GB2336143A

GB2336143A - Mooring system

Info

Publication number: GB2336143A
Application number: GB9903565A
Authority: GB
Inventors: Malcolm Bowie
Original assignee: VICTORIA OILFIELD DEV
Current assignee: VICTORIA OILFIELD DEV
Priority date: 1998-03-04
Filing date: 1999-02-18
Publication date: 1999-10-13
Anticipated expiration: 2019-02-18
Also published as: GB2336143B; GB9903565D0

Abstract

A vessel (10) is moored to the seabed by a pair of anchor spreads at bow and stern. Each anchor spread consists of a number of anchor cables (16) terminating in a mooring node (18) which in turn is connected to the vessel (10) by a pendant (20). A number of flexible risers (12) extend up the side of the vessel (10) for transport of fluids. The arrangement is such that the mooring nodes (18) are located below the water surface (22) at all times during anticipated cargo and weather conditions.

Description

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29 30 31 2336143 "Mooring System" This invention relates to a mooring system for use with marine vessels engaged in the exploitation of offshore minerals such as oil and gas.

The invention is particularly, but not exclusively, relevant to floating production and storage facilities in offshore oilfields.

It is well known in such facilities to moor a vessel by means of a single point mooring or a turret mooring about which the vessel may weathercock. This usually requires the use of a relatively complex swivel arrangement to permit flowline risers to communicate with the vessel.

Our British Patent GB 2,296,904B discloses a mooring and flowline system which avoids the need for rotary conduit joints. While this system offers considerable technical advantages, it is inherently limited to use with a single, or only a few, risers.

It is also well known in the field of mooring vessels generally to make use of a spread mooring in which the vessel is moored at bow and stern by a number of anchor cables deployed in varying directions. Spread moorings have not hitherto always been considered suitable for floating production and storage facilities, since it is difficult to bring a transport vessel into a safe

2 4 6 7 8 11 12 13 14 is 16 17 18 19 20 21 22 23 24 26 27 28 29 30 31 32 33 34 36 offloading relationship.

The present invention provides a mooring system for a vessel, comprising a forward mooring spread for attachment to the bow of the vessel and an aft mooring spread for attachment to the stern of the vessel; each of the mooring spreads comprising a plurality of anchors for engagement in the sea bed, a respective anchor cable extending from each anchor to a node at which the anchor cables of the spread are secured together, and a top pendant extending from the node to said single point on the bow or stern; and each spread being arranged and dimensioned such that its node is located a predetermined depth beneath the water surface.

From another aspect, the invention provides a system for storage and transport of oil or gas, the system comprising a floating storage vessel moored at a desired location by a mooring system, the vessel. having bow, stern and midships sections and being connected by one or more flexible risers to one or more underwater pipelines; the riser(s) being routed up one side of the vessel; and the mooring system comprising a forward mooring spread for attachment to the bow of the vessel and an aft mooring spread for attachment to the stern of the vessel; each of the mooring spreads comprising a plurality of anchors for engagement in the sea bed, a respective anchor cable extending from each anchor to a node at which the anchor cables of the spread are secured together, and a ton pendant extending from the node to the bow or stern; and each spread being arranged and dimensioned such that its node is located a predetermined depth beneath the water surface.

Preferably, the vessel is adapted for mooring a second 3 1 2 3 4 5 6 7 8 9 10 11 12 13 14 is 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 vessel alongside the side opposite to the riser (s).

Each anchor cable will typically be a chain cable, but for deep waters chain or wire rope or textile rope, or a mixture of these, may be used.

Each of the nodes is preferably formed by a plate of generally triangular form, and having apertures along one edge to which anchor cables are shackled, and an aperture at an opposing vertex to which the top cable is shackled.

Alternatively, the node may be in the form of a ring.

Each of the top pendants may be a single cable, typically of chain or wire rope, extending from the respective node to a single point at the bow or stern of the vessel. Alternatively, each of the top pendants may comprise twin cables extending from the node to the bow or stern. In the latter case, the cables may conveniently utilise existing hawse pipes, chain stoppers, bollards, windlasses and the like.

The system will most commonly have a number of risers in the midships area of said one side of the vessel, but could be disposed at the bow or stern regions.

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

Figure 1 is a plan view of a floating production and storage system forming one example of the present invention; Figure 2 is a side elevation of the system of Figure 1; 4 1 3 4 6 7 8 9 10 11 12 13 14 is 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 Figure 3 is an end elevation of the same system; and Figure 4 is a view, to an enlarged scale, of a node member used in the system.

