GB2143191A - Mooring system - Google Patents

Abstract

A mooring system is provided for a floating structure such as a semi-submersible oil drilling/production platform (1). The mooring system comprises an anchor (3), an anchor chain (4), a pennant wire (7), a rope (11) and a buoy (12). The anchor chain (4) connects the anchor (3) to the platform (1), the pennant wire (7) having one end attached to the anchor. The other end of the pennant wire (7) is attached to one end of the rope (11), and the other end of the rope is attached to the buoy (12). <IMAGE>

Description

SPECIFICATION Mooring system This invention relates to a mooring system for a floating structure such as a semi-submersible oil drilling/production platform.

A semi-submersible platform (rig) of this type is anchored to the sea bed by a plurality of anchors, each of which is connected to the rig by means of an anchor chain or an anchor cable. Throughout this specification, the term "anchor chain" will be used for the item connecting an anchor to a rig, but it will be appreciated that an anchor cable could be used, and that the term "anchor chain" should be construed accordingly. In cases where the sea bed does not provide sufficient anchorage, the anchors may each be provided with one or more back-up (piggy-back) anchors. Each piggy-back anchor is attached to its neighbouring anchor by means of a ground wire (such as a steel cable) and a tail chain.

In the adverse conditions which prevail in the North Sea, it is necessary to check and adjust the tension in the anchor chains of such a semi-submersible rig very frequently.

Indeed, it is frequently necessary for a service ship to go round and round a rig checking and adjusting the tension in its anchor chains almost continually during the hours of daylight. In order to adjust the tension in an anchor chain, it is necessary to drag up the anchor, and to re-position it on the sea bed. It is also desirable to be able to retrieve anchors for inspection and/or repair.

There are two known types of mooring sytem which permit anchor retrieval and anchor chain tension adjustment, namely the use of anchor crown buoys or what is known as chain chasing". In the former case, each of the end anchors (the main anchor where there are no piggy-back anchors, or the end piggy-back anchor where there are) is connected to an anchor crown buoy by a steel cable (pennant wire). When a given anchor is to be retrieved, or the tension in its anchor chain needs adjustment, a service ship hauls the appropriate anchor crown buoy on board, and winches up that anchor using the pennant wire. The anchor can then be inspected and/or repaired, or be replaced on the sea bed, at the correct position to tension the associated anchor chain by the required amount, by winching out the pennant wire.A pennant wire is usually at least 1.5 times (and often 2 times) as long as the depth of the water. Moreover, the pennant wire must be a thick steel cable. Typically, the pennant wire will be of 225 metres of 64 millimetre steel cable, and this will have a total weight in sea water of 3.25 tonnes. This means that the associated anchor crown buoy must have a reserve buoyancy of at least this amount, and this results in the use of large, heavy buoys.

Until recently, anchor crown buoys have been made of steel. However, because of their size and weight, they are a danger to small ships such as fishing vessels if they break away. Unfortunately, because pennant wires chafe on the sea bed (as a result of the surface motion of their buoys), they have a high risk of failure, particularly during periods of severe weather. Heavy steel buoys are also difficult to manoeuvre onto the decks of service vessels, particularly in adverse sea conditions, so the tensioning of anchor chains is a difficult, time-consuming and often dangerous procedure.

In order to reduce the dangers to fishing vessels from anchor crown buoys which have come adrift, it has recently been proposed to make the buoys from relatively soft material such as fibre glass or plastics material. Even, then however, there is still a danger to small ships, since the buoys are still large and heavy. Moreover, the problems associated with hauling the buoys onto the decks of service vessels still remain.

The chain chasing system was designed to do away with the need for anchor crown buoys, and so to avoid the problems outlined above. This system utilises a large steel collar for each anchor chain. Each collar is supported by a pennant chain and is permanently located around the associated anchor chain.

Initially, an anchor is carried to the drop location, by a service vessel, with the collar seated between the flukes and shank of the anchor. At the desired drop point, the anchor is lowered to the sea bed, and the collar is returned to the rig along the anchor chain.

