GB2439295A

GB2439295A - Method and vessel for laying reeled articles at sea

Info

Publication number: GB2439295A
Application number: GB0612260A
Authority: GB
Inventors: Jean-Baptiste Pose; Stewart Kenyon Willis
Original assignee: Acergy UK Ltd
Current assignee: Acergy UK Ltd
Priority date: 2006-06-21
Filing date: 2006-06-21
Publication date: 2007-12-27
Also published as: GB0612260D0

Abstract

A pipe laying vessel (100, Fig 1) has the form of a semi-submersible lift ship, having a submersible load deck (104, Fig 1)). The ship is ballasted temporally to a depth where a reel-carrying barge 220 can be floated over the load deck, and then de-ballasted to carry the barge and its load to a working location. Pipe handling apparatus (120, Fig 1) is positioned to receive a prefabricated pipeline (206, Fig 2) from the reel carried on the barge. Auxiliary barges 230 may be provided for termination modules 242, such as tees. Two reel barges 200 and 220 may be provided to permit continuous laying and re-stocking without return to base. Both barges may be carried on the load deck in a different orientation, for transit between operating regions. The combination is self contained and not limited to any particular region for deployment.

Description

PIPE LAYING VESSEL, AUXILIARY VESSELS AND

METHODS OF USE THEREOF

The invention relates to pipe laying vessels, particularly of the type which is stocked with pre-fabricated pipeline stored on one or more reels.

Reel lay vessels are well-known in the art for laying steel pipe in a range of diameters to the seabed. In known vessels such as Technip's Deep Blue, or the older Apache, the largest load capacity is only achieved by fixing the reel permanently to the vessel, with as low a centre of gravity as possible. An advantage of reel lay is that the pipe can be laid at a good rate, without the delay of fabricating the pipeline from shorter sections.

Depending on the radius of the reel and diverter structures in the pipelay system, pipes of significant diameter can be reeled, unreeled, straightened and laid to the seabed in this way.

To stock the reel, the vessel must attend a shore base where the pipeline is being fabricated, which adds considerably to the expense and delay of many operations, and may in practice limit the regions in which the vessel can operate. To mitigate this drawback, different measures have been tried. An obvious measure is to increase the capacity of the reel, but this cannot be done without engineering difficulties.

Accordingly, the vessel Deep Blue features a pair of upright reels, as described in US 6371694, allowing a larger load to be carried in manageable units. This makes for a large and expensive vessel, however. Another approach is described in different forms in US 6328502 (Global) and WO 04/068012 (StoltiAcergy). In these proposals, a large reel is mounted horizontally on a lay barge together with a pipeline factory, allowing the reel to be stocked from pipe sections without returning to a spool base. The Global example provides for demounting the reel when not required, but not while it is laden.

Similarly, in WO 03/004915 (Torch Offshore), two reels are provided side by side, and the tower carrying the pipe laying apparatus translates from side to side to receive pipe from one then the other, while the reel not in use for laying is re-stocked with pipe from an on-board pipeline factory.

The inventors have recognised that a submersible lift ship, such as is occasionally used to recover frigates that have run aground in foreign parts, can form the basis for a modular pipelay system which keeps the simplicity of conventional reel lay, but substantially eliminates the normal delay in re-spooling. In its normal use the lift ship is submerged and a vessel is floated over the deck. The lift ship is then de-ballasted so that the vessel is lifted clear of the water and is carried as deck cargo on the lift ship. A known example of such a lift ship is Dockwise's Blue Marlin, famous for recovering stricken US naval vessels, and delivering large oil production vessels from the shipyard to their working location.

The inventors have recognised that, by adapting such a vessel to pipe laying operations, barges pre-laden with reeled pipe and/or modules such as end terminations can be floated onto and off the vessel, avoiding the delays associated with reeling on pipeline at a shore base.

The invention provides a pipe laying vessel in the form of a semi-submersible lift ship, the lift ship having a submersible load deck and ballasting means operable firstly to submerge the load deck to a depth where a reel-carrying barge can be floated over the load deck and then to float the ship to carry the barge and its load to a working location, the lift ship being adapted for pipe laying operations by the provision of pipe handling apparatus, said apparatus being positioned in association with said load deck for receiving a pre-fabricated pipeline from a reel carried on such a barge and controlling the paying out of said pipeline to the seabed.

