| China,as the world's third largest frozen country,is widely distributed in permafrost regions,accounting for 3/4.There are abundant oil and gas resources in these regions,especially in permafrost regions.With the continuous development of China's economy and society,buried gas pipeline laying in the permafrost area as the current and future long distance transportation of the main oil and gas resources will inevitably pass through these areas in consideration of buried pipeline has the advantages of large volume,small occupation area,short construction period and so on.However,laying buried in permafrost areas,especially in the Tibetan Plateau average high altitude,which is characterized by high average altitude,wide-distributed permafrost,large thickness,poor natural conditions and complex geological conditions,there is no mature experience for reference,furthermore the research on the interaction mechanism between the buried pipeline and the surrounding permafrost has been in a gradual exploration stage.The mechanical properties,thermal stability and engineering characteristics of the permafrost remain as the key problems in the study of pipeline engineering in Permafrost Regions.In this paper,the positive temperature gas pipeline and the surrounding soil in a section of Qinghai Tibet Plateau Permafrost region work as the research object.Based on the pipeline diseases caused by the degradation of permafrost along the section,considering future climate warming conditions,the characteristics of the permafrost zone was combined.Heat transfer theory and the finite element numerical calculation software,ANSYS were also applied.The numerical calculation of the unsteady temperature field of the soil around the pipeline under the condition of ice water phase change process was carried out under different buried depths of the center line of the pipeline,different delivery temperatures.The temperature distributions of the soil around the pipeline under different conditions were obtained.At the same time,the thermal mechanical coupling numerical calculation of the pipeline and surrounding soil is carried out by using ANSYS thermal couple unit to obtain the distribution of equivalent stress of the pipeline under different conditions.The main conclusions of this paper are as follows.(1)Based on the numerical analysis of the soil temperature field around the buried gas pipeline in high altitude permafrost regions under different conditions,it is found that the thermal stability of permafrost is quite poor and puts great threats to the environment of permafrost around the pipeline to maintain stable.(2)The buried depth of the pipeline has a great influence on the temperature fielddistribution around the pipeline,generally speaking,when the temperature is certain,the buried depth of the pipeline center is larger,and the thawing depth of the pipeline at the bottom of the pipe is larger in the calculation time.The center of pipeline buried depth were2.0m,2.5m,3.0m,in conveying the temperature of 10 ? and 16 ? permafrost thaw depth calculation of fiftieth year of the bottom of the pipe are respectively 2.76 m,3.21 m,3.68 m,2.84 m,3.35 m,3.76 m.In the design and construction of the buried gas pipeline,the depth of the pipeline center should be both economical and reasonable.(3)With the transfer temperature increases,the bottom of the pipe soil under the maximum thawing depth is greater.When the buried depth of the pipeline center is 2.0m,under the conditions of the delivery temperature are 10? and 16?,the maximum thawing depth under the bottom of the pipe are 43 cm and 50cm;when the buried depth of the pipeline center is 2.5m,under the conditions of the delivery temperature are 10 ? and 16 ?,the maximum thawing depth under the bottom of the pipe are 38 cm and 47cm;when the buried depth of the pipeline center is 2.5m,under the conditions of the delivery temperature are 10?and 16?,the maximum thawing depth under the bottom of the pipe are 35 cm and 43 cm.In the actual operation,the delivery temperature should be controlled to avoid high delivery temperatures.(4)The use of 45 mm polyurethane foam as the external wall insulation measures can effectively reduce the thermal disturbance around the pipeline caused by the heat of the positive temperature of gas pipeline.It is shown that the melting range of the permafrost around the pipe is smaller than that without the insulation measures.When the buried depth of the pipeline's center is 2.0m,under the conditions of the delivery temperature is 10 ? and16?,the maximum thawing depth of the soil under the bottom of the pipeline are 0mm and43.7mm;when the buried depth of the pipeline's center is 2.5m,under the conditions of the delivery temperature is 10?and 16 ?,the maximum thawing depth of the soil under the bottom of the pipeline are 0mm and 35.3mm;when the buried depth of the pipeline's center is3.0m,under the conditions of the delivery temperature is 10 ? and 16 ?,the maximum thawing depth of the soil under the bottom of the pipeline are 0mm and 30.5mm.The results show that the 45 mm polyurethane foam used as the heat preservation measure can effectively reduce the heat loss of the liquid natural gas in the pipeline,and reduce the disturbance of the pipeline to the surrounding permafrost so as to protect the permafrost around the pipeline,ensure the safe operation of the pipeline,and reduce the operation cost and maintenance cost.(5)In the first 10~20 years after the laying and operation of the buried gas pipeline,the thermal disturbance to the permafrost around the pipeline is the biggest and attention shouldbe paid to strengthening the temperature monitoring around the pipeline and maintenance of the pipeline during this period.(6)Under different conditions,the maximum equivalent stress produced by the pressure in the pipeline,gravity,the upper part and the change of the surrounding soil temperature in the pipe is located at the bottom of the pipe.Under different conditions,the maximum equivalent stress produced by the pressure inside the pipeline,the weight of the soil,the weight of the overlying soil and the change of the surrounding soil temperature in the pipeline are located at the bottom of the pipe.(7)The equivalent stress at the bottom of the pipeline increases gradually with the increase of calculation time,and the maximum equivalent stress is calculated in the thirtieth year.(8)Delivery temperature is 16 ? with no insulation measures in case of pipeline,the greater of depth of the pipeline is,the internal equivalent pressure of the pipeline is greater,caused by the internal pressure,gravity and the overlying soil due to temperature change of surrounding soil load.In the calculation of thirty years,the depth of the center of buried pipeline is 3.0m,the equivalent stress of the pipe inside is 230 MPa,while the depth of pipeline center buried of 2.5m and 2.0m,the equivalent stress were 172 MPa and 149 MPa respectively,which is the central depth of a buried pipe is 2.0m,the 2.5m equivalent stress of 1.36 times and 1.18 times.(9)The buried depths of pipeline center are the same,when temperature was 16? and take 45 mm polyurethane foam insulation measures can effectively reduce the internal equivalent pressure of the pipeline,caused by the internal pressure,gravity and the overlying soil due to temperature change of surrounding soil load.The maximum internal equivalent pressure of the pipeline is 135 MPa when the depth of center is 3.0m,and when the depth of the pipeline center is 2.0m and 2.5m,the equivalent stress is 78.3MPa and 94.7MPa respectively,which are the equivalent stress' s 52.6%,55.1% and 58.7% of no insulation measures. |
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