The collars are stored on the rig. When a given anchor chain is to be retrieved (for example for re-tensioning), the collar is run down that anchor chain until it is firmly seated on the flukes of the associated anchor.

This anchor can then be lifted using the pennant chain, after which it can be repositioned to provide the desired anchor chain tension.

Unfortunately, the use of chain chasers leads to a number of disadvantages. In particular, chain chasers cannot be used where piggy-back anchors are used to supplement the anchoring force of a main anchor. This is because a chain chaser cannot be run down further than the main anchor, so that the piggy-back anchors cannot be lifted. Moreover, chain chasers cannot be used with anchor chains in the form of steel cables (wires) as these quickly cut through the steel collars of chain chasers. If toughened, wear-resistant steel is used for making the collars, the anchor wires themselves can be damaged.Other disadvantages of using chain chasers are the risk of damaging the associated anchors, difficulties in correctly positioning the anchors due to the anchor stabilising bars being fouled by the pennant wire of the chasers, and the need to provide special equipment on the service vessel to handle chasers.

The aim of the invention is to provide a mooring system for a semi-submersible rig which permits anchor retrieval without the use of large, heavy buoys or chain chasers, and which is effective even where piggy-back anchors are used.

The present invention provides a mooring system for a floating structure, the mooring system comprising an anchor, an anchor chain, a pennant wire, a rope and a buoy, the anchor chain connecting the anchor to the floating structure, the pennant wire having one end attached to the anchor, the other end of the pennant wire being attached to one end of the rope, and the other end of the rope being attached to the buoy.

Throughout this specification the term "rope" should be taken to mean any lightweight elongate member such as a wire, rope or cable, and the term "wire" when used in the phrase "pennant wire" should be taken to mean any heavyweight elongate member such as a cable or a chain.

Advantageously, the anchor is connected to said one end of the pennant wire by means of a tail chain.

In one preferred embodiment, said other end of the pennant wire is attached to said one end of the rope by means of an intermediate elongate member. Advantageously, the intermediate elongate member has a length of 40 metres and is a chain. In this case, the entire length of the pennant wire lies along the sea bed when the mooring system is in use. The intermediate elongate member is also arranged to lie partly along the sea bed.

Consequently, any chafing of the sea bed is done by the intermediate elongate member, and not by the pennant wire.

Where the sea has a depth of 1 50 metres, the pennant wire may have a length of 225 metres and the rope may have a length of 140 metres.

In another preferred embodiment, a submersible buoy is attached to the pennant wire part-way therealong. Preferably, the submersible buoy is so positioned, in use, that any movement of the first mentioned buoy resulting from sea level changes is taken up by that portion of the pennant wire which lies between said other end and the submersible buoy, said portion forming a loop hanging down from the submersible buoy and said other end of the pennant wire. In this case, part of the pennant wire lies along the sea bed. I However, this part does not chafe the sea bed even when the surface buoy moves up and down with the sea, as all this movement is taken up by the looped portion of the pennant wire.

In another preferred embodiment, said other end of the pennant wire is attached directly to said one end of the rope, the system being such that, in use, the major part of the length of the pennant wire lies on the sea bed, and such that the connection point between said other end of the pennant wire and said one end of the rope lies a sufficient distance above the sea bed to prevent said point chafing on the sea bed in the severest expected sea conditions.

Preferably, the first-mentioned buoy is a small glass fibre spar buoy of about 500 kilogram displacement. Alternatively, the firstmentioned buoy may be of the foamed elastomer type, and has a roping bollard fixed to its base. In either case, the rope may be a polypropylene rope of strength 23.4 tonnes.

Such a rope is buoyant in sea water, and so is easy to haul out of the sea.

The anchor may be a main (sole) anchor.

Alternatively, the anchor may be a piggy-back anchor, in which case the piggy-back anchor is attached to the anchor chain via another (main) anchor. In this case, the piggy-back anchor is attached to the main anchor by a ground wire and a tail chain.

