Disease Mechanism Analysis And Numerical Simulation Of Positive Temperature Gas-pipe In Segregated Frozen Ground Based On Hydrothermal Coupling Effect

Pipeline transportation is one of the most commonly used methods for oil and gas resources.There are lots of oil and gas pipelines located in permafrost regions in China.Because the transportation medium is in the state of positive temperature,and the pipeline and the frozen soil are in direct contact,interaction will inevitably occur.The non-uniformity of frozen soil melting and frost heaving will cause deformation of the pipeline locally,causing stress and strain.If it exceeds a certain limit,the pipeline will be damaged when the pipeline exceeds the limit state.This phenomenon is the result of thermal interaction between pipe and soil.The coupling element is mainly temperature.That is,the soil temperature influences the heat transfer process of the oil pipeline,and at the same time,the heat transfer process of the pipeline affects temperature distribution of soil around a certain range of pipe.The final stable temperature distribution determines the pipeline strain,stress value.Soil freezing is a complex process involving multiple couplings of temperature field,moisture field and stress field.The constant change of the moisture field will make the moisture content of soil various,and the time-variance property of soil moisture content leads to dramatic changes in the thermal physical parameters of the soil,such as volumetric specific heat capacity,thermal conductivity,etc.Since the ice thermal conductivity is 4 times of liquid water,the influence of the ice content of the soil is the most important.Finally,all kinds of action on the soil make the soil freeze and melt a huge change.Some studies which have been conducted on the positive temperature gas-pipeline in segregated frozen ground in northwest China are demonstrated as follows:(1)Actual measurement is taken to acquire the exact temperature filed of ground.The actual measurement and data analysis of ground temperature shows that,for measuring section,heat flux ascending and descending are nearly equivalent.The frozen soil is unstable and easy to degrade with the thermal disturbance of pipeline.The durations of melt season and frozen season are 7 and 5 months.The horizontal range of influence the pipeline brings to soil is above 1.5m.Frozen soil upper limit is 1.5~2.0 m in the natural condition and the maximum thawed depth goes over 7.0 m.(2)Conduct some soil tests on the on thawed soil of segregated frozen ground.The soil physical and mechanical parameters such as specific gravity,maximum dry density,liquid and plastic limit,coefficient of permeability,compression modulus,shear strength and c,? were obtained.(3)In order to study the temperature field of surrounding pipe and the effect of applying the thermal insulting layer in different medium temperature to positive temperature oil-gas pipeline in the high altitude cold zone,The equation of the unsteady temperature field considering the phase transition was established,and the Abaqus numerical simulation was used to simulate the soil temperature field at the beginning of the pipeline and after 50 years.On that basis,to the soil under pipe,the rate of thawing,characteristic of thaw bulb and the maximum thawed depth are analyzed,and for three medium temperature,three formulas are established to calculate the maximum depth of the soil under pipe in fifty years.Studies show: To the soil of the pipeline without thermal insulting layer,the rate of thawing decays exponentially and the maximum thawed depth deepens negative exponentially,and both approaching to one constant respectively.It can achieve the goal preventing and treating the disease through reducing the medium temperature or setting thermal insulting layer,but the latter is more effective.(4)The partial differential equations of hydro-thermal coupled field of unsaturated soil considering the water seepage and moisture convection heat transfer were established.The PDE module in COMSOL Multiphysics was used to simulate the hydro-thermal coupled field.The unfrozen moisture field and the frost heaving ratio field were obtained,and the migration of unfrozen water was observed.