| The interaction between pipeline and permafrost becomes the most difficult problem when laying buried pipeline transporting oil and gas resources at positive temperature on permafrost foundation.With the global warming,the seasonal active layer of permafrost gradually thickens and the upper limit of permafrost gradually decreases.Due to the gradual freezing of the seasonal active layer and the shallow buried depth of the pipeline in winter,the permafrost interacts with the pipeline under the influence of the special terrain and the surrounding soil.After a long period of freeze-thaw cycles,the frost heaving force of Permafrost on the surface of buried pipeline is prone to differential freeze-thaw,which leads to deformation and even damage of buried pipeline due to excessive deflection.In order to accurately calculate the stress and deformation of buried pipelines in permafrost regions under different freeze-thaw conditions,analyze the influence of various factors on buried pipelines,and scientifically evaluate the safety of buried pipelines under different freeze-thaw conditions,this paper takes the field monitoring of buried pipelines in a permafrost region in Northwest China as the background,and mainly carries out the following research:(1)Numerical calculation of temperature field.Based on the knowledge of permafrost thermodynamics,the three-dimensional finite element model of buried pipeline in permafrost region is established by considering the global warming and permafrost phase change and combining with the field monitoring and investigation results.The transient analysis method in finite element software is used to calculate and simulate the temperature field of pipe soil system in 50 years.The results show that the soil under the pipeline has the largest melting depth,which can reach 6.5m.In permafrost regions,buried pipelines change with time in four different stages.By comparing the calculated results with the measured results,it is found that the errors are small and basically consistent,which can better simulate the long-term variation of the temperature field of the pipe soil system in this area.(2)Thermal mechanical coupling finite element calculation.According to the calculation results of temperature field,considering the influence of frost heave coefficient,thaw settlement coefficient and physical and mechanical parameters of pipe soil material,the pipe soil displacement and stress distribution of buried pipeline in permafrost region under different freeze-thaw conditions are calculated.At the same time,the pipe diameter,transmission medium pressure,pipe wall thickness and other pipe material parameters are calculated under different working conditions.The results show that the equivalent stress at the top of the pipeline reaches 449 mpa,which is very close to the yield stress of the pipe(450MPa)under the condition of extra strong frost heaving.The buried pipeline is easy to be damaged without protective measures.Increasing the pipe radius can control the stress and deformation of the pipe;At a certain internal pressure of about 6Mpa,the equivalent stress of the pipeline is significantly reduced compared with that of the pipeline without internal pressure.The wall thickness of pipeline has little influence on the stress and deformation of buried pipeline.(3)Safety evaluation.After the comprehensive analysis of the effect of various factors,this paper uses two kinds of safety evaluation methods of buried pipeline under different freeze-thaw conditions in frozen soil area.The first method obtains the relationship between the maximum equivalent stress and the vertical displacement at the top of the pipeline after analyzing the numerical simulation results,and puts forward the evaluation method to judge the safety state of the pipeline based on the vertical displacement of the pipeline.The second method uses analytic hierarchy process(AHP)and fuzzy mathematics theory to comprehensively evaluate the safety performance of buried pipeline,establishes a two-level fuzzy evaluation model,adds 12 main factors affecting the safety of pipeline into the model,and gives the membership degree and limit value of each influencing factor.Based on the engineering conditions and the location of a pipeline in the Golmud Lhasa line,the safety evaluation calculation is carried out.The results show that the state of the buried pipeline in this paper is generally safe to relatively safe,and the state of a certain point in the Kunlun mountain section of the Golmud Lhasa line is generally unsafe to generally safe. |