The invention also provides a floating structure whenever anchored to the sea bed by one or more mooring systems of the type defined above. Preferably, the floating structure is a semi-submersible oil rig.

The invention further provides a method of adjusting a mooring system comprising an anchor, an anchor chain, a pennant wire, a rope and a buoy, the anchor chain connecting the anchor to a floating structure, the pennant wire having one end attached to the anchor, the other end of the pennant wire being attached to one end of the rope, and the other end of the rope being attached to the buoy, the method comprising the steps of lifting the buoy out of the water, winching the rope out of the water, winching the pennant wire out of the water, thereby lifting the anchor off the sea bed, and re-siting the anchor on the sea bed so that the anchor chain has a predetermined tension.

The invention also provides a buoy for use with the mooring system defined above, wherein the buoy is of the foamed elastomer type, and has a roping bollard fixed to its base.

The invention further provides a method of making a buoy as defined above, the method comprising the steps of fabricating a roping bollard onto a central tie tube, foaming polyurethane around the central tie tube, fixing sheets of polystyrene around the foamed polyurethane using an adhesive, and spraying an elastomer onto the polystyrene.

Three forms of mooring system, each of which is constructed in accordance with the invention, will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a schematic side elevation of the first form of mooring system; Fig. 2 is a schematic side elevation of a modified version of the first mooring system; Fig. 3 is a schematic side elevation of the second form of mooring system; Fig. 4 is a schematic side elevation of the third form of mooring system; Fig. 5 is a side elevation of a buoy forming part of the third form of mooring system; Fig. 6 is a partial view of the buoy looking in the direction of the arrow A shown in Fig.

5; Fig. 7 is a partial view showing the top of a rope to be attached to the buoy; Fig. 8 shows an alternative type of lifting eye; and, Fig. 9 shows a shackle for connecting the rope to the buoy.

Referring to the drawings, Fig. 1 shows a semisubmersible oil rig 1 provided with a plurality of anchor means 2 (only one of which is shown). The anchor means 2 are positioned to radiate outwardly from the rig 1, and are substantially equispaced. Each anchor means 2 has an anchor 3 and an anchor chain 4. Each anchor chain 4 is connected to its anchor 3 by means of a swivel joint 5.

Each anchor 3 is connected to a 10 metre length of tail chain 6; which, in turn, is connected to a pennant wire 7 by a shackle 8. Each pennant wire 7 is connected to a 40 metre length of 25 millimetre open link chain 9 by a shackle 10. Each chain 9 is connected to a length of polypropylene rope 11 which, in turn, is connected to a glass fibre spar buoy 1 2. The rope 11 is 44 millimetres in diameter, and has a strength of 23.4 tonnes. It is buoyant in sea water.

In the arrangement shown, if the sea depth is 1 50 metres, the pennant wire 7, which is a 64 millimetre diameter steel cable, has a length of 225 metres (that is to say 1.5 times the sea depth), and the rope 11 has a length of 1 40 metres. Normally, therefore, the connection point between the chain 9 and the rope 11 is 10 metres above the sea bed (identified by the reference numeral 13).

In order to moor the rig 1, the anchors 3 are positioned on the sea bed 1 3 in the normal way by means of a service vessel (not shown). Once a given anchor 3 is positioned, its pennant wire 7 is laid along the sea bed 13, and its spar buoy 1 2 (which is already connected to the pennant wire by means of the respective chain 9 and shackle 10) is then put into the sea at a position substantially above the end of the pennant wire remote from the rig 1.

In order to retrieve a given anchor 3, for example so that its anchor chain 4 can be retensioned, the service vessel hauls the appropriate buoy 1 2 onto its deck. As the buoys 1 2 are relatively light, this is easy to accomplish, even in adverse sea conditions. The rope 11 associated with that buoy 1 2 is then winched up, carrying the chain 9 and the pennant wire 7 with it. When the pennant wire 7 emerges from the sea, it is attached to the winch and hauled up. This detaches the associated anchor 3 from the sea bed 1 3. This anchor 3 can then be hauled up to the service vessel for re-siting on the sea bed at the correct position to tension the associated anchor chain 4 by the required amount.

