GB2250006A

GB2250006A - Vessel anchor system with extendible load limit device

Info

Publication number: GB2250006A
Application number: GB9106764A
Authority: GB
Inventors: Heijst Willem Jan Van
Original assignee: Bluewater Terminal Systems NV
Current assignee: Bluewater Terminal Systems NV
Priority date: 1990-11-20
Filing date: 1991-03-28
Publication date: 1992-05-27
Anticipated expiration: 2011-03-28
Also published as: GB9106764D0; GB9025155D0; GB2250006B; AU7435891A

Abstract

An anchor system for a vessel (10) comprises a plurality of anchor chains/cables (15) to extend between the vessel and spaced anchorages (17) on the seabed. At least one anchor chain/cable has an extendible load relieving device (18) capable of limited extension when subjected to an instantaneous load in excess of a predetermined level to provide a limited extension to the chain when subjected to said excess load. The device (18) may be at the seabed end of the anchor chain/cable, at the vessel end of the chain/cable or intermediate the ends thereof. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO VESSEL ANCHOR SYSTEMS This invention relates to an anchor system for a vessel for anchoring the vessel to the seabed. The term "vessel" is intended to include static and self-propelled vessels such as semi-submersibles, tankers and the like. Anchoring systems for providing a permanent mooring for a vessel are known in which a plurality of anchor chains or cables extend radially from the vessel in catenary form to fixed or conventional anchorages (semi-submersibles are usually anchored with heavy holding marine anchors) on the seabed.

British Patent Specification No. 1382946 discloses such an arrangement in which the seabed anchorages include guides through which the anchor chain can pass freely. Thus the chains can pull through the guides on the anchorage devices when a certain tension in the chain below the breaking strain is reached whereby the buoyant member attached to the chain is pulled deeper into the water so that a more or less constant tension in the chain results. The disadvantage of this arrangement is that the buoyant chamber itself exerts an upward lift on the seabed anchorage requiring the anchorage to be more firmly fixed at the seabed than would otherwise be required. Moreover such an arrangement cannot be combined with a conventional marine anchor. U.S.

Patent Specification No. 3430597 shows a similar arrangement with a single anchor chain and single float in this case at the surface. European Patent Publication No. 0147176 discloses a motion compensator for use under water in a mooring of a vessel to an underwater anchorage point comprising a pair of telescopically acting members for connection to the anchorage and to the vessel respectively to resist movement of the vessel.

This invention provides an anchor system for a vessel comprising a plurality of anchor chains/cables to extend between the vessel and spaced anchorages on the seabed, at least one anchor chain/cable having an extendable load relieving device capable of limited extension when subjected to a load in excess of a predetermined level to provide a limited extension to the chain/cable when subjected to said excess load.

The following is a description of some specific embodiments of the invention, reference being made to the accompanying drawings in which: Figure 1 is a diagrammatic view of a vessel anchored using a catenary mooring system having a number of anchor chains each with an extendable hydraulic jack; Figure 2 is a diagrammatic view of one of the hydraulic jacks and its associated pressure supply circuit; Figure 3 is a diagrammatic view of part of a hawser leading an anchor chain into the vessel; Figure 4 is a diagrammatic view of an alternative construction in which the hydraulic jacks for the anchor chains are mounted on the vessel; Figure 5 is a similar view to Figure 2 showing a modified hydraulic circuit; and Figures 6 to 9 show the application of different forms of the invention to anchoring vessels and in this case semi-submersibles.

Referring firstly to Figure 1 of the drawings, there is shown a part of a vessel 10 into the hull of which is built a turret 11 rotatable about a vertical axis 12. The turret has a lower end 13 located generally level with the bottom of the hull of the vessel and having a number of outwardly angled hawser ports 14 (see Figure 3) through which anchor chains 15 extend. The upper ends of the chains are secured in the turret using conventional chain stoppers not shown. Typically six such chains are provided being equally spaced around the axis of the turret and extending downwardly and outwardly of the turret in catenary form to fixed anchorages 16 piled into the seabed.

