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 calledHibernia. 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
- 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.