It will be apparent that the mooring system described above avoids the disadvantages of the crown anchor buoy system. In particular, it utilises small, light, glass fibre spar buoys 1 2 instead of large, heavy crown anchor buoys. Thus, if a buoy 1 2 does break away, it is far too small and light to damage even the smallest ship. Moreover, by lying the pennant wires 7 along the sea bed 13, there is no danger of them chafing on the sea bed in varying sea conditions. Instead, the chains 9 which connect the pennant wires 7 to their ropes 11 will chafe on the sea bed 1 3.

However, because buoys 1 2 of this type have a small water plane area, they are affected only to a minimum extent by sea surface conditions. Consequently, the chafing to which the chains 9 are subjected is minimal, and so the risk of the buoys 1 2 breaking away is negligible. However, as mentioned above, even if this does happen, there is no danger to shipping from such small and light buoys. In this case, the mooring system described above has the additional advantage of requiring replacement of the relatively cheap chain rather than the expensive pennant wires.

Moreover, the mooring system described above is easily modified to accommodate anchor means which incorporate one or more piggy-back anchors, so that this system (unlike the chain chaser system) can be used when piggy-back anchors are necessary. Thus, Fig. 2 shows a modified version of the mooring system of Fig. 1, this version being associated with anchor means which includes a piggy-back anchor. This figure shows part of one of the anchor means 20 of an oil rig (not shown). The anchor means 20 has a main anchor 21 connected to an anchor chain 22 by means of a swivel joint 23. The anchor 21 is connected to a ground wire 24 by a tail chain 25. The tail chain 25 is attached to a piggy-back anchor 26 by a swivel joint 27.

The piggy-back anchor 26 is attached to a tail chain 28, and this tail chain is attached to a pennant wire 29. As with the arrangement of Fig. 1, the pennant wire 29 is attached to a 40 metre length of chain 30, and this is connected to a spar buoy (not shown but similar to the spar buoy 12) by means of a length of polypropylene rope (not shown).

With this modified arrangement, the rig is moored by positioning the main anchors 21 and piggy-back anchors 26 on the sea bed in the normal way, by lying the pennant wire 29 on the sea bed, and then by releasing the spar buoys. Anchor retrieval is achieved in a similar manner as for the embodiment of Fig.

1. The only difference here is that, as a pennant wire 29 is winched up, the associated piggy-back anchor 26 is lifted from the sea bed first, after which the associated main anchor 21 is lifted.

In both of the arrangements described above with reference to Figs. 1 and 2, the chains 9 and 30 are shown to have a length of 40 metres so as to be capable of accommodating the most severe sea conditions nor- mally encountered in the North Sea. Thus, in slack water conditions, 10 metres of the chains 9 (or 30) are supported above the sea bed, and in storm conditions 30 metres of the chains are supported. In more severe conditions, all 40 metres of the chains would be supported, so even under conditions worse than "storm condition", there is no danger of the pennant wires chafing the sea bed. Moreover, if conditions get even worse, the arrangement is such that the spar buoys 1 2 will submerge rather than iift the associated pennant chains.This is because of the relatively small reserve buoyancy of the spar buoys.

Consequently, no matter how severe the sea conditions, there is no danger of the pennant wires chafing.

Fig. 3 shows the second form of mooring system. Here an oil rig 40 is provided with a plurality of anchor means 41 (only one of which is shown). As with the embodiment of Fig. 1, the anchor means 41 radiate outwardly from the rig 40, and are substantially equispaced. Each anchor means 41 has an anchor 42 connected to the rig 40 by an anchor chain 43. A tail chain 44 connects the anchor 42 to a pennant wire 45. The other end of the pennant wire 45 is attached to a length of polypropylene rope 46, and this is connected to a spar buoy 47. A submersible buoy 48 is attached to the pennant wire 45 part-way therealong, so that the pennant wire takes up the configuration shown.

