Submarine pipeline
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]
Submarine pipeline characteristics[edit]
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]
Environmental and legal issues[edit]
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]