There are many structures that have been built and are to be assembled offshore for purposes connected with the offshore oil and gas industries. Such structures commonly comprise a supporting structure of some kind which is mainly below sea level but extends up to a height above sea level, and a superstructure, often referred to as a deck, supported above sea level on the structure. The structure commonly stands on the seabed and is usually referred to as a jacket. Typically, it comprises a plurality of legs extending upwardly from the seabed to the top of the jacket and diagonal and cross bracing that together hold the legs against relative lateral movement; the vertical load carried by the jacket is borne principally by the legs. An alternative arrangement is one where the support structure is generally in the form of an elongate buoyant body floating in the sea with its longitudinal axis substantially vertical and a plurality of tethers, which may be chains, extending from the buoyant body and anchored to the seabed. The tethers not only restrain the support structure against horizontal movement but also against

vertical movement. Such arrangements involving buoyant bodies held down by tethers are commonly referred to as spars and are particularly suitable for use in deep water.

The nature of the deck is dependent upon the purpose of the structure. For example, it would commonly comprise principally a drilling rig but might consist exclusively of accommodation for workers on an adjacent rig. During installation, the support structure is commonly located in position on the seabed first and the deck thereafter placed on top of the support structure. The deck may be built as a single unit onshore, taken out to sea and placed on top of the support structure, or it may be built as a number of separate modules that are taken separately to the support structure and assembled only as they are placed on the structure.

It will be appreciated that the form of superstructure and the form of the supporting structure vary considerably from one structure to another and the terms"deck", "jacket"and"spar"as used herein need to be understood as correspondingly broad.

Leaving aside the problem of installation there may often be considerable benefit in building an entire deck onshore and then taking the deck as a single unit to the supporting structure and installing it on the structure.

One method that may be adopted for installation is to use a vessel with a large crane to lift the deck from a barge on

which it has been brought from the shore and place it on the supporting structure. Such a method is convenient when the deck is not too large, but for decks having a mass or more than, say, 10,000 tons there are very few crane vessels available to lift the deck onto the support structure.

An alternative method for installing a deck on a support structure is that often referred to as a floatover method. In this method, the deck is carried to the support structure on one or more barges and the deck and barges are arranged so that the deck can be brought into a position directly over the support structure. The deck can then be lowered onto the support structure. Such a method can avoid the need for a large crane vessel but requires one or more specially designed barges, which is liable to make the method expensive.

It is an object of the invention to provide an improved method of installing a deck on an offshore structure.

It is a further object of the invention to provide an improved method of removing a deck from an offshore structure.

It is a still further object of the invention to provide a combination of vessels suitable for carrying out the methods of the invention.

According to the invention there is provided a method of installing a deck on an offshore support structure, the method including the following steps: transferring the deck onto a first vessel, bringing the first vessel into the vicinity of a second vessel, connecting the first vessel to the second vessel, bringing the combination of the first and second vessels into a position in which the support structure is located adjacent to the second vessel, transferring the deck from the first vessel to the second vessel, transferring the deck from the second vessel to the support structure, and removing the first and second vessels from the vicinity of the support structure.

Although using two vessels to convey the deck to the offshore support structure introduces some complications, we have also found that it has considerable advantages.

For example, the first vessel may be designed to be able to carry the deck in shallow water, thereby enabling the first vessel to take the deck directly from the quayside; on the other hand, the second vessel need not be required to carry out that task and may therefore be designed with a considerably greater minimum draft; it may also rely partly on the buoyancy of the first vessel to provide adequate

support for the deck, even when the deck is supported wholly on the second vessel. Also, one of the vessels, preferably the first vessel, need not be specially designed for use in the method of the invention but rather may be an existing vessel that has other uses also. In such a case, even if the second vessel has to be designed and built specially to fulfil the purposes of the invention, the overall capital cost of supplying the equipment to carry out the method of the invention may be less than that which applies using known floatover techniques.

Preferably, the step of transferring the deck from the first vessel to the second vessel is by substantially horizontal movement of the deck.

The method is preferably arranged such that the installation procedure can be reversed (for example because of unexpected bad weather) right up to a stage in the step of transferring the deck from the second vessel to the support structure where significant load is transferred to the support structure. Such is the case in an embodiment of the invention described below.