The pipe handling apparatus may comprise a tower structure carrying a pipe diverter and tensioning apparatus above the level of the load deck. The tower structure may be tiltable. It will be appreciated that the particular form of the pipe laying apparatus is not essential to the invention, and many known types are possible. The term tower structure' as used here is intended to encompass all manner of elevated structures, including ramps.

The tower structure may have an open lower portion, to permit modules to be fitted to the pipeline below said tensioning apparatus.

In one type of embodiment, the tensioning apparatus comprises one or more track tensioners. Alternatives such as moving clamps are also possible. The pipe laying apparatus may include a pipe straightener located between the diverter and the tensioning apparatus.

The lift ship may be provided in combination with at least one reel barge, being a floating vessel having at least one pipe reel mounted on it so as to feed pre-fabricated pipeline to the pipe laying apparatus when the reel barge is carried on the load deck of the lift ship.

The reel barge can be simply that, without pipe laying, apparatus and without pipeline fabrication apparatus. The reel barge may carry two or more reels. Each reel may have a load capacity in excess of 1 500t.

The combination may further include an auxiliary barge which can be accommodated on the load deck of the lift ship simultaneously with the reel barge, for carrying modules to be fitted to the pipeline during laying.

In a preferred arrangement, the pipe laying apparatus is located at one end of the load deck, for example at the stern of the lift ship. The reel barge and auxiliary barge may be carryable on the load deck with the auxiliary barge located between the reel barge and the pipe laying apparatus. in an alternative arrangement, one could imagine the pipe laying apparatus is located between two separate parts of the load deck (or two separate load decks), the reel barge and auxiliary barge then being accommodated on opposite sides of the apparatus.

The combination may further include a second reel barge, such that one of said reel barges can be re-stocked with pipeline separately from the lift ship, while the other is carried on the load deck of the lift ship.

The reel barges may be dimensioned so that both can be carried on the load deck for transit between operating regions. The reel barges and load deck may be dimensioned so that one barge only can be carried in a working orientation to feed the pipe laying apparatus, while both barges can be carried together in a different orientation for transit between operations. The combination is thus self-contained and not limited to any particular region for deployment.

It is envisaged that the pipe handling apparatus is permanently mounted on the vessel.

In principle, however, the apparatus would itself be carried on a barge so as to be separable from the lift ship when not required.

The ballasting means will typically comprise a number of ballast tanks distributed within the vessel and pumps for removing water from said tanks to de-ballast the vessel. The ballasting means may further comprise pumps for flooding the tanks in a controlled manner to ballast the vessel, although valves coupling the tanks to the surrounding water would in principle be sufficient, if controlled safely enough. Other forms of moveable ballast could in principle be used instead of or in combination with seawater, but the convenience of floodable ballast tanks makes them the natural choice.

The invention further provides various methods of operation based on the concept of the vessel mentioned above. These methods may be conducted in different territories, even though they are part of one larger commercial operation. For this reason, different methods are set forth here and claimed independently.

The invention provides a method of loading a pipe laying vessel with pre-fabricated pipeline stored on a reel, wherein the pipe laying vessel is provided in the form of a semi-submersible lift ship having a submersible load deck and having pipe handling apparatus for receiving a pre-fabricated pipeline from an associated storage location above the load deck and controlling the paying out of said pipeline to the seabed, the method comprising: (a) providing a reel-carrying barge with at least one reel loaded with said prefabricated pipeline; (b) ballasting the vessel to submerge the load deck below water; (c) manoeuvring the reel barge and vessel such that the load-carrying barge is positioned over the load deck; (d) de-ballasting the vessel to carry the reel barge on the load deck, thereby presenting the loaded reel at the storage location associated with the pipe handling apparatus.

The vessel in step (b) may be already carrying a second reel barge on its load deck, the method further including a step (bi) in which the second load carrying barge is floated clear of the load deck before step (c). The second reel barge may be of similar form to the first-mentioned barge, having at least one reel which is depleted of pipeline (or loaded with an unsuitable type for the job in hand). The method may further comprise (e) re-loading the reel of the second reel barge with pre-fabricated pipeline while the vessel with the first reel barge is engaged in pipe laying operations elsewhere. These steps allow rapid re-stocking of the vessel in the course of pipe laying operations, since the vessel does not need to attend the manoeuvring site for the duration of the reeling operation.

