【作者】 丁仕风；

【导师】 张圣坤； 唐文勇；

【作者基本信息】 上海交通大学 ， 工程力学， 2010， 博士

【摘要】 大型LNG船具有航速高、结构复杂、航行海域广和装载工况多样的特点,为确保船舶航行安全需要克服LNG超低温和晃荡载荷的作用。薄膜型货舱是目前主流的LNG船围护系统,其技术专利还都掌握在少数造船强国手中,提高液化天然气船自主研发水平是保障我国能源安全的切实需要。本论文以工程实际运用为研究背景,考虑漫长服役期中可能出现的各种海洋环境、潜在的事故工况等,对大型LNG船体结构在超低温作用下的安全性问题展开研究:分析了超低温作用下船舶系统的热传递问题,评估了材料性能随温度的非线性变化对结构强度计算的影响,研究了多种环境下船体结构强度问题,预报了LNG船潜在的事故工况及对结构强度的危害,分析了超低温、晃荡联合作用对LNG船结构强度的影响。论文开展的主要工作如下:(1)简要介绍了天然气产业现状、LNG海运市场潜力、LNG船技术难点、课题意义等,并针对LNG船结构安全研究所涉及的不同学术领域,介绍了目前国内外主要科研成果和研究进展。(2)提出了采用等效对流换热耦合系数来模拟船体内外壳之间热传递的思路。分析了多个环境温度下典型局部船舶系统的热传递规律,计算了船体内壳温度、封闭空间的热对流系数,修正了环境变化、加筋等因素对热对流系数的影响,开发了用于模拟船体内外壳之间对流关系的专用计算模块,并研究了船体不同部位处的等效对流耦合系数,研究结果表明采用搜索等效对流耦合系数的方法可以较准确模拟船体内外壳之间空气的传热作用。(3)提出了计及低温下材料非线性对LNG船结构热应力计算结果影响的修正方法。归纳总结了大量工程材料在温度变化情况下的极限强度、屈服强度、杨氏模量、热膨胀系数、导热系数等材料特性的变化规律,分析了影响LNG船强度计算的关键材料参数,采用非线性热-弹塑性耦合计算理论,对多工况下船体局部结构进行非线性分析,进而研究了材料温度非线性变化对结构强度计算结果的影响,回归得到线性热应力计算结果的修正公式。(4)考虑了货舱内温度分布变化和环境温度变化,研究了超低温作用下LNG船结构温度场和热应力问题。基于对货舱围护系统的热分析,研究了装载工况对船舶货舱内温度的影响,计算了多装载工况下货舱内温度分布规律,并推导了不同装载工况下货舱内不同位置处的温度计算公式,以大型LNG船常见的装载工况为研究条件,计算了船舶压载、满载情况下船体温度场分布,在此基础上研究了船体热应力问题,并对最终计算结果进行材料温度非线性修正。(5)假定了多种LNG船事故工况,研究了事故工况下的船体结构安全问题。根据货舱绝热层载荷、结构特点,分析了可能出现的局部绝热层失效形式,在此基础上假定了一系列潜在的事故工况,研究了绝热层失效对船体结构安全性的影响,模拟了LNG泄漏、超低温直接作用下船体结构响应问题,分析了参数变化对结构超低温响应的影响,并进一步研究了船体隔舱加热设备故障、薄膜损坏主绝热层失效等其他事故对船体结构安全的影响。(6)基于货舱内液体晃荡模拟,研究了超低温和晃荡联合作用下船体结构安全问题。根据LNG船的自身特点及航行规律,选取了4种纵向变速晃荡激励条件,采用有限体积法(VOF),模拟了不同激励条件下满载、压载时货舱内液体的晃荡问题,并以特定激励条件下的液体晃荡、超低温为研究背景,分析了二者联合作用下的大型LNG船结构安全问题,评估了晃荡和超低温因素对结构强度的影响。从理论和工程实际相结合的角度,论文针对LNG船服役期中超低温作用下可能面临的各种结构安全问题展开了深入研究,其研究方法、重要结论可以为我国自主研发LNG新船型、开发新型货舱围护系统提供有益参考。更多还原

【Abstract】 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.更多还原