**2. Materials and Methods**

#### *2.1. Proposed Solution for Remote Arctic Oil and Gas Facilities*

The proposed solution for the creation of industrial infrastructure facilities is a pile foundation, mounted in wintertime with pre-strengthening of soils, see Figure 3.

The concept of creating modular pile foundations developed at Mining University [36,37] implies optimal placement and year-round operation of equipment production infrastructure facilities in the allocated technological zones. To implement these solutions, new types of piles or traditional foundations, which are widely used in construction work in areas of permafrost soils, can be used.

**Figure 3.** Prototyping of production infrastructure facilities on a pile foundation.

The Arctic Research Center of Mining University developed design documentation for modular pile foundations (Figure 4) for placing drilling rigs in zones of permafrost spreading, subject to seasonal flooding. As a result of the research and development, it was found that the cost of constructing modular pile foundations is on average 50% less than the cost of constructing and maintaining sand dumps [38].

**Figure 4.** The modular pile base of a drilling rig. (**a**) for exploration drilling, (**b**) for production drilling.

#### *2.2. Physical and Mathematical Modeling of Geotechnical Solutions for the Location of Arctic Oil and Gas Facilities under Climate Change*

To select the best technological solutions, it is first necessary to understand how climate change in the Arctic zone affects the bearing capacity of the piles. For this purpose, according to statistical data, three warming scenarios for the period from 2031 to 2050 were developed as initial characteristics for modeling: (1) positive with a temperature increase of 2.2 ◦C (0.1 ◦C per year); (2) neutral with a temperature increase of 3 ◦C (0.16 ◦C per year); (3) negative with a temperature increase of 4.8 ◦C (0.24 ◦C per year); (4) locally negative with a temperature increase of 9.6 ◦C (0.5 ◦C per year). The initial data for modeling were obtained as a result of engineering surveys at a remote field in the Russian Arctic zone. The characteristics of soils for modeling the bearing capacity of foundations are summarized in Table 1.


**Table 1.** Soil characteristics for modeling the bearing capacity of foundations.

The physical and mechanical properties of soils are determined from laboratory data. Additional parameters such as heat capacity and thermal conductivity of the material in the thawed and frozen state were calculated according to the Russian standardization document [39]. The average active layer, according to the surveys, was 0.5 m.

The initial meteorological data are average monthly and annual air temperature and wind speed. The data are shown in Table 2.

**Table 2.** Air temperature and wind speed (average monthly and annual data).


Modeling the distribution of soil temperatures around the pile and calculating the bearing capacity of the pile foundations was carried out in the Frost 3D software. In this researce we considered the air temperature increase to model active layer depth. However, there are other factors such as content of water [40–42], air content [42], organic matter [43], etc. Taking these factors into account is a direction for further research.

#### *2.3. Proposed Solution for the Stability of Pile Foundations in Permafrost*

Thermosyphons are an effective means of temperature stabilization of permafrost soils, the main elements of which are an evaporator and a condenser. In cold seasons, ground cooling is performed by natural convection of low-boiling refrigerant (ammonia, refrigerant, carbon dioxide, etc.) in a thermosyphon, with heated coolant flowing from the buried evaporator to the condenser, which dissipates the heat into the atmosphere, and cooled coolant back to the evaporator. During the warm season, when the ground temperature is lower than the atmospheric air temperature, the thermosiphon does not work. Thus, capacity of the ground temperature stabilization system should be calculated so that accumulated winter "cold" was enough to maintain the necessary ground temperature until the new cold season comes.

However, the accelerated global warming in recent years complicates the task of thermal stabilization of pile foundations. At the construction stage it is required to install more productive thermosiphons or a larger number of them. Also at the stage of operation it's required to provide for the possibility of installing new thermosiphons, taking into account the global warming. In this regard, there are developments providing not only passive cooling of the soil by thermosiphons, but also its active cooling. For example, the thermosiphon can have a second circuit, closed to the cooling machine, which allows to cool the soil adjacent to the pile in the warm season [44]. In another variant, the thermosiphon can have one circuit; however, when the warm season comes, the liquid refrigerant should be pumped out, and then the cold air should be circulated by means of an air turbo-cooling machine [45]. In [46], a combined thermal stabilization unit is proposed, which in addition to passive ground cooling by thermosiphon provides for active cooling, carried out by injection into the thermosiphon, cooled by throttling the refrigerant through a special removable nozzle of the thermosiphon. The method of year-round ground cooling by means of thermosiphon and compressor-condenser unit is also known, according to which the thermosiphon condenser is simultaneously a refrigerating machine evaporator [47]. Issues of using thermoelectric modules for year-round ground stabilization based on the Pelte effect are covered in [22]; however, the results of pilot operation by enterprises Fondamentproekt and Gazprom VNIIGAZ showed the complexity of providing the required ground temperature and high-power consumption of this technology.