The vessel in step (b) may be already carrying the first and second reel barges together in a transit configuration, step (bi) including floating off both barges. This allows the vessel and both reel barges to sail as a self-contained unit between widespread regions of operation. The barges in step (b) may be in a different orientation to that required to present a loaded reel at said storage location, the method further comprising re-orienting the first reel barge prior to step (d).

The first reel barge in said transit configuration may be already loaded with said pre-fabricated pipeline. Alternatively, the first reel barge may arrive in the transit configuration with both reel barges depleted of pipeline, the method then including loaded the first reel barge with pipeline before the vessel and first reel barge sail for pipe laying operations. The step (a) in that case may include loading the first reel barge with pipeline before or after any of steps (b), (c) and (d).

The steps (c) and (d) may include loading the vessel with a stock of one or more pipeline accessories such as end terminations, to be fitted to the pipeline during laying operations.

The pipeline accessories may be loaded on board the first reel barge, or directly on the load deck (after floating the vessel in step (d), of course).

Alternatively the pipeline accessories may be carried by an auxiliary barge floating separately from the first reel barge, the method further comprising step (ci) in which the auxiliary barge loaded with accessories is also manoeuvred into a position over the load deck. In a preferred embodiment, the auxiliary barge is placed on the load deck at a position generally between the reel barge and the pipe handling apparatus.

In an embodiment where the vessel arrives for step (b) in a transit configuration with first and second reel barges carried on the load deck, the auxiliary barge may also be carried on said load deck. In a preferred embodiment, there are first and second reel barges and first and second auxiliary barges, all of which can be accommodated on the load deck in said transit configuration. Reel barges cannot be stacked (unless the reels are demountable). The first auxiliary barge may be stacked on top of the second auxiliary barge in the transit configuration.

In a preferred embodiment, described below, each of the first and second reel barges has two comparable reels. It is well known that smaller reels for auxiliary pipes, cables arid the like may be provided. These are generally movable with cranes, whether empty or laden, and are not the features constraining design and operational flexibility.

Auxiliary reels may be carried on the reel barges, on the auxiliary barges or both.

In the step (c) (and (bi) where present), one of the reel barge and the pipe laying vessel may be manoeuvred while the other holds a fixed position, or both may move.

Manoeuvring of each may be by on-board propulsion devices, separate tugs or a combination of both. The pipe laying vessel will typically have a comprehensive array of thrusters and dynamic position control system, while the barges may have no on-board propulsion, relying entirely on tugs and moorings for their positioning.

The invention further provides a method of laying at sea a pre-fabricated pipeline stored on at least one reel, the method being performed using a pipe laying vessel in the form of a semi-submersible lift ship having a submersible load deck and having pipe handling apparatus, wherein said reel is carried on a first reel barge which in turn is carried temporarily on the submersible load deck such that the loaded reel is presented at the storage location associated with the pipe handling apparatus, the method comprising the step (f) using said pipe laying apparatus to receive pipeline from said reel and control the paying out of said pipeline to the seabed into the sea.

The method may further include (g) fitting one or more pipeline accessories such as end terminations to the pipeline during said paying out. The pipeline accessories prior to fitting may be carried on board the first reel barge, or directly on the load deck.

Alternatively the pipeline accessories may be carried by an auxiliary barge carried on the load deck. In a preferred embodiment, the auxiliary barge is placed on the load deck at a position generally between the reel barge and the pipe handling apparatus.

The method may further comprise the steps of (h) interrupting pipe laying operations, (I) ballasting the vessel to submerge the load deck and float the first reel barge free of the vessel; (j) manoeuvring the first reel barge clear of the storage location and substituting a second reel barge pre-loaded with pipeline; (k) de-ballasting the vessel to raise the load deck and support the second reel barge on the load deck at said storage location; and (1) resuming pipe laying operations using said pipe laying apparatus to receive pipeline from a reel on the second reel barge and control the paying out of said pipeline to the seabed into the sea.

In step (j) the first reel barge may be moved entirely clear of the vessel. The method may further include a step (m) of re-stocking the reel on the first reel barge concurrently with performance of step (1). The steps (j) to (I) may then be repeated with the roles of first and second reel barges reversed, thereby to re-stock the vessel as many times as desired, with minimal interruption.

