Kun Huang
Jiali Wu
Hongfang Lu
Southwest Petroleum University
Chengdu, China
Yi Jiang
CNOOC Energy Technology & Services Oilfield Construction Engineering
Tianjin, China
Liangxue Cai
Southwest Petroleum University
Thermal insulation properties of fiber-reinforced (FRP) plastic crude oil pipelines are superior to seamless steel pipelines. Operating a crude oil pipeline at too low a temperature can lead to wax precipitation, a major economic and safety problem. Not only does wax precipitation lead to increased energy consumption during operation, but it can also initiate pipeline gelling. On the other hand, too high a temperature can cause serious scaling. Scaling not only affects the quality of the crude oil, but also causes pipeline corrosion and reduces service life.
In the last 5 years studies on crude pipeline operating temperatures have narrowed their focus to oil temperature in pipelines, heat loss from pipelines, heat transfer coefficients, and the temperature field around pipelines during their normal operation.1-5 One study, focused on temperature drop after shutdown of buried hot oil pipelines, used Fluent software to simulate the soil-temperature field distributions at different air and oil temperatures and for different coefficients of thermal conductivity and also obtained temperature field distributions under the influence of different conditions.
This study simulated non-steady states based on steady states and obtained temperature field distributions after shutdown.6 It examined the Hong-Ke fiber-reinforced plastic oil pipeline running from the Honglian pump station of the Hongshanzui field to a nearby thin-oil processing station. It establishes physical and mathematical models for non-steady state heat transfer between the pipeline system and the soil and defines boundary conditions, using Fluent for numerical simulations. Pipephase software was used for simulation analysis of the temperature drop along the Hong-Ke pipeline. A field experiment on the Hong-Ke pipeline validated the results' accuracy and reliability. The study used air temperature, period of operation, pipeline material, oil flow temperature, buried depth, and distance to the pipeline wall as influencing factors.
Basic theory
Soil temperature generally remains constant below the surface zone. This article does not consider phase changes in the earth or changes in its thermal parameters under natural conditions and makes the following assumptions:
• Only pure heat conduction exists between the pipeline and soil.
• Thermal contact resistance between the pipeline and soil can be (and is) ignored.
• Soil surrounding the pipeline is a homogeneous medium.
• Changes in elevation along the pipeline are also ignored.
• The pipeline's thermal influence zone ranges from -10 m to 10 m on its horizontal axis and from -8 m to 0 m vertically.
These assumptions led to a rectangular heat dissipation model with the following dimensions: a pipeline length of 50 m, an axial cross section of 20 × 8 m, and a coverage depth of 1.0 m (Fig 1). A convective heat transfer boundary condition was specified on the earth's surface, with adiabatic boundary conditions at X = -10 m and X = 10 m and Y = -8 m defined as the constant temperature boundary condition.
Gambit software established a 3D model of the soil surrounding the buried pipeline, generating the mesh used in Fluent (Fig. 2).
Mathematical model
Fluent uses different control equations for different heat transfer boundaries when simulating the temperature field of soil. The non-steady state conditions of buried hot oil pipelines involve the mass conservation equation, momentum conservation equation, energy conservation equation for fluid flows inside pipelines, the heat conduction equation for soil outside pipelines, and the standard k-ε two-equation model. Reynolds averaged Navier-Stokes (RANS) methods calculate turbulence.7 8
Equation 1 is the continuity equation, Equation 2 the momentum conservation equation, and Equation 3 the energy conservation equation.
The temperature field of the soil surrounding a pipeline can be regarded as the temperature field of a homogeneous semi-infinite solid under periodic boundary conditions, and can be described using a differential equation. Equation 9 is the 3D unsteady heat transfer equation for soil.
Boundary conditions
The assumptions of the physical model led to the boundary conditions defined by Equations 10-14. Equation 10 represents the convective heat exchange between soil and atmosphere and Equations 11 and 12 the conductive heat exchange between soil and the FRP pipeline. Equation 13 defines the adiabatic boundaries of the vertical planes 10 m away from the pipeline to both the left and right and Equation 14 the constant temperature of the boundary 8 m below the surface.
