GB2160278A

GB2160278A - Adjusting axial length of a marine platform tether line

Info

Publication number: GB2160278A
Application number: GB08414990A
Authority: GB
Inventors: Brian Robert Brockway
Original assignee: Vickers PLC
Current assignee: Vinters Ltd
Priority date: 1984-06-12
Filing date: 1984-06-12
Publication date: 1985-12-18
Also published as: GB8414990D0

Abstract

Apparatus for adjusting the axial length of a tether line (1) of a marine platform at a location exposed to tension load in said line comprises upper and lower clamping members (5, 7) engageable with the line (1) to resist the tension thereon and each unidirectionally movable along the said line (1) against the load direction when the load is taken by the other member. Jacking means (3) operatively connected to the upper member (5) moves it upwardly to transfer the load to the upper clamping member (5), move the line (1) upwards and permit the lower clamping member (7) to move downwards, and on release of said jacking means permits the upper clamping member (5) to move downwards along the line (1). <IMAGE>

Description

SPECIFICATION Apparatus for adjusting the axial length of a marine platform tether line The present invention relates to apparatus for adjusting the length of a tether line of a marine platform.

The use of marine tethers to connect a marine platform to an anchoring chamber is described in Norwegian Patent Specification No. 812435 (Conoco). Methods for releaseably attaching the lower end of the tether line to a seabed anchoring chamber are described in our British Patent No. B 2068320 and British Patent Application No. 8132218.

The present invention enables length adjustment of the tension legs to be made in order to take up tolerances in foundation position on the seabed relative to the surface.

Broadly stated the invention comprises apparatus for adjusting the axial length of a tether line of a marine platform at a location exposed to tension load in said line comprising upper and lower clamping members engageable with the line to resist the tension thereon and each movable along the said line against the load direction when the load is taken by the other member and jacking means operatively connected to the upper member to move it upwardly to transfer the load to the upper clamping member, move the line upwards and permit the lower clamping member to move downwards, and on release of said jacking means to permit the upper clamping member to move downwards along the line.

An embodiment of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a vertical section through the region where a tether line for a marine platform of the tension leg type is supported in the platform; and Figure 2 is a plan view.

In the drawing, a tether line 1 extends between the platform 10 and a foundation position on the seabed. The lower end of the tether line 1 may be releaseably latched into a so-called foundation template on the seabed as described in our Patent specification No.

GB8132218. Its upper end passes through an aperture 11 in the lower face of the platform and thence through an annular elastomeric spherical bearing or so-called "flexjoint assembly" 2. The assembly 2 has an annular upper flange 1 2 and an annular lower flange 1 4 of somewhat larger diameter which are interconnected by a elastomeric member 1 3 which is in compression and serves to transmit the tension load in the line 1 as a downward and outward compressive load on the platform 10. A compliant mounting 15 is interposed between the lower flexjoint flange 14 and the metal of the platform 10. The flexjoint not only serves to take up the tension load but also permits limited angular travel of the shaft 1 from the vertical axis about a centre 16.

The shaft 1 has an upper region 1 a that passes through the flexjoint 2 and is formed with annular grooves 1 6. A lower collet 7 which is permanently in place fits between the region 1 a of the shaft and the upper flange 1 2. It has a downwardly convergent frustoconical outer surface that wedges against a complementary inner surface of the flange 1 2 and annular grooves 1 7 on its inner surface that are complementary with the grooves 1 6 in region 1 a of the shaft. Although the collet 7 is in principle capable of clamping on a smooth shaft 1, it is preferably formed with shallow V-form grooves as shown in Fig. 1 to enhance friction.If necessary the lower flanks of the grooves 1 6 on the shaft and the upper flanks of the grooves 1 7 of the collet could be downwardly directed so that the interengaging grooves were of buttress form so as to exert a rachet action whereby the collet 7 could "walk" downwards along the region 1 a but could not move in the other direction.

To shorten the shaft 1 a shaft tensioning device generally denoted by the reference numeral A is lowered over the free end of the shaft 1. It comprises upper and lower annular members 1 8, 19 interconnected by vertical guide bars 4 and jacks 3. The lower annular member 19 mates with an upturned spigot 20 on the upper flange 1 2 on which it sits so that the tensioning device is held in position during jacking. The upper member 18 cooperates with an upper collet 5 that is similar to the collet 7 and likewise exhibits a rachet-like walking action along the region 1a.

In operation hydraulic pressure is applied to the jacks 3 to transfer the tension load from the lower collet 7 to the upper collet 5, raising the upper part of the assembly on the guide bar 4. The collet 5 locks into the annular grooves 1 6 in the shaft so that upward travel of the member 1 8 is transmitted to the shaft 1. The lower collet 7 is free from tension load and walks a corresponding distance down the shaft 1. When the jacks 3 have completed their stroke the hydraulic pressure can be released, returning the load to the collet 7 which retains its new position, and permitting the member 1 8 to move downwardly. The cycle can be repeated until the loaded portion of shaft 1 has been shortened the required distance, after which the device A is removed.

To detension the shaft 1, the device A is replaced and the jacks 3 are activated. The collets 7 are lifted out of engagement with the shaft 1 by means of air cylinders 21 connected thereto and the pressure in the jacks 3 is relieved to permit the shaft 1 to move downwardly the required distance. Then the collet 7 is re-engaged with the shaft 1 and the device A is removed. Air cylinders 22 also control the upper collet 5 if shaft detensioning requires more than one "stroke" of the jacks 3.

Thus the length adjustment apparatus of the invention permits axial length adjustment of a shaft or line under tension. After adjustment to the correct length the tensioning apparatus is removed, leaving the shaft or line locked at the correct length. The shaft can be permitted to move from the vertical axis by means of an elastomeric spherical bearing whose outer flange sits in a support bracket or the like on the platform.

Claims

1. Apparatus for adjusting the axial length of a tether line of a marine platform at a location exposed to tension load in said line comprising upper and lower clamping members engageable with the line to resist the tension thereon and each unidirectionally movable along the said line against the load direction when the load is taken by the other member and jacking means operatively connected to the upper member to move it upwardly to transfer the load to the upper clamping member, move the line upwards and permit the lower clamping member to move downwards, and on release of said jacking means to permit the upper clamping member to move downwards along the line.

2. Apparatus according to Claim 1 wherein the upper and lower clamping members are collets that have wedge-like outer surfaces and are held by annular supports having complementary inner surfaces that urge the collets against the line when the collets are under axial load.

3. Apparatus according to Claim 1 or 2 wherein the line is formed with annular grooves on its outer surface that cooperate with complementary grooves in the collets to exert a rachet-like action.

4. Apparatus according to claim 3 wherein the load on the lower member is transmitted to the platform via a spherical bearing.

5. Apparatus according to Claim 4 wherein the spherical bearing comprises an annular upper member that supports the collet connected to an annular member that is supported on the platform through an annular elastomeric member.

6. Apparatus according to Claim 5 wherein the jacking means comprises upper and lower thrust rings, the upper thrust ring locating the upper collet and the lower thrust ring seating on the upper member of the spherical bearing.

7. Apparatus according to Claim 6 wherein the jacking means comprises an hydraulic jack or jacks.

8. Apparatus for adjusting the axial length of a tether line of a marine platform substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.