Buoyancy-supported struts for ocean platforms

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Exemplary embodiments of the invention will now be described, with reference to the accompanying drawings, in which: FIG.
1 is a pictorial view of a tension-leg-platform; FIG.
2 is a sectional view of a section of a tether of the TLP of FIG.
1: FIG.
3 is a close-up partly sectional pictorial view of part of the tether of FIG.
2; FIG.
4 is a sectional view of an alternative tether; FIG.
5 is a close-up of part of a tether similar to that of FIG.
4; but slightly modified; FIG.
6 is sectional view on line 6--6 of FIG.
5; and FIG.
7 is a diagrammatic view of the tether of FIG.
2 during deployment.
DESCRIPTION OF THE CONSTRUCTIONAL DETAILS The TLP of FIG.
1 comprises a platform 3 supported by four floats 4.
On the ocean-bed are four hold-fast anchors 5.
Tethers 6 (four at each corner, i.
e.
sixteen in all) extend from the platform 3 to the anchors 5.
The platform 3 is jacked down the tethers 6, against the action of the floats 4, to create a permanent state of tension in the tethers 6.
Such a construction provides a platform of great stability, which makes it a suitable construction for platforms that are to be left on the same site more or less permanently and an especially suitable construction where the water is very deep (of the order of 1500 m).
SOLID TETHER Part of one of the tethers 6 is shown in FIG.
2.
A section 7 of the tether 6 is made of high-strength steel, and the section is shaped with a bulbous, female threaded, upper end 8, and a male threaded lower end 9.
The remaining major portion of the length of the section is comparatively slender.
(Typically its diameter is 300 mm and it will support a nominal tensile force of around 3000 tonnes).
The section 7 is provided with two donut-shaped buoyancy canisters 43, 45.
The canisters 43, 45 are nominally identical, and each is closed at the top and open at the bottom.
A tube 46 passes up the length of the canister and has a port 47 near the bottom of the canister.
The tube 46 acts as a conduit to convey air that enters the port 47 upwards and into the next canister above