Research On Structure Safety Of Large LNG Carrier Under Ultra-low Temperature

High speed, complex structure, wide navigation area are the major characteristic of LNG carriers. In order to ensure the safety of the ship it is essential to overcome ultra-low temperature and sloshing problem. At present the core-technology of cargo maintenance system is controlled by several powerful shipbuilding countries. It is necessary to research on the LNG carrier technology independently to make sure the state energy security. This thesis combined theory with engineering practice and analysed the structure safety of LNG carrier under different marine environments and potential accident conditions.The main research contents of this thesis are summarized as below:(1) Briefly introduced the status of natural gas industry, the potential of natural gas shipping market, technical difficulties of LNG carriers, independent research achievement of LNG transport in China, etc. Then an overview of resent research and process in academic fields related to the LNG carrier structure was presented.(2) An equivalent coupling convection coefficient is proposed to simulate the heat transfer between outer and inner hull. Thermal analysis of LNG carrier under different ambient temperatures was carried out based on thermodynamics method. Inhull temperature and heat convection coefficient in closed ship structure was calculated taking into account the affect of environmental change and stiffened plate. Then an automatic calculation module was developed in software to calculate the equivalent coupling convection coefficient. The results show that equivalent coupling convection coefficient method was an effective way to simulate the heat transfer from outer hull to inner space.(3) A correction method to LNG structure strength is proposed to analyze the influence of the material nonlinearity at low temperatures. The variation of engineering material properties caused by temperature change (ultimate strength, yield strength, Young's modulus, thermal expansion coefficient, thermal conductivity coefficient, etc.) was summarised. Then a study on the materials properties which would affect the linear strength result was carried out. According to thermoelastoplastic method, a partial LNG carrier model was built to evaluate the influence of temperature nonlinear material properties on the structure strength analysis result. At last a correction formula was presented to revise the actual engineering calculation.(4) Taking account the change of temperature distribution in the cargo hold and the environmental temperature, the analysis on the temperature field and thermal stress of the LNG carrier structure under ultra-low temperature was done. The changed temperature field in cabin under different loading cases and different ambience temperatures was studied. Then the calculation formula of temperature in LNG cabin which was an important boundary condition of ship thermal analysis was concluded. And the temperature field and thermal stress of LNG carrier under different loading cases was calculated using finite element method. The result of thermal stress was revised by nonlinear material properties correction formula too.(5) Some potential accidents were assumed to study structure safety problem of the LNG carrier under special sailing conditions. Considering the structure characteristics and loads of the insulation, a strength analysis on insulation box was carried out. Then several partial insulation failure assumptions were presented to study the strength of LNG carrier with incomplete insulation. The partial model was built to study the structural response under the direct impact of ultra-low temperature caused by leakage of LNG. How the structural response as parameters changed was also studied. At last, the analysis on structure strength under two worse assumed accident conditions, including the fault of bulkhead heater and the failure of whole major insulation, was carried out to measure the safety of LNG carriers.(6) Considering the action of both ultra-low temperature and sloshing, the research on LNG carrier structure strength is carried out. Based on volume-of-fluid method (VOF), a computational fluid dynamics model was built to simulate the sloshing in LNG cabin under different loading cases and excitation conditions. Then a fitting formula on LNG sloshing pressure was generalized. Taking the sloshing pressure and ultra-low temperature as boundary conditions, the strength analysis of LNG carrier was carried out. At last an influence study on structure safety caused by sloshing and ultra-low temperature showed that LNG carrier strength was closely related to LNG motion and temperature.Base on theory and practical engineering, this thesis deeply worked on structural safety problem under loading conditions, ultra-low temperature, environment temperature, LNG sloshing, etc. The method and result of this thesis could be helphul for future work on developing new LNG carrier and new cargo maintenance system.