Tension-leg platform

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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). Also 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 wellheads on deck (connected directly to the subsea wells by rigid risers), instead of on the seafloor. This makes for a cheaper well completion and gives better control over the production from the oil or gas reservoir.

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.

The deepest (E)TLPs measured from the sea floor to the surface are^[1]:

4,674 ft (1,425 m) Magnolia ETLP. Its total height is some 5,000 feet (1,500 m).
4,300 ft (1,311 m) Marco Polo TLP
4,250 ft (1,295 m) Neptune TLP
3,863 ft (1,177 m) Kizomba B TLP
3,863 ft (1,177 m) Kizomba A TLP
3,800 ft (1,158 m) Ursa TLP. Its height above surface is 485 ft (148 m) making a total height of 4,285 ft (1,306 m)^[2].
3,350 ft (1,021 m) Allegheny TLP
3,300 ft (1,006 m) W. Seno A TLP