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depth over 100m.
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 meters (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. Tension Leg
Platforms (TLPs) are floating facilities that are tied down to the
seabed by vertical steel tubes called tethers. This characteristic
makes the structure very rigid in the vertical direction and very
flexible in the horizontal plane. The vertical rigidity helps to tie in
wells for production, while, the horizontal compliance makes the
platform insensitive to the primary effect of waves. It has large
columns and Pontoons and a fairly deep draught.
The Tension Leg Platform (TLP) consists of a floating
structure (semi-submersible) with excess buoyancy which is
restrained by tautly moored vertical or inclined cables (tension
legs). The cable network limits platform motions and provides
restoring force to maintain position. The economical applicability
of the tension 1eg platform concept begins at locations where
conventional platforms cannot be used.
Although still in the preliminary stages, the tension-leg
platform (TLP) concept appears a viable means of affording
offshore oil and gas production while remaining relatively
economically insensitive to water depth. In many cases, the TLP
offers distinct cost advantages over fixed-platform installations,
yet it retains many of the platforms' desirable operational

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