An oil platform is a large structure used to drill and produce oil and/or natural gas from the seas or oceans. Depending on the circumstances, the platform may be floating or an artificial island or attached to the ocean floor. A typical platform may have around thirty well heads located on the platform and directional drilling allows reservoirs to be accessed at both different depths and at remote positions up to 8 kilometres from the platform.
The world s largest oil platform is called Hibernia. Many platforms also have remote well heads attached by umbilical connections, these may be single wells or a manifold centre for multiple wells.
Larger lake and sea-based oil platforms and oil rigs are some of the largest moveable man-made structures in the world. There are several distinct types of platforms and rigs:
- Fixed Platforms, built on concrete and/or steel legs anchored directly onto the seabed, supporting a deck with space for drilling rigs, production facilities and crew quarters. Such platforms are, by virtue of their immobility, designed for very long term use (for instance the Hiberniaplat form). Various types of structure are used, steel jacket, concrete caisson, floating steel and even floating concrete. Steel jackets are vertical sections made of tubular steel members, and are usually piled into the seabed. Concrete caisson structures, pioneered by the Condeep concept, 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. Fixed platforms are economically feasible for installation in water depths up to about 1,700 feet.
- Compliant Towers platforms, consist of narrow, flexible towers and a piled foundation supporting a conventional deck for drilling and production operations. Compliant towers are designed to sustain significant lateral deflections and forces, and are typically used in water depths ranging from 400 and 900 meters.
- 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 the use of steerable thrusters. Semi-submersible can be used in depths from 200 to 1800 meters.
- 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, used in relatively low depths, are designed to move from place to place, and then anchor themselves by deploying the jack-like legs.
- Ship-board Rigs platforms. 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.
- Floating production systems are large ships equipped with processing facilities and moored to a location for a long period. The main types of floating production systems are FPSO (floating production, storage, and offloading system), FSO (floating storage and offloading system), and FSU (floating storage unit).
- Tension-leg Platforms, consist of floating rigs tethered to the seabed in a manner that eliminates most vertical movement of the structure. Tension Leg Platforms are used in water depths up to about 1500 meters.
- Seastars platforms are mini Tension Leg Platforms of relatively low cost, used in water depths between 200 and 1000 meters. They can also be used as utility, satellite or early production platforms for larger deep water discoveries.
- Spar Platforms, moored to the seabed like the Tension Leg Platforms , but whereas the Tension Leg Platforms has vertical tension tethers the Spar has more conventional mooring lines. Spars have been designed in three configurations: the “conventional” one-piece cylindrical hull, the “truss spar” where the midsection is composed of truss elements connecting the upper buoyant hull (called a hard tank) with the bottom soft tank containing permanent ballast, and the “cell spar” which is built from multiple vertical cylinders. The Spar may be more economical to build for small and medium sized rigs than the Tension Leg Platforms , and has more inherent stability than a Tension Leg Platform since it has a large counterweight at the bottom and does not depend on the mooring to hold it upright.