusions are obtained: bilge keel installed in vessel with a large block coefficient has limited effect on the reduction of roll motion owing to its small width;filling level affects add mass in the mode of surge,sway and yaw,potential damping in the mode of roll and global motion response in the mode of roll and yaw;a low filling level leads to a higher pressure comparing to a high liquid level and maximum shows in the vicinity of free surface;a lower liquid density still works in reducing roll amplitude;compared with the GTT type tank,a MOSS type tank shows a disadvantage in yaw drift force apart from sway,heave,roll and pitch motions,but a remarkable reduction in bulkhead pressure is achieved.In addition,the coupling responses of FLNG vessel/SPM has been studied through the methodology of model tests and numerical simulations.The effect of sloshing in time domain were studied by model test while the stress analysis of mooring line was executed by numerical simulations.The conclusions are as follows: in survival condition,horizontal motions contributes more to line stress as the increasing of peak period of wave spectrum;in 100-year return condition of South China Sea,the maximum values of top tensions on the hawsers is 8344 kN,accounting for 44.1% of the designed breaking load,which indicates the safety of the mooring line system;for the whole system,overall load achieves a maximum in a moderate filling level in which the line stress bears more stress than those in a lower or higher filling condition.In summary,this paper has conducted investigations on the effect of inner-tank sloshing on the global motion of FLNG and some other critical hydrodynamic characters,which provides valuable reference for further application of FLNG in South China Sea in the future. |
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