It is envisaged that the de-ballasting and swapping of barges may need to be performed at a location distant from the pipe laying for a number of reasons. This may be so that the manoeuvres can be conducted in a more sheltered location, but primarily because it will be more economic to bring the lay vessel to an inshore base than to bring barges, tugs etc. to an offshore location where pipe laying is being performed. Step (1) in this context may include the vessel sailing between the location of the de-ballasting operation and the location where pipe laying is performed. Concurrency of steps (m) and (I) therefore is intended to encompass includes re-stocking the first reel barge while the vessel is in to or from the laying location, not only strictly during laying operations.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, by reference to the accompanying drawings, in which: Figure 1 shows the main components of a pipe laying vessel in the form of a semi-submersible lift ship, according to a first embodiment of the present invention; Figure 2 shows the vessel of Figure 1 carrying a first reel barge and auxiliary barge and engaged in pipe laying operations; Figure 3 shows the vessel with the stock of pipes and accessories depleted and a second reel barge and second auxiliary barge ready for substitution; and Figure 4 shows the vessel and barges in a transit configuration.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring to Figure 1 initially, there is shown a pipe laying vessel 100 in the form of a semi-submersible lift ship. The lift ship comprises a hull 102, an extensive load deck 104, forward superstructure 106 (including accommodation for crew, control rooms etc.) and thrusters 108 providing propulsion. Further thrusters may be provided such that the vessel has good speed in transit between operating regions across the world, and also a dynamic positioning (DP) thruster system with automatic controls so that the vessel can hold position and manoeuvre accurately independently of any tugs, moorings etc. A crane 110 is provided near the stern for handling loads onto and off the vessel.

The hull 102 of the vessel includes substantial ballast tanks which can be flooded or emptied of seawater by means of pumps and suitable control systems. The result of this is that the vessel can be raised or lowered in the water specifically to the extent of submerging the load deck 104 completely below the surface of the water. Other vessels, particularly various barges to be described, can then be floated over the vessel before it is de-ballasted, resulting in the vessel being able to sail with the barge and associated load carried safely on the load deck.

The above features are well known in the art and commonly provided on a vessel of this type. What is unusual for a semi-submersible lift ship, is the provision of pipe laying apparatus overhanging the stern of the vessel, aft of the load deck. The apparatus 120 in this example comprises a tiltable tower structure 122 which carries at its upper end a diverter 124 in the form of a large radius wheel, suitable for diverting continuous steel pipe from a storage position above the load deck onto a lay axis aligned with the tower, while protecting the pipe from excessive bending. At the foot of the tower, a hold-off clamp 126 is provided, carried on a retractable support structure 128. Above the clamp, a retractable working platform 130 is also provided. The tower 122 is open in its lower portion, acting as an A frame manoeuvrable over the working platform 130 and providing working space around the lay axis when required. In the upper part of the tower, aligned on the lay axis between. the diverter 124 and the clamp 126, are a straightener 132 for removing the bend acquired by the pipe during reeling and two track-type tensioners 134, 136 for controlling the paying out of the pipe under very high tension. Omitted from the diagram but obviously present in practice is a framework supporting these elements 132-136 on the tower. At the outboard side of the tower at its top is a sheave 138 supporting a lifting block 140, which can be used for abandonment and recovery operations, supporting loads of the pipe line when it is not supported by the tensioners 134 and 136 or the clamp 126.

The tower 120 and associated components are similar to those that might be found on many pipe laying vessels, yet the vessel as shown in Figure 1 has no means for storage or fabrication of pipe line to be laid. The vessel in this case is intended to be operated in combination with a suite of barges, including reel barges and auxiliary barges.

Figures 2 to 4 serve to illustrate the different configurations of the vessel and suite of barges. These configurations support various modes of operation, as will now be described.

Two reel barges 200, 220 are provided in this example, each fitted with two reels 202/222, 204/224 capable of carrying a large quantity of finished pipeline. Each reel may have a capacity of, say 2000t, 3000t or even 3500t of pipe which is wound on by plastically deforming the pipe. Two further barges 230, 240 are used to carry accessories 232/242 such as pipe line end terminations (PLETs) and mid-line fittings that are to be installed in the pipelines.

in a typical operating sequence, the first reel barge 200 is moored at an inshore spool-base where it is aligned with a pipeline fabrication factory in which individual pipes are welded together to form a pre-fabricated continuous pipeline 206. There may be two parallel production lines each supplying a different reel on the same barge. The first PLET barge 230 can be loaded with accessories 232 at the same time but at a different location.

