During the construction of buildings and structures in the Far North, permafrost thawing in the immediate vicinity results in ground settlement and deformation of foundations. This issue is particularly topical for pipelines and tanks containing petroleum products with elevated temperatures. The most effective method for providing foundation stability is by establishing control over the ground thermal regime through the operation of seasonal or year-round cooling units. It is impossible to formulate an accurate plan for the collocation of cooling devices or assess ground freezing potential without computer simulation.
To predict the ground thermal regime for passive refrigeration under an oil tank, it is necessary to use specialized software — Frost 3D.
In order to create a computer model according to the geometric magnitudes of the tank and system of cooling units, a 90х90х33 m computational domain is created. The geological and lithological structure of the soils in the considered domain – sand, peat, loam and sandy loam – is reconstructed on the basis of geotechnical boreholes by means of interpolation.
2D contour map of the tank bottom (left) and section view of the tank and cooling units (right), both constructed according to the data from the geotechnical boreholes
The surfaces on which boundary conditions are subsequently specified are determined on the reconstructed 3D geometry of the simulation area.
Reconstructed 3D model for simulation with cooling units and soil basement under oil tank
The 3D simulation model is discretized into an irregular hexahedral computational mesh.
Discretized 3D simulation area
For different soil layers such as sand, peat, loam and sandy loam, the following thermophysical properties are specified: volumetric heat capacity of soil in thawed and frozen states, thermal conductivity of soil in thawed and frozen states, moisture content of soil, freezing point, and an empirical parameter in the equation that approximates the quantity of ice content for given temperatures.
Heat transfer with the air is specified on the upper boundary of the computational domain by means of the heat transfer coefficient and changes in air temperature over time. To consider the influence of snow cover on the heat transfer between soil and air, the change of snow cover thickness over time is specified.
Specifying boundary conditions for convective heat transfer
Specifying thermophysical properties of ground
Export of air temperature dependence on time
Specifying snow cover thickness on time dependence
Tank bottom temperature and heat transfer coefficient are specified for the soil basement of oil tank.
A constant temperature equal to -1.7oC is specified on the lower boundary of the computational domain, and on the side boundary – heat flow is equal to zero.
Heat flow on the evaporating section of the system of cooling devices is automatically calculated on the basis of form factors of the cooling system.
The initial temperature distribution is specified in the form of dependence over depth.
Specifying initial temperature distribution
After all the input data is entered, the simulation for the required period of time is conducted. The resulting predictions for the ground thermal regime are shown below as color distribution of sections of the simulation area at different moments in time.
Simulation results of ground thermal regime after 90 days – longitudinal section of
simulation area along the plane of cooling device installation
Simulation result of ground temperature distribution after 1 year
Result of thermal regime simulation in the form of temperature isolines
along the longitudinal section of the simulation area
On conclusion of the simulation process, design engineers obtain full information regarding the dynamics of a 3D temperature field in the ground during specified time intervals.