AU3538789A

AU3538789A - Rotating lug anchor connector

Info

Publication number: AU3538789A
Application number: AU35387/89A
Authority: AU
Inventors: George Walter Peppel
Original assignee: Lockheed Corp
Current assignee: Shell USA Inc
Priority date: 1988-05-20
Filing date: 1989-04-17
Publication date: 1989-12-12
Anticipated expiration: 2009-04-17
Also published as: DE68914662T2; JPH0686235B2; EP0417123A4; NO302350B1; NO905005D0; EP0417123B1; AU638209B2; WO1989011415A1; EP0417123A1; NO905005L; JPH03504221A; DE68914662D1

Description

ROTATING LUG ANCHOR CONNECTOR

TECHNICAL FIELD

This invention relates to offshore oil production, and particularly to a connector for the tether of a tension leg platform (TLP).

BACKGROUND OF THE INVENTION

Offshore petroleum drilling and production has become a significant industry worldwide. Many techniques have been developed to achieve what, at first, seemed impossible; the drilling and production of petroleum reserves from beneath the sea floor to a platform on the surface.

One significant design which has found great success in offshore petroleum production is the tension leg platform (TLP). In this design, a platform is literally secured to the sea floor through a number of tethers which extend vertically from the sea floor to the platform floating on the surface. The tethers are kept in tension by the buoyancy of the platform. Tidal motion and wave action are compensated for by lateral movement of the platform and tethers. Vertical movements, normally associated with heave, pitch and roll motions of the sea, are eliminated by the combined buoyancy of the platform and tethers. Typically, latching structure is permanently mounted on the sea floor for receiving the tethers with some mechanism to accommodate pivotal movement of the tethers. The platform is then put in place with the tethers latched to the sea floor structure. The platform can remain in place for many years during drilling and production, but it is anticipated that the platform and tethers will eventually be unlatched irom the sea floor mounted latching structure for reuse elsewhere, or scrapping. Several designs have been proposed for latching mechanisms to secure tethers to the sea floor receptacle mechanisms. One is disclosed in U.S. Patent No. 4,498,814 issued February 12, 1985 and assigned to Vickers. The design includes a collet configuration with shoulder blocks which are deployed into contact with a mooring sleeve at the sea floor. A flexible joint permits angular or torsional movement of the tether. However, this design requires hydraulic actuation to unlatch the tether. Hydraulic actuation of necessity requires a pressurized hydraulic line to extend from the connector at the sea floor to the surface where the hydraulic pump and control circuitry is situated. The hydraulic line is typically routed through the hollow interior of the tether. If portions of the tether interior are designed to be dry to increase buoyancy, it requires substantial effort to seal the line at the bottom bulkhead perforation at the lower end of the tether. If inspection tools are run through the tether interior, they can foul and damage the hydraulic line going to the connector. This reliance on hydraulic operation creates a question as to the reliability of releasing the connector over the long service life demanded of this type of system. Any seals employed can easily deteriorate and fail over a span which could be as long as 30 years. As the working components of the connector are internal within the latch body and hidden from exterior view, the analysis or identification of any mechanical or hydraulic problems by visual inspection become virtually impossible. Another design for a TLP connector is disclosed in U.S. Patent No. 4,439,055, issued March 27, 1984 and assigned on its face to Vetco Offshore, Inc. The design of this patent relies upon a series of latch dogs with attached shoulder blocks. The latch dogs are mounted at the lower end of the tether and are held in the retracted position as the lower end of the tether is stabbed into a cavity in the sea floor mounted structure. Each of the dogs is pivoted to the tether. As the tether is stabbed into the sea floor structure, a running tool releases the dogs to pivot against a receptacle load _ng to secure the tether. J release the tether, a release tool is run down the bore of the tether to retract the dogs. The design is incompatible with a tether having a dry interior, since the release tool must move from the surface to the connector within the tether. Again, visual inspection and verification of the latch is difficult.

To add further complications to the connector design, the industry requires a secondary unlatch technique to exist, if the primary unlatching technique fails.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, an anchor connector is provided for releasably securing the tether of a tension leg platform to the sea floor. The anchor includes a first member secured to the sea floor and a second member secured to the lower end of the tether. A ring is provided which has a central vertical axis. The ring has at least one lug, preferably at least hree, extending horizontally therefrom along a radius centered on the vertical axis. A cylindrical receptacle is provided which also has a central vertical axis. The receptacle includes a primary guide, a vertical slot, a secondary guide and a notch. The ring is mounted to one of the members, while the cylindrical receptacle is mounted to the other of the members. The ring and receptacle are mounted on the members to permit relative rotation between the ring and receptacle about the central axis of the tendon.