When the first reel barge 200 and the first PLET barge 230 have been loaded, the reel barge is towed into an area of the harbour with sufficient depth, where the lift ship is submerged and the reel barge 200 and PLET barge 230 are floated over the top. The lift ship is then de-ballasted so that the load deck 104 rises, lifting the reels and the PLETs.

The whole assembly then takes on the aspect of a reel-lay vessel as seen in Figure 2.

The vessel thus equipped goes to the work area, which may be some distance offshore, and lays pipe. Meanwhile, the second reel barge 220 is moored at the end of the production lines and pipelines are wound on to the reels 222, 224 for a next phase of laying. The second PLET barge 240 is also loaded.

At the completion of laying the pipe 206 from the first reel barge, the vessel returns to harbour where the second reel barge and second PLET barge are waiting, as seen in Figure 3. It then ballasts down and floats the barge 200 with the empty reels off and replaces it with the barge 220 with full reels and does the same for the PLET barges 230, 240. (These movements are indicated generally by arrows in Figure 3.) This operation, which may take only 4 to 6 hours, provides a very great advantage by avoiding having the reel-lay vessel tied up while stalks of pipe that have been produced in the production lines are joined together and wound on to the reels. This would normally take 4 to 6 days, which might otherwise be used for laying pipe, and transit to and from the lay site.

The sequence of operations shown in Figures 2 and 3 can be repeated for as many times as it takes to complete the desired lengths and types of pipeline in one region.

Figure 4 shows the vessel and barges in transit to work in another location, possibly an entirely different region of the world. While the semi-submersible reel-lay vessel is laying the final barge load in one part of the world, the unused, empty barge (say, 220) can be loaded with pipe for the next region. The vessel 100 then returns to harbour and the barge just used (200) is floated off and turned through 90 degrees, so that its longitudinal axis is athwart the lift ship. The newly laden barge (220) is floated in alongside the empty barge so that both barges span the deck of the lift ship. The PLET barges 230, 240 are stacked and thus can both be carried in the usual position. The vessel is de-ballasted with the result that all the barges needed to repeat the operations at another location are carried in a self-contained manner on the lift ship. This allows the suite of barges and reels to be transported for long distances at commercially

acceptable speeds.

On arrival at a spoolbase in the new region the lift ship is ballasted down and the barges are floated off. The barge 220 with the laden reels is re-aligned so that its longitudinal axis is on the centreline of the lift ship, as the storage location appropriate for operation of the pipe laying apparatus 120. The vessel is then de-ballasted, and is then immediately free to lay pipe and the spool base can load the empty reels on barge 200, in the configuration shown in Figure 2.

Many variations of the suite of vessels and the modes of operation can be envisaged within the spirit and scope of the present invention, including and beyond those mentioned in the introduction and the specific description above. The numbers of reels and barges that may be useful in combination, and the particular maimer of fitting them onto the load deck or decks of the vessel can be varied in several ways according to the capacity of the reels, type of work involved, the size of an available lift ship and so on. 1-, Ii

The pipe lay apparatus itself may be detachable from the lift ship 100, in order that the ship not be committed permanently to pipe lay operations.

While the swapping of barges and reels facilitates the laying of a single pipeline of great length with minimal interruption, it will be appreciated that different types of pipe might be reeled in different loads, or even between the two reels of one load. When preloading for transit to a new job, the type of pipe to be laid may be different from that currently being laid. It is of course possible for the vessel and its suite of barges 200, 240 to travel in the transit mode with none of the reels laden, depending on the economics of supply and demand of the vessel, and of suitable pipe d each location.

The skilled reader will recognise that many details of a typical pipe laying operation have been omitted from this description and drawings, in the interests of clarity and conciseness. Typically, numerous auxiliary reels and accessories will be present, for laying auxiliary pipes, cables and umbilicals, for example. The positioning of the reels on the barges, and the shape and orientation of the barges in operational and transit modes can be varied according to particular requirements. Reels on a barge may be arranged side by side, for example with an angle between their axes according to the patent US 63710694, mentioned in the introduction. Alternatively or in addition, parts of the lay tower may be made moveable from side to side, so as to receive pipe from reels as different positions across the deck. In principle, however, the facility to exchange reel barges and avoid the delays associated with reeling-on of pipe line during its production, should allow a vessel of very high capacity and mobility to be made and operated without resorting to those more complicated known arrangements.

