Study On The Sloshing Problems Of Liquid Cargo Tanks

With the development of world economy, Liquefied natural gas (LNG),liquefied petroleum gas (LPG) and other energy consumer demand are growingrapidly. So that the LNG transport technology continues to improve, and LNGships and VLGC ships become larger and larger. Large-scale liquid cargo ships havegreater load capacity because of large width and loading depth, but at the same time,it also brings security risks that may occur during the voyage, large impact pressurecaused by severe sloshing has a hazard for bulkheads and hull structures. So sloshingloads has become an important part of safety assessment of liquid cargo. How toreduce the sloshing loads and how to ensure the safety of the ship structure is anurgent problem.Following work is done in this thesis:(1) The basic governing equations and corresponding numerical methods ofnumerical simulation of liquid sloshing are deeply discussed(2) Tank model sloshing test including testing processes, analysis and discussionof the test results. Sloshing tests of LNG and VLGC model tanks are designed inthe study. Through statistical analysis of test data, the influence of filling level andexcitation frequency on the impact pressure is also discussed, and some regularityconclusions are made.(3) A numerical model of Very Large Gas Carriers(VLGC) tank is established.The influence of horizontal grider and swash bulkheads on the impact pressure dueto sloshing and liquid natural frequency are studied.(4) With the LNG tank sloshing numerical model, the parameters of the tank internal fluid density, viscosity and gas pressure on the sloshing impact pressure arestudied.(5) An LNG tank model test including the pump tower structure is designed. Thetest results showed that the pump tower will not affect the natural frequency og theliquid within the tank. At the same time, the force on the top of the pump tower wasmeasured. The strength analysis of pump tower is made based on Morison formulaand numerical method.Through this study, the conclusions can be made as follow:(1) LNG tank model sloshing test is designed. The effect of filling level,excitation frequency on the impact pressure and liquid natural frequency are studied.Through the analysis of the impact pressure, the bulkhead above70%H height ofLNG tank is the greatest danger area withstand the maxium sloshing impact. Thefilling level of70%H is the most dangerous under pitch condition; The filling levelof70%H is the most dangerous under roll condition, when the excitation frequencyis near the liquid natural frequency in the tank, violent sloshing will be occurred.(2) VLGC tank model sloshing test is designed. Based on the numerical modeltank, the effect of horizontal grider and swash bulkheads on the impact pressure andliquid natural frequency are studied. The results show that the internal structure cannot only reduce the sloshing impact pressure on the bulkhead, but also change theliquid natural frequency. The tank with internal structures may have larger sloshingthan the tank without internal structures if the excitation frequency is nearer theliquid natural frequency of the tank with internal structures than that of the tankwithout internal structures.(3) An LNG tank model test including the pump tower structure is designed. Bymeasuringthe pressure and bearing force on the top of pump tower, the effect ofpump tower on sloshing and the response of the pump tower under sloshing loads areanalyzed. The numerical method based on Morison formula and numerical sloshingtank model are used for the analysis of the pump tower. By comparison with testresults, the accuracy of the numerical method is verified.