Storage Characteristics And Safety Technology Of FPSO-LNG Tank

LNG-FPSO involves nature gas liquefaction, storage and offloading, with the characters of low investment cost, small development risk, and convenient for migration, high security and so on, which has a unique advantage in the offshore oil and gas development. As LNG (Liquefied Natural Gas) is multi-component mixture and stored in the environment of atmospheric pressure and low temperature, in addition to complicated marine environment, the components and temperature changes will inevitably lead to instability in LNG thermal equilibrium, resulting in many problems, such as storage tank pressure suffering, LNG evaporating and releasing issues. In order to solve these problems, many factors causing LNG evaporation need to be studied, including initial fill rate, environment temperature, initial pressure, insulation materials, wind speed, solar radiation, LNG component and cargo sloshing and so on; furthermore,heat transfer process, pressure and evaporation rate changes in LNG storage tank are also studied.This paper studied on the pressure and evaporation rate variation during low-temperature storage,taking the tank of the capacity of 30000 m3 as object, LNG as medium.On the basis of experiment, a simulation model which can predict pressure and evaporation rate was established and was proved by experiment results, based on the principle of mass conservation and energy conservation.The results showed that the simulation model was relatively accurate and reliable. Factors which can influence pressure and evaporation rate inside the closed tank were analyzed using simulation model. The results show that: There exists"optimum diameter","optimum initial fill rate"and"the filling scope of security"in the closed tank; The bigger the thermal conductivity of tank insulation is, the pressure of the closed LNG tank will increase more quickly, and the safety storage time will become shorter; The higher the environment temperature is, the pressure of the closed LNG tank will increase more quickly, and the safety storage time will become shorter; Nitrogen content and wind speed have little effect on pressure in storage tank; The higher the nitrogen content is, the lower the evaporation rate is, and evaporation rate can be effectively reduced by filling nitrogen in LNG storage tank; The bigger initial pressure is, the faster the evaporation rate increases, while the evaporation rate increases and the increase extent slows down as initial pressure increases. Through analysis of regression,the major factors influencing on pressure and evaporation rate in storage tank were obtained, including initial fill rate, storage time and the heat transfer coefficient.With the help of Fluent software, the sloshing situations in different initial velocities, initial fill rates and different viscosities were simulated, which were based on the VOF model and combined UDF, and the methods were found to reduce sloshing. The study results provide technical support for LNG storage tank design and operation.