GB2096963A - An off-shore mooring system - Google Patents

Abstract

An apparatus for mooring a floating vessel 22 adjacent to an offshore platform 10 stabilized at sea by a plurality of cables 13 anchored to the sea floor is disclosed. The apparatus comprises a buoyant module 21, at least one tension line 19 having one end connected to the module and the other end connected to one of the cables, and means 23, 24, 25 for pivotally connecting one end of the vessel to the module. <IMAGE>

Description

SPECIFICATION A multipurpose mooring system This invention relates to mooring a floating vessel adjacent to a platform at sea, and more particularly to a method and apparatus for mooring.

In the petroleum industry the search for new sources of oil and gas has been extended to deep bodies of water which present numerous problems, one of which is mooring a floating vessel adjacent to a platform at sea. Due to waves, currents, and wind, it is difficult to moor the vessel close enough to the platform to be accessible for transferring cargo and similar activities without colliding with the platform. Heretofore, such mooring has been accomplished by using apparatus which includes a tension line connected to a submerged buoy anchored to the ocean floor (U.S. Patent No. 4,239,417 granted to A. Statten, et al., German Pat. No. 26 10 812 granted to A.

G. Weser and "Safer Mooring on UK Field" by C.

Lykes, et al, in Offshore Services, February 1979) or a tension line connected to the anchor or base of a platform (U.S. Pat. No. 4,000,713 granted to S. Heien). Such apparatus is undesirable because the tension line, which extends the full depth of the ocean, is too long and consequently not stiff enough to stabilize the horizontal position of the vessel to prevent collision thereof with the platform. The former apparatus is further undesirable because extra anchors in addition to those securing the platform, are required to hold down the submerged buoy.

Some offshore platforms are positioned or stabilized at sea by a plurality of circumferentially positioned cables, each having one end connected to the platform and the other end anchored to the sea floor. The cables are designed to maintain the platform within certain horizontal limits even in the most severe environments. Consequently, the cables are extremely strong and, in lesser environments, have reserve strength which can be utilized for other purposes. The present invention is based on the discovery of a mooring method and apparatus which can take advantage of the reserve strength of these cables by connecting one or more tension lines to separate cables at a shallow depth below the surface of the sea.As a result, the tension lines are sufficiently short and stiff so that the translational motion of the vessel can be maintained within an acceptable range to prevent collision with the platform. Furthermore, because the tension line is shorter, the couplings associated with it do not have to be as strong as those associated with a longer tension line and the buoyancy required to support it does not have to be as large as that required for a longer tension line.

The invention is described with reference to the drawings wherein: FIGURE 1 is a schematic representation of mooring apparatus in accordance with the invention FIGURE 2 is a perspective view of a connector between the tension line and the cable of FIGURE 1; FIGURE 3 is a perspective view of another connector between the tension line and the cable of FIGURE 1; and FIGURE 4 is a schematic representation of another mooring apparatus in accordance with the invention.

Referring now in more detail to FIGURE 1, a platform 10 floats on a sea 11 and is anchored to the sea floor 12 by cables 1 3 which are attached to the platform 10 and which extend downwardly and radially outward therefrom to the sea floor 12 in a generally catenary configuration where they are attached to anchors 14 embedded in the sea floor 12. The cables 1 3 can be, for example, wire rope, synthetic rope, clump weights, chain, or a suitable combination thereof. They should be sufficiently strong to maintain the platform 10 within certain horizontal limits in the most severe environments. Consequently, the cables 1 3 are extremely strong and, in lesser environments, relatively motionless with respect to the floor 1 2 of the sea 11.The anchors 14 are spaced circumferentially around the platform 10. Only two cables 13 are shown in FIGURE 1, but substantially any number of cables 13 may be attached around the periphery of the platform 10.

