Marine Tether Anchoring Device

The invention relates to marine tether anchoring devices.

Various methods are known for fixing tethers of drilling platforms, and the like, in position and it is an object of the present invention to provide an improved marine tether anchoring device which can be released and re-established if necessary and as required.

According to the invention, there is provided a marine tether anchoring device comprising an anchor body, a segmented spring collet surrounding said body movable axially relative thereto and arranged to be expanded at its upper end by wedging means attached to said anchor body as said anchor body moves downwardly relative to said collet so that the expanded upper end of the collet lockingly engages the wall of an anchoring chamber and the lower end of a tether line attached to said wedging means cannot escape from said anchoring chamber, and controllable actuator means in said body operable to move the collet downwardly relative to said body so that the upper end of the collet is released from the wedging means and can retract away from the wall of the anchoring chamber to provide an unlocked condition of said device.

The device is self-locking under gravity since the weight of the anchor body and tether line will automatically pull the wedging means downwards and expand the collet. But when the tether line is under tension the collet is locked in its expanded state since it is only free to expand when the collet is spaced from the locking surface at the mouth of the anchoring chamber.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a cut-away view of a marine tether anchoring device;

FIG. 2 is a part-sectional elevation of the device in an operational or locked condition; and

FIG. 3 is a sectional elevation of one half of the device in a release condition and with the tether line at an angle to the vertical.

In the drawings an anchor body 1 extends upwards to a locking ring 20 to which it is secured by means of studs 14. The ring 20 has on its exterior an upper frustoconical surface 21 of larger diameter and a lower frustoconical surface 22 of smaller diameter interconnected by a surface 23. The interior surface of ring 20 is connected by a so-called flexijoint assembly 37 of elastomer and metal to a flared lower end 24a of a tether line 24. The end 24a has a cup-shaped inner surface that is supported by a spherical seat 4 bolted to the body 1. The purpose of the flexijoint assembly which is resiliently deformable is to permit limited angular movement of the tether line 24 relative to the anchor body 1 as shown in FIG. 3. The body 1 has a series of radial fins 26 that support a cam ring 9 and an abrasion ring 10. A spring collet 2 surrounds the body 1 and extends from a base 27 at its lower end to flanged fingers 28 at its upper end. The fins 26 protrude through slots defined between adjacent fingers 28. The hooked upper ends of the fingers 28 either engage the upper surface 21 or define a radially expanded state of the fingers 28 or engage the lower surface 22 to define a collapsed state thereof. The top ends of the fingers 28 are also formed with bearing surfaces 29.

The tether line 24 is formed with a central passage that communicates with an axial bore 36 through the seat 4. Fluid under pressure pumped down the central passage presses on the top face of a piston or so-called "thrust column" 11 that is supported in the body 1 in upper and lower bearings 8, 7 and terminates at its lower end in a flange 12.

The structure that is secured to the seabed includes a mooring sleeve 30 having an abutment ring 31 at its upper end. A so-called template insert 38 that defines an anchoring chamber is a loose fit within the mooring sleeve 30 and has at its upper end a locking ring 3 that seats against the abutment ring 31 so that the template insert cannot be withdrawn from the mooring sleeve. According the template insert 38 can move up and down within the mooring sleeve 30 between its operational (FIG. 2) and release (FIG. 3) states but is permanently retained. The mooring sleeve 30 is provided with internal guide ribs 50 that locate the template insert, and similarly the template insert 38 is provided with guide ribs 51 that locate the assembly of anchor body and collet. A buffer 39 at the lower end of the collet 2 locates against a support 40 on the floor of the template insert to arrest downward travel of the anchor body and collet.

In FIG. 3 the anchoring device is shown in an unlocked condition with the template insert 38 at its release position within the mooring sleeve 30 so that locking ring 3 is spaced from abutment ring 31. The tether line is lowered so that the assembly of anchor body 1 and collet 2 enters the template insert 38. The collet is maintained in its collapsed state by the cam ring 9 that fits over an external shoulder 17 on the collet. The assembly moves downwards until the buffer 39 touches the support 40 after which no further downward movement of the collet 2 takes place. But the anchor body 1 continues to move downwardly unde the weight of the tether line 24, disengaging ring 9 from shoulder 17 to permit partial expansion of the collet which is then fully expanded as fingers 28 travel over surface 23 of the locking ring until their hooked ends locate onto its upper surface 21. As this happens, the base 27 of the collet thrusts the column or piston 11 from the position shown in FIG. 3 upwardly to the position shown in FIG. 2. By this means the device reaches its locked state. As tension is applied to the tether line 24 the body 1 rises taking the collet 2 upwards with it until the surface 21 engages the corresponding surface of the template insert. Thereafter the end 23 of the tether line cannot escape from the template insert and the ends of the fingers 28 are held between the locking ring 20 and the upper end of the template insert so that they cannot disengage. Continued raising of the tether line brings the locking ring 3 into contact with the abutment ring 31 as shown in FIG. 2.

The procedure for unlocking the device is as follows. Tension is removed from the tether line 24 to allow the template insert 38 to move downwards, normally under its own weight from the position shown in FIG. 2 an the body 1 and collet 2 to move downwardly into the template insert. Then fluid is pumped down the interior of the line 24 and flows through the central hole 36 in spherical seat 4 and drives the piston or thrust column 11 downwardly. This forces the collet 2 downwards disengaging fingers 28 from the upper surface 21 of the locking ring. Continued downward movement of the collet causes the ramp 16 to travel past cam ring 9 which finally engages the shoulder 17 to retain the collet in its fully collapsed state in which the fingers 28 engage lower surface 22 of the locking ring 20. The collet 2 will then pass out of the template insert 38 when tension is again applied to the tether line 24.

Thus a relatively simple releasable tether connection is provided. The planar mating surfaces 21 between the locking ring 20 and the fingers 28 in the locked condition is preferably at approximately 45.degree. to the axis of the body 1.