Submarine pipeline
A submarine 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 many factors, such as the offshore ecology, geohazards and environmental loading – they are often undertaken by multidisciplinary, international teams.
Route selection
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.Physical factors
The primary physical factor to be considered in submarine pipeline construction is the state of the seabed – whether it is smooth or uneven. 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 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.File:Pipeline_protection_-_burial_below_seabed_gouge.svg|thumb|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:
- Seabed mobility: Sand waves and megaripples are features that move with time, such that a pipeline that was supported by the crest of one such feature during construction may find itself in a trough later during the pipeline's operational lifespan. The evolution of these features is difficult to predict so it is preferable to avoid the areas where they are known to exist.
- Submarine landslides: They result from high sedimentation rates and occur on steeper slopes. They can be triggered by earthquakes. When the soil around the pipe is subjected to a slide, especially if the resulting displacement is at high angle to the line, the pipe within it can incur severe bending and consequent tensile failure.
- Currents: High currents are objectionable in that they hinder pipe laying operations. For instance, in shallow seas tidal currents may be quite strong in a strait between two islands. Under these circumstances, it may be preferable to bring the pipe elsewhere, even if this alternative route ends up being longer.
- Waves: In shallow waters, waves can also be problematic for pipeline laying operations and, subsequently, to its stability, because of the water's scouring action. This is one of a number of reasons why landfalls are particularly delicate areas to plan.
- Ice-related issues: In freezing waters, floating ice features often drift into shallower waters, and their keel comes into contact with the seabed. As they continue to drift, they gouge the seabed and can hit the pipeline. Stamukhi can also damage this structure, either by exerting high local stresses on it or by causing to soil around it to fail, thereby inducing excessive bending. Strudel are another pipeline hazard in cold waters – water gushing through them can remove the soil from below the structure, making it vulnerable to overstress or vortex-induced oscillations. Pipeline route planning for areas where these risks are known to exist has to consider laying the pipeline in a back-filled trench.
Other uses of the seabed
- Other pipelines: If and where the proposed pipeline intersects an existing one, which is not uncommon, a bridging structure may be required at that juncture in order to cross it. This has to be done at a right angle. The juncture should be carefully designed so as to avoid interferences between the two structures either by direct physical contact or due to hydrodynamic effects.
- Fishing vessels: Commercial fishing makes use of heavy fishing nets dragged on the seabed and extending several kilometers behind the trawler. This net could snag the pipeline, with potential damage to both pipeline and vessel.
- Ship anchors: Ship anchors are a potential threat to pipelines, especially near harbors.
- Military activities: Some areas still have mines originating from former conflicts but that are still operational. Other areas, used for bombing or gunning practices, may also conceal live ammunition. Moreover, at some locations, various types of instrumentation are laid on the seafloor for submarine detection. These areas have to be avoided.
Submarine pipeline characteristics
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, 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
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, 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.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. 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.- Surface tow: In this configuration, the pipeline remains at the surface of the water during tow, and is then sunk into position at lay site. The line has to be buoyant – this can be done with individual buoyancy units attached to it. Surface tows are not appropriate for rough seas and are vulnerable to lateral currents.
- Near-surface tow: The pipeline remains below the water surface but close to it – this mitigates wave action. But the spar buoys used to maintain the line at that level are affected by rough seas, which in itself may represent a challenge for the towing operation.
- Mid-depth tow: The pipeline is not buoyant – either because it is heavy or it is weighted down by hanging chains. In this configuration, the line is suspended in a catenary between two towing vessels. The shape of that catenary is a balance between the line's weight, the tension applied to it by the vessels and hydrodynamic lift on the chains. The amount of allowable sag is limited by how far down the seabed is.
- Off-bottom tow: This configuration is similar to the mid-depth tow, but here the line is maintained within 1 to 2 m away from the bottom, using chains dragging on the seabed.
- Bottom tow: In this case, the pipeline is dragged onto the bottom – the line is not affected by waves and currents, and if the sea gets too rough for the tow vessel, the line can simply be abandoned and recovered later. Challenges with this type of system include: requirement for an abrasion-resistant coating, interaction with other submarine pipelines and potential obstructions. Bottom tow is commonly used for river crossings and crossings between shores.