An oil platform, offshore platform, or (colloquially) oil
rig is a large structure with facilities to drill wells, to extract and process
oil and natural gas, or to temporarily store product until it can be brought to
shore for refining and marketing. In many cases, the platform contains
facilities to house the workforce as well.
Depending on the circumstances, the platform may be fixed to
the ocean floor, may consist of an artificial island, or may float. Remote
subsea wells may also be connected to a platform by flow lines and by umbilical
connections. These subsea solutions may consist of one or more subsea wells, or
of one or more manifold centres for multiple wells.
TYPES OF OIL RIGS
Fixed platforms
A fixed platform base under construction on a Louisiana
river.These platforms are built on concrete or steel legs, or both, 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 Hibernia
platform). 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. To see
more details regarding Design, construction and installation of such platforms
refer to
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 520 m (1,710 ft).
Compliant towers
These platforms consist of slender, flexible towers and a
pile 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 370
to 910 metres (1,210 to 2,990 ft).
Semi-submersible platform
These platforms have hulls (columns and pontoons) of
sufficient buoyancy to cause the structure to float, but of weight sufficient
to keep the structure upright. Semi-submersible platforms can be moved from
place to place and can be ballasted up or down by altering the amount of
flooding in buoyancy tanks. They are generally anchored by combinations of
chain, wire rope or polyester rope, or both, during drilling and/or production
operations, though they can also be kept in place by the use of dynamic positioning.
Semi-submersibles can be used in water depths from 60 to 3,000 metres (200 to
10,000 ft).
Jack-up drilling rigs
Jack-up Mobile Drilling Units (or jack-ups), as the name
suggests, are rigs that can be jacked up above the sea using legs that can be
lowered, much like jacks. These MODUs (Mobile Offshore Drilling Units) are
typically used in water depths up to 120 meters (390 ft), although some designs
can go to 170 m (560 ft) depth. They are designed to move from place to place,
and then anchor themselves by deploying the legs to the ocean bottom using a
rack and pinion gear system on each leg.
Drillships
A drillship is a maritime vessel that has been fitted with
drilling apparatus. It is most often used for exploratory drilling of new oil
or gas wells in deep water but can also be used for scientific drilling. Early
versions were built on a modified tanker hull, but purpose-built designs are
used today. Most drillships are outfitted with a dynamic positioning system to
maintain position over the well. They can drill in water depths up to 3,700 m
(12,100 ft)
Floating production systems
The main types of floating production systems are FPSO
(floating production, storage, and offloading system). FPSOs consist of large
monohull structures, generally (but not always) shipshaped, equipped with
processing facilities. These platforms are moored to a location for extended
periods, and do not actually drill for oil or gas. Some variants of these
applications, called FSO (floating storage and offloading system) or FSU
(floating storage unit), are used exclusively for storage purposes, and host
very little process equipment. This is one of the best sources for having
floating production.The world's first floating liquefied natural gas (FLNG)
facility is currently under development. See the section on particularly large
examples below.
Tension-leg platform
TLPs are floating platforms tethered to the seabed in a
manner that eliminates most vertical movement of the structure. TLPs are used
in water depths up to about 2,000 meters (6,600 feet). The
"conventional" TLP is a 4-column design which looks similar to a
semisubmersible. Proprietary versions include the Seastar and MOSES mini TLPs;
they are relatively low cost, used in water depths between 180 and 1,300 metres
(590 and 4,270 ft). Mini TLPs can also be used as utility, satellite or early
production platforms for larger deepwater discoveries.
Gravity-based structure
A GBS can either be steel or concrete and is usually anchored
directly onto the seabed. Steel GBS are predominantly used when there is no or
limited availability of crane barges to install a conventional fixed offshore
platform, for example in the Caspian Sea. There are several steel GBS in the
world today (e.g. offshore Turkmenistan Waters (Caspian Sea) and offshore New
Zealand). Steel GBS do not usually provide hydrocarbon storage capability. It
is mainly installed by pulling it off the yard, by either wet-tow or/and
dry-tow, and self-installing by controlled ballasting of the compartments with
sea water. To position the GBS during installation, the GBS may be connected to
either a transportation barge or any other barge (provided it is large enough
to support the GBS) using strand jacks. The jacks shall be released gradually
whilst the GBS is ballasted to ensure that the GBS does not sway too much from
target location.
Spar platforms
Spars are moored to the seabed like TLPs, but whereas a TLP
has vertical tension tethers, a spar has more conventional mooring lines. Spars
have to-date been designed in three configurations: the
"conventional" one-piece cylindrical hull; the "truss
spar", in which 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 has more inherent stability than a
TLP since it has a large counterweight at the bottom and does not depend on the
mooring to hold it upright. It also has the ability, by adjusting the mooring
line tensions (using chain-jacks attached to the mooring lines), to move horizontally
and to position itself over wells at some distance from the main platform
location. The first production spar was Kerr-McGee's Neptune, anchored in 590 m
(1,940 ft) in the Gulf of Mexico; however, spars (such as Brent Spar) were
previously used as FSOs.
Eni's Devil's Tower located in 1,710 m (5,610 ft) of water
in the Gulf of Mexico, was the world's deepest spar until 2010. The world's
deepest platform is currently the Perdido spar in the Gulf of Mexico, floating
in 2,438 metres of water. It is operated by Royal Dutch Shell and was built at
a cost of $3 billion.The first truss spars were Kerr-McGee's Boomvang and
Nansen.[citation needed] The first (and only) cell spar is Kerr-McGee's Red
Hawk
Condeep platforms
Condeep platform got its breakthrough in the summer of 1973.
When a contract was signed between the Norwegian Contractors (NC) and Mobil on
the construction of Beryl A and the Shell Brent B. It had not been implemented
on bold building projects in Norway since the Rjukan development. The Norwegian
Condeep platforms attracted international attention, and the construction was
awarded the prize for the technological innovation in the world's largest oil
conference in Houston in 1975.Internationally Condeep is the epitome of oil
operations in the North Sea. Condeep platforms have been the most significant
independent contributions to the offshore industry. The latest and greatest
Condeep platform is Troll A, which was submitted to Shell in 1995.
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