Fu Junpeng Ma Guiyang, School of Petroleum and Natural Gas Engineering, Liaoning University of Petroleum and Chemical Engineering, Fushun 113001, Liaoning Fujunpeng, etc. , 2012, 1 analysis, obtained the general law of uneven distribution and change of stress and strain of the pipeline across the frozen soil area. Due to the heat dissipation of the pipeline, the structure and composition of the surrounding frozen soil layer are destroyed, the pore water of the soil is reduced, and the heat storage is lower than the melted soil. At the same time, the heat is removed with the heat and mass transfer of water to make the soil drier. These changes affect the soil deformation and stress status: the soil strain and stress above the pipeline change drastically; the strain around the pipeline gradually increases and the stress continues to decrease, which is prone to thawing; the strain of the unfrozen frozen soil below the pipeline is smaller than that of the above melted soil and decreases in layers The soil is strengthened and the stress is constantly increasing. The diameter of the Sino-Russian crude oil pipeline is 813mm, and the Chinese section is 965km. It crosses the Greater Xing'an Mountains, Songnen Water System, the permafrost area and the seasonally frozen area at the northern end of the Northeast Plain. The status of frozen soil, including temperature, water content and water migration, has a huge impact on pipeline safety and threatens the smooth operation of the pipeline. In previous studies on the coupling of water and heat in frozen soil, no visual representation of the three-field changes in the frozen soil was given. Based on this, a systematic analysis of the temperature field, moisture field, strain and stress field in the frozen soil area was conducted to study the general laws of the mutual restriction and influence of temperature, moisture and soil mechanical characteristics. Preventive programs provide technical support. 1 Mathematical model 1.1 Water-heat coupling control equation for saturated frozen soil porous media Soil porosity refers to the ratio of the mass of water to the mass of solid particles when the pores are completely filled with water. Assuming that all phases of the soil are homogeneous and continuous, the fluid density change during the phase transition process conforms to the Boussinesq hypothesis. At low-velocity seepage, the water migration conforms to Darcy's law, ignoring the speed change caused by the phase transition melting. According to the theory of finite volume, the equations of conservation of mass, conservation of momentum and conservation of energy are established, which are the permeability of the porous medium, m2; the average particle diameter, mm; the porosity; the pore pressure, Pa; 6 = 3.5 (1 s) / (Ma s3), is the inertial loss coefficient, 1 / m; is the hydrodynamic viscosity, Pas; a is the fluid expansion coefficient, 1 / K, Zm is the solid-liquid paste zone constant, used to reflect the shape of the frozen front; Liquid volume fraction. Factory is the fluid temperature, K7 "is the fluid reference temperature, K; 7; is the solidification temperature, K is the melting temperature, K. The formula is: energy conservation equation: enthalpy of the skeleton, / kg; keff is the effective thermal conductivity, W / (M'K); rk is the thermal conductivity of liquid phase and solid phase, W / (m'K); p is the thermal conductivity of porous media skeleton, W / (mK); s is the density of solid phase media, kg / m3; p is the density of the porous media skeleton, kg / m3. 1.2 Mathematical model of stress and strain saturated saturated frozen soil consists of porous media soil skeleton, pore water and unfrozen water. When the temperature of the frozen soil changes, the stress and strain change accordingly . The temperature change process does not calculate the structural change factors, and the soil water-heat coupling temperature field is used as the basis for frost heave analysis. To calculate the frost heave stress, the temperature change effect must be considered in the physical equation, that is: =) = is the node node displacement coefficient; q is the element node displacement. Substituting the above formula into the equivalent nodal force formula gives: nodal load, which is called the equivalent nodal force vector of element temperature change. Substitute the expression of initial strain into the above formula: lF = iBYWaTl 2 Numerical simulation analysis 2.1 Calculate the boundary condition is less = 0: hour: dx work =, work = especially heat transfer coefficient, take 117, W / (m2.K) ; Tw is the crude oil temperature in the pipe, K.2.2 calculation parameter wall thickness 15mm, pipeline buried depth 2.4m, soil density 1680kg / m3. Unfrozen soil specific heat capacity 2403 / (kg'K), thermal conductivity 1.211W / (m'K) , Elastic modulus 16MPa, Poisson's ratio 0.3, thermal expansion coefficient 1.0X10-5. Oil delivery temperature 288K, ground temperature 268K, ground wind speed 1.5m / s, calculation area 15mX10m. Ignore the axial temperature drop of the pipeline, establish a two-dimensional unsteady state Heat transfer model. 