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. In some cases, the pipeline is mostly on-land but in places it crosses water expanses, such as small seas, straights and rivers. Submarine pipelines are used primarily to carry oil or gas, but transportation of water is also important. A distinction is sometimes made between a flowline and a pipeline. 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. Sizeable pipeline construction projects need to take into account a large number of factors, such as the offshore ecology, geohazards and environmental loading – they are often undertaken by multidisciplinary, international teams.

One of the earliest and most critical tasks in a submarine pipeline planning exercise is the route selection. 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. 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.

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. 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. 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. 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.

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