The invention is particularly applicable to the case in which the offshore support structure is a buoyant structure supported in the water by its buoyancy. Such a structure may be held in position by a plurality of tethers extending from the support structure and anchored to the seabed.

The step of transferring the deck from the first vessel to the second vessel preferably takes place after the step of bringing the combination of the first and second vessels into a position in which the support structure is located adjacent to the second vessel. It should be noted, however, that an alternative possibility is to transfer the deck from the first vessel to the second vessel before the vessels are brought adjacent to the support structure. Thus the sequence of steps described above does not in that respect represent a fixed order in which the steps must be carried out, although the remaining steps are usually carried out in the order in which they are listed.

As indicated above, it is an advantage of the invention that the deck can be transferred onto the first vessel at a quayside. The step of transferring the deck onto a first vessel preferably comprises the step of skidding the deck from a quay onto the first vessel. Where reference is made herein to"skidding"it should be understood that a generally horizontal movement of one body over the top of another body is being referred to; the use of the term does not, however, imply that surfaces of the two bodies that are in contact with one another are forced to slide over one another; such sliding movement may be avoided by the use of rollers or the like.

Preferably the first and second vessel are connected in such a manner that relative vertical movement of the vessels at the connection is precluded. Thus any vertical forces applied to one vessel can be transferred partly to the other vessel. Preferably the first and second vessels are connected in such a manner as to allow limited relative pivotal movement about a horizontal axis lying approximately in a vertical plane through the interface of the first and second vessels. By allowing such pivotal movement, the stresses arising in the adjoining portions of the first and second vessels can be reduced considerably.

Nonetheless, because vertical forces are able to be transferred through the connection the first vessel can still provide support to the second vessel to assist it in supporting the weight of a deck. Such support may not be necessary for a relatively light deck but is advantageous for heavier decks. Furthermore the first vessel can add stability to the second vessel by virtue of its connection to the second vessel; thus the second vessel may be unable to support a given deck in a stable manner on its own, but may become capable of so doing by virtue of its connection to the first vessel. Such features further reduce the design requirements (and therefore the cost) of the second vessel. Preferably said limited relative movement of the first and second vessels is the only movement that is allowed.

When the combination of the first and second vessels are brought into the position in which the support structure is located adjacent to the second vessel, the support structure is preferably located at least partly within the envelope of the second vessel, when viewed in plan. The second vessel preferably has two hulls which, when the combination of the first and second vessels are brought into the position in which the support structure is located adjacent to the second vessel, are positioned adjacent to, and on opposite sides of, the support structure. The second vessel preferably has a transverse portion which extends between the two hulls and which, when the combination of the first and second vessels are brought into the position in which the support structure is located adjacent to the second vessel, is also positioned adjacent to the support structure. The transverse portion may itself contribute to the buoyancy of the vessel and may have ballast tanks.

The step of transferring the deck from the first vessel to the second vessel preferably comprises a first step of skidding the deck from a first position on the first vessel to a second position, a second step of transferring the load of the deck from a first set of supports to a second set of supports and a third step of skidding the deck from the second position to a third position on the second vessel. The second position of the

deck will usually be one in which the deck is close to the interface between the first and second vessels. Preferably the second position of the deck is one in which the deck is at least mainly supported, preferably wholly supported, by the second vessel.

During the step of transferring the deck from the second vessel to the support structure, the second vessel is preferably ballasted. In the case where the support structure is a buoyant structure floating in the water, the support structure may be de-ballasted during this step.

Support elements provided on the second vessel for supporting the deck may be retracted during this step.

The invention is especially useful for installing massive decks on support structures. For example, while it is envisaged that the method may be used when the deck has a mass of more than 1,000 tons, it is likely to be of greater advantage in a case where the deck has a mass of more than 10,000 tons and may be used for decks having a mass of 30,000 tons or more.

As indicated above, the minimum draft of the first vessel when carrying the deck is advantageously substantially less than the minimum draft of the second vessel when carrying the deck. For example, the minimum draft of the first vessel when carrying the deck may be less than 7m and the minimum draft of the second vessel when carrying the deck may be more than 7m.

