Development of offshore hydrocarbon (HC) fields is today’s oil and gas industry priority in the Russian Federation. Water areas of the Arctic shelf are considered to be potential offshore HC production regions. When designing pipelines for such fields it is necessary to take into account the impact of specific Arctic conditions including hazardous ice impact (ice gouging), possible presence of permafrost on the seabed, lithological and geomorphological distinctive features of bottom soils. All main parameters ensuring safety of offshore pipelines must be determined and validated at the early design stage.
The paper reviews one of the main conditions that influences reliable operation of underwater pipeline systems, namely, stable position of the underwater pipeline at design reference marks.
Calculations of offshore pipeline stability on the seabed use the following main conditions:
geotechnical conditions of the seabed;
bathymetrical conditions (water depth);
pipeline parameters (diameter, wall thickness).
Criteria of pipeline stability on the seabed include:
floating up or submerging of pipeline;
Soils with weak strength properties, especially when they are used for backfilling, may be potentially dangerous due to liquefaction under hydrodynamic forces. It is especially dangerous in the first years of operation when the soil is not consolidated enough. Relief in a local zone of diluted soil causes longitudinal stresses in pipeline, which may result in offshore pipeline stability loss. Liquefied soil potential depends also on backfilling process technology. This operation is performed by special ships – dredgers. Such ship has two pipes, one for soil suction, and the other equipped with water injection nozzles - for washing out and backfilling. When a trench is backfilled with controlled soil flow, "front" of backfill material is formed under pipe working head and a layer of fluidized material appears in the upper part of this "front". Therefore, if weak soil is used as backfill material, a size of liquefied soil layer will be considerable, as well as its impact on the pipeline. This process may lead either to floating up or submerging of pipeline into the soil. To stabilize offshore pipelines position the following measures can be taken: backfilling with soil not subject to liquefaction; pipeline laying below the layer of liquefied soil to eliminate risks related to soil liquefaction; using different methods of ballasting.