2.3 Results analysis According to the movement law of the frozen soil melting phase transition interface at different operating time after the first year of pipeline production, as the pipeline operating time increases, the frozen soil melting speed directly under the pipeline is significantly reduced. Because the surrounding soil is baked by hot oil pipeline, the temperature As the temperature rises and becomes dry, the thermal conductivity of the soil decreases as the water content decreases. At the same time, the thermal conductivity of the molten soil increases as the temperature increases, and the thermal storage capacity of the molten soil increases. The interface movement simulation diagram is based on the soil moisture migration vector diagram at different running times after the pipeline is put into production. During the process of establishing a stable temperature field around the pipeline, the soil loses moisture due to heat, so that a dry layer of ring-shaped soil is formed around the pipeline. Thermal conductivity is smaller than the original soil. The heat dissipation of the pipeline in the soil depends not only on the heat conduction of the soil, but also on the convective heat and mass transfer of moisture. Because the soil near the pipe is much hotter than the soil away from the pipe, the moisture in the soil around the pipe tends to move upward due to the lower density. During its natural convection, the heated water continuously leaves the pipe, taking away heat and making the soil more dry. Therefore, the thickness of the annular soil layer is proportional to the running time of the pipeline. (A) Vector diagram of frozen soil moisture transport around the pipeline at different operating times for one month. It is assumed that the thermal stress of the pipeline in the calculation area only affects the soil, that is, only the soil is displaced, and the pipeline is not deformed. According to the displacement map of freezing and thawing soil at different operation time after the pipeline is put into production (MX represents the maximum value, MN represents the minimum value), the heat storage of the upper layer of the pipeline changes rapidly, the humidity changes significantly, and the water migration effect increases the deformation; the soil below the pipeline increases As the depth increases, the heat transfer between the soil and the external environment weakens. The moisture content and gas content decrease due to heat absorption from the pipeline. The depth is compacted and the strength increases and the heat is uniform. Therefore, the deformation is relatively small. 003 Different operations The frozen soil displacement map around the pipeline is based on the stress field map of the frozen and thawed soil at different operating times after the pipeline is put into production. With the extension of the operating time, the distribution of soil stress around the pipeline generally changes regularly at three locations above the pipeline, around the pipeline and below the pipeline. . The stress of the surface soil directly above the pipeline is continuously increasing, because the surface soil is strongly affected by changes in atmospheric temperature. The frozen soil around the pipeline intensifies into thawing soil, which leads to a decrease in soil stress. The frozen soil under the pipeline (excluding the melted soil near the pipeline) loses moisture due to the baking of the pipeline, and the porosity decreases, resulting in the structure of the frozen soil becoming tighter and the soil stress increasing significantly. However, as the time increases, the range of molten soil near the pipeline becomes smaller, and the probability of pipeline melting increases. (A) Conclusions and recommendations of the soil stress field at different operating times for 1 month The calculation results for saturated water-containing frozen soil show that: water content affects the heat storage capacity of the frozen soil, and water movement increases the diffusion and convection of heat in the soil. Temperature affects hydrothermal motion and causes uneven changes in strain and stress. Through the simulation of the two-dimensional unsteady heat transfer model, the general law of uneven distribution and change of stress and strain in the frozen soil area is obtained, which can provide a basis for the prevention and cure of the pipeline's frost heave and thaw. At present, the research on frozen soil tends to take the whole micro-analysis model as the object. In the future, the three-field coupling calculation method that can be applied to the particle should be explored.
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