GB2321290A

GB2321290A - Laying of underwater pipeline from a vessel-mounted drum

Info

Publication number: GB2321290A
Application number: GB9700915A
Authority: GB
Inventors: Alexander Craig Lang
Original assignee: J Ray McDermott SA
Current assignee: J Ray McDermott SA
Priority date: 1997-01-17
Filing date: 1997-01-17
Publication date: 1998-07-22
Anticipated expiration: 2017-01-17
Also published as: GB2321290B; GB9700915D0; MX9710449A; NO975386D0; NO975386L; CA2227163A1; AU725405B2; AU5038198A; BR9706045A; ID19569A

Abstract

The pipe carrying drum (30), mounted with its axis parallel or right-angles to the fore-aft direction of the vessel hull, has a curved launch structure (36) with pipe engaging clamps (46 and 48) engaging the pipe along a curved path, and reciprocally moveable to tension the pipe. The curved path has an angle of curvature which is approximately equal to the residual curvature of the pipe drawn from the drum. The launch angle can also be adjusted by moving the angle or position of the curved path within a plane perpendicular to the drum axis by a telescopically movable launch arm 58 with curved and straight portions (58a,b) or by an angularly movable cantilever arm. An abandonment and recovery system comprising a winch and pulleys is mounted on the launch structure, which is moveable alongside the drum as a level wind, as a carriage 40, portal frame or crane with an arm across the drum.

Description

IMPROVEMENTS RELATING TO THE LAYING OF UNDERWATER PIPELINE This invention relates to the laying of pipeline underwater, for example, on the sea bed, by a pipe laying vessel or barge. The invention is particularly suitable for laying large diameter steel pipeline in deep water, but it is not limited exclusively to this.

International (PCT) Patent Application Publication No. WO-A95/22484 discloses a conventional type of pipe laying vessel. The pipeline is coiled around a vertically orientated drum located amidships, and is launched over a ramp at the stern of the vessel. The ramp carries the necessary pipe conditioning units, namely means defining a bending path of fixed curvature for imparting a fixed curvature to the pipe, a straightener for imparting a reverse curvature to straighten the pipe after the bending path, a pipe tensioner, and a pipe clamp for securing the end of the pipe. The ramp is pivoted at its lower end and can be raised or lowered to achieve a desired pipe launch angle during pipe-laying, or a desired (flat) loading angle for loading pipe on to the drum.

A steep launch angle (60-90 degrees) is needed for laying pipe in deep water (for example, to a depth of 500 metres or greater). When the ramp is elevated steeply, the pipe has to be bent through almost 60 degrees as it passes around the bending path. Since the pipe is pulled around the bending path by the action of the tensioner (or by the weight of pipe hanging in the water once pipelaying is underway), and the drum is braked and/or driven in "reverse" to maintain a back tension, substantial stresses occur in the pipe as it passes around the bending path. This severely limits the maximum diameter of steel pipe which could be handled without the pipe buckling as it passes around the bending path.

Alternative designs have been proposed in International (PCT) Patent Application Publication No. WO-A-94/06678 with a view to reducing the distance between the point of launch and the drum, reducing the bending curvature of the pipe on its way to the point of launch, and reducing the number of times that the pipe is bent plastically (since each plastic bending step increases the risk of elastic buckling and collapse, as well as increasing the work done and therefore the energy required to lay the pipe). The pipe carrying drum is mounted longitudinally in a vessel and the pipe is launched over the side of the vessel as the vessel is propelled crab-wise (sideways).

In one proposal, an overhead gantry is movable in a direction along the vessel, and acts as a levelwind supporting the pipe-launching components.

However such proposals leave only a small amount of room on a launch platform for mounting a pipe straightener, a pipe tensioner, and a pipe-end securing clamp. Furthermore, it can still remain difficult to avoid stresses in large diameter pipes for steep launch angles.

The present invention has been devised bearing in mind the problems discussed above.

