Numerical Analysis Of The Temperature Field And Three-dimensional Pipe-soil Thermal-mechanical Coupling For Buried Normal Temperature Gas Pipeline With Thermosyphon In Permafrost Region

Pipeline transportation,as a transportation tool that can realize automatic control,is widely used in long-distance transportation of oil and natural gas due to its advantages in liquid and gas energy transportation.Pipeline transportation can leave out the transit section of water transportation or land transportation,shorten the cycle of transportation,reduce the cost of transportation and improve the efficiency of transportation.Materials of transportation in the right temperature state,and for safety reasons,most of the pipe buried underground and contact with the soil,when the pipeline crossing the permafrost region,the pipe and soil will exchange heat and then affect the morphology change of permafrost: during warm,pipe heat melting permafrost are temperature,in the cold season,soil refreezing,the cycle of freezing and thawing lead to stress accumulation,deformation of pipe is too large,eventually destroy the pipe.Therefore,it is very important to study the temperature field and stress field of the soil around the pipe to prevent pipeline diseases.Based on the actual buried positive temperature gas pipeline project located in a certain section of permafrost region,the following research was carried out in this paper:(1)In order to obtain the actual state of the frozen soil temperature field at the location of the pipe section,a temperature monitoring section was set up for long-term observation.It can be seen that the maximum thawing depth at the center of the pipe is about 6.5m,and the maximum influence range of the pipeline on the horizontal distance of the surrounding frozen soil at the center of the pipe is about 2m.(2)The explicit heat capacity method was used to establish the governing equation of three-dimensional unsteady temperature field considering phase transition,and ANSYS was used to calculate the temperature field of soil in the 50 years after the pipeline started to operate under different material temperatures.On this basis,the variation of soil temperature around the pipeline with different material temperature,the variation of thawing circle and the maximum thawing depth at the bottom of the pipeline are analyzed,and the occurrence mechanism of pipeline diseases in permafrost regions is revealed.(3)In view of the problem of frozen soil diseases along the pipeline,in order to reduce the disturbance of permafrost around the pipeline caused by treatment measures,the measures of placing hot rods are considered for the treatment of pipeline diseases.Based on the numerical simulation results under field conditions,the effects of two hot rod layouts on the cooling effect of soil around the pipe and the effects of different material temperatures on the cooling effect of hot rod are analyzed.The analysis shows that the cooling effect of hot rod on the soil around the pipe is significant,and the material temperature is positively correlated with the cooling effect of hot rod.(4)According to the principle of elastic mechanics,considering the characteristics of the gas pipeline construction,based on the stress and deformation of pipeline and soil of the finite element control equation,combined with the calculation result of temperature field,indirect coupling analysis model is established,under different working conditions,to a certain pipeline were analyzed,and the vertical stress gets the maximal vertical stress occurs at the bottom of the pipeline and the temperature reached the material pipe under the influence of the vertical stress and heat pipe under the influence of vertical stress.