The combination of flow and pipe diameter determined inlet velocity within a preselected inlet boundary condition. The pipeline's outlet boundary condition was set as free outflow.
Project profile
The Xinjiang Petroleum Investigation Design and Research Institute designed the Hong-Ke FRP oil pipeline, which began operation end-2011. The pipeline is 31.7 km long with a nominal diameter of 200 mm. It lies 1 m below the surface and operates at a safe pressure of 5.5 MPa. The soils in the areas along the pipeline are primarily loam and light loam. The climate is characterized by cold winters, hot summers, and large temperature differences between night and day.
Table 1 details physical parameters of both Hong-Ke and the surrounding soil, as used in Fluent calculations. Properly reflecting data changes requires using dense meshes at the wall of the pipeline. Sparse meshes are used in the large area outside the pipe wall to reduce the size of the model. The crude in the pipeline and the pipewall were displayed primarily as hexahedral meshes, and the soil outside the pipeline primarily as triangular meshes. Figs. 2-3 show meshing results.
Numerical simulation
Fig. 4 shows the temperature distribution of soil under natural conditions and Fig. 5 the temperature distribution of the surrounding soil when the FRP oil pipeline reaches steady state at an air temperature, tf, of 12 °C. and an oil temperature, tw, of 40 °C. Under natural conditions the soil exhibited a linear temperature field, in stark contrast to the curved temperature field shown while under the influence of the hot oil pipeline.
Fig. 6 shows the soil's temperature field in isotherms, highlighting the curved distribution in place while the pipeline is operating. According to the temperature values in the figure, as the soil depth increases, the effect of the hot oil pipeline on the temperature field lessens.
Validation
Validating the established numerical simulation model of the temperature field required comparison with Hong-Ke. The pipeline's length was problematic when using Fluent, as it can suffer errors when simulating models with a high length-width ratio. Pipephase, therefore, was used for simulation validation of the temperature changes along the pipeline. The temperature value of the soil at the buried depth calculated by Fluent was used as the ambient temperature in Pipephase simulations, which were in turn used to validate the accuracy of the temperature value of the soil.
FRP pipe wall roughness measured 0.00533 mm, with a thermal conductivity coefficient 0.4 w/(m·°C.). The thermophysical parameters of the soil and flows, pressures, and temperatures at the inlet and outlet of the pipeline were set to the same values as the operating parameters set in the Fluent simulation. Table 2 shows simulated soil temperatures at 1 m based on air temperature changes in August 2014. Fig. 7 illustrates the temperature-drop curve along the Hong-Ke pipeline Aug. 1, 2014.
The temperature-drop curve shows the rate of decline decreasing as distance grows. Table 3 and Fig. 8 show temperature drop along the Hong-Ke pipeline for 30 days in August, including errors. Comparing the data obtained through the software simulation with the data from the field operation shows a consistent trend in temperature drop and some errors in values.
When Fluent simulated soil temperature, the air temperature outside the soil was assigned the daily mean temperature instead of changing with time. The thermal conductivity coefficient of soil also changes as air temperature changes, but was set to a fixed value during Fluent simulation, causing the values obtained through simulation to differ from those obtained in the field. The simplified simulation pipeline showed high accuracy and validated the correctness of the model.
Air temperature
Fig. 9 shows 2014 simulation results for soil temperature at depths of 0.5-4.0 m as air temperature changes. Natural temperature curves for the soil unaffected by the pipeline appeared as a group of smooth cosine curves. As depth of the soil increased, the amplitude of the cosine curves decreased gradually until approaching zero. Temperature changes also shrank as depth increased. Temperature field results under natural conditions simulated by Fluent were consistent with reality.
Operation period
Simulated changes in soil and crude oil temperatures occurred for periods ranging from 0 to 2,650 hr (Fig. 10). The pipeline constantly released heat during its operation. Affected by the hot pipeline, the soil's initial temperature field was redistributed. The soil near the pipeline initially was affected greatly by the hot oil pipeline. As pipeline operation continued, heat diffused. The soil temperature field did not immediately reach steady state due to delays in its temperature waves.