Each of the anchor chains 15 can incorporate an extendable hydraulic ram 18 lying on the seabed adjacent the anchorage 17 to extend when the respective anchor chain is subjected to an excessive load. Each ram comprises a cylinder 19 connected to one point of the chain and a plunger 19a connected to the other part of the chain. The ram is connected to a pair of pressurised gas (e.g. nitrogen) operated hydraulic fluid pressure accumulators 20 by means of a circuit indicated at 21. The circuit has parallel branches 22 and 23 connected to the ram, the branch 22 having an adjustable throttle valve 24 and a non-return valve 25 to allow release of fluid from the ram to the accumulator 20 and the branch 23 having a non-return valve 26 and adjustable throttle valve 27 to allow return of hydraulic fluid from the accumulators to the ram.The pressure in the accumulator is set to a pre-determined level to hold the ram contracted with a predetermined force of, for example, 400 tons so that the ram remains contracted until the chain load exceeds that level. When the level is temporarily exceeded, the ram extends delivering hydraulic fluid via branch 22 to the accumulators thereby avoiding overloading of the chain. Once the load on the chain has taken to a normal level, pressure in the ram falls below said pre-determined level and hydraulic fluid pressure from the accumulators contracts the ram to its original state.

Thus each of the anchor chains in the mooring system which could be subject to overload incorporates a hydraulic ram to relieve temporary excess load applied to the respective chains by movement of the ship at the surface. The loads of which the rams are set to extend are calculated in accordance with the anticipated loads which would be imposed on the anchor chain in the event of an extreme storm.

In the case where only certain anchor chains in an anchor system are likely to be subjected to excess load, only those chains are equipped with extendable rams. For example, in the case of a vessel moored near to a coast line, only those anchor chains holding the vessel away from the shore against the prevailing wind may be equipped with load relieving rams.

Figure 3 of the drawings shows an alternative arrangement in which the extendable rams are mounted in the turret on the vessel rather than lying on the seabed. To that end the head 30 of the turret provides mounting for the upper ends of the cylinders 19 of the ram and the ends of the anchor chains are to passing through the ports 14 in the lower end of the turret are connected to the plungers 20 of the rams.

As illustrated in Figure 3, each anchor chain 15 has a stopper in the form of a plate 31 in the length of the anchor chain adjacent the turret so that when the plunger of the ram to which the chain is connected is in the retracted condition, the stop plate 31 abuts against the outwardly inclined face of the turret around the port 14 from which the chain emerges.

Referring now to Figure 5 of the drawings, there is shown an alternative form of hydraulic system in which the adjustable throttle valves 24 and 27 are omitted and the branch 22 to the back of accumulators by a pre-set hydraulic overload valve 40 such that a higher pressure is created in the fluid during the outgoing stroke whilst the return stroke via branch 23 takes place at the pressure level of the accumulators.

Figures 6 to 9 of the drawings illustrate the various forms of the invention described above applied to a semi-submersible vessel rather than a conventional ship.

Claims

1. An anchor system for a vessel comprising a plurality of anchor chains/cables to extend between the vessel and spaced anchorages on the seabed, at least one anchor chain/cable having an extendable load relieving device capable of limited extension when subjected to an instantaneous load in excess of a predetermined level to provide a limited extension to the chain when subjected to said excess load.

2. An anchor system as claimed in Claim 1, wherein the extendable device includes means automatically to retract the device when the excess load has been removed.

3. An anchor system as claimed in Claim 1 or Claim 2, wherein the extendable device comprises a hydraulic jack connected to a hydraulic fluid accumulator means activated by pressurised gas to hold the jack against extension up to said predetermined load and to allow the jack to extend when said predetermined load is exceeded.

4. An anchor system as claimed in any of the preceding claims wherein the or each extendable device is/are located in the respective anchor chains/cables adjacent the connection to the seabed anchorage.

5. An anchor system as claimed in any of Claims 1 to 3, wherein the or each extendable device is/are connected to the end of the respective anchor chain to be secured to the vessel to be anchored.

6. An anchor system as claimed in Claim 5 and in the case where the vessel has a downwardly facing rotatable turret mounted in the vessel hull to receive the anchor chains, wherein the or each extendable device is/are adapted to be mounted on the turret to receive the respective anchor chain/cable.

7. An anchor system as claimed in any of Claims 1 to 3, wherein the extendable damping devices are provided partway along the or each chain/cable.

8. An anchor system substantially as described, with reference to and as illustrated in Figures 1 and 2 or Figures 3 and 4 of the accompanying drawings.

9. An anchor system substantially as described, with reference to and as illustrated in Figures 1 to 4 as modified by Figure 5.

10. An anchor system substantially as described, with reference to and as illustrated in Figures 6 to 9 of the accompanying drawings.