Referring to Figures 1 to 3, a floating production and storage facility is provided in the form of a ULCC or other tanker 10 which is connected by a number of flexible risers 12 to corresponding seabed pipelines (not shown). The flexible risers 12 may be of any suitable known type, and may as shown be provided with flotation 14. The risers 12 are secured in a parallel array at the midships area of the tanker 10.

The tanker 10 is moored in position by a mooring system which will now be described.

A mooring spread is provided at the bow and at the stern of the tanker 10. Each of the mooring spreads consists of four chain cables 16 extending from a seabed anchor (not shown) to a mooring node 18, and a single top chain 20 extending from the node 18 to a strong point on the bow or stern of the tanker 10. The arrangement is such that, under normal conditions, the node 18 is disposed at a predetermined depth beneath the water surface 22. A suitable depth may be, for example, 30 feet. When the vessel is loaded by exceptional conditions of wind or wave action, one or both of the nodes 18 may be pulled closer to the water surface 22 and may even break through it.

The effect of this arrangement is that a shuttle tanker 28, as indicated in broken outline in Figure 1, may readily be manoeuvred alongside the tanker 10 to berth against the side of the tanker 10 opposite to that 2 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 occupied by the risers 12, and can do so without danger of fouling the anchor cables 16 which are below the draught of any shuttle tanker 28 except in a very limited area adjacent the bow and stern into which the shuttle tanker 28 would not enter.

Figure 4 illustrates one suitable form of node 18 which comprises a generally triangular steel plate having four apertures along one side 24 for engagement with the anchor cables 16, and an aperture 26 in the area of the opposite vertex for engagement by the top anchor chain 20. Other forms of node may be used, such as a plain forged ring.

The tanker 10 requires minimal modification for use in the system. Suitable strong points may already be available at bow and stern, or may be specially provided. Typically, it will also be necessary to provide non-standard fairleads at bow and stern to accommodate the top chains 20; these fairleads may be in the form of swivelling tubular members, or simple bellmouth fairleads with reasonably large curvature both vertically and athwartships.

The present invention therefore allows a floating production of storage facility, or like floating vessel, to be moored fore and aft to a spread mooring in such a manner that another vessel can be brought alongside in a simple and safe way.

Modifications and improvements may be made to the foregoing within the scope of the present invention.

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Claims

CLAIMS.

1. A mooring system for a vessel, comprising a forward mooring spread for attachment to the bow of the vessel and an aft mooring spread for attachment to the stern of the vessel; each of the mooring spreads comprising a plurality of anchors for engagement in the sea bed, a respective anchor cable extending from each anchor to a node at which the anchor cables of the spread are secured together, and a top pendant extending from the node to said single point on the bow or stern; and each spread being arranged and dimensioned such that its node is located a predetermined depth beneath the water surface.

2. A system for storage and transport of oil or gas, the system comprising a floating storage vessel moored at a desired location by a mooring system, the vessel having bow, stern and midships sections and being connected by one or more flexible risers to one or more underwater pipelines; and the mooring system comprising a mooring system in accordance with claim 1.

3. A system according to claim 2, the riser(s) being routed up one side of the vessel.

4. A system according to claim 3, in which the vessel is adapted for mooring a second vessel alongside the side opposite to the riser(s).

5. A system according to any preceding claim, in which each anchor cable is a chain cable, wire rope or textile rope, or a mixture of these.

6. A system according to any preceding claim, in which each of the nodes is formed by a plate of generally 7 1 triangular form, and having apertures along one edge to which anchor cables are shackled, and an aperture at an opposing vertex to which the top cable is shackled.

2 3 4 5 6 7. A system according to any of claims 1 to 6, in which each of the nodes is in the form of a ring.

7

8 8. A system according to any preceding claim, in which each of the top pendants is a single cable extending from the respective node to a single point at the bow or stern of the vessel.

9

10 11 12 13 14 9. A system according to any of claims 1 to 7, in which each of the top pendants comprises twin cables extending from the node to the bow or stern.

is 16 17 18 19 20 21 22 23 24 25 26 10. A system according to claim 9, in which the cables utilise existing hawse pipes, chain stoppers, bollards, windlasses of the moored vessel.

11. A system according to any preceding claim, in which said predetermined depth is such that the anchor cables, in normal conditions, are disposed at a depth greater than the draught of a predetermined shuttle tanker in an area providing a clear approach to berthing alongside the vessel.