Study On Analysis Method Of Liquid Tank Sloshing And Ultra-low Temperature Field Of LNG-FSRU And Its Local Structure Optimization

As an advanced technical equipment in the field of offshore oil engineering,the high value-added and high-tech LNG-FSRU(Liquefied Natural Gas-Floating Storage and Regasification Unit)can completely replace the LNG receiving terminals in the coastal land and raise the economical and environmental benefits of the LNG receiving terminals.Meanwhile,the LNG receiving terminal with the low cost is flexible and mobile,so it is now widely used in the field of foreign LNG projects and has become one of the hot issues in the field of national ocean engineering in China.In the research background of large-scale LNG-FSRU,the calculation method of the systematic steady temperature field of LNG under the ultra-low temperature is analyzed in this paper with key security issues in the process of design and construction as the research objective.Meanwhile,combined with the possible failure conditions of wall layers and heating equipment in the whole lifecycle of LNG-FSRU,this paper also makes an analysis of steady temperature field distribution calculation and conducts an in-depth study of security issues in the invalid working condition.Considering the liquid tank sloshing of LNG-FSRU,first-order and second-order sloshing of LNG needs to be intensively studied.In addition,the optimization design method of the local structure of LNG-FSRU is discussed under the interaction of low temperature and tank sloshing load.In this paper,the principal research content and the research work are summarized as follows:Initially,the heat transfer of air layers in the closed square cavity between internal and external shells of LNG-FSRU is calculated after summarizing basic theory of thermodynamics.Specifically,the influence of radiation,frame and frosting at low temperature between internal and external shells on heat transfer is converted into the change of convective heat transfer coefficient,therefore correcting the convective heat transfer coefficient.In accordance to the thermodynamic energy conservation law,the analytic calculation approach for the temperature of the local structure of LNG-FSRU on the basis of the simplified model.Based on the theoretical research,the heat transfer between the internal and external shells is calculated and analyzed by numerical simulation method with the equivalent convective coupling boundary conditions,and the steady temperature field of LNG-FSRU is simulated at different ambient temperatures.At the same time,given the long service period and a variety of potential assumptions of conditions,this paper analyzes the temperature distribution under the valid working conditions of primary and secondary wall layers and malfunction of compartment heater,which provides the theoretical basis for the LNG-FSRU system affecting its structural safety.Secondly,combined with the hydrodynamic characteristics of the hull,the sloshing of LNG-FSRU is studied.In terms of studying the rolling of the double-row-tank LNG-FSRU,the influence of the swing axle position on sloshing load distribution provides the reference for the structural design for the cargo tank.In this paper,the excitation amplitude of extreme hull motion is selected by the first-order natural frequency,and the distribution of the sloshing load of LNG-FSRU at different liquid loading rates is analyzed,which shows that the sloshing load is the biggest at the liquid loading rate of 20%.On this basis,the second-order sloshing is analyzed,as well as the theoretical derivation for the three-dimensional second-order sloshing in the framework of the potential flow theory combined with perturbation theory,so the theoretical solution of the second-order resonance is obtained and the numerical verification is carried out.Finally,the optimal design of the systematic local structure of LNG-FSRU in the interaction of extreme tank sloshing load and low temperature.Introducing the buffer action of wall layers in the calculation,the buffer coefficient is used to substitute the effect of buffer action in order to obtain the actual distribution of stress and displacement and optimize the design for the local structure.This method provides a useful reference for the optimal design for the local structure of LNG-FSRU.