In contrast to the prior art technique of using a pipe tensioner acting on a straight length of pipe, one aspect of the present invention is to use a tensioner which is operable to tension the pipe in a curved path. Such a tensioner provides two immediate advantages. Firstly, it reduces the stresses on the pipe because the tensioner can guide the pipe directly along the curved path, instead of applying tension to the pipe downstream of the curve. Secondly, the tensioner can be positioned upstream of the launch ramp or platform, leaving more room for other pipe conditioning components, such as the pipe straightener and the securing clamp, on the launch platform itself.

The tensioner may, for example, comprise a tracked tensioner and suitable guide means defining a curved path for the tensioner tracks.

However, a preferred feature of the invention is that the tensioner comprises at least one reciprocating clamp guided back and forth on an arcuate path, the clamp being actuated to grip the pipe as the clamp moves in one direction (such that the pipe moves with the clamp), and to release the pipe as the clamp is moved back in the reverse direction (such that the clamp moves back relative to the pipe). More preferably, the tensioner comprises at least two such clamps (two in the preferred embodiment) one downstream of the other, and operating in antiphase (i.e. one clamp being actuated to pull the pipe while the other moves back, with a degree of overlap when both clamps engage the pipe so as to synchronise prior to release).

Each clamp can be supported on a respective carriage movable along a curved track defining the arcuate path. The curvature of the path may either be fixed or be adjustable. For example, an adjustable curvature track may be formed by track segments which are pivoted relative to each other, and coupled by adjustable means for anchoring adjacent segments at a desired relative angle defining the curvature. The adjustable means may comprise a hydraulic cylinder or jack coupled at one end to one segment and at the other end to an adjacent segment.

In contrast to using a pivoted launch structure of fixed length, another aspect of the present invention is to use a telescopically extendible launch structure.

Preferably, the launch structure defines an arcuate path having a constant radius of curvature, and the arcuate path is telescopically extendible to vary the launch angle (by varying the length of the arcuate portion of the path). Such a structure is advantageous because the position of the upstream end of the launch structure is not dependent on the launch angle. The upstream end can remain in a predetermined optimum position for feeding pipe from the drum.

The telescoping portion of the launch structure may be received within an outer pipe carrying portion, or it may simply be arranged alongside a fixed pipe carrying portion (such that the fixed portion and the telescoping portion overlap each other). The structure may comprise a single telescoping stage, or multiple telescoping stages.

A further aspect of the invention is to guide the pipe along a path of constant curvature between the drum and the launch deck or platform, the angle of curvature being approximately the same as the residual curvature of the pipe as it is drawn off the drum.

By this aspect of the invention, it has been appreciated that steel pipe drawn from the drum will tend to have a residual curvature caused by plastic deformation during the winding of the pipe on to the drum. By using a launch path of the same curvature, the stresses on the pipe and on the pipe handling machinery can be significantly reduced. The risk of pipe buckling, particularly important for large diameter pipes, can be reduced, enabling large diameter steel pipes to be laid.

In another aspect, a launching structure defines a curved feed path extending overhead of a pipe carrying drum, the pipe being supported along the curved feed path by the launch structure, and the launching angle is adjustable by varying the position of the curved feed path in a plane substantially perpendicular (transverse) to the drum axis.

Preferably the curved path has a substantially constant angle of curvature, for example, equal to the residual curvature of the pipe drawn from the drum.

The curved path may, for example, be raised or lowered relative to the drum. Alternatively, or additionally, the angle of the curved path could be varied relative to the drum. In at least one of the preferred embodiments, the curved path is movable about the drum axis.

Another aspect of the invention is to provide an articulated structure comprising articulated members defining a feed path along which the weight of the path is supported by the members, the articulation angle between adjacent members being adjustable to define a path of desired curvature.

A further aspect of the invention relates to the abandonment/recovery system normally used on pipelaying vessels for handling the end of a pipeline. In contrast to the conventional technique of deck mounting the winch of such a system, another aspect of the present invention is to mount the winch so that it is movable with a launching structure acting as a levelwind. Such a technique can provide a much simpler design of abandonment/recovery system than in the prior art.

Although the above aspects of the invention can be used independently, significant advantages can be achieved by using certain aspects in combination.