In operation, the tether is lowered vertically until the lug contacts the primary guide. With subsequent lowering of the tether, the primary guide rotates the lug into and along the vertical slot and into contact with the secondary engagement guide. With further downward movement of the tether, the secondary engagement guide will rotate the lug approximately halfway to the notch. Subsequent upward movement of the tether causes the lug to rotate the remaining distance to the notch to engage the notch to secure the tether to the sea floor. During this operation, the tether is only moved vertically, and is not required to rotate or pivot about its central axis. The movement necessary to guide the lug into the notch is completely accommodated by the rotation of the ring and lug. To release the tether from the sea floor, the tether is again lowered, bringing the lug into engagement with the release guide. With further downward movement of the tether, the release guide rotates the lug approximately halfway to a second vertical slot. Upward movement of the tether rotates the lug the remaining distance and pulls the lug through the second vertical slot and releases the first member from the second member.

In accordance with another aspect of the present invention, the ring is mounted on the second member for rotation about a vertical axis, and has one or more lugs, preferably at least three lugs, spaced uniformly about its outer circumference. The elastomeric element is bonded between the ring and the second member to permit pivotal motion of the tether about a center of rotation lying on the vertical central axis of the tether. The cylindrical receptacle is rigidly secured to the first member and has sufficient primary guides, vertical slots and secondary guides to allow all the lugs to be engaged in notches to secure the tether to the sea floor.

In accordance with another aspect of the present invention, the lugs can be mounted on the first member at the sea floor. The guides, slots and retention notches can be mounted on the second member at the lower end of the tether. In this configuration, either, or both, of the members can mount the rotating structure permitting the lugs to engage and disengage the retention notches.

If desired, multiple rows of lugs can be provided, which interact with multiple rows of notches. Further, the sequence of mounting the elastomeric elements, lugs and notches between the tether and sea floor can be altered as required to suit a specific application. In the event that fouling occurs which prohibits free rotation of the ring and lugs in relation to its adjoining members, the tether can be released by moving it vertically and permitting the entire tether to rotate and release the tether and latch assembly from the sea floor mounted receptacle assembly. This provides a reliable secondary release mechanism to satisfy this requirement of the industry.

o

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which:

EIGURE 1 is a perspective view of a rotating lug anchor connector forming a first embodiment of the present invention;

FIGURE 2 is a vertical cross sectional view of the latch assembly used in the connector of FIGURE 1;

FIGURE 3 is a top plan view of the receptacle assembly of FIGURE 1; and

FIGURE 4 is a flattened view of the interior configuration of the receptacle assembly.

DETAILED DESCRIPTION with reference now to FIGURE 1, a rotating lug anchor connector 10 for anchoring a tether 12 of a tension leg platform (TLP) 14 to the sea floor is illustrated. The connector includes an anchor latch assembly 18 which forms the lower end of the tether 12 and a receptacle assembly 20 which is permanently mounted on the sea floor 16.

With reference now to FIGURE 2, further details of the latch assembly 18 will be described. The latch assembly 18 can be used with any tether design permitting vertical upward and downward movement of the assembly. However, the tether 12 illustrated has a tubular shape with a hollow dry interior which is designed to result in near neutral buoyancy for the tether when installed. For example, the tether may have a main diameter along substantially its whole length of about 40 inches. The tether preferably necks down in portion 22 on which the assembly is secured to reduce the stresses in the tether as it pivots relative to the connector 10 and sea floor 16 (see FIGURE 1). For example, the portion 22 can neck down to a final diameter of 27 inches from the diameter specified previously for the main portion of the tether. However, such a neck down or alternate design is generally determined by the overall system design and not by the operation of the present invention.

Secured to the portion 22 is a center shaft 24. The lower end of the shaft 24 ends in a hemispherical concave surface 26 which is formed about a radius R centered at point 28 on the vertical axis of symmetry 30 of the tether 12 and latch assembly 18. The surface 26 is formed in part by a flange 32 which defines a semi- spherical surface 34 on its outer surface having a radius of curvature R₂ and centered on point 28 as well. An elastome_^ic assembly 36 is bonded o_A. its inner surface 38 to the surface 34 of the flange 32. The elastomeric assembly preferably is an assembly of a number of individual elastomeric elements 40 of elastomeric material formed into semi-spherical sections with radii centered on point 28 bonded together to form the assembly 36. If desired, rigid reinforcing elements 42, each being a semi-spherical section having its radius centered on point 28, can be bonded between the elements to reinforce the elastomeric assembly 36. A collar 44 having a semi-spherical inner surface 46 with its radius centered on point 28 is bonded to the outer surface 48 of the elastomeric assembly 36.

