Underwater pipeline

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Course of the Langeled Pipeline, one of the largest underwater pipelines in the world

An underwater pipeline or offshore pipeline is a pipeline that is laid on the bed of the water or below it in a trench. Offshore pipelines are mostly used to transport oil , natural gas or water.

technology

Subsea pipelines range in diameter from three inches for gas pipelines to 1.83 meters for large oil pipelines. The wall thicknesses range between 10 and 75 mm. The pipe can be designed for high pressures and temperatures. The pipelines are usually made of a material with high tensile strength and good weldability . The pipe is usually provided with a coating or sheathing as corrosion protection, which consists of bitumen or epoxy resin . The corrosion protection is usually supplemented by a sacrificial anode . Concrete or glass fiber reinforced plastic is also used as mechanical protection . The use of concrete in particular makes sense if the pipeline would experience buoyancy from a conveyed medium with a low density. An internal coating of the pipeline is not necessary when transporting oil; however, if corrosive substances or salt water are pumped, the pipeline is coated on the inside with epoxy, polyurethane or polyethylene .

Since leakages must not occur in the oil industry and the internal pressures are up to 10 MPa, the pipeline segments are welded through; mechanical connections, for example flange connections, also occur. To check the pipeline for pressure testing and cleaning are usually pigs used.

Pipeline construction

The pipeline is built in two steps: assembling a large number of pipe segments to form the pipeline and laying them along the planned route. There are various methods of pipeline construction, some of which differ greatly from one another and are selected depending on factors such as environmental conditions, water depth, pipeline length and diameter and costs.

The laying methods can be roughly divided into four different types: the pulling / towing system, the S-method, J-method and R-method. They are used alone, but also in combination for larger pipeline projects.

Pulling / towing system

With the towing / towing system, the underwater pipeline is manufactured on land, then towed or pulled to the laying site and installed there.

It is made parallel to or at an angle to the shore line. When building parallel to the bank line, part of the testing of the pipeline is already possible on land. However, the length of the pipeline is limited by local conditions. When building at an angle to the bank, the pipeline segments are welded on land, but the actual pipeline is pulled onto the water by a tug. This usually allows for greater lengths.

The pipeline can be towed or pulled at different depths:

  • Towing on the surface: When towing on the surface, the pipeline floats on the water. For this purpose, floating bodies are attached to the pipeline. This type of laying is only suitable for calm waters without surface currents .
  • Towing below the surface: This is where the pipeline is located below the surface of the water. Floats that are on the surface of the water are also used. The pipeline is less susceptible to waves, but the floating bodies are exposed to the waves and surface currents.
  • Trolling at medium depths: The pipeline has no buoyancy here because it is either heavy enough or is weighed down with weights. With this towing method, the line forms a chain line between the two tugboats. The possible sagging of the pipeline is primarily determined by the water depth. In addition, precise coordination of the tugs is necessary in order not to damage the pipeline.
  • Towing above the bottom of the water: The process is similar to towing at medium depth. In contrast to this, the line is dragged just a few meters above the ground. Chains are attached to the pipeline and are dragged across the ground.
  • Towing on the bottom of the water: When towing on the bottom, the line is pulled over the bottom of the water. This eliminates the influence of currents and waves on the pipeline. Another advantage is the fact that the tugboat can detach itself from the pipeline in problematic seas and can be pulled on after recovery. The problem, however, is the damage to the fauna and flora of the water bed, the risk of damage to other pipelines and the necessary protection (for example by means of an abrasion-resistant coating) of the pipeline. Therefore, this procedure is mostly only used when crossing rivers or between two banks.
The Solitaire , one of the largest pipelayers in the world (S-Lay)

The S procedure

With the S-method or S-Lay, the pipeline is assembled on site. Special work boats , so-called pipelayers , are used for this. This is done by welding the pipe segments, testing the welded joints and coating. The designation S-Lay is based on the S-shaped course of the pipeline in the laying process, since the pipeline is welded horizontally on the ship.

One of the largest pipelayers in the world in J-Lay, the Saipem 7000

The J procedure

The J-method or J-lay corresponds in the work steps and the procedure to the S-method with the difference that the pipeline is welded more or less vertically and thus a J-shaped course occurs during laying. The welding and the other work steps take place in the so-called J-Lay tower.

The R method

The Aegir , a pipelayer for R- and J-Lay

With the R process, R-Lay or Reel-Lay, the pipeline is not welded on the ship, but on land and wound onto a kind of drum on the ship. The advantage of the process is the production of the pipeline on land - and therefore independent of the environmental influences at sea - and the fact that production and laying can take place separately from one another. The ship can thus lay the pipeline and is then supplied with a newly spooled pipeline at sea. Due to the winding of the pipeline, however, the process is limited to small and medium diameters. In addition, the plastic deformation and the realignment when laying represent a high material load.

See also

literature

  • Bai Y. & Bai Q .: Subsea Engineering Handbook . Gulf Professional Publishing, New York 2010.
  • P. Barrette: Offshore pipeline protection against seabed gouging by ice: An overview, Cold Regions Science and Technology ( http://www.sciencedirect.com/science/article/pii/S0165232X11001091 ), 2011.
  • RJ Brown: Past, present, and future towing of pipelines and risers . In: Proceedings of the 38th Offshore Technology Conference (OTC) . Houston, USA (2006).