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c d

Figure 6.36 – Building of SPAR

6.3 Tension-Leg Platform Drilling

In the class of compliant structures the tension leg (or tethered) platforms are

important developments as alternatives to the fixed platforms that become

extremely expensive for water 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

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