A rotating ring 50 is fit around the flange 32 and collar 44, as shown. The ring 50 includes a centering ball 52 with a convex he i-spherical surface 54 of radius approximately R^ centered on point 28 which may bear against the surface 26 of center shaft 24 to guide pivotal motion between the elements about point 28. The collar 44 contacts a shoulder 56 on the ring 50 to hold the ring on the collar 44, but permits the ring 50 to rotate about the axis 30 relative to the flange 32, and tether 12. Three lugs 58 extend radially outward from the outer surface 60 of the ring on radii extending perpendicular the axis 30. As best seen in FIGURE 1, the lugs may have a diamond shape, including upper slanted surfaces 62 and 64 and lower slanted surfaces 66 and 68. The lugs are oriented at equal spacing from each other about the outer surface 60 which, for three lugs, translates to 120 degrees separation between a lug and the adjacent lugs.

With reference now to FIGURES 1, 3 and 4, the details of the receptacle of assembly 20 will be described. The assembly 20 includes a cylinder 70 which is rigidly, and permanently mounted to the sea floor 16. The mounting can ^e made by any acceptable 1 hnique. On the interior surface 72 of cylinder 70 are mounted three members 74, 76 and 78. Each member defines slanted primary guide surfaces 80 and 82, side walls 84 and 86, lower slanted surfaces 88 and 90, and a notch 92 in its interior. Spaced underneath the members 74, 76 and 78 is an annular ring 94 which define a series of secondary guide surfaces 96 which alternate between secondary engaging guide surfaces 98 and secondary release guide surfaces 100. Associated with each of the guide surfaces 96 is a facing vertical surface 102 for stopping the rotation of lugs 58 at the proper spacing.

With particular reference to FIGURE 4, the operation of the connector can be described. The tether, and anchor latch assembly 18, are lowered proximate with the receptacle assembly 20. At some point, the lugs 58 will enter the opening of cylinder 70 and move into close proximity to•the primary guide surfaces 80 and 82. It can be seen that the guide surface 80 of one member and the guide surface 82 of the adjacent guide member cooperate to rotate ring 50 and direct the lugs contacting one of the surfaces into vertical slots 104 defined between the side walls 84 and 86 of the adjacent members. Each of the three or more lugs 58 will contact one of the surfaces 80 or 82 at about the same time. The lugs then slide downwardly along the surfaces and the ring 50 rotates about the axis 30 to align the lugs with the vertical slots 104. To accommodate the motion, the shoulder 56 will rotate against the collar 44 so that no rotation of the tether is necessary about its vertical axis 30 to engage the connector.

As the tether continues its downward motion, the lower slanted surface 68 of each lug will come into engagement with the secondary engaging guide surface 98 at the bottom ot each vertical slot 104. Cuxitinued downward movement will cause the ring 50 to rotate about the axis 30 relative to the tether until the lugs impact against stops 102. At this point, the tether can not move any further downward. At the surface, the stoppage of the downward movement of the tether provides the signal to raise the tether to perform the final steps of engaging the latch assembly within the receptacle assembly. As the tether is raised, the upper slanted surface 64 of each lug will engage the surface 90 of a member and rotate the ring about axis 30 sufficiently to align each lug with a notch 92 to secure each lug within a notch to complete the connection. The tension in the tether 12 caused by the buoyancy of the platform will hold the lugs within notches 92.

Once connected, wave and tidal action acting on the platform can be accommodated by pivotal motion of the tether 12 about the pivot point 28 permitted by the deflection of the elastomeric assembly 36. In a typical application, multiple tethers and connectors will be employed for each platform.

When it becomes necessary to disconnect tether 12 from the assembly 20, whether for inspection, maintenance or removal of the platform from the site, the tether 12 is moved downwardly so that each lug contacts a secondary release guide surface 100. Further downward movement of the tether will cause the lugs to slide down surfaces 100, with the ring 50 again rotating relative to the tether about the axis 30 until the lug motion is halted by the stops 102 facing the surfaces 100. The cessation of movement is again sensed at the surface, and provides the signal to begin raising the tether.

