In the operation of buried pipelines laid in permafrost, the main hazard is represented by the formation of thaw bulbs around the pipes. This phenomenon can provoke thaw settlement of long distance.
Without appropriate engineering measures, this inevitably leads to the cost for fixing the construction zone around the pipeline. A thaw bulb that grows in the lateral direction can also be hazardous for objects situated close to the pipeline: transmission towers, side roads, etc.
Simulation of thermal processes in ground
The best way to assess the reliability of engineering measures is via computer simulation. To ensure the accuracy of the computation of the thermal impact of an oil pipeline on permafrost, the following critical factors require consideration:
- actual geometry of the simulated objects
- inhomogeneous geological and lithological structure of the ground, including ice wedges
- changes in thermophysical ground properties as a result of phase transitions
- ground thawing and freezing velocity
- changes in meteorological conditions over time
- presence of thermal insulation materials
- thickness and type of thermal pipe insulation
- temperature and velocity of oil pumped through the pipe
- structural features of the trench in which the pipeline is located
Frost 3D by Simmakers is unlike any other software solution on the market because it allows to predict ground thaw bulb formations around pipelines, factoring in all of the above mentioned parameters.
Frost 3D addresses all of the requirements of modern users conducting thermal analysis in terms of accuracy, speed, and presentability of results.
Sample Computation of the Thermal Impact of an Oil Pipeline on Permafrost in Frost 3D
To calculate the development of a thaw bulb caused by the thermal impact of the pipeline on ice wedges for the section of the Eastern Siberia – Pacific Ocean (ESPO-1) main oil pipeline, for 2 years from September 2011.
The section of the ESPO-1 pipeline. This section is in the permafrost zone with an average ground temperature of –1.1oС, and underground ice deposits with the thickness of up to 4 m.
To determine the geocryological danger, expressed numerically as the magnitude of thaw bulb around the underground pipeline lying in terrain containing ice wedges.
Numerical estimation in a three-dimensional recreation of the thaw bulb around the pipeline lying in terrain with ice wedges, with simultaneous consideration of multiple factors: variation of meteorological conditions, velocity and temperature of pumped oil, thickness and type of thermal pipe insulation and structural features of the trench in which the pipeline is located.
Implemented mathematical model and numerical method
The solution of the problem required employment of the nonlinear heat equation. For more details, please read the whole article Computer Simulation of Artificial Ground Freezing.
Numerical simulation of thaw bulb
The computation was performed using the following configuration: a pipeline running through a layout with ice wedges.
Configuration of mutual arrangement of the pipeline and the ice wedges
The following geometric parameters were factored in:
- linear dimensions of the simulation area: 25×25 m in the horizontal plane and 15 m in depth
- ice depth: 0.7 m, thickness: 4.3 m
- pipe wall thickness: 10 mm
- pipe insulation thickness: 70 mm
The discretization of the simulation area was carried out on a hexahedral computational mesh consisting of 2,905,980 nodes.
The thermophysical characteristics of soil layers around the trench (with installed pipeline) were specified in accordance with the table below:
|No. of soil layer||Ground||Layer thickness, m||Thermal conductivity coefficient of thawed and frozen ground λT/λF , W/(m∙K)||Volumetric heat capacity of thawed and frozen ground СТ/СF , kJ/(m3∙K)||Volumetric water content, m3/m3|
|1||Peat||0.7||0.5 / 2||3600 / 1300||0.33|
|2||Peat, predominantly decomposed, plastic frozen, icy. Contains ice wedges||4.3||0.5 / 2||3600 / 1300||0.33|
|3||Sandy loam soil, icy||2.2||1.45 / 2.3||2867 / 2030||0.57|
|4||Sandy loam soil with high ice content||7.8||1.25 / 2.3||3160 / 205||0.42|
The ice wedges were assigned with a heat capacity equal to 1860 kJ/(m3∙K), and thermal conductivity – 2.25 W/(m∙K).
It was assumed that the oil was pumped through the pipeline with a temperature of +8oС, and the average flow was 30 tons per year.
According to the computations for two years, it was determined that the maximum ground thawing around the pipeline formed in the directions in which the heat insulator was absent, and did not exceed 0.9 m. The amplitude of annual fluctuations of the temperature field in the ground in the vicinity of the pipeline varied between 3 to 5oС.
This case only necessitated the results for two years, but the Frost 3D software package is well capable of forecasting the evolution of the thaw bulb over 5, 10 and 30 years.
3D temperature field calculated in the Frost 3D software
Results of the calculation of the thermal field in the form of isolines in the cross section of the simulation area in August
Simulation results of the ground thawing area around the oil pipeline
Ground thaw bulb around the pipeline in the YZ plane in August
Ground thaw bulb around the pipeline in the YZ plane in January
It is also worth mentioning that the thermal impact of the pipeline did not lead to the melting of the ice wedges beneath it. This is primarily due to the fact that the pipeline is located in a trench where the underground ice was removed and insulating material was placed.