Tension-leg platform
Encyclopedia
A Tension-leg platform or Extended Tension Leg Platform (ETLP) is a vertically moored floating structure normally used for the offshore production of oil or gas
, and is particularly suited for water depths greater than 300 metres (about 1000 ft) and less than 1500 meters (about 4900 ft). Use of tension-leg platforms has also been proposed for wind turbines.
The platform is permanently moored by means of tethers or tendons grouped at each of the structure's corners. A group of tethers is called a tension leg. A feature of the design of the tethers is that they have relatively high axial
stiffness
(low elasticity
), such that virtually all vertical motion of the platform is eliminated. This allows the platform to have the production wellhead
s on deck (connected directly to the subsea wells by rigid risers), instead of on the seafloor. This allows a simpler well completion and gives better control over the production from the oil
or gas
reservoir
, and easier access for downhole intervention operations.
TLP's have been in use since the early 1980s. The first Tension Leg Platform was built for Conoco's Hutton field
in the North Sea in the early 1980s. The hull was built in the dry-dock at Highland Fabricator's Nigg yard in the north of Scotland, with the deck section built nearby at McDermott's yard at Ardersier. The two parts were mated in the Moray Firth in 1984.
Larger TLP's will normally have a full drilling rig on the platform with which to drill and intervene on the wells. The smaller TLPs may have a workover rig, or in a few cases no production wellheads located on the platform at all.
The deepest (E)TLPs measured from the sea floor to the surface are: Magnolia ETLP. Its total height is some 5000 feet (1,524 m). Marco Polo TLP Neptune TLP Kizomba A
TLP Ursa TLP
. Its height above surface is 485 ft (147.8 m) making a total height of 4285 ft (1,306.1 m). Allegheny TLP W. Seno A TLP
and the National Renewable Energy Laboratory
first explored the concept of TLPs for offshore wind turbines in September 2006. Earlier offshore wind turbines cost more to produce, stood on towers dug deep into the ocean floor, were only possible in depths of at most 50 feet (15.2 m), and generated 1.5 megawatts for onshore units and 3.5 megawatts for conventional offshore setups. In contrast, TLP installation was calculated to cost a third as much. TLPs float, and researchers estimate they can operate in depths between 100 and 650 feet (198.1 m) and farther away from land, and they can generate 5.0 megawatts.
TLPs cost less to make and install because they are assembled onshore, are towed to their destination, and can be moved. Paul Sclavounos, an MIT professor of mechanical engineering
and naval architecture who was involved in the design, said, "You don't pay anything to be buoyant."
Computer simulation
s project that in a hurricane TLPs would shift three to six feet and the turbine blades would cycle above wave peaks. MIT and NREL researchers say dampers could be used to reduce motion in the event of a natural disaster
.
MIT and NREL researchers plan to install a half-scale prototype south of Cape Cod
. Sclavounos said, "We'd have a little unit sitting out there to show that this thing can float and behave the way we're saying it will."
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...
, and is particularly suited for water depths greater than 300 metres (about 1000 ft) and less than 1500 meters (about 4900 ft). Use of tension-leg platforms has also been proposed for wind turbines.
The platform is permanently moored by means of tethers or tendons grouped at each of the structure's corners. A group of tethers is called a tension leg. A feature of the design of the tethers is that they have relatively high axial
Axial
Axial may mean:* Along the same line as an axis of rotation in geometry* A type of modal frame in music* One of several anatomical directions in an animal body* Axial age, the period from 800 to 200 BC in China, India and the western world...
stiffness
Stiffness
Stiffness is the resistance of an elastic body to deformation by an applied force along a given degree of freedom when a set of loading points and boundary conditions are prescribed on the elastic body.-Calculations:...
(low elasticity
Elasticity (physics)
In physics, elasticity is the physical property of a material that returns to its original shape after the stress that made it deform or distort is removed. The relative amount of deformation is called the strain....
), such that virtually all vertical motion of the platform is eliminated. This allows the platform to have the production wellhead
Wellhead
A wellhead is a general term used to describe the component at the surface of an oil or gas well that provides the structural and pressure-containing interface for the drilling and production equipment....
s on deck (connected directly to the subsea wells by rigid risers), instead of on the seafloor. This allows a simpler well completion and gives better control over the production from the oil
Petroleum
Petroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...
or gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
reservoir
Oil reservoir
A petroleum reservoir, or oil and gas reservoir, is a subsurface pool of hydrocarbons contained in porous or fractured rock formations. The naturally occurring hydrocarbons, such as crude oil or natural gas, are trapped by overlying rock formations with lower permeability...