As the tether is raised, the surfaces 64 on the lug 58 contact the surfaces 88 on each of the members 74, 76 and 78 to rotate the ring 50 and allow the lugs to move up through the vertical slots 104 to discon_^^ct the anchor latch assembly 18 from the receptacle assembly 20.

If a remote operated submersible vehicle (R.O.V.) is available, the tether and receptacle assembly can be marked with various axial and circumferential markings viewable by the R.O.V. to monitor the latching and unlatching operation. For example, proper insertion of the tether into the receptacle assembly can be assisted and confirmed by the R.O.V. by providing axial marks on the tether and receptacle assembly which move into alignment upon proper insertion of the tether. The R.O.V. can also be used to assist and confirm relative axial motion of the ring 50 and receptacle assembly to various axial function points by alignment of circumferential marks on the tether and receptacle assembly.

One significant advantage of the present invention over prior designs is the ability to provide a reliable secondary release mechanism. As noted previously, in normal operation, the tether 12 need never be rotated about its vertical axis in latching or unlatching the tether. The rotation necessary to engage lugs 58 with the notches 92, and subsequently disengage those lugs, is accommodated by rotation of the ring 50 relative to the tether. However, should normal operation of the connector be impossible, due to corrosion, damage or other factor, the lugs can be moved relative to members 74, 76 and 78 by making provisions to rotate the tether 12 about its vertical axis 30 and disengage the lugs from the notches.

While three lugs 58 and cooperating notches 92 are illustrated in the FIGURES and described above, it will be clear that the advantages of the present invention can be realized by using a greater or lesser number of cooperating lugs and notches. While the lugs have been shown to have a diamond shape, any other shape which cooperates with the guide surfaces and notches could be used. For example, the lugs can have a round configuration. It is preferred to shape the notches and the notch engaging surfaces of the lugs similarly to distribute the forces transferred between the lugs and notches as uniformly as possible. Further, the lugs have been illustrated as being elastically mounted to the lower end of the tether. However, the lugs can be mounted permanently on the sea floor either within cylinder 70, or mounted on other structure, and members 74, 76 and 78 elastically secured at the lower end of the tether to cooperate with the lugs in substantially the same manner. In such a configuration, either the lugs, or members 74, 76, and 78 can be mounted for rotation about axis 30. In fact, the lugs and members 74, 76 and 78 can all be mounted for rotation about axis 30. If desirable, the elastomeric assembly can form part of the permanent sea floor installation and connected to the associated members or lugs as desired.

In the figures, and the description above, only one row of lugs and notches have been used. However, it could be desirable to use multiple rows of lugs and notches to provide greater force transferring capability. In such a design, the cylinder 70 could be extended in height to mount multiple rows of members 74, 76 and 78, stacked in vertical rows, but defining common vertical slots. The ring could mount multiple rows of lugs, with each lug in one row stacked vertically above or below a corresponding lug in the adjacent row. The members 74, 76 and 78 would be stacked sufficiently apart vertically so that a lug in a given row could rotate between surface 90 of a given member and the surface 82 of the member below it to engage the notch in the given member. In this design, the lowest row of lugs would contact the uppermost members 74, 76 and 78 to orient the ring and lugs into alignment with the vertical slots. As the lugs move downward, the lowest row of lugs would engage surfaces 98 to rotate the lugs in all rows into alignment with the notches designed to receive them. Upward vertical motion of the tether would then engage all lugs simultaneously with their associated notches. In disengaging the lugs, the surface 68 of each lug and surface 80 of the member just below it could engage simultaneously with the lowest row of lugs engaging surface 100 to align the lugs for removal.

While one embodiment of the present invention has been illustrated in the accompanying drawings, and described in the foregoing Detailed Description, it will be understood that the invention is not limited to the embodiment disclosed, but is capable of numerous rearrangements, modifications and substitutions of parts and elements without departing from the spirit of the invention.

