Development of offshore hydrocarbon (HC) fields is today's oil and gasindustry priority in the Russian Federation. Water areas of the Arctic shelfare considered to be potential offshore HC production regions. When designingpipelines for such fields it is necessary to take into account the impact ofspecific Arctic conditions including hazardous ice impact (ice gouging), possible presence of permafrost on the seabed, lithological andgeomorphological distinctive features of bottom soils. All main parametersensuring safety of offshore pipelines must be determined and validated at theearly design stage.
The paper reviews one of the main conditions that influences reliable operationof underwater pipeline systems, namely, stable position of the underwaterpipeline at design reference marks.
Calculations of offshore pipeline stability on the seabed use the followingmain 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 forbackfilling, may be potentially dangerous due to liquefaction underhydrodynamic forces. It is especially dangerous in the first years of operationwhen the soil is not consolidated enough. Relief in a local zone of dilutedsoil causes longitudinal stresses in pipeline, which may result in offshorepipeline stability loss. Liquefied soil potential depends also on backfillingprocess technology. This operation is performed by special ships - dredgers. Such ship has two pipes, one for soil suction, and the other equipped withwater injection nozzles - for washing out and backfilling. When a trench isbackfilled with controlled soil flow, " front" of backfill material is formedunder pipe working head and a layer of fluidized material appears in the upperpart of this " front". Therefore, if weak soil is used as backfill material, asize of liquefied soil layer will be considerable, as well as its impact on thepipeline. This process may lead either to floating up or submerging of pipelineinto the soil. To stabilize offshore pipelines position the following measurescan be taken: backfilling with soil not subject to liquefaction; pipelinelaying below the layer of liquefied soil to eliminate risks related to soilliquefaction; using different methods of ballasting.