Research On The Mechanism Of Stratification And Rollover Phenomenon Of LNG Cargo Under Sloshing Conditions

The safety of shipborne LNG has always attracted the attention of the shipping industry.In recent years,with the vigorous development of offshore LNG transportation,LNG ship loading sources have shown a diversified trend,which has led to the occurrence of several offshore LNG rollover accidents.The occurrence of a rollover accident is accompanied by a sharp increase in pressure in the cargo tank,which seriously threatens the safety of the LNG ship.Therefore,it is of great significance to carry out research on LNG stratification and rollover under sloshing conditions for the management and prevention of offshore LNG rollover accidents.In this paper,by analyzing the heat-mass transfer mechanism of the LNG carrier tank,the physical model of LNG stratification and rollover was reasonably simplified,and the model including free surface evaporation and double-diffusive convection was established.In order to further explore the mechanism of driving LNG rollover under static and sloshing conditions,the influence of factors such as heat leakage intensity,layer height,number of layers,rolling period,and superheat accumulated in the lower layer on the rollover process is mainly considered.The hazards of rollover under two working conditions are analyzed.The main work and conclusions of this paper are as follows:(1)Considering the influence of concentration diffusion on heat transfer,the process of LNG stratification and rollover is described according to the continuity,momentum,energy and component transportation equations,and the accuracy of the numerical model is verified by comparing experimental data.(2)Under the same concentration difference,the heat leakage intensity is the main factor that determines the evolution time of rollover under static conditions.The change of the layer height of the lower layer and the-number of layers has little effect on the evolution time of LNG rollover.The hazard of LNG rollover mainly depends on the migration speed of the interface during stage 3 and the amount of superheat accumulated in the lower layer before stage 3.The increase in the number of layers,the decrease in the height of the lower layers,and the decrease in the heat leakage intensity will all reduce the hazards of LNG rollover.(3)Under sloshing conditions,thermodynamic factors are no longer the main reason that drives the concentration difference and temperature difference between LNG layers to rapidly decrease.The main factor that affects the mixing of the layers of LNG during the initial stage of sloshing is the dynamic factor of the tank motions.As time goes by,the evaporation and condensation of the free surface gradually become prominent,and the two mechanisms jointly promote the mixing between the LNG layers.(4)The peak value of BOR under sloshing conditions mainly depends on the superheat accumulated in the lower layer.