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Concept of Subsea Pipelines:

How are subsea gas pipelines builtSea can go several kilometers deep. Laying a pipe at the bottom is a challenge. However, 6,000kilometers of pipelines are laid at the bottom of the North Sea, some of which have been therefor 40 years already.

The worlds largest vessel, Solitaire, is 300 meters long and about40 meters wide. This particular vessel is involved in the construction of the Nord Stream gas

pipeline.

Searching for obstacles

Subsea gas pipelines account for 45 per cent of natural gas exports to Europe. Seabed is carefullysurveyed along its entire route before laying the gas pipeline. Specialists must identify all

potential obstacles including sunk ships, ammunition, and simply large boulders. These obstaclesare either removed or bypassed. At this stage, specialists also identify locations where the

pipelines should be buried or backfilled.

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All pipes for future gas pipeline are subject to a special treatment. From the inside, they aretreated with an antifriction coating which is reducing resistance during gas transmission.Externally, pipes are treated with an anticorrosion coating and then with a negative buoyancyconcrete coating.

Floating housesPipes are laid on the seabed by special pipelaying vessels . Pipelaying vessels are huge floating

platforms which can accommodate several hundreds of people at a time.

As a rule, several vessels participate in the process of pipe laying. Special barges continuouslysupply pipes to a pipelaying vessel, which is preceded by a vessel monitoring the seabed. Thesupplied pipes are unloaded on storage sites located directly on the deck of the pipelaying vessel.They should have a stock of pipes for 12 hours of work.

How pipes are laid

Special conveyor is installed on the pipelaying vessel, which receives pipes that are welded hereas well. Each welded joint is then ultrasonically tested for defects. Anticorrosion coatingis applied to all joints after welding. Welded pipes are moving over the conveyor towards the aft.Stinger, a special boom, immersed in the water at an angle, over which the pipes are graduallylowered onto the seabed, is located here. It defines the required deflection of the top part of thegas pipeline, which prevents metal deformation.

As a rule, pipes lie on the seabed under their own weight. They dont need to be fixed, becauseeach pipe weighs up to several tons after the concrete coating is applied. Pipes are laid in specialtrenches and backfilled with ground only in some locations, e.g. at landfalls, to ensure stability

of the pipe.

From sea to shore

The process of laying a subsea gas pipeline normally starts not on the shore, as one could think, but in the sea. Gas pipeline can consist of several sections built at different times from differentvessels and then connected to each other. Gas pipeline should be able to withstand different

pressures in different sections, for this purpose pipes with different wall thickness are used.Upon the completion of the subsea section construction, pipes are drawn on shore using a specialwinch installed on firm ground which is connected to the pipe with steel ropes and is slowlydrawing it from the sea. The pipeline is then connected with its onshore section a tie-in

is made.

Hydro testing of the gas pipeline is a mandatory stage. For that, the pipeline is filled with waterunder requisite pressure and left for some time to identify possible defects. Condition of the gas

pipeline is carefully monitored after its commissioning as well. Special electronic devices for in- pipe inspection are used in this process.

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