A submarine pipeline (also known as marine, subsea or offshore pipeline) is a pipeline that is laid on the seabed or below it inside a trench.[1][2] In some cases, the pipeline is mostly on-land but in places it crosses water expanses, such as small seas, straits and rivers.[3] Submarine pipelines are used primarily to carry oil or gas, but transportation of water is also important.[3] A distinction is sometimes made between a flowline and a pipeline.[1][3][4] The former is an intrafield pipeline, in the sense that it is used to connect subsea wellheads, manifolds and the platform within a particular development field. The latter, sometimes referred to as an export pipeline, is used to bring the resource to shore.[1] Sizeable pipeline construction projects need to take into account many factors, such as the offshore ecology, geohazards and environmental loading – they are often undertaken by multidisciplinary, international teams.[1]

Example of a submarine pipeline route: the Langeled pipeline.

Route selection

One of the earliest and most critical tasks in a submarine pipeline planning exercise is the route selection.[5] This selection has to consider a variety of issues, some of a political nature, but most others dealing with geohazards, physical factors along the prospective route, and other uses of the seabed in the area considered.[5][6] This task begins with a fact-finding exercise, which is a standard desk study that includes a survey of geological maps, bathymetry, fishing charts, aerial and satellite photography, as well as information from navigation authorities.[5][6]

Physical factors

Interaction between a submarine pipeline and the seabed onto which it rests (four possible scenarios).

The primary physical factor to be considered in submarine pipeline construction is the state of the seabed – whether it is smooth (i.e., relatively flat) or uneven (corrugated, with high points and low points). If it is uneven, the pipeline will include free spans when it connects two high points, leaving the section in between unsupported.[2][7] If an unsupported section is too long, the bending stress exerted onto it (due to its weight) may be excessive. Vibration from current-induced vortexes may also become an issue.[7][8] Corrective measures for unsupported pipeline spans include seabed leveling and post-installation support, such as berm or sand infilling below the pipeline. The strength of the seabed is another significant parameter. If the soil is not strong enough, the pipeline may sink into it to an extent where inspection, maintenance procedures and prospective tie-ins become difficult to carry out. At the other extreme, a rocky seabed is expensive to trench and, at high points, abrasion and damage of the pipeline's external coating may occur.[7][8] Ideally, the soil should be such as to allow the pipe to settle into it to some extent, thereby providing it with some lateral stability.[7]

One of a number of reasons why submarine pipelines are buried below the seabed: to protect them against the gouging action of drifting ice features, such as icebergs.

Other physical factors to be taken into account prior to building a pipeline include the following:[2][7][8][9][10][11]

Other uses of the seabed

Proper planning of a pipeline route has to factor in a wide range of human activities that make use of the seabed along the proposed route, or that are likely to do so in the future. They include the following:[2][8][13]

Submarine pipeline characteristics

Submarine pipelines generally vary in diameter from 3 inches (76 mm) for gas lines, to 72 inches (1,800 mm) for high capacity lines.[1][2] Wall thicknesses typically range from 10 millimetres (0.39 in) to 75 millimetres (3.0 in). The pipe can be designed for fluids at high temperature and pressure. The walls are made from high-yield strength steel, 350-500 MPa (50,000-70,000 psi), weldability being one of the main selection criteria.[2] The structure is often shielded against external corrosion by coatings such as bitumastic or epoxy, supplemented by cathodic protection with sacrificial anodes.[2][14] Concrete or fiberglass wrapping provides further protection against abrasion. The addition of a concrete coating is also useful to compensate for the pipeline's positive buoyancy when it carries lower density substances.[2][15]

The pipeline's inside wall is not coated for petroleum service. But when it carries seawater or corrosive substances, it can be coated with epoxy, polyurethane or polyethylene; it can also be cement-lined.[2][14] In the petroleum industry, where leaks are unacceptable and the pipelines are subject to internal pressures typically in the order of 10 MPa (1500 psi), the segments are joined by full penetration welds.[2][14] Mechanical joints are also used. A pig is a standard device in pipeline transport, be it on-land or offshore. It is used to test for hydrostatic pressure, to check for dents and crimps on the sidewalls inside the pipe, and to conduct periodic cleaning and minor repairs.[1][2]

Pipeline construction

Pipeline construction involves two procedures: assembling many pipe segments into a full line, and installing that line along the desired route. Several systems can be used – for a submarine pipeline, the choice in favor of any one of them is based on the following factors: physical and environmental conditions (e.g. currents, wave regime), availability of equipment and costs, water depth, pipeline length and diameter, constraints tied to the presence of other lines and structures along the route.[2] These systems are generally divided into four broad categories: pull/tow, S-lay, J-lay and reel-lay.[16][17][18][19]

Simplified drawings showing three configurations used to tow subsea pipelines offshore to the planned installation site (not to scale).

