A Tension-leg platform
or Extended Tension
Leg Platform (ETLP)
is a vertically moored floating
structure normally used for the offshore production of oil
, 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
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
), such that virtually all
vertical motion of the platform is eliminated. This allows the
platform to have the production wellheads
on deck (connected directly to the subsea wells by rigid risers),
instead of on the seafloor
. This allows a
simpler well completion
better control over the production from the oil
or gas reservoir
, and easier access for downhole
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.
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 .
- Marco Polo TLP
- Neptune TLP
- Kizomba B TLP
- Kizomba A TLP
- Ursa TLP. Its height
above surface is making a total height of .
- Allegheny TLP
- W. Seno A TLP
Use for wind turbines
Massachusetts Institute of
Technology and the National Renewable Energy
Laboratory designed the first 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, 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 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
architecture who was involved in the design, said, "You don't pay
anything to be buoyant
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
NREL researchers plan to install a half-scale prototype south of
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."
- Alexander's Gas & Oil Connections - Shell's
Ursa deep water TLP begins production
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