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ABSTRACT
This article presents a study of water-heat coupled problems of frozen soil along the Mohe–Daqing crude oil pipeline with thermosyphons. It is known that the Mohe–Daqing pipeline goes through four different kinds of frozen soil classified by ice content in soil. To determine the region with high tendency of permafrost thawing, the temperature field and water content field of the four types of frozen soil were first investigated in this paper. Taking freezing and thawing risk, typical geological condition and other factors into consideration, the soil temperature field of pipeline located at the permafrost swamp area was calculated numerically. Comparative study was also carried out for soil temperature field with and without thermosyphons to investigate the cooling effect of thermosyphons. It is evident that thermosyphons have great influence on temperature drop of permafrost, and this influence is especially magnificent in winter. In contrast to soil temperature without thermosyphons, installing the thermosyphons could reduce the soil temperature right beneath the pipeline by approximately 4°C. Numerical results, which have been validated by field data, are of great guiding significance for construction and operation safety of pipelines buried in cold regions.
Nomenclature
| A0 | = | amplitude of atmospheric temperature, dimensionless |
| C | = | inertia coefficient, dimensionless |
| ci | = | heat capacity of ice, J/(kg·°C) |
| cj | = | heat capacity of the jth layer, namely wax deposition layer, pipe wall and insulation layer from inside to outside, J/(kg·°C) |
| cm | = | heat capacity of whole cold soil region, J/(kg·°C) |
| co | = | heat capacity of crude oil, J/(kg·°C) |
| cs | = | heat capacity of soil skeleton, J/(kg·°C) |
| cw | = | heat capacity of water, J/(kg·°C) |
| D | = | inner diameter of pipeline, m |
| Dt | = | diameter of thermosyphon, m |
| f | = | Darcy friction coefficient, dimensionless |
| g | = | gravity acceleration, m/s2 |
| H | = | heat influence region in the y-direction, m |
| H0 | = | pipeline buried depth, m |
| k | = | permeability of frozen soil, m2 |
| L | = | half of heat influence region in the x-direction, m |
| Li | = | latent heat of water-ice phase change, J/kg |
| Lt | = | length of evaporator section of thermosyphon, m |
| P | = | average pressure along the pipeline, Pa |
| p | = | moisture migration driving pressure, Pa |
| Qt | = | heat flux of thermosyphon, W/m2 |
| q0 | = | heat flux density of crude oil along the pipeline, W/m2 |
| R0 | = | inner radius of wax deposition layer, m |
| R3 | = | outer radius of insulation layer from inside to outside, m |
| Rj | = | outer radius of the jth layer, namely wax deposition layer, pipe wall and insulation layer from inside to outside, m |
| r | = | radial direction, m |
| T0 | = | temperature of inner wall of wax deposition layer, °C |
| T1 | = | temperature of wax deposition layer, °C |
| Ta | = | temperature of atmosphere, °C |
| Taver | = | annual average temperature of atmosphere, °C |
| Tb | = | freezing temperature of water in soil, °C |
| Tj | = | temperature of the jth layer, namely wax deposition layer, pipe wall and insulation layer from inside to outside, °C |
| Tn | = | temperature of constant temperature layer, °C |
| To | = | temperature of crude oil, °C |
| Tp | = | thawing temperature of ice in soil, °C |
| Ts | = | temperature of frozen soil, °C |
| u | = | velocity of unfrozen water in the x-direction, m/s |
| V | = | average velocity of oil flow in the pipeline, m/s |
| v | = | velocity of unfrozen water in the y-direction, m/s |
| vc | = | infiltrating velocity of surface water, m/s |
| x | = | horizontal direction, m |
| y | = | vertical direction, m |
| z | = | axial direction, km |
| Δz | = | grid spacing in the axial direction, km |
Greek Symbols
| αa | = | heat transfer coefficient at the ground surface, W/(m · °C) |
| αo | = | heat transfer coefficient of oil flow in the pipeline, W/(m · °C) |
| βo | = | expansion coefficient of crude oil, °C−1 |
| βw | = | expansion coefficient of water, °C−1 |
| θ | = | circumferential direction, dimensionless |
| ϵ | = | parameter defined by Eq. (Equation5 |
| λ | = | thermal conductivity of whole cold soil region, W/(m · °C) |
| λ1 | = | thermal conductivity of wax deposition layer, W/(m · °C) |