Embodiments of the invention are now described by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a schematic view of a first embodiment of pipe launching structure; Fig. 2 is a plan view of the structure of Fig. 1; Fig. 3 is a schematic view of the tensioner clamps used in Fig. 1; Fig. 4 is a schematic view of the abandonment and recovery system used in the structure of Fig. 1; Fig. 5 is a schematic view of a second embodiment of launch structure; Fig. 6 is a schematic view of the articulated arm used in Fig. 5; Fig. 7 is a schematic view of a third embodiment of launch structure; Fig. 8 is a schematic view of a fourth embodiment of launch structure; Fig. 9 is a schematic view of a fifth embodiment of launch structure; Figs. 10 and 11 show schematically a sixth embodiment of launch structure; Fig. 1 2 illustrates schematically a drive system used in the structure of Figs 10 and 11; Figs. 1 3a and 1 3b are schematic views of alternatives of a seventh embodiment of launch structure; Fig. 14 is a schematic view of an eighth embodiment of launch structure; Fig. 1 5 is a schematic view of a ninth embodiment of launch structure; and Fig. 16 is a schematic view of a tenth embodiment of launch structure.

Referring to Figs. 1-4, a steel pipe carrying drum 30 is mounted within the hull 32 of a pipe laying vessel. In this embodiment, the drum 30 is mounted within an open topped circular-shaped tank 34 which can be flooded with sea water to make the drum 30 at least partly buoyant, and so relieve the weight which has to be borne by bearings (not shown) supporting the drum 30. However, in other embodiments, the drum may simply be supported by large bearings attached directly to the hull 32 of the vessel.

The drum 30 is mounted longitudinally within the hull 32, i.e. with its axis parallel with the bow-stern axis of the hull 32. The pipe is laid over the side of the hull by means of a launch structure 36 while the vessel is driven crab-wise (i.e. sideways) by suitable thrusters (not shown) or by pulling on anchor lines. Such a side-laying technique is described in WO-A-94/06678 referred to herein before.

The launch structure 36 consists of an arcuate upper member 38 mounted above a carriage 40 by fixed struts 42. The carriage 40 is movable on a pair of guide rails 44 arranged on the deck alongside the drum 30, such that the launch structure is movable as a levelwind.

The angle of curvature of the upper member 38 is made substantially equal to the average residual curvature of the pipe drawn from the drum.

This residual curvature depends in a known way on the diameter and thickness of the steel pipe, and on the diameter of the winding on the drum.

By making the curvature of the upper member 38 equal to the residual curvature, the pipe drawn off the drum naturally follows the curve of the upper member 38 without requiring additional bending, or with reduced bending and consequently lower resistance. The weight of the pipe is borne by rollers 45 or on lubricated pads (not shown) mounted on the upper member 38.

The pipe is tensioned on the launch structure by means of a tensioner arrangement consisting of first and second gripper clamps 46 and 48 carried on first and second reciprocating clamp carriages 50 and 52, respectively.

Referring to Fig. 3, the clamp carriages 50 and 52 are slidable on an arcuate track 54 mounted on the upper member 38. The first clamp carriage 50 moves within a first portion 54a of the track 54, and the second clamp carriage 52 moves within a second portion 54b of the track downstream of the first portion 54a. Driven by hydraulic cylinders 56, the carriages 50 and 52 move in opposite directions. Each clamp is actuated to grip the pipe as the clamp carriage moves forward from right to left (in the drawings), and to release the pipe as the clamp carriage moves back from left to right. The pipe is therefore pulled along the arcuate path by the two clamps 46 and 48 in a "hand by hand" manner. For added clamping reliability and increased lay speed, the carriages are driven back slightly faster than they move forward, such that the beginning of the forward cycle of one clamp always overlaps the end of the forward cycle of the other clamp. This ensures that pipe slippage does not occur at the time of clamp changeover, by providing a degree of overlap when both clamps are gripping the pipe.

The positioning of the tensioning clamps 50 and 52 along the arcuate path helps to reduce the stress applied to the pipe, compared to the prior art technique of tensioning the pipe downstream of the curved path, because the tensioner acts to guide the pipe along the arcuate path. This means that much larger diameters of steel pipe can be handled with much reduced risk of buckling.