The submersible buoy 48 is such as to remain at a position of neutral buoyancy irrespective of sea surface conditions. Consequently, the pennant wire 45 will not chafe on the sea bed no matter how severe the sea conditions. Obviously, the buoy 48 must be positioned along the pennant wire so as to leave a loop of wire below the lower end of the rope 46 which is long enough to accommodate any variations in sea level. In the arrangement shown, where the water depth is 1 50 metres and the length of the pennant wire is 225 metres, the distance AB is 20 metres, the distance BC is 50 metres, the distance CD is 100 metres, and the distance DE is 55 metres.

It wil be apparent that the mooring system of Fig. 3 has the same advantages as that of Fig. 1, in that it prevents pennant wire chafing, whilst utilising light-small spar buoys.

Moreover, the system of Fig. 3 can also be used with anchors provided with piggy-back anchors, so can be used where chain chasers cannot. The system of Fig. 3 has the additional advantage, when compared with the system of Fig. 1, that there is no chafing at the sea bed level at all, and so no need for chain replacement. However, the system of Fig. 3 is more complicated to set up. as it requires careful positioning of the submersible buoy.

Another advantage of both systems is that the buoys 1 2 and 47 can be of rectangular cross-section. Consequently, they can be laid in stable positions on the deck of a support vessel during re-tensioning activities. They are, therefore, unlikely to roli around, even in heavy seas, and so constitute little or no danger to the crew.

Fig. 4 shows the third form of mooring system. This system is similar to that shown in Fig. 1. Accordingly, like (but primed) reference numerals have been used for like parts, and only the modified parts will be described in detail. The third mooring system differs from that shown in Fig. 1 only in that the pennant wire 7' is connected directly to the polypropylene rope 11' by a shackle 10' which is positioned at a predetermined distance X above the sea bed 1 3' (rather than lying on the sea bed). This distance X is so chosen that the shackle 10' will not chafe on the sea bed 13' even under the severest expected sea conditions.

Figs. 5 to 9 show. in detail, the spar buoy 12'. As shown in Fig. 5, the buoy 12' has a central tie tube 51, a pair of end plates 52 and 53, a lifting eye 54, a roping bollard 55 and a synthetic plastics body 56. The parts 51, 52, 53, 54 and 55 are made of steel, and are welded together before the body 56 is formed thereon (in the manner described above).

The roping bollard 55 has a central, vertical tubular member 57 and a pair of diverging fins 58 which are also welded to the tie tube 51 and the end plate 53. A plate 59 is welded to the base of the tubular member 57, the plate 59 having a tubular mooring eye 60 welded thereto. This mooring eye 60 is connected to a rope 61 (such as the rope 11' of the mooring system of Fig. 4-see Fig. 7), by means of a shackle 62 (see Fig. 9). The lifting eye 54 is generally T-shaped (see Fig. 6).

Alternatively, this eye could be as shown in Fig. 8.

When in use, with the mooring system of Fig. 4, the buoy 12' occasionally needs to be removed from the sea. In order to do this, a service vessel manoeuvres alongside, and the buoy 12' is lassood with a loop of wire. The loop of wire engages a fin 58 of the roping bollard 55, so that the buoy 12' can be lifted out of the water.

This procedure is relatively simple, and is to be contrasted with the equivalent procedure for known buoys not provided with roping bollards. Thus, when a known buoy is lassood, the heavy wire lassoo sinks, and it is extremely difficult for the lassoo then to grip the buoy. Obviously, the buoy 12' could be used instead of the buoys 1 2 and 47 in the mooring systems of Figs. 1 to 4.

Claims (22)

1. A mooring system for a floating stucture, the mooring system comprising an anchor, an anchor chain, a pennant wire, a rope and a buoy, the anehor chain connecting the anchor to the floating structure, the pennant wire having one end attached to the anchor, the other end of the pennant wire being attached to one end of the rope, and the other end of the rope being attached to the buoy.