Cables 1 5 can be attached to the platform 10 and anchored to the sea floor 12 to maintain the vertical position of the platform 10 with respect to the floor 12. However, it is to be understood that the inventon is not limited to the floating platform 10 described above, but can also be practised in other embodiments such as, for example, a platform (not shown) vertically supported above sea level by legs embedded in the sea floor 12 and horizontally restrained by the cables 1 3.

Referring now to both FIGURES 1 and 2, connecting means each comprise a first universal joint 1 6 having one yoke mounted on a clamp 17 which is attached to a fixed position on the cable 1 3 and stabilized thereon by a lumped weight 18 attached to the clamp 1 7. The other yoke of the univeral joint 1 6 is attached to one end of a tension line 19 which can be, for example, a chain, a rope, or other shackling device. The other end of each tension chain 19 is attached to one yoke of a second universal joint 20, the other yoke of which is mounted on the lower end of a buoy 21 comprising a cylindrical body 22 and a cylindricaltube or pendant 23 of lesser diameter extending axially therefrom to above the surface of the sea 11.The body 22 of the buoy 21 is completely submersed and stabilized near the surface of the sea 11 by the pendant 23. Accordingly, the buoy 21 stretches the tension chains 1 9 upward toward the surface of the sea 11. Referring only to FIGURE 1, the pendant 23 supports a swivel connector 24 to which the hawser 25 of a vessel 26 can be attached. When the vessel 26 is moored by the hawser 25 to the swivel connector 24, the vessel 26 can swing freely around the buoy 21 without colliding with the platform 10. The free end of the vessel 26 can be moored to the platform 10 so that the vessel 26 is accessible for transferring cargo and similar activities (not shown).

Another embodiment of means for connecting the tension chain 1 9 to the cable 1 3 in accordance with the invention is shown in Figure 3. The catenary cable comprises an upper and lower portion, 27 and 28 respectively, held together by a metal plate 29. The cables 27 and 28 are connected to pins 30 and 31 supported by swivels 32 and 33 which are attached to the opposing faces of the connecting plate 29. The end of each cable, 27 and 28, is attached to the corresponding pin, 30 and 31 respectively, by zinc poured or epoxied socket connections 34. The lumped weight 18 and the universal joint 1 6 are attached to the vertically opposing faces of the connecting plate 29.Because of the swivels 32 and 33, the connecting plate 29 is rotatable about the axis through the cables 27 and 28 to further reduce the wear thereon that is associated with the motion of the lump weight 1 8 and the buoy 21 (FIGURE 1).

In order that the stern of the vessel 26 does not collide with the platform 10, the clamps 1 7 are attached to the cables 1 3 at positions sufficiently low thereon to hold the buoy 21 generally equidistant over the clamps 1 7 at a distance from the platform 10 of at least the length of the vessel 26. Additionally, the clamps 1 7 are attached to the cables 13 at a depth sufficiently shaliow below the surface of the sea 11 to keep the tension chains 1 9 taut, thereby reducing the translational motion of the buoy 21.Because the tension chains 1 9 are taut, the buoy 21 prevents the stern of the vessel 26 from colliding with the platform 10, while at the same time allowing the vessel 26 to weathervane, i.e., pointing the moored bow of the vessel 26 into the environment. However, to ensure that the stern of the vessel 26 is close enough to the platform 10 for mooring and accessibility, the length of the hawser 25 or the buoyancy of the buoy 21 is adjustable to position the vessel 26 within a safe distance from the platform 10. Other buoys can be attached to other cables in a similar fashion so that the vessel 26 can be moored to a stabilized buoy at many positions around the periphery of the platform 10 and weathervane without having the free end collide with the platform.

Referring now to FIGURE 4, it is also to be understood that the buoy 21 can be held in position by only one tension chain 19' which is connected to the yoke of a universal joint 16', the other yoke of which is mounted on a clamp 17'.