The downstream end of the launch structure 36 consists of a launch member 58 telescopically mounted relative to the upper member 38 and supported on a pivoting strut 60. As best seen in the extended position of the launch member (broken line in Fig. 1), the launch member includes a curved portion 58a having the same angle of curvature as the upper member 38, and a straight end portion 58b upon which are mounted a conventional pipe straightener 62 (formed by an upper presser roller) and an end clamp 64. The launch member is movable between a retracted position (shown by full lines in Fig. 1) and an extended position (shown by broken lines in Fig.

1) to enable a desired launch angle to be set. The launch member 58 can be moved relative to the upper member 38 by a suitable hydraulic cylinder.

Alternatively, with the end clamp 64 gripping the end of the pipe, the tensioner clamps 46 and 48 can be operated to drive the pipe to push the launch member 58 outwardly, or to pull the launch member 58 back.

A primary advantage achieved by this embodiment is that the curvature of the launch path is independent of the launch angle, and a wide range of launch angles is possible with very little stress on the pipe.

Moreover, the curvature of the launch path is constant, and is made equal to the residual curvature of the pipe.

Referring to Fig. 4, an abandonment and recovery (A/R) system is mounted on the launch structure 36 for laying and recovering an end of the pipe. The A/R system consists of a winch 66 mounted on the carriage 40, intermediate pulleys 68 mounted on the fixed struts 42, and an end pulley 70 mounted on a swinging arm (not shown) so that it can be swung into position over the launch member 58. In use, a line 71 with a buoy can be attached to a bullnose or padeye fitted to the end of the pipe for laying down the end of the pipe underwater, or for later recovery. By having the entire A/R system including the winch 66 mounted on the launch structure 36 which moves as a level wind, the design of the A/R system can be significantly simplified compared to prior designs.

A second embodiment is illustrated in Fig. 5. The main difference in the second embodiment is that the launch structure 36 consists of a gantry 80 straddling the drum 30. The gantry 80 moves as a level wind, fore and aft, on deck rails 82 in the same way as the launch structure 36 of the first embodiment. As indicated schematically in parts of Fig. 5, the gantry is constructed from interconnected girders. The gantry 80 supports a central bridge 84 which is movable up and down on vertical racks (not shown) on the uprights of the gantry. The bridge 84 carries the tensioner clamps 46 and 48, the arcuate track 54, and the support rollers 45 described hereinbefore. A launch platform 86 carrying the straightener 62 and the end clamp 64 is supported by a lateral extension 88 of the gantry 80.

In the second embodiment, the launch angle is determined by a combination of the height of the bridge 84 and the angle of the launch platform 86. As shown by the two positions in Fig. 5, the launch platform is movable between a maximally raised position 90 used for loading the pipe on the drum and for shallow lays, to a maximally depressed position 92 for deep water lays. In the depressed position 92, the arcuate curve is maintained downstream of the clamps 46 and 48 by means of an articulated arm 94. Referring to Fig. 6, the arm 94 consists of pivotally interconnected segments 96. Adjacent segments 96 are also coupled by means of a hydraulic cylinder 98 which is used to set the angle of one segment relative to the next, and hence create a desired curved or straight path.

The same design of articulated arm can also be used to form the curved track 54 on which the clamp carriages 52 and 54 discussed above slide. The advantage of using such an articulated track is that the curvature of the track can be set to match differing residual pipe curvatures. As explained above, the curvature of the launch path should be made equal to the residual curvature, the latter depending on the pipe diameter, the wall thickness, the material yield stress, and the diameter of the pipe windings around the drum.

As a modification, the telescoping launch member 58 of the first embodiment could replace the articulated arm 94 and launch platform 86 shown in Fig. 5 as an alternative technique for achieving variable launch angles.

Although not illustrated explicitly in the drawings, in this embodiment and the following embodiments, an A/R system similar to that described previously is mounted on the launch structure 36 movable relative to the drum 30 as a levelwind.

A third embodiment is illustrated in Fig. 7.