Whilst the invention has been described to this point in connection with a method of installing a deck on a support structure, it can also be employed as a method of removing a deck from an offshore support structure.

Accordingly the present invention also provides a method of removing a deck from an offshore support structure, the method including the following steps: bringing a first vessel into the vicinity of a second vessel, connecting the first vessel to the second vessel, bringing the combination of the first and second vessels into a position in which the support structure is located adjacent to the second vessel, transferring the deck from the support structure to the second vessel, transferring the deck from the second vessel to the first vessel, removing the combination of the first and second vessels from the vicinity of the support structure, separating the first vessel and the deck from the second vessel, and removing the deck from the first vessel.

It will be understood that when removing a deck from a support structure the method for installing a deck that is described above can be carried out mutatis mutandis in reverse. Thus the various features referred to above in

connection with the method of installing a deck may be applied mutatis mutandis to a method of, \. removing a deck from an offshore support structure.

Preferably, the step of transferring the deck from the second vessel to the first vessel is by substantially horizontal movement of the deck.

The present invention further provides a combination of a first vessel and a second vessel, for use in installing a deck on an offshore support structure and/or removing a deck from an offshore structure, the first and second vessels being connectable together, the first vessel preferably being capable of supporting a deck having a mass of more than 10,000 tons and the first and second vessels preferably being arranged to accommodate skidding of a deck of more than 10,000 tons from the first vessel to the second vessel, when the first and second vessels are connected together.

The combination of vessels may be suitable for employment in any of the methods described above and in particular the connection between the vessels may be of the character described above. Thus the design requirements (and therefore the cost) of the second vessel can be reduced. It will also be understood that in certain applications the deck may be loaded directly onto the second vessel and never be supported (directly) on the first vessel ; the first vessel is nonetheless essential to

the invention in providing support and/or stability to the second vessel.

By way of example, a method of installing a deck on a spar will now be described with reference to the accompanying schematic drawings, of which: Fig. 1 shows a first vessel carrying the deck and ready for docking with a second vessel, Fig. 2 shows the first and second vessels after docking and with the vessels connected together, Fig. 3 shows the combination of the first and second vessels, with the deck supported on the first vessel, having been brought into the vicinity of the spar, Fig. 4 shows the combination of the first and second vessels, with the deck supported on the first vessel, after the combination has docked with the spar, Fig. 5 shows the combination of the first and second vessels, after the deck has been skidded to a position on the second vessel in which it is positioned over the spar, Fig. 6 shows a part of one possible arrangement for transferring the load of the deck from the second vessel to the spar, and

Fig. 7 shows the combination of the first and second vessels, after they have been towed clear of the spar.

Referring first to Fig. 1, there is shown a first vessel 1, a second vessel 2 and a deck 3. Both the vessels 1 and 2 are barges and rely on tugs to be manoeuvred as required. In the drawings a main towing tug is designated by numeral 4, an auxiliary towing tug by numeral 5 and steering tugs by numeral 6.

The vessel 1 is a conventional barge of a kind generally known per se and which may be used for various general purposes apart from those relating to the invention. In this particular example, it is the barge known as the Saipem S44 barge. The barge 1 is able to operate with a reasonably small draft even when carrying a heavy load. It can therefore be loaded at a quayside where the deck 3 is skidded as a single unit onto the barge 1 in a reasonably central position on the barge as shown in Fig.

1.

The barge 1 is then taken to deeper water and manoeuvred into an appropriate position for docking with the vessel 2, that situation being shown in Fig. 1.

The vessel 2 is also a barge but is specially built for the invention. It may be referred to as an "articulated load transfer unit". Because the general purpose barge 1 is already provided with various standard

equipment and accommodation, the specially built barge 2 need only be provided with facilities essential for its own specialised purposes. It has a pair of hulls 7A and 7B connected together at one end by a transverse portion 8.

Thus the barge 2 is generally'U'shaped. The hulls 7A and 7B are spaced apart by a distance slightly greater than the width of the barge 1 and the transverse portion 8 is inset from the ends of the hulls 7A and 7B so that, when the barges 1 and 2 are docked, the hulls 7A and 7B overlap part of the barge 1, as shown in Fig. 2. The minimum draft of the barge 2 when carrying a heavy load is substantially greater than that of the barge 1, but that is not a problem, because the barge 2 is not required to dock at a quayside when loaded. Whilst such docking is of course possible at a deep quayside, for a shallower quayside the barge 1 is the only barge that need dock at the quayside.

