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, straits 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. 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, straits 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. Other physical factors to be taken into account prior to building a pipeline include the following: 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: Submarine pipelines generally vary in diameter from 3 inches (76 mm) for gas lines, to 72 inches (1,800 mm) for high capacity lines. 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. The structure is often shielded against external corrosion by coatings such as bitumastic or epoxy, supplemented by cathodic protection with sacrificial anodes. Concrete or fiberglass wrapping provides further protection against abrasion. The addition of a concrete coating is also useful to compensate for the pipeline's negative buoyancy when it carries lower density substances. 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. 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. 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. Pipeline construction involves two procedures: assembling a large number of 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. These systems are generally divided into four broad categories: pull/tow, S-lay, J-lay and reel-lay. 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. A significant advantage with the pull/tow system is that pre-testing and inspection of the line are done onshore, not at sea. It allows to handle lines of any size and complexity. 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.