The main difference in the third embodiment is that instead of supporting a raisable bridge, the gantry 80 supports an angularly movable arcuate launcher 100, two positions of which are shown in Fig. 7. The launcher 100 is carried on two concentric racked rails 102 and 104, one end 106 of the launcher 100 being guided by a toothed wheel 108 engaging the lower rail 102, and the other end 110 of the launcher 100 being guided by a toothed wheel 11 2 engaging the upper rail 104. A supporting structure (not shown) for the lower rail 102 is provided on the lower part of the gantry 80 near the end 106, with a similar structure on the other side of the drum.

The launcher 100 carries the tensioner assembly (described hereinbefore), the straightener and the end clamp. The launcher 100 is movable between a maximally raised position 114 for pipe loading and for shallow lays, and a maximally depressed position 116 for deep water lays.

In order to set the desired launch angle, the toothed wheels 108 and 11 2 are driven to advance the launcher 100 along the rails 102 and 104, until the end 110 of the launcher 100 is at the correct launch angle.

A fourth embodiment is illustrated in Fig. 8. This is similar to that shown in Fig. 7 except that the upper part of the gantry is omitted, leaving an arch shaped gantry 120 upon which the launcher 100 is mounted. The launcher 100 is angularly movable on rails 1 22 on the periphery of the arch 120.

Fig. 9 shows a fifth embodiment which is very similar to that shown in Fig. 8. The only significant difference is that the rim 124 of the tank 34 is built up and is strengthened to take the weight of the gantry 120 and launcher 100. The gantry 1 20 slides on rails 126 mounted on the rim 124 instead of on deck rails. This can provide advantages in saving deck space around the drum 30, leaving more room in which the pipe handling crew can work.

In common with the first and second embodiments, the third, fourth and fifth embodiments all employ a launch path of constant curvature, and tensioning clamps 46 and 48 acting along the arcuate path, so that large diameter steel pipe can be handled.

A sixth embodiment is shown in Figs. 10 and 11. This is slightly different from the third, fourth and fifth embodiments, in that the launch structure 130 is carried on rails 132 of a cage 134 surrounding the drum 30.

The launch structure 130 is driven slidably along the rails as a levelwind, in the fore-aft direction. As before, the launch structure 130 carries the tensioning assembly, the straightener 62 and the end clamp 64.

In order to set a desired launch angle the cage 134 is rotated with the drum 30 (so that there is no unwinding movement of the pipe from the drum). Once the desired launch angle is reached, the cage 134 is anchored in that orientation relative to the vessel hull. Thereafter, the drum 30 can be rotated relative to the cage 134 to enable pipe to be drawn from the drum 30 or wound on to the drum 30. The cage 134 has an open region 1 36 through which pipe passes from the drum to the launch structure 130.

The drum 30 and the cage 134 are rotated by a drive system 138 located on the deck at one end of the drum 30. Referring to Fig. 12, the drive system comprises a motor 140 which drives through a gearbox 142.

A first clutch 144 couples the gearbox output shaft to a gear 146 which engages the rim 148 of the cage 1 34. A second clutch 1 50 couples the gearbox output shaft to a gear 152 which engages a rack 154 on the rim end flange of the drum 30. By engaging both clutches 144 and 150 the drive system rotates the drum 30 and the cage 134 in unison, and by engaging the second clutch 150 only, the drum 30 can be rotated relative to the cage 134, or back tension maintained during pipe laying.

A slightly modified seventh embodiment is illustrated in Fig. 13a. In this embodiment, the size of the cage has been reduced, such that it no longer completely surrounds the drum 30. Instead, the cage 134' merely straddles the drum 30 on either side. However, this embodiment is the same as the sixth embodiment in other respects. In Fig. 13b, instead of a surrounding cage, an arcuate carriage 135 is used to support the launcher 130. The carriage is rotatably mounted on the end flanges 137 of the drum, allowing relative rotation between the drum and the carriage 135. The launch angle is controlled by a drive winch 139 coupled by lines to the carriage for angularly moving the carriage around the periphery of the end flanges 137.