The clamp 17' is attached to the cable 13' at a position sufficiently low thereon to hold the buoy 21 substantially over the clamp 17' at a distance from the platform 10 of at least the length of the vessel 26. However, the clamp 17' is connected at a depth sufficiently shallow below the surface of the sea 11 to reduce the translational motion of the buoy 21 in a fashion similar to that described hereinabove with respect to two tension lines.

As mentioned above, the tension chain 1 9 could also be a tension wire, rope, or other suitable shackling device. When the mooring apparatus comprises tension wire ropes (not shown), the ropes can be wound on drums or reels (not shown) mounted on the buoy 21. The reels have motors (not shown) for rotating the reels to take up or let out the ropes as required. It will be apparent that other changes may be made in details of construction from those shown in the attached drawings and discussed in conjunction therewith without departing from the spirit and scope of this invention as defined in the appended claims. For example, different types of universal joints other than the Hooke-type joint 1 7 could be used. Also, different buoy designs other than the cylindrical buoy 21 could be used.

Claims (11)

1. An apparatus for mooring a floating vessel adjacent to an offshore platform stablized at sea by a plurality of cables, each having one end attached to the platform and the other end extending radially outward therefrom and anchored to the sea floor, said apparatus comprising: a buoyant module; a tension line having a lower end and an upper end; first means for connecting the upper end of said tension line to said module; second means for connecting the lower end of said tension line to one of the cables at a position sufFiciently low thereon to hold said module substantially over the lower end of said tension line at a distance from the platform of at least the length of the vessel; and, third means for connecting one end of the vessel to said module, whereby the free end of the vessel can rotate to a position adjacent to the platform without colliding therewith.

2. An apparatus as claimed in claim 1 wherein there are at least two said tension lines, there are first means for connecting each of said tension lines to said module; and wherein there are second means for connecting each of said tension lines to a separate cable.

3. An apparatus as claimed in either of claims 1 and 2, wherein the or each said second connecting means comprises a clamp attached to the cable, and a universal joint having one yoke mounted on said clamp and the other yoke attached to one end of said tension line.

4. An apparatus as claimed in any one of the preceding claims, wherein the clamp is rotatable about the axis of the cable.

5. An apparatus as claimed in either of claims 3 and 4 which additionally comprises a weight attached to the cable.

6. An apparatus as claimed in any one of the preceding claims, wherein said buoyant module comprises a submerged cylindrical body and a cylindrical tube of lesser diameter extending therefrom above the surface of the sea.

7. An apparatus as claimed in any one of the preceding claims, wherein said third connecting means comprises a connector pivotally mounted on the end of said cylindrical tube extending above the surface of the sea, and a hawser connected between said connector and the vessel.

8. An apparatus as claimed in any one of the preceding claims which comprises a universal joint connected between said buoyant module and the or each of said first connecting means.

9. An apparatus for mooring a floating vessel adjacent to an offshore platform stabilized at sea by a plurality of anchor lines, said apparatus comprising: a buoy; at least one mooring line having one end connected to said buoy and the other end attached to one of the anchor lines at a position thereon to hold said buoy at a distance from the platform of at least the length of the vessel; and a pendant having one end attached to said buoy and the other end pivotally connected to one end of the vessel, whereby the free end of the vessel can rotate to a position adjacent to the platform without colliding therewith.

10. A method of mooring a floating vessel adjacent to an offshore platform stabilized at sea by a plurality of cables, each having one end attached to the platform and the other end extending radially outward therefrom and anchored to the sea floor, said method comprising: connecting one end of a tension line or each end of a plurality of tension lines to a buoyant module; connecting the other end of the tension lines to one of the cables or connecting each of the other ends of the tension lines to a separate cable at a position or positions thereon to hold the buoyant module at a distance from the platform of at least the length of the vessel; and, pivotally connecting one end of the vessel to the buoyant module, whereby the free end of the vessel can rotate to a position adjacent to the platform without colliding therewith.

11. An apparatus according to claim 1 substantially as hereinbefore described with reference to the drawings.