Study On The Thermal Evolution Of Permafrost Around The China-Russia Crude Oil Pipeline And The Application Of Crushed-stone Pipe Embankment

Under the background of global warming,permafrost degradation in permafrost area has brought great harm to all kinds of engineering buildings and structures.Different from other projects,the disturbance of buried oil and gas pipeline to permafrost is more intense because of its own characteristics.Under the influence of buried depth and operating temperature,the thawing consolidation settlement and differential frost heave of permafrost under buried oil and gas pipeline are very significant.In order to solve the problem of uneven frost heave and thaw settlement caused by environmental temperature change and high temperature crude oil pipeline along the pipeline,a lot of field monitoring and numerical simulation analysis have been carried out so far.Due to the normal temperature operation of the pipeline all year round,there are serious problems such as thawing settlement of frozen soil around the pipeline,water accumulation in the pipe trench,differential settlement of the pipeline,etc.Therefore,it is very important to grasp the thermal state of frozen soil around the oil and gas pipeline in the process of operation for the study of pipe soil interaction and the prevention and control of pipeline diseases.This paper takes China-Russia crude oil pipeline(CRCOP)as the research background,based on the existing field monitoring data of the China-Russia crude oil pipeline and the scale of the pipeline,the model tests of the conventional buried oil pipeline and the crush stone pipe embankment are carried out.During the experiment,the heat exchange process and low of water transfer between the pipeline and the surrounding frozen soil were studied systematically by monitoring the temperature and unfrozen water of the frozen soil around the pipeline.On this basis,the heat transfer model and fluid-solid coupling model are used to study the long-term thermal effect of buried pipeline and block stone pipe embankment on the permafrost under it.The main conclusions are as follows:(1)The results of the model test of buried oil pipeline: the permafrost around the pipeline has significant warming and melting under the influence of pipeline heat,and the maximum melting depth after six cycles can reach 0.84 m.The maximum melting depth fluctuates periodically with the temperature fluctuation of the pipe,the thawed interlayer has formed near the pipe,and the thawed interlayer thickness increases with the increase of the running time of the pipe.Under the influe nce of pipeline heat,the unfrozen water content at the bottom of the pipeline has significant moisture migration,but with the increase of pipeline operation,the water dissipates gradually.During the operation of the pipeline,the surface has significan t frost heaving and thawing deformations,but the overall subsidence tends to be higher the closer the subsidence deformations are to the pipeline.(2)The results of the model test of the crushed-stone pipe embankment show that there is a significant cooling process in the frozen soil at the bottom of the pipe embankment,which is because the rock layer blocks the direct heat exchange between the pipe and the frozen soil at the bottom.The variation range of the unfrozen water content at the bottom of the pipe embankment is obviously lower than that of the buried section,and there is no obvious water migration,because the frozen soil at the bottom of the pipe embankment is always in a stable cooling and cooling process.The surface deformation of the fill at the top of the massive rock layer is higher than that of the buried type and decreases gradually from the middle to both sides.(3)The results of conventional buried numerical simulation show that the heat exchange intensity between the pipeline and the surrounding frozen soil can be significantly reduced by wrapping the insulation layer.The maximum melting depth of the pipeline bottom can reach 7 m(4 cm),6 m(8 cm),5.5 m(12 cm)within 50 years of operation under different thickness of the insulation layer,and the descending degrees are 0.1 m/a,0.08 m/a and 0.07 m/a respectively.Through the analysis of the change process of the melting circle around the pipe under different thickness of the insulation layer,it can be seen that when the thickness of the insulation layer reaches 8 cm,the increase speed of the melting circle has little difference with the increase of the thickness of the insulation layer.(4)The numerical simulation results show that: under the open boundary condition,the cooling effect of the embankment is better than that of the closed boundary,and the cooling effect is further improved after the insulation layer is wrapped.At the beginning of the pipeline operation,there is a significant cooling process in the frozen soil at the bottom of the embankment,and then the temperature of the frozen soil at the bottom of the embankment increases with the increase of pipeline operation time.The temperature field of the frozen soil at the bottom of the open boundary asymmetry distribution is significant,but it decreases with the increase of pipeline operation time.