Referring to Fig. 14, in an eighth embodiment, the launch structure consists of a jib crane 160 which is movable parallel to the drum 30 on deck rails 162, to act as a levelwind. The crane boom carries the tensioning clamps, the pipe straightener and the end securing clamp described hereinbefore, and defines a curved launch path from the drum 30 to the point of launch. The jib can be raised and lowered to set the desired launch angle. A raised position of the jib is suitable for pipe loading and for laying pipe in shallow water, and a lowered position is suitable for laying pipe in deep water. In the lowered position, the jib can rest on the deck of the vessel, or be propped on the deck, to relieve the stress on the jib raising cables. This embodiment may be particularly suitable to implement by converting an existing vessel having a hold and a loading jib crane. The design may also be attractive to ship owners because the crane can be used for other purposes, allowing the vessel to be used, for example, for transporting cargo when not in pipe laying service.

An alternative "crane" type embodiment is illustrated in Fig. 1 5. In this embodiment, the launch structure consists of a quadrant arm 164 pivotally mounted on a carriage 166 which is slidable on rails 168 mounted on the upper surface of the tank 34. The quadrant arm 164 carries the tensioner clamps 46 and 48, the pipe straightener 62 and the end securing clamp 64 described previously, and defines a curved launch path, of constant curvature, from the drum 30 to the point of launch. The launch angle is set simply by pivoting the quadrant arm 1 64 about the carriage 166, and the carriage can be driven along the rails 166 to act as a levelwind.

Referring to Fig. 16, in a tenth embodiment, the quadrant arm 164' is mounted on a carriage 166' movable athwartships on a bridge or gantry 170 to increase clearance between the quadrant 164' and the vessel for high launch angles. The gantry 1 70 is movable on deck rails 172 as a levelwind.

It will be appreciated that the embodiments described above provide structures for laying pipeline from a reel or drum, in which the pipe is guided on a uniform curved path from the drum to the point of launch. The use of a tensioner arrangement acting on the curved length of pipe, and the angle of curvature being made roughly equal to the residual curvature in the pipe, result in very little stress being applied to the pipe as it passes through the launch structure. Large diameter pipes can therefore be handled without risk of buckling, and the pipes can be launched at very steep launch angles needed for pipe laying in deep water.

Typically, in the above embodiments, the launch angle can be varied from about 10 degrees (below horizontal) which would be used during loading of a pipeline from shore and for laying pipeline in very shallow water, to over 60 degrees (below horizontal) which would be used for laying pipeline in very deep water. Typically, a launch angle of about 65 degrees would be practicable, but it should be possible to go up to 90C (although 75 -80 is probably the practical maximum).

Although the drum is mounted in the described embodiments with its axis parallel to the fore-aft direction of the vessel, in other embodiments, the drum may be mounted with its axis transverse to the vessel. For example, the drum could be mounted transversely across the stern of the vessel, and the pipe laid over the stern by the launching structure. Alternatively, the drum could be arranged amidships and the hull split (in a similar manner to a dredger hull) to provide an open gap through which the pipe is laid, or the pipe could be launched through a moonpool.

Whilst endeavouring in the foregoing description to draw attention to those features of the invention believed to be of particular importance, it should be understood that the Applicant claims protection in respect of any patentable feature of combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

1. A vessel for laying pipeline underwater and comprising a launching structure for launching the pipeline from the vessel, the launching structure defining a path along which the pipeline is fed, and the structure comprising pipe tensioning means operable to engage the pipe along a curved portion of the path.

2. A vessel according to claim 1, wherein the tensioning means comprises at least one clamp operable to grip the pipe, the clamp being reciprocally movable along an arcuate path.

3. A vessel according to claim 2, comprising at least two of said clamps operating in a complementary manner, one clamp being operable to grip the pipe to tension the pipe in the direction of movement of the clamp, while the other clamp moves in the opposite direction without gripping the pipe.

4. A vessel according to claim 2 or 3, further comprising an arcuate track or channel defining the movement path of the clamp or clamps.

5. A vessel according to claim 4, wherein the or each clamp is mounted on a respective carriage movable along the track or channel.

6. A vessel according to 4 or 5, wherein the curvature of the track or channel is adjustable.

7. A vessel according to any preceding claim, further comprising a drum, and wherein the position of the launch structure is adjustable in a plane substantially perpendicular to the axis of the drum.