Claims

1. A connector for releasably securing the tether of a tension leg platform to the sea floor, comprising: a first member secured to the sea floor; a second member secured to the lower end of the tether; a ring having a central vertical axis, said ring having at least one lug extending horizontally therefrom along a radius centered on the vertical axis; a cylindrical receptacle having a central vertical axis, said cylindrical receptacle having a primary guide, a vertical slot, a secondary engaging guide and a notch; the ring being attached to one of said members and the cylindrical receptacle attached to the other of said members, the attachment of a selected one of said ring and cylindrical receptacle to the associated member permitting rotation of said selected one about the vertical axis relative to the associated member; and the lowering of the tether causing the lug to contact the primary guide surface, subsequent lowering of the tether causing the primary guide to guide the lug into and along the vertical slot and into contact with the secondary engaging surface, the secondary engaging guide surface guiding the lug under the notch, upward movement of the tether engaging the lug and notch to secure the tether to the sea floor.

2. The connector of Claim 1 wherein a selected one of said ring and cylindrical receptacle, and associated member, are connected by an elastomeric assembly permitting pivotal motion of the tether about a pivot point centered on the vertical axis of the tether.

3. The connector of Claim 1 wherein three lugs extend from the ring spaced uniformly apart about the ring, the cylindrical receptacle including three members, each member defining a notch for receiving one of said lugs.

4. The connector of Claim 1 wherein the cylindrical receptacle further includes a secondary release guide, lowering of the tether causing the lug to disengage from the notch, contact the secondary release guide and rotate about the central vertical axis beneath a second vertical slot, subsequent upward movement of the tether causing said lug to move through the second vertical slot to release the tether.

5. A connector for releasably securing the tether of a tension leg platform to the sea floor, comprising: a receptacle assembly secured to the sea floor, said receptacle assembly including a cylinder with first, second and third members secured on the inner wall of the cylinder, each member defining a primary guide surface thereon and a notch, vertical slots being defined between each of said members, said receptacle assembly further comprising an annular ring mounted on the inner surface of the cylinder beneath the members, said annular ring defining upwardly facing secondary guides; an anchor latch assembly mounted on the tether, said anchor latch assembly including a center shaft defining a hemi-spherical concave surface centered on a first point on the vertical axis of the tether and a flange defining a semi-spherical surface also centered on the first point, said anchor latch assembly further including a rotating ring having a centering ball defining a convex hemi-spherical surface in sliding engagement with the hemi-spherical concave surface on the center shaft, the hemi-spherical surfaces having generally identical radii of curvature, said anchor latch assembly further having an elastomeric assembly operably positioned between the semi-spherical surface of the flange and the rotating ring, said elastomeric assembly permitting pivotal motion of the tether about the first point, said rotating ring further having first, second and third lugs extending horizontally therefrom; and lowering the tether onto the receptacle assembly engaging said lugs with the primary guide and rotating the lugs and rotating ring about the vertical axis to align the lugs with the vertical slots between said members, the anchor latch assembly and lugs moving downward within the receptacle assembly and into contact with the secondary guides, the secondary guides guiding the lugs and rotating the rotating ring to position each lug beneath a notch, subsequent upward movement of the tether engaging the lugs and notches to secure the tether to the sea floor.

6. The connector of Claim 5 wherein the lugs have a diamond shape, including first and second slanted upper surfaces and first and second slanted lower surfaces.

7. The connector of Claim 5 wherein each of said members has an inverted U-shape including first and second slanted primary guides, first and second lower slanted surfaces, vertical side walls and the notch.

8. The connector of Claim 5 wherein the elastomeric assembly is formed of a plurality of elastomeric elements bonded together, each element being formed of a semi-spherical section having a radius centered on the first point.

9. The connector of Claim 8 wherein the elastomeric assembly includes rigid reinforcing elements shaped into semi-spherical sections having a radius centered on the first point.

10. A method for securing and releasing a tether of a tension leg platform from the sea floor, comprising the steps of: moving the tether downward to engage an anchor latch assembly mounted on the tether with a receptacle assembly mounted on the sea floor, the anchor latch assembly including a ring rotatably mounted on the tether for rotation about the vertical axis of the tether, said ring having at least one lug extending horizontally therefrom, said receptacle assembly defining a primary guide, a vertical slot, a secondary guide and a notch; lowering the tether and anchor latch assembly to engage the lug with the primary guide to rotate the ring and align the lug with the vertical slot and downward along the vertical slot into engagement with the secondary guide for movement along the guide beneath the notch; and raising the tether to engage the lug and notch to secure the tether to the sea floor.

11. The method of Claim 10 further comprising the step of: lowering the tether to move the lug into engagement with a secondary release guide to move the lug toward a second vertical slot; and raising the tether to move the lug into and through the second vertical slot and release the tether from the sea floor.