Larger lake- and sea-based offshore platforms and
drilling rig for oil. •
1) &
2) Conventional fixed platforms •
3) Compliant tower •
4) &
5) Vertically moored tension leg and mini-tension leg platform •
6) Spar •
7) &
8) Semi-submersibles •
9) Floating production, storage, and offloading facility •
10) Sub-sea completion and tie-back to host facility Note that jack-up drilling rigs, drillships, and gravity-based structures are not pictured here.
Fixed platforms . These platforms are built on
concrete or
steel legs, or both, anchored directly onto the seabed, supporting the 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 structures are used: steel jacket, concrete
caisson, floating steel, and even
floating concrete. Steel jackets are structural 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: and.
Concrete caisson structures, pioneered by the
Condeep concept, often have in-built oil storage in tanks below the sea surface. 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 .
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 .
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 . The "conventional" TLP is a 4-column design that looks similar to a semisubmersible. Proprietary versions include the Seastar and MOSES mini TLPs; they are relatively low cost, used in water depths between . Mini TLPs can also be used as utility, satellite or early production platforms for larger deepwater discoveries.
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 in the Gulf of Mexico; however, spars (such as
Brent Spar) were previously used as FSOs.
Eni's
Devil's Tower located in of water in the Gulf of Mexico, was the world's deepest spar until 2010. The world's deepest platform as of 2011 was 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. The first (and, as of 2010, only) cell spar is Kerr-McGee's Red Hawk.
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 .
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 in production. See the section on
particularly large examples below.
Jack-up drilling rigs ,
China 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 , although some designs can go to depth. They are designed to move from place to place, and then anchor themselves by deploying their 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 .
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's 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.
Normally unmanned installations (NUI) These installations, sometimes called toadstools, are small platforms, consisting of little more than a
well bay,
helipad and emergency shelter. They are designed to be operated remotely under normal conditions, only to be visited occasionally for routine maintenance or
well work.
Conductor support systems These installations, also known as
satellite platforms, are small unmanned platforms consisting of little more than a
well bay and a small
process plant. They are designed to operate in conjunction with a static production platform which is connected to the platform by flow lines or by
umbilical cable, or both. ==Particularly large examples==