The barge 1 is provided with skid beams 10A and 10B of a kind known per se and the barge 2 is provided with similar skid beams 11A and 11B which, as can be seen in Fig. 2 align with the skid beams 10A and 10B when the barges 1 and 2 are docked.

The barge 2 also has further skid beams 12A and 12B located laterally outwardly of the skid beams 11A and 11B respectively and extending the whole length of the barge 2.

Once the barges 1 and 2 have docked, they are connected together by a connection system (not shown) that

prevents all relative movement of the barges 1 and 2 apart from limited pivotal movement about a horizontal axis H, marked by a dotted line in Fig. 2. Some minor modification of the barge 1 is required in order to provide components of a suitable connection system on it.

The combination of the barges 1 and 2 is then navigated to the vicinity of the support structure on which the deck 3 is to be supported. In the example of the invention described herein that support structure is a buoyant elongate body 14 of substantially circular cross- section as shown in Fig. 3. The body 14 floats in the sea with its longitudinal axis substantially vertical and is held in its selected position by a plurality of chains 15 which extend downwardly and outwardly from the body 14 and are anchored to the seabed. The support structure is referred to herein as a spar; such spars are already well- known in the offshore industry and the particular construction of the spar 14 is not a part of the present invention and will not be described in detail herein.

Once the combination of the barges 1 and 2 reaches the spar 14 it is manoeuvred into an appropriate position and orientation relative to the spar and mooring lines 16 are connected between the spar 14 and the barge combination, as shown in Fig. 3. Also at this stage, the spar 14 is ballasted and the chains 15 hauled in so that the spar is floating lower in the sea than is usual.

The combination of barges is then brought up to the spar to take up the position shown in Fig. 4. The hulls 7A and 7B are spaced apart sufficiently that they can pass on opposite sides of the spar 14 and the transverse portion 8 has a curved profile on its inner face to enable it to be brought adjacent to the spar across the whole of the space between the hulls 7A and 7B. Thus the spar 14 is accommodated wholly within the envelope of the barges 1, 2, when viewed from above. Suitable fendering and mooring systems are provided to keep the spar and the barges correctly positioned relative to one another.

At the stage shown in Fig. 4 and in the particular example being described, the deck 3 is still supported wholly by the barge 1. Once the barges 1,2 are secured to the spar 14, the deck 3 is skidded substantially horizontally over the barges to the position shown in Fig.

5; that skidding has three distinct steps: in a first step the deck 3 is skidded substantially horizontally along the skid beams 10A, 10B of the barge 1 and then also along the skid beams 11A and 11B of the barge 2, until the deck 3 is supported only on the skid beams 11A and 11B of the barge 2; in a second step the vertical load of the deck 3 carried on the skid beams 11A and 11B is transferred to the skid beams 12A and 12B, without any horizontal movement of the deck 3; in the third and final step the deck 3 is skidded substantially horizontally along the skid beams 12A, 12B

until it is positioned vertically above the spar 14 (with a small vertical gap between the ballasted spar 14 and the deck 3).

The skidding of the deck as just described and the transfer of load from the skid beams 11A, 11B to 12A, 12B may be accomplished by any suitable means. Usually, each of the first and second steps will be accompanied by appropriate redistribution of ballast within a barge and/or some overall ballasting or de-ballasting. The transfer of load from the skid beams 11A, 11B to the beams 12A, 12B is facilitated because both sets of beams are on the same barge 2 and may involve extension/retraction of jacking systems associated with the deck 3 and/or the barge 2.

Once the deck 3 is positioned in the correct position above the spar 14, it then remains to transfer the load of the deck 3 to the spar 14. Such a transfer can be carried out using one or more known techniques; those techniques are likely to include lowering the level of the deck 3 by ballasting the barge 2 and/or by retracting a jacking arrangement between the deck and the barge, and also de- ballasting of the spar 14. Further de-ballasting of the spar 14 may be carried out even after all the load has been transferred to the spar, in order to increase the freeboard of the spar back to an operating freeboard. Also the barge 2 may be further ballasted to increase the vertical clearance between the barge and the spar 14.

