By the present time, vast experience has been gained in construction and operation of gas pipelines in the Far North areas, which makes it possible to evaluate possibilities of pipeline laying under various permafrost conditions and schedules of gas supply and transportation.
In the permafrost environment, all three methods of pipeline laying are applied: subsurface (underwater), surface and above-ground .
The pipeline laying method is selected taking into account the transported product temperature, type of locality and permafrost environment of the pipeline route.
Subsurface pipeline laying is used for cold, warm and hot sections of pipelines. The pipeline is laid into the preliminary prepared trench, the width of which at the bottom is on average 1.5 pipe diameters, and the pipe top burial depth is 0.8-1.0 meters. The trench bottom is usually leveled with a sand layer 5-7 centimeters thick.
As application of the subsurface method results in formation of a loose ground zone around the pipeline, in which the filtration and erosional factors are significantly higher compared to that of the surrounding ground, even at insignificant surface slopes good conditions for underground water drainage are formed. To prevent development of erosion processes, water-tight and erosion protection jumpers are arranged across the trench, these jumpers made of non-filtering materials (clay concrete, ground mixture with synthetic resins, ground treatment by polymers, etc.).
Cold pipeline sections laid in thawed, potentially heaving soil, and also warm sections laid in frozen soil with plenty of ice, are usually equipped with heat protection shields or seasonal cooling devices (SCD). Heat protection shields can be flat or have a cylindrical shape. Flat heat protection shields are assembled on the trench bottom, and in some cases – on its sides. Cylindrical shields are assembled directly over the pipe in the form of a shell. Application of heat insulation helps to slow down the process of freezing of the thawed soil in cold sections and thawing of the frozen soil in warm sections, but does not prevent these processes. Reinforced heat insulation in the course of long-term pipeline operation does not provide the desired effect, but complicates the technical solutions and increases the pipeline construction cost. Therefore, application of heat insulation is usually combined with other actions: controlled cooling or heating of the product, installation of warm or cold heat tracers, application of SCD, etc.
To prevent pipeline buoyancy in water-flooded and water-logged route sections, and also in dry areas, where soil weight is insufficient to hold the pipeline, the pipeline is ballasted by special weights, weighting coating of the pipe or by installation of anchors, including frozen-in ones.
Subsurface pipeline laying is most popular in the Far North regions. Its advantages include the following:
Pipeline reliability at external shock actions;
Thermal stability of the pipeline environment and, therefore, reduction of costs related to its control;
Its disadvantages include a large volume of earthwork, complexity of repair, necessity of special actions to ensure the pipe stability and expenses on environmental safety.
Surface pipeline laying