An oil platform is a large structure used to house workers and machinery needed to drill and then produce oil and natural gas in the ocean. Depending on the circumstances, the platform may be attached to the ocean floor, consist of an artificial island, or be floating.

Generally, oil platforms are located on the continental shelf though as technology improves, drilling and production in ever deeper waters becomes both feasible and economic. A typical platform may have around thirty wellheads located on the platform and directional drilling allows reservoirs to be accessed at both different depths and at remote positions up to maybe 5 miles (8 kilometres) from the platform. Many platforms also have remote wellheads attached by umbilical connections, these may be single wells or a manifold centre for multiple wells.

Larger lake and sea-based oil rigs are some of the largest moveable man-made structures in the world. There are at least five distinct types of rig platform:

• Immobile Platforms, a rig built on concrete and/or steel legs anchored directly onto the seabed. Such platforms are, by virtue of their immobility, designed for very long term use (for instance the Hibernia platform).
• Semi-submersible Platforms having legs of sufficient buoyancy to cause the structure to float, but of weight sufficient to keep the structure upright. Semi-submersible rigs can be moved from place to place; and can be lowered into or raised by altering the amount of flooding in buoyancy tanks; they are generally anchored by cable anchors during drilling operations, though they can also be kept in place by active steering.
• Jack-up Platforms, as the name suggests, are platforms that can be jacked up above the sea, by dint of legs than can be lowered like jacks. These platforms are designed to move from place to place, and then anchor themselves by deploying the jack-like legs.
• Ship-board Rigs. Active steering of ships, especially based on Global Positioning System measurements, enables certain drilling operations to be conducted from a ship which holds its position relative to the drilling point, within the parameters for movement acceptable in a given circumstance — i.e. within the point at which movement of the ship would cause the drill string to break.
• Tension-leg Platforms, a rig tethered to the seabed in a manner that eliminates most vertical movement of the structure.

Various types of structure are used, steel jacket, concrete caisson, floating steel and even floating concrete. The concrete caisson structures often have in-built oil storage in tanks below the sea surface and these tanks were often used as a flotation capability, allowing them to be built close to shore (Norwegian fjords and Scottish firths are popular because they are sheltered and deep enough) and then floated to their final position where they are sunk to the seabed. Steel jackets are fabricated on land and towed by barge to their destination where a crane is used to upright the jacket and locate it on the seabed. Steel jackets are usually piled into the seabed.

A typical oil production platform is self-sufficient in energy and water needs, housing electrical generation, water desalinators and all of the equipment necessary to process oil and gas such that it can be either delivered directly onshore by pipeline or to a Floating Storage Unit and/or tanker loading facility. Elements in the oil/gas production process include wellhead, production manifold, production separator, glycol process to dry gas, gas compressors, water injection pumps, oil/gas export metering and main oil line pumps. All production facilities are designed to have minimal environmental impact.

The Petronius platform is an oil and gas platform in the Gulf of Mexico, which stands 610 metres (2,000 feet) above the ocean floor. This structure is partially supported by buoyancy. Depending on the criteria it may be the world's tallest structure.

In practice, these larger platforms are assisted by smaller ESVs (emergency support vessels) like the British Iolair that are summoned when something has gone wrong, e.g. when a search and rescue operation is required. During normal operations, PSVs (platform supply vessels) keep the platforms provisioned and supplied, and AHTS vessels can also supply them, as well as tow them to location and serve as standby rescue and firefighting vessels.

The nature of their operation — extraction of volatile substances sometimes under extreme pressure in a hostile environment — has risk and not infrequent accidents and tragedies occur. In July 1988, 167 people died when Occidental Petroleum's Alpha offshore production platform, on the Piper field in the North Sea, exploded after a gas leak. The accident greatly accelerated the practice of housing living accommodation on self-contained separate rigs, away from those used for extraction.

Further risks are the leeching of heavy metals that accumulate in buoyancy tanks into water; and risks associated with their disposal. There has been concern expressed at the practice of partially demolishing offshore rigs to the point that ships can traverse across their site; there have been instances of fishery vessels snagging nets on the remaining structures. Proposals for the disposal at sea of the Brent Spar, a 137-metre-tall storage buoy (another true function of that which is termed an oil rig), was for a time in 1996 an environmental cause célèbre in the UK after Greenpeace occupied the floating structure. The event led to a reconsideration of disposal policy in the UK and Europe, though Greenpeace, in hindsight, admitted some inaccuracies leading to hyperbole in their statements about Brent Spar.

In British waters, the cost of removing all platform rig structures entirely was estimated in 1995 at £1.5 billion, and the cost of removing all structures including pipelines — a so-called "clean sea" approach — at £3 billion.

See Also

• Oil rig
• Floating oil production system
• Petroleum

External links

• Oil Rig Disposal Caution: PDF (http://www.parliament.uk/post/pn065.pdf) — Post note issued by the UK Parliamentary Office of Science and Technology.

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