In light of the above modifications and many other that will occur to the skilled person upon reading the present disclosure, it is to be understood that the scope of protection is not limited to the examples shown or discussed, but rather is defined by the full scope of the appended claims.

Claims

CLAIMS

1. A pipe laying vessel in the form of a semi-submersible lift ship, the lift ship having a submersible load deck and ballasting means operable firstly to submerge the load deck to a depth where a barge can be floated over the load deck and then to float the ship to carry the barge and its load to a working location, the lift ship being adapted for pipe laying operations by the provision of pipe handling apparatus, said apparatus being positioned in association with said load deck for receiving a pre-fabricated pipeline from a reel carried on such a barge and controlling the paying out of said pipeline to the seabed.

2. A vessel as claimed in claim I wherein the pipe handling apparatus comprises a tower structure carrying a pipe diverter and tensioning apparatus above the level of the load deck.

3. A vessel as claimed in claim 2 wherein the tower structure is tiltable.

4. A vessel as claimed in claim 2 or 3 wherein the tower structure has an open lower portion, to permit modules to be fitted to the pipeline below said tensioning apparatus.

5. A vessel as claimed in claim 2, 3 or 4 wherein the tensioning apparatus comprises one or more track tensioners.

6. A vessel as claimed in claim 2, 3, 4 or 5 wherein the pipe laying apparatus includes a pipe straightener located between the diverter and the tensioning apparatus.

7. A vessel as claimed any preceding claim wherein the pipe laying apparatus is located at one end of the load deck.

8. A vessel as claimed in claim 7 wherein the pipe laying apparatus is located at the stern of the lift ship.

9. A vessel as claimed in any preceding claim in combination with at least one reel barge, the real barge being a floating vessel having at least one pipe reel mounted on it so as to feed pre-fabricated pipeline to the pipe laying apparatus when the reel barge is carried on the load deck of the lift ship.

10. A combination of vessels as claimed in claim 9 further including an auxiliary barge which can be accommodated on the load deck of the lift ship simultaneously with the reel barge, for carrying modules to be fitted to the pipeline during laying.

11. A combination as claimed in claim 10 wherein the reel barge and auxiliary barge may be carryable on the load deck with the auxiliary barge located between the reel barge and the pipe laying apparatus.

12. A combination as claimed in claim 9, 10 or 11 further including a second reel barge, such that one of said reel barges can be re-stocked with pipeline separately from the lift ship, while the other is carried on the load deck of the lift ship.

13. A combination as claimed in claim 12 wherein the reel barges are dimensioned so that both can be carried on the load deck for transit between operating regions.

14. A combination as claimed in claim 13 wherein the reel barges and load deck are dimensioned so that one barge only can be carried in a working orientation to feed the pipe laying apparatus, while both barges can be carried together in a different orientation for transit between operations.

15. A vessel or combination as claimed in any preceding claim wherein the ballasting means comprises a number of ballast tanks distributed within the vessel and pumps for removing water from said tanks to de-ballast the vessel.

16. A method of loading a pipe laying vessel with pre-fabricated pipeline stored on a reel, wherein the pipe laying vessel is provided in the form of a semi-submersible lift ship having a submersible load deck and having pipe handling apparatus for receiving a pre-fabricated pipeline from an associated storage location above the load deck and controlling the paying out of said pipeline to the seabed, the method comprising: (a) providing a reel-carrying barge with at least one reel loaded with said prefabricated pipeline; (b) ballasting the vessel to submerge the load deck below water; (c) manoeuvring the reel barge and vessel such that the load-carrying barge is positioned over the load deck; (d) de-ballasting the vessel to carry the reel barge on the load deck, thereby presenting the loaded reel at the storage location associated with the pipe handling apparatus.

17. A method as claimed in claim 16 wherein the vessel in step (b) is already carrying a second reel barge on its load deck, the method further including a step (bi) in which the second load carrying barge is floated clear of the load deck before step (c).