Fig. 6 shows one possible arrangement that may be employed to effect the transfer of the deck 3 from the barge 2 to the spar 14. Referring to Fig. 6, the deck 3 has a plurality of feet 18 which are positioned on the deck in positions corresponding to sockets 19 within which the feet are to be received and supported. One of the feet 18 and its corresponding socket 19 is visible in Fig. 6. A plurality of jacking assemblies 20 are provided on each of the hulls 7A and 7B and one of those is shown in Fig. 6 supporting the deck 3 adjacent to the foot 18 that is shown in Fig. 6. Each jacking assembly 20 has a base part 21 mounted on the barge 2 and a top part 22 which supports the deck 3 and which is slidably received in a cylindrical part 23 of the base part 21. Sliding movement of the top part 22 relative to the cylindrical part 23 is controlled by six piston and cylinder assemblies 24 equiangularly spaced around the parts 22 and 23 and each connected at a lower end to the base part 21 and at an upper end to a flange on the top part 22.

Immediately prior to the procedure of transferring the load of the deck 3 from the barge 2 to the spar 14, the parts are in the position shown in Fig. 6: the piston and cylinder assemblies 24 are extended and the deck 3 is positioned with the feet 18 over, but above and spaced from, the sockets 19. When it is decided to commence transfer of the load, the piston and cylinder assemblies 24

are retracted. It will be appreciated that even following full engagement of the feet 18 in the sockets 19, transfer of load continues to be progressive because as the load is transferred to the spar 14 it tends to sink lower into the water and as the load is removed from the barge 2 it tends to rise up from the water. Thus by controlling the rate of extension of the piston and cylinder assemblies 24 and, if desired, ballasting of the barge 2 and/or de-ballasting of the spar 14, the rate of load transfer can be controlled, as desired. It may be noted that right up until towards the end of this procedure, sea-fastenings between the deck and the second vessel may be kept in place, thereby enabling the operation to be aborted and the vessels 1,2 with the deck 3 removed from the vicinity of the spar even at a very late stage in the procedure.

After the load transfer is complete and the barge is well clear of the deck 3, the combination of barges 1,2 can be unmoored from the spar 14 and towed clear of the spar 14, on which the deck 3 is now supported. Thereafter the barges 1,2 can be disconnected from one another and made available for other use.

In a particular example of the invention, where it is intended to install a deck having a mass of about 30,000 tons, the barges 1,2 may be as follows: overall length of barge 1: 190m overall width of barge 1: 50m

approximate minimum draft of barge 1 when carrying deck: 5m overall length of barge 2: lllm overall width of barge 2: 86m approximate minimum draft of barge 2 when carrying deck: 8.4m free space between hulls 7A and 7B: 54m As will be understood, although a procedure for installing a deck on a support structure has been described above, the barges 1,2 can also be used for removing a deck from a support structure, by carrying out the steps referred to above in reverse. Thus, the method may involve the following steps, which are described only briefly in view of the description already provided above: (a) connect together barges 1,2; (b) bring the barges to the spar 14 and deck 3 and moor them to the spar 14; (c) transfer the load of the deck 3 from the spar 14 to the barge 2 and lift the deck 3 clear of the spar 14; (d) skid the deck 3 along the barge 2 on the skid beams 12A and 12B; (e) transfer the load of the deck 3 from the skid beams 12A and 12B to the skid beams 11A and 11B ; (f) skid the load of the deck 3 first along the skid beams 11A and 11B and then along the skid beams 10A and 10B to transfer the deck 3 to the barge 1;

(g) release the mooring of the combination of barges 1, 2 to the spar 14 and tow the barges away from the spar; (h) disconnect the barge 2 from the barge 1 ; (i) tow the deck 3 on the barge 1 to a quayside; and (j) transfer the deck 3 from the barge 1 to the quay.

As is mirrored in the case of installing the deck 3 on the spar 14, steps (d) to (f) may instead be carried out after step (g), if preferred.