Research On Structure Design Of Independent Cargo Tank Type C For Small And Medium LPG Vessel

LPG is not only an important industrial raw material,but also a clean energy source.With the increasingly strict requirements for environmental protection in China,the great demand for LPG is brought.The LPG cargo tank type C is the main type of the small and medium LPG transport ship,which has high safety and economy.The LPG cargo tanks suffer the combined effect of the inner vapour pressure and the dynamic load induced by the motion of the hull,which is complicated.According to the three-dimensional acceleration ellipsoid method,the load value at each position of the tank is evaluated,and then the thickness of cylinder plate is designed according to the pressure vessel code.The sloshing phenomenon poses a great threat to the structural strength of cargo tanks.Based on the inner fluid’s inherent period and the ship’s movement cycle,sloshing risk is evaluated,and the design of anti-sloshing bulkhead is carried out according to the calculated sloshing load.The saddle section and the middle section of LPG cargo tank(type C)belong to the dangerous section.The stress state of above sections is evaluated by studying the applied load of the cargo tank.The external pressure calculation and the buckling evaluation of the outer shell are also carried out for the strength evaluation of the cargo tank.A comprehensive structural strength assessment of the cargo tank can be conducted through the structural finite element analysis method.The key point is the simulation of pressed wood.The temperature load is another important load of the LPG cargo tank,which is applied on the cargo tank together with the design vapour pressure and the dynamic load induced by ship motion.Low temperature is an important feature of LPG cargo tank,which is different from conventional pressure vessels.Therefore,the adjacent hull structures of saddle will be at low temperature,which need higher steel grades to ensure the safety of related structures.Based on the basic theory of heat transfer and finite element analysis software,the temperature field of saddle and adjacent hull structure of cargo tank is calculated.According to the requirement of Classification Society for the minimum temperature of steel grade and the distribution of temperature field,the selection of steel grade in adjacent area is determined.The temperature field analysis is one of the important differences between the LPG transport ship and the conventional ship.Insulation design can minimize the transferred heat from environment into the LPG tank,which can reduce the rate of evaporation,increase the time of pressure storage and ensure the economy and safety of the cargo tank.By studying the influence of insulation layer thickness and cargo tank’s parameters on evaporation rate,the thickness of the thermal insulation layer of the cargo tank is determined.And the evaporation rate and pressure storage time of the cargo tank under this thickness are calculated.Through studying the structural design and strength evaluation of a 3800 m~3 LPG cargo tank,the key design technology of independent cargo tank(type C)is grasped.The paper lays a foundation for future design and development of lower temperature and more complicated cargo tanks.