The pull/tow system

In the pull/tow system, the submarine pipeline is assembled onshore and then towed to location. Assembly is done either parallel or perpendicular to the shoreline – in the former case, the full line can be built prior to tow out and installation.[20] A significant advantage with the pull/tow system is that pre-testing and inspection of the line are done onshore, not at sea.[20] It allows to handle lines of any size and complexity.[18][21] As for the towing procedures, a number of configurations can be used, which may be categorized as follows: surface tow, near-surface tow, mid-depth tow and off-bottom tow.[22]

Simplified drawings of three common systems used for the construction and installation of subsea pipelines (not to scale): S-lay, J-lay and reel.
The Solitaire, one of the largest pipe-laying ships in the world.
The DCV Aegir, a pipelay vessel designed for J-lay and reel-lay.
The Saipem 7000, a semi-submersible crane vessel equipped with a J-lay pipe-laying system.

The S-lay system

In the S-lay system, the pipeline assembly is done at the installation site, on board a vessel that has all the equipment required for joining the pipe segments: pipe handling conveyors, welding stations, X-ray equipment, joint-coating module, etc.[25] The S notation refers to the shape of the pipeline as it is laid onto the seabed. The pipeline leaves the vessel at the stern or bow from a supporting structure called a stinger that guides the pipe's downward motion and controls the convex-upward curve (the overbend). As it continues toward the seabed, the pipe has a convex-downward curve (the sagbend) before coming into contact with the seabed (touch down point). The sagbend is controlled by a tension applied from the vessel (via tensioners) in response to the pipeline's submerged weight. The pipeline configuration is monitored so that it will not get damaged by excessive bending.[25] This on-site pipeline assembly approach, referred to as lay-barge construction, is known for its versatility and self-contained nature – despite the high costs associated with this vessel's deployment, it is efficient and requires relatively little external support.[26] But it may have to contend with severe sea states – these adversely affect operations such as pipe transfer from supply boats, anchor-handling and pipe welding.[25] Recent developments in lay-barge design include dynamic positioning and the J-lay system.[25][27]

The J-lay system

In areas where the water is very deep, the S-lay system may not be appropriate because the pipeline leaves the stinger to go almost straight down. To avoid sharp bending at the end of it and to mitigate excessive sag bending, the tension in the pipeline would have to be high.[28] Doing so would interfere with the vessel's positioning, and the tensioner could damage the pipeline. A particularly long stinger could be used, but this is also objectionable since that structure would be adversely affected by winds and currents.[28] The J-lay system, one of the latest generations of lay-barge, is better suited for deep water environments. In this system, the pipeline leaves the vessel on a nearly vertical ramp (or tower). There is no overbend – only a sagbend of catenary nature (hence the J notation), such that the tension can be reduced. The pipeline is also less exposed to wave action as it enters the water.[29] However, unlike for the S-lay system, where pipe welding can be done simultaneously at several locations along the vessel deck's length, the J-lay system can only accommodate one welding station. Advanced methods of automatic welding are used to compensate for this drawback.[30]

The Reel-lay system

In the reel-lay system, the pipeline is assembled onshore and is spooled onto a large drum typically about 20 metres (66 ft) x 6 metres (20 ft) in size,[31] mounted on board a purpose-built vessel. The vessel then goes out to location to lay the pipeline. Onshore facilities to assemble the pipeline have inherent advantages: they are not affected by the weather or the sea state and are less expensive than seaborne operations.[21] Pipeline supply can be coordinated: while one line is being laid at sea, another one can be spooled onshore.[32] A single reel can have enough capacity for a full length flow line.[32] The reel-lay system, however, can only handle lower diameter pipelines – up to about 400 mm (16 in).[33] Also, the kind of steel making up the pipes must be able to undergo the required amount of plastic deformation as it is bent to proper curvature (by a spiral J-tube) when reeled around the drum, and straightened back (by a straightener) during the layout operations at the installation site.[34]

Stabilisation

Several methods are used to stabilise and protect submarine pipelines and their components. These may be used alone or in combinations.[35]

Trenching and burial

Simplified drawing showing a typical jetting system for trenching below a submarine pipeline that is lying on the seafloor.

A submarine pipeline may be laid inside a trench as a means of safeguarding it against fishing gear (e.g. anchors) and trawling activity.[36][37] This may also be required in shore approaches to protect the pipeline against currents and wave action (as it crosses the surf zone). Trenching can be done prior to pipeline lay (pre-lay trenching), or afterward by seabed removal from below the pipeline (post-lay trenching). In the latter case, the trenching device rides on top of, or straddles, the pipeline.[36][37] Several systems are used to dig trenches in the seabed for submarine pipelines:

″A buried pipe is far better protected than a pipe in an open trench.″[41] This is commonly done either by covering the structure with rocks quarried from a nearby shoreline. Alternatively, the soil excavated from the seabed during trenching can be used as backfill. A significant drawback to burial is the difficulty in locating a leak should it arise, and for the ensuing repairing operations.[42]

