Submerged pipelines can be constructed on pipelaying vessels by stovepipe welding of pipe sections stored on the deck or by the reel method by which the pipeline produced onshore is wound onto reels beforehand. Winding onto the reel is possible for up to 90 kilometers of 100 to 400 millimeters diameter pipes. During pipeline laying the reel constantly rotates and the pipe is laid down to a 300 meter depth at a speed of up to 4 kilometers per hour. During lowering of the pipeline prepared on a deck a special device (a stinger) supports it to prevent from major bends when moving it from the vessel. The maintenance of all pipelaying vessel systems and welding equipment is performed by a computer (sea depth, wave and wind speeds are taken into account; pipelaying vessel stability is ensured). As a rule, several vessels participate in the process of pipe laying. Special barges continuously supply pipes to a pipelaying vessel, which is preceded by a vessel monitoring the seabed. The supplied 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. Special conveyor is installed on the pipelaying vessel, which receives pipes that are welded here as well. Each welded joint is then ultrasonically tested for defects. Anticorrosion coating is 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 gradually lowered onto the seabed, is located here. It defines the required deflection of the top part of the gas pipeline, which prevents metal deformation. There are three main ways that subsea pipe is laid -- S-lay, J-lay and tow-in -- and the pipelay vessel is integral to the success of the installation. Buoyancy affects the pipelay process, both in positive and negative ways. In the water, the pipe weighs less if it is filled with air, which puts less stress on the pipelay barge. But once in place on the sea bed, the pipe requires a downward force to remain in place. This can be provided by the weight of the oil passing through the pipeline, but gas does not weigh enough to keep the pipe from drifting across the seafloor. In shallow-water scenarios, concrete is poured over the pipe to keep it in place, while in deepwater situations, the amount of insulation and the thickness required to ward of hydrostatic pressure is usually enough to keep the line in place.

• published: 04 Apr 2020
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