, and easier access for downhole intervention operations.
TLP's have been in use since the early 1980s. The first Tension Leg Platform was built for Conoco's Hutton field
Hutton oilfield
The Hutton oil field, located on the UK continental shelf, was the location for the first ever production Tension Leg Platform .-History:...
in the North Sea in the early 1980s. The hull was built in the dry-dock at Highland Fabricator's Nigg yard in the north of Scotland, with the deck section built nearby at McDermott's yard at Ardersier. The two parts were mated in the Moray Firth in 1984.
Larger TLP's will normally have a full drilling rig on the platform with which to drill and intervene on the wells. The smaller TLPs may have a workover rig, or in a few cases no production wellheads located on the platform at all.
The deepest (E)TLPs measured from the sea floor to the surface are: Magnolia ETLP. Its total height is some 5000 feet (1,524 m). Marco Polo TLP Neptune TLP Kizomba A
Kizomba deepwater project
The Kizomba deepwater project, is an oil drilling project owned and operated by ExxonMobil, situated off the coast of northern Angola. It is named after the Kizomba Angolan dance.-Kizomba A:...
TLP Ursa TLP
Ursa tension leg platform
The Ursa tension leg platform is an oil platform with a tension leg structure located at about southeast of New Orleans in the Gulf of Mexico. It is operated by Shell Oil Company. It has a total height from the seabed to its top of ....
. Its height above surface is 485 ft (147.8 m) making a total height of 4285 ft (1,306.1 m). Allegheny TLP W. Seno A TLP
Use for wind turbines
The Massachusetts Institute of TechnologyMassachusetts Institute of Technology
The Massachusetts Institute of Technology is a private research university located in Cambridge, Massachusetts. MIT has five schools and one college, containing a total of 32 academic departments, with a strong emphasis on scientific and technological education and research.Founded in 1861 in...
and the National Renewable Energy Laboratory
National Renewable Energy Laboratory
The National Renewable Energy Laboratory , located in Golden, Colorado, is the United States' primary laboratory for renewable energy and energy efficiency research and development. The National Renewable Energy Laboratory is a government-owned, contractor-operated facility; it is funded through...
first explored the concept of TLPs for offshore wind turbines in September 2006. Earlier offshore wind turbines cost more to produce, stood on towers dug deep into the ocean floor, were only possible in depths of at most 50 feet (15.2 m), and generated 1.5 megawatts for onshore units and 3.5 megawatts for conventional offshore setups. In contrast, TLP installation was calculated to cost a third as much. TLPs float, and researchers estimate they can operate in depths between 100 and 650 feet (198.1 m) and farther away from land, and they can generate 5.0 megawatts.
TLPs cost less to make and install because they are assembled onshore, are towed to their destination, and can be moved. Paul Sclavounos, an MIT professor of mechanical engineering
Mechanical engineering
Mechanical engineering is a discipline of engineering that applies the principles of physics and materials science for analysis, design, manufacturing, and maintenance of mechanical systems. It is the branch of engineering that involves the production and usage of heat and mechanical power for the...
and naval architecture who was involved in the design, said, "You don't pay anything to be buoyant."
Computer simulation
Computer simulation
A computer simulation, a computer model, or a computational model is a computer program, or network of computers, that attempts to simulate an abstract model of a particular system...
s project that in a hurricane TLPs would shift three to six feet and the turbine blades would cycle above wave peaks. MIT and NREL researchers say dampers could be used to reduce motion in the event of a natural disaster
Natural disaster
A natural disaster is the effect of a natural hazard . It leads to financial, environmental or human losses...
.
MIT and NREL researchers plan to install a half-scale prototype south of Cape Cod
Cape Cod
Cape Cod, often referred to locally as simply the Cape, is a cape in the easternmost portion of the state of Massachusetts, in the Northeastern United States...
. Sclavounos said, "We'd have a little unit sitting out there to show that this thing can float and behave the way we're saying it will."