Comparison also shows the soil temperature not initially reaching stability. After 10 hr of operation, the pipeline had only had a small effect, heat diffusion limited to the pipe wall. An obvious soil temperature increase only appeared after 500 hr.
Pipeline material
Different pipeline materials have different thermophysical parameters, different coefficients of thermal conductivity, and different thermal insulation properties. This article studied both FRP and seamless steel pipelines. Table 4 shows their performance parameters.
Fig. 11 shows temperature distribution of the soil after crude transport through both an FRP pipeline and a seamless steel pipeline, with a steady state reached at an oil flow temperature of 40 °C. At this state the temperature of soil surrounding the seamless steel pipeline reached 37 °C., while that of soil surrounding the FRP pipeline was 25 °C.
The FRP pipeline had a smaller effect on the temperature field of the surrounding soil than the seamless steel pipeline and an increasing number of oilfields in China are using FRP pipelines to transport crude oil.
Oil flow temperature
Oil flow temperature studies used tw of 20 °C., 30 °C., 40 °C., and 50 °C. to analyze changes in soil temperature with time. A certain point in the soil surrounding the pipeline was studied and any changes in the temperature at this point with changes in the air temperature and oil flow temperature noted (Fig. 12).
Oil flow temperature inside the pipeline significantly affected the temperature of the surrounding soil. The thermal insulation performance of the FRP pipeline and delay in the soil's temperature waves caused temperature changes in the soil to lag changes in air temperature. The lowest soil temperature occurred 55 days after the lowest air temperature.
Buried depth
Ensuring all other transportation conditions were unchanged, simulation used a buried pipeline depth of 0.8-1.6 m and an oil flow temperature of 30 °C. to obtain soil temperature changes. Fig. 13 shows the results
Fig. 13 shows soil temperature as influenced by pipeline depth. As the air temperature changed, the soil temperature increased at a slower rate as the buried depth of the pipeline increased. When the buried pipeline depth was shallow, moreover, the soil temperature field was significantly affected by the environment, especially when air temperature was high.
Distance to pipeline wall
Fig. 14 shows simulation results regarding temperature change over time for certain points in a horizontal direction from the pipeline and below the pipeline. Distance from the pipeline influences soil temperature, and the temperature increases at different rates. Temperatures of soil near the pipeline increased rapidly, whereas those of the soil far from the pipeline increased at a slower rate.
In the horizontal direction, temperatures at points 3.5 m away from the pipeline's center almost reached stable values. The temperature field of soil 4.5 m vertically away from the pipeline's center was nearly unaffected by the hot oil pipeline. Temperature changes of soil outside the pipeline were concentrated primarily within a certain range and the assumption of the simplification of an infinite area of soil into a finite area was validated (Fig. 14).
References
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The authors
Kun Huang ([email protected]) is a professor at the School of Petroleum Engineering, Southwest Petroleum University of China, Chengdu. He specializes in oil and gas gathering and processing technology theory. He holds an MA in computer application technology from University of Electronic Science and Technology of China, Chengdu.
Jiali Wu ([email protected]) is a postgraduate student at the School of Petroleum Engineering, Southwest Petroleum University of China, Chengdu. She specializes in oil and gas storage and transportation security theory and technology. She holds a BA in oil and gas storage and transportation engineering from Southwest Petroleum University. She is a member of Society of Petroleum Engineers (SPE).
Hongfang Lu ([email protected]) is a postgraduate student at the School of Petroleum Engineering, Southwest Petroleum University of China, Chengdu. He specializes in pipeline stress analysis technology. He holds a BA in oil and gas storage and transportation engineering from Southwest Petroleum University. He is a member of American Society of Civil Engineers (ASCE).
Yi Jiang ([email protected]) is an assistant engineer at CNOOC Energy Technology & Services Ltd., Tianjin. She specializes in oil and gas gathering and processing technology theory. She holds an MA in oil and gas storage and transportation engineering from Southwest Petroleum University.
Liangxue Cai ([email protected]) is a lecturer at the School of Petroleum Engineering, Southwest Petroleum University of China, Chengdu. He specializes in oil and gas storage and transportation engineering construction technology. He holds a PhD in oil and gas storage and transportation engineering from China University of Petroleum, Beijing.