Mattresses

Concrete mattresses on barge

Mattresses may be laid over the pipeline, or both under and over it depending on the substrate.[35]

Ground anchors

Clamps holding the pipeline to piles may be used to prevent lateral movement.[35]

Saddle blocks

Precast concrete saddle blocks may be used to provide lateral support and hold the pipeline down more firmly.[35]

Sandbags and groutbags

These may be packed at the sides or under a pipeline to provide vertical and/or lateral support.[35]

Gravel dumps

Gravel may be dumped over parts of a pipeline to reduce scour and help stabilise against lateral movement.[35]

Environmental and legal issues

The Espoo Convention created certain requirements for notification and consultation where a project is likely to have transboundary environmental effects. Scholars are divided on how effective Espoo is at mitigating environmental harm. Law of the Sea concepts involved in the construction of transboundary pipelines concern territorial waters, continental shelves, exclusive economic zones, freedom of the high seas and protection of the environment. Under international law the high seas are open to all states to lay underwater pipelines and for various other types of construction.[43]

Underwater pipelines pose environmental risk because pipelines themselves may become damaged by ship's anchors, corrosion, tectonic activity, or as a result of defective construction and materials. Stanislav Patin has said that study on the effects of natural gas on underwater ecosystems, fish and other marine organisms has been limited. Researchers found a cause-effect relationship between mass fish mortality and natural gas leaks after drilling accidents in the Sea of Azov in 1982 and 1985.[43]

Concerns about the environmental risks of underwater pipelines have been raised on numerous occasions. There have been at least two serious incidents involving oil pipelines on the UK's continental shelf. There have also been several "minor spills and gas leaks" involving other North Sea pipelines. In 1980 a pipeline was damaged by a ship's anchor and in 1986 a pipeline valve failed due to pressure changed. Both incidents resulted in oil spills. Several Baltic countries expressed concerns about the Nord Stream pipeline. The route of the 1,200 km underwater pipeline would travel through fishing areas of the Baltic Sea, as well as area where chemical weapons from World War II had been discarded.[43]

See also

References

  1. ^ a b c d e f Dean, p. 338-340
  2. ^ a b c d e f g h i j k l Gerwick, p. 583-585
  3. ^ a b c Palmer & King, p. 2-3
  4. ^ Bai & Bai, p. 22
  5. ^ a b c Palmer & King, p. 11-13
  6. ^ a b Dean, p. 342-343
  7. ^ a b c d e Palmer &King, p. 13–16
  8. ^ a b c d Dean, Sect. 7.2.2
  9. ^ Palmer & Been, p. 182–187
  10. ^ Croasdale et al. 2013
  11. ^ Cardenas, I.C.; et al. (2022). "Marine geohazards exposed: Uncertainties involved". Marine Georesources and Geotechnology: 1–31. doi:10.1080/1064119X.2022.2078252. hdl:11250/3058338.
  12. ^ Barrette 2011
  13. ^ Palmer & King, p. 16-18
  14. ^ a b c Ramakrishnan, p. 185
  15. ^ Ramakrishnan, p. 186
  16. ^ Dean, p.347-350
  17. ^ Palmer & King, Chap.12
  18. ^ a b Bai & Bai, p. 910-912
  19. ^ Wilson, Chap.1
  20. ^ a b c Brown, p. 1
  21. ^ a b Palmer & King, p. 412
  22. ^ Palmer & King, 12.4
  23. ^ Palmer & King, p. 415
  24. ^ Palmer & King, p. 417
  25. ^ a b c d Gerwick,15.2
  26. ^ Palmer & King, p. 395
  27. ^ Palmer & King, p. 397
  28. ^ a b Palmer & King, p. 401
  29. ^ Palmer & King, p. 402
  30. ^ Gerwick, p. 615
  31. ^ Bai & Bai, p. 145
  32. ^ a b Gerwick, p. 611
  33. ^ Bai & Bai, p. 144
  34. ^ Gerwick, p. 610
  35. ^ a b c d e f g h i Bevan, John, ed. (2005). "Section 1.7". The Professional Divers's Handbook (second ed.). Alverstoke, GOSPORT, Hampshire: Submex Ltd. p. 34. ISBN 978-0950824260.
  36. ^ a b c Palmer & King, sect. 12.5.1
  37. ^ a b Ramakrishnan, p. 212
  38. ^ Palmer & King, p. 420
  39. ^ Ramakrishnan, p. 214
  40. ^ Palmer & King, p. 421
  41. ^ Palmer & King, p. 424
  42. ^ Palmer & King, p. 425
  43. ^ a b c Karm, Ellen (June 2008). "Environment and Energy: The Baltic Sea Gas Pipeline". Journal of Baltic Studies. 39 (2): 99–121. doi:10.1080/01629770802031200. S2CID 144484018.

Bibliography