| λi | = | thermal conductivity of ice, W/(m · °C) |
| λj | = | thermal conductivity of the jth layer, namely wax deposition layer, pipe wall and insulation layer from inside to outside, W/(m · °C) |
| λm | = | thermal conductivity of whole cold soil region, W/(m · °C) |
| λs | = | thermal conductivity of soil skeleton, W/(m · °C) |
| λw | = | thermal conductivity of water, W/(m · °C) |
| μ | = | dynamic viscosity of unfrozen water, Pa · s |
| ρi | = | density of ice, kg/m3 |
| ρj | = | density of the jth layer, namely wax deposition layer, pipe wall and insulation layer from inside to outside, kg/m3 |
| ρm | = | density of whole cold soil region, kg/m3 |
| ρo | = | density of crude oil, kg/m3 |
| ρs | = | density of soil skeleton, kg/m3 |
| ρw | = | density of water, kg/m3 |
| τ | = | time, s |
| φ | = | porosity of frozen soil, dimensionless |
| ϕ | = | water content in soil pores, dimensionless |
| ϕa | = | initial phase in equation of atmospheric temperature, dimensionless |
Acknowledgements
The study is supported by the National Science Foundation of China (No. 51176204, No. 51325603 and No. 51636006), and the Science Foundation of China University of Petroleum, Beijing (No.2462012KYJJ0404).
Additional information
Notes on contributors
Lichao Fang
Lichao Fang is a Master student in Oil & Gas Storage and Transportation Engineering at the University of Petroleum (Beijing), China. She received her B.S. degree in Oil & Gas Storage and Transportation Engineering from the same university. She is currently working on numerical simulation for heat transfer, water flow and mechanical structure of frozen soil around pipeline.
Bo Yu
Bo Yu is a professor of Mechanical Engineering and Oil & Gas Storage and Transportation Engineering at Beijing Institute of Petrochemical Technology, China. He received his Ph.D. in 1999 from Xi'an Jiaotong University, China. He worked at Kyushu University from June 1999 to March 2001 as a postdoctoral fellow and at the National Institute of Advanced Industrial Science and Technology, Japan, from April 2001 to March 2005 as a special research associate. He is a senior member of the Chinese Society of Theoretical and Applied Mechanics and a member of the Society of Petroleum Engineers. He is an academic committee member of several international symposiums and the Key Laboratory of Railway Vehicles & Thermal Technique of Ministry of Education, China. Since 1999, he has published more than 100 papers in archival journals and conference proceedings and obtained four national and provincial natural science and technology awards. His current research interests include long distance transportation technology of waxy crude oil, turbulent drag-reducing flow, and computational fluid dynamics.
Jingfa Li
Jingfa Li is a Ph.D. student at China University of Petroleum, Beijing, China, under the supervision of Prof. Bo Yu. He received his B.S. degree in Oil & Gas Storage and Transportation in 2012 from Southwest Petroleum University, Chengdu, China. He is currently working on the large-eddy simulation of turbulent drag reduction by surfactant additives.
Yu Zhao
Yu Zhao is a Ph.D. student in Petroleum Engineering at the University of Tulsa, USA. He received his B.S. degree and M.S. degree in Oil & Gas Storage and Transportation Engineering from the University of Petroleum (Beijing), China. He is currently working on assisted history matching, production optimization and numerical simulation for complex reservoirs.
Guojun Yu
Guojun Yu is currently a lecturer in Department of Thermal Energy and Power Engineering, Shanghai Maritime University, China. He received his Ph.D. in Petroleum Storage and Transportation Engineering (a branch of Petroleum Engineering) from China University of Petroleum (Beijing) in 2015. His research has concentrated in three areas: pipeline transportation technology, thermal management of marine and numerical modeling for engineering problems. Additional research interests include numerical methods for heat transfer with complex phase change, heat transfer in porous media and convective heat transfer of non-Newtonian fluid.
Weitao Zhao
Weitao Zhao is an intermediate Engineer in Jiagedaqi Oil & Gas Pipeline Company, China. He received his B.S. degree in Automation Engineering from the Xi'an Petroleum University. He is currently working on pipeline risk analysis and preventive measures of pipeline damage.