18. A method as claimed in claim 17 further comprising a step (e) of re-loading the reel of the second reel barge with pre-fabricated pipeline while the vessel with the first reel barge is engaged in pipe laying operations elsewhere.

19. A method as claimed in claim 17 or 18 wherein the vessel in step (b) is already carrying the first and second reel barges together in a transit configuration, step (b 1) including floating off both barges.

20. A method as claimed in claim 19 wherein the barges in step (b) are in a different orientation to that required to present a loaded reel at said storage location, the method further comprising re-orienting the first reel barge prior to step (d).

21. A method as claimed in claim 19 or 20 wherein the first reel barge in said transit configuration is already loaded with said pre-fabricated pipeline.

22. A method as claimed in claim 19 or 20 wherein the first reel barge arrives in the transit configuration with both reel barges depleted of pipeline, the method then including loaded the first reel barge with pipeline before the vessel and first reel barge sail for pipe laying operations.

23. A method as claimed in claim 22 wherein the step (a) in that case includes loading the first reel barge with pipeline before or after any of steps (b), (c) and (d).

24. A method as claimed in any of claims 17 to 23 wherein the steps (c) and (d) include loading the vessel with a stock of one or more pipeline accessories such as end terminations, to be fitted to the pipeline during laying operations.

25. A method as claimed in claim 22 wherein the pipeline accessories are carried by an auxiliary barge floating separately from the first reel barge, the method further comprising step (ci) in which the auxiliary barge loaded with accessories is also manoeuvred into a position over the load deck.

26. A method as claimed in claim 25 wherein the auxiliary barge is placed on the load deck at a position generally between the reel barge and the pipe handling apparatus.

27. A method as claimed in any of claims 17 to 26 wherein the vessel arrives for step (b) in a transit configuration with first and second reel barges carried on the load deck, and wherein the auxiliary barge is also carried on said load deck in the transit configuration.

28. A method as claimed in claim 27 wherein there are first and second reel barges and first and second auxiliary barges, all of which can be accommodated on the load deck in said transit configuration.

29. A method of laying at sea a pre-fabricated pipeline stored on at least one reel, the method being performed using a pipe laying vessel in the form of a semi-submersible lift ship having a submersible load deck and having pipe handling apparatus, wherein said reel is carried on a first reel barge which in turn is carried temporarily on the submersible load deck such that the loaded reel is presented at a storage location associated with the pipe handling apparatus, the method comprising the step (f) using said pipe laying apparatus to receive pipeline from said reel and control the paying out of said pipeline to the seabed into the sea.

30. A method as claimed in claim 29 which further includes (g) fitting one or more pipeline accessories such as end terminations to the pipeline during said paying out.

31. A method as claimed in claim 30 wherein the pipeline accessories are carried by an auxiliary barge carried on the load deck.

32. A method as claimed in claim 31 where before fitting, the auxiliary barge is placed on the load deck at a position generally between the reel barge and the pipe handling apparatus.

33. A method as claimed in claim 29, 30, 31 or 32 which further comprises the steps of (h) interrupting pipe laying operations, (i) ballasting the vessel to submerge the load deck and float the first reel barge free of the vessel; ) manoeuvring the first reel barge clear of the storage location and substituting a second reel barge pre-loaded with pipeline; (k) de-ballasting the vessel to raise the load deck and support the second reel barge on the load deck at said storage location; and (1) resuming pipe laying operations using said pipe laying apparatus to receive pipeline from a reel on the second reel barge and control the paying out of said pipeline to the seabed into the sea.

34. A method as claimed in claim 33 wherein in step (j) the first reel barge is moved entirely clear of the vessel.

35. A method as claimed in claim 34 which further includes a step (m) of re-stocking the reel on the first reel barge concurrently with performance of step (I).

36. A method as claimed in claim 33, 34 or 35 wherein the steps (j) to (I) are then repeated at least once with the roles of first and second reel barges reversed, thereby to re-stock the vessel.

37. A pipe laying vessel substantially as described herein with references to the accompanying drawings.

38. A combination of pipe laying vessel and a plurality of reel barges substantially as described herein with reference to the accompanying drawings.

39. A method of laying pipe from a vessel and re-stocking the vessel with reeled pipeline, substantially as described herein with reference to the accompanying drawings.