8. A vessel for laying pipeline underwater and comprising a launching structure for launching the pipeline from the vessel, the launching structure comprising a first part which is telescopically mounted relative to a second part, the pipeline launching angle being determined by the position of the first part relative to the second part.

9. A vessel according to claim 8, wherein the launching structure defines a curved path along which the pipe is fed to be launched, and wherein the arcuate length of the path is determined by the relative position of the first part of the launch structure with respect to the second part.

10. A vessel according to claim 9, wherein the second part of the launch structure is angularly movable with respect to the first part.

11. A vessel according to claim 8, 9 or 10, wherein the first part of the launch structure is mounted adjacent to a drum around which the pipeline is wound, and wherein the second part of the launch structure carries pipe straightener means, and a pipeline-end securing clamp.

12. A vessel according to claim 11, wherein the first part of the launching structure carries pipe tensioner means.

13. A vessel according to claim 12, wherein the pipe tensioner means is operable to engage the pipe along a curved path.

14. A vessel for laying underwater-pipeline of a predetermined type, the vessel comprising a drum around which the pipeline is wound, and a launching structure for launching pipeline drawn from the drum into the water, the launching structure defining a feed path of substantially constant curvature extending from the drum to a pipe straightener prior to launch, the curvature of the path being substantially equal to the residual curvature associated with said type of pipe, caused by winding around the drum.

15. A vessel according to claim 14, further comprising pipe tensioning means for tensioning the pipe along the curved path.

16. A vessel according to claim 14 or 1 5, wherein the launching structure is movable to vary the position of the curved path in a plane substantially perpendicular to the axis of the drum, to vary the launch angle.

17. A vessel according to claim 14 or 1 5, wherein the launching structure comprises first and second telescoping parts, the length of the curved path being determined by the position of the first part relative to the second part.

18. A combination of a vessel as defined in any of claims 14 to 17, and a pipeline of a predetermined type wound around the drum of the vessel.

1 9. A vessel for laying pipeline underwater, the vessel comprising a drum around which the pipe is wound, a launching structure for launching pipeline drawn from the drum into the water, the launching structure defining a curved feed path extending overhead of the drum and along which the pipeline is supported by the launch structure, and the launch structure being movable to vary the position of the curved path in a plane transverse to the axis of the drum, in order to control the pipe launching angle.

20. A vessel according to claim 19, wherein the curved path has a substantially constant radius of curvature.

21. A vessel according to claim 19 or 20, wherein the launching structure comprises means for raising or lowering the curved path relative to the drum.

22. A vessel according to claim 19, 20 or 21, wherein the launching structure comprises means for varying the angle of the curved path relative to the drum.

23. A vessel according to claim 22, wherein the launching structure comprises means for varying the angular position of the curved path about the axis of the drum.

24. A vessel according to claim 23, wherein the launching structure comprises a cage in which the drum is received, the cage being rotatable about the drum.

25. A vessel according to claim 19, 20, 21, 22 or 23, wherein the launching structure comprises a gantry straddling the drum, the gantry being movable in a direction substantially parallel to the drum axis.

26. A vessel according to claim 19, 20, 21, 22 or 23, wherein the launching structure comprises a jib crane.

27. A vessel for laying pipeline underwater, the vessel comprising a launching structure for launching the pipeline from the vessel, the launching structure comprising articulated members defining a feed path along which the weight of the pipe is supported by the members, the articulation angle between adjacent members being adjustable to define a path of a desired curvature.

28. A vessel according to claim 27, wherein adjacent articulated members are pivotally coupled to each other, and are coupled to opposite ends of a respective variable extension means for varying the angle of articulation.

29. A vessel for laying pipeline underwater, the vessel comprising a drum around which the pipeline is wound, a launching structure for launching pipe drawn from the drum into the water, the launching structure being movable in a direction parallel to the axis of the drum, and an abandonment/recovery system for use in handling a pipeline end, the abandonment and recovery system comprising a winch movable with the launching structure in said direction substantially parallel to the drum axis.

30. A vessel according to claim 29, wherein the abandonment and recovery system is mounted on the launching structure.

31. A vessel for laying pipeline underwater, the vessel being substantially as herein before described with reference to any of the accompanying drawings.