At the same time, if the object is far away from the centralized power supply, which is quite common in relation to objects of the oil and gas sector, the organization of power supply of the refrigeration machine may present certain difficulties. In addition, it is possible that the fight against the effects of global warming actually aggravates the climate problem—if the refrigeration machine uses installations based on fossil fuels, especially diesel, fuel oil, etc. For these reasons, the idea of using renewable energy sources (RESs) to stabilize permafrost has been developed. The possibility of using RESs for active thermal stabilization is mentioned in [45–48].

Mining University proposed to use a combined system based on a two-circuit thermosiphon and a refrigeration machine connected to its second circuit. Power supply of the refrigeration machine is provided by RESs, for example—wind power or photovoltaic station, also there are storages of electricity and a reserve source to increase the reliability of power supply. The peculiarity of the proposed device is the placement of electric power storage, backup source, control system and other devices, sensitive to the temperature regime, in a thermally insulated container. The container is heated in the cold season at the expense of the heat removed from the thermosiphon condenser with an additional circuit with a coolant. During the warm season, when the thermosiphon is not in operation, heating of the thermally insulated container is usually not required, otherwise its own thermostat system can be used. The functional diagram of the proposed system is shown in Figure 5.

The heat capacity required to heat the equipment container was determined by Formula (1):

$$P = \frac{V \cdot \Delta T \cdot K}{860},\tag{1}$$

where *V* is the volume of the heated room, Δ*T* is the difference between the ambient air temperature and the desired temperature in the heated room, *K* is the coefficient of heat losses, 860 is the coefficient to convert kcal/h to kW.

For the calculation, the following geometric dimensions of the insulated container were taken: length 6 m, width 2.5 m, height 2.6 m. Such dimensions of the container allow to place electric energy accumulators, reserve source, control unit, electric energy conversion devices and circulation pump of the container heating system. The coefficient of heat losses is taken equal to 2, which corresponds to an average level of thermal insulation. So, in order to have 0 ◦C (the lower temperature limit for Russian-made lithium-ion energy storage units) inside the container in the coldest winter month (according to Table 2, February, temperature is −24.6 ◦C), heat power supply of about 2.23 kW is needed. It is also necessary to take into account thermal losses, which in each case will differ depending on the design of the system. Assuming the coefficient, taking into account heat losses by pipelines of hot water supply systems, equal to 1.15 kW, we obtain that in the coldest winter month heat capacity of 2.57 kW is required.

In the above mentioned Frost 3D software package the heat removed from the thermosiphon condensers is calculated, which can be used for heating the insulated container with equipment in the cold season.

Figure 6 shows a graph of the power of 14 thermosiphons under the considered conditions, a graph of the required thermal power to heat the container and a graph of the ambient air temperature.

**Figure 6.** Annual graph of monthly averaged capacity of thermosyphon without application of jet grouting technology.

In accordance with the results of simulation of cooling capacity in the program Frost 3D and numerical simulation it was found that under the considered conditions, when using 14 thermosiphons, the required air temperature inside the container is maintained 9 months out of 12, and is not provided in March, April and May. The possibility of heat recovery when using a backup power generation source was not considered. For 3 out of 12 months of the year, the temperature inside the container must be maintained by other means of thermostatting. It should be noted that it is possible to use smaller containers.

The issues of thermostatting the bases of remote Arctic objects become even more relevant when placing gas-chemical complexes [50] with high energy flows in the areas of permafrost spreading. In any case, the number of thermosiphons is determined based on the year-round provision of the bearing capacity of piles, and the possibility of effective use of thermal energy of thermosiphon condensers is determined after fulfilling the conditions of mechanical stability of pile foundations.