2. A mooring system as claimed in claim 1, wherein the anchor is connected to said one end of the pennant wire by means of a tail chain.

3. A mooring system as claimed in claim 1 or claim 2, wherein said other end of the pennant wire is attached to said one end of the rope by means of an intermediate elongate member.

4. A mooring system as claimed in claim 3, wherein the intermediate elongate member has a length which is substantially equal to the difference in the sea level at slack water and the expected severest sea condition likely to be encountered.

5. A mooring system as claimed in claim 4, wherein the intermediate elongate member has a length of 40 metres.

6. A mooring system as claimed in any one of claims 3 to 5, wherein a chain constitutes the intermediate elongate member.

7. A mooring system as claimed in claim 1 or claim 2, wherein a submersible buoy is attached to the pennant wire part-way therealong.

8. A mooring system as claimed in claim 7, wherein the submersible buoy is so positioned, in use, that any movement of the firstmentioned buoy resulting from sea level changes is taken up by that portion of the pennant wire which lies between said other end and the submersible buoy, said portion forming a loop hanging down from the submersible buoy and said other end of the pennant wire.

9. A mooring system as claimed in claim 1 or claim 2, wherein said other end of the pennant wire is attached directly to said one end of the rope, the system being such that, in use, the major part of the length of the pennant wire lies on the sea bed, and such that the connection point between said other end of the pennant wire and said one end of the rope lies a sufficient distance above the sea bed to prevent said point chafing on the sea bed in the severest expected sea conditions.

1 0. A mooring system as claimed in any one of claims 1 to 6, wherein the pennant wirehas a length of 225 metres and the rope has a length of 140 metres.

11. A mooring system as claimed in any one of claims 1 to 10, wherein the firstmentioned buoy is a small glass fibre spar buoy of about 50 kilogram displacement.

12. A mooring system as claimed in any one of claims 1 to 10, wherein the firstmentioned buoy is of the foamed elastomer type, and has a roping bollard fixed to its base.

1 3. A mooring system as claimed in any one of claims 1 to 12, wherein the rope is a polypropylene rope.

14. A mooring system as claimed in any one of claims 1 to 13, wherein the anchor is a piggy-back anchor, and wherein the piggyback anchor is attached to the anchor chain via another (main) anchor.

1 5. A mooring system as claimed in claim 14, wherein the piggy-back anchor is attached to the main anchor by a ground wire and a tail chain.

16. A mooring system substantially as hereinbefore described with reference to, and as illustrated by, Fig. 1, Fig. 1 as modified by Fig. 2, Fig. 3 or Fig. 4 of the accompanying drawings.

1 7. A floating stucture whenever anchored to the sea bed by one of more mooring systems of the type claimed in any one of claims 1 to 16.

1 8. A floating stucture as claimed in claim 17, wherein the floating structure is a semisubmersible oil rig.

1 9. A method of adjusting a mooring system comprising an anchor, an anchor chain, a pennant wire, a rope and a buoy, the anchor chain connecting the anchor to a floating structure, the pennant wire having one end attached to the anchor, the other end of the pennant wire being attached to one end of the rope, and the other end of the rope being attached to the buoy, the method comprising the steps of lifting the buoy out of the water, winching the rope out of the water, winching the pennant wire out of the water, thereby lifting the anchor off the sea bed, and resiting the anchor on the sea bed so that the anchor chain has a predetermined tension.

20. A buoy for use with the mooring system as claimed in any one of claims 1 to 18, wherein the buoy is of the foamed elastomer type, and has a roping bollard fixed to its base.

21. A method of making a buoy as claimed in claim 20, the method comprising the steps of fabricating a roping bollard onto a central tie tube, foaming polyurethane around the central tie tube, fixing sheets of polystyrene around the foamed polyurethane using an adhesive, and spraying an elastomer onto the polystyrene.

22. A buoy substantially as hereinbefore described with reference to, and as illustrated by, Figs. 5 to 9 of the accompanying drawings.