GB2316147A

GB2316147A - Pipe laying vessel with reel and diverter structure

Info

Publication number: GB2316147A
Application number: GB9716168A
Authority: GB
Inventors: Robert George Martin
Original assignee: Coflexip Stena Offshore Ltd
Current assignee: Technip UK Ltd
Priority date: 1996-08-03
Filing date: 1997-08-01
Publication date: 1998-02-18
Also published as: ZA976917B; WO1998005894A1; AU3776197A; GB9716168D0; GB9616395D0

Abstract

The vessel has a main pipe storage reel (26) mounted for rotation about a vertical axis, and the diverter structure (30) around which the pipe (28) is bent to a required launch angle as it is unspooled from the reel over the side of the vessel, so as to reduce pitching movements transferred to the pipe from the vessel, as compared with conventional stern-launching vessels. The vessel may also include an additional reel (31) for storing flexible pipe (32), which may be launched via the same diverter structure as pipe from the main reel. The structure (30) may be a vertical wheel, a single endless conveyor or a series of rollers or roller track assemblies.

Description

"Pipe Laying Vessel and Method" This invention relates to the laying of submarine pipelines.

Known methods of submarine pipelaying fall into two broad classes.

First, there are methods in which pipe lengths are shipped on a vessel such as a lay barge and are welded end to end as laying proceeds ("stovepiping"). These methods can be divided into "S lay" where the pipe is welded up in a horizontal path and laid over a stinger, and "J lay" where the pipe is launched from the vessel in a substantially vertical direction and each succeeding pipe length must be erected into a substantially vertical position for welding to the last section.

Each of these has problems and limitations. S lay requires a cumbersome and expensive stinger and can operate only in relatively shallow water. J lay can cope with deeper water, but handling pipe lengths into the vertical and welding them in that position is difficult and time consuming.

The second broad category of known method is reel pipelaying. This has the great benefit of allowing the bulk of the welding to be carried out in factory conditions ashore to form a continuous length of pipe which is then spooled onto a storage reel on the vessel, prior to being unspooled during pipelay operations. This requires a complex, specialised lay vessel for larger diameter pipelines. Reel pipelaying using existing arrangements is generally preferable to stovepiping on economic grounds for rigid pipelines having an o.d. of 16" or less. However, this is not an absolute limit and may vary depending upon detailed operational requirements.

Another form of reel pipelaying involves the use of specially constructed "flexible pipeline", which can be of relatively large diameter but which is inherently flexible and does not require straightening after unspooling from its storage reel, as is the case with rigid pipe of comparable diameter. Flexible pipeline is expensive compared to conventional rigid pipeline and its use is generally restricted to applications justifying the additional expense.

It will be understood that, as used herein, "pipeline" means a pipe of such diameter and wall thickness that, when formed from conventional rigid materials such as steel, it is plastically deformed when spooled onto a storage reel and requires straightening to remove the plastic deformation during unspooling, as distinct from small bore tubing which can generally be spooled without plastic deformation. "Flexible pipeline" means a specially constructed pipeline of such a diameter that a comparable pipeline of conventional construction would require plastic deformation during spooling.

The present invention is concerned with reel pipelaying.

Early reel pipelay systems used a lay barge having a reel mounted for rotation about a vertical axis, together with appropriate straightening and tensioning apparatus, as exemplified by US-A-3680342. The use of such systems is restricted to relatively shallow lay depths for the same reasons as "S lay" stovepiping systems, owing to the horizontal orientation of the pipe leaving the reel. Subsequently, a reel pipelay system was proposed in which the reel is mounted for rotation about a horizontal axis, allowing the pipeline to be diverted to a variable, relatively steep launch angle prior to being straightened and launched from the vessel and thus permitting pipelay operations at much greater water depths than was possible with earlier reel systems. This concept is disclosed in GB-A1507959 and was implemented in the reelship "Apache", as disclosed for example in U.S. Patents Nos. 4 230 421, 4 269 540, 4 297 054, 4 340 322 and 4 345 855.

Whilst the "Apache" has been operated with great success, systems based on horizontal-axis reels capable of carrying substantial payloads of pipe have certain disadvantages. The loaded reel results in a high centre of gravity and, ideally (as in the case of the Apache), requires the reel to be set in a well extending below deck, which in turn requires either a purpose-built vessel or substantial modification of an existing vessel. In this respect, a vertical axis reel is preferable since the centre of gravity is lower for a given load and the reel may be mounted on the deck of the vessel, simplifying the conversion of existing vessels for pipelaying and re-conversion for other purposes.

Also, launching the pipe from the stern of the vessel, as in the case of existing reel systems whether of the vertical or horizontal axis type, maximises the pitching motions of the vessel which are transferred to the pipe as it is launched. This obviously creates certain operational difficulties and limitations.

The present invention is primarily concerned with the provision of a reel pipelay system which avoids one or more of the above mentioned problems. The preferred embodiment combines the advantages of vertical and horizontal axis reel systems, whilst avoiding their associated disadvantages. In a particularly preferred embodiment, the invention also accommodates a flexible pipelay system (existing flexible pipelay systems are exemplified by WO-A-91/15699).

In accordance with the present invention, there is provided a reel pipelaying vessel having a pipe storage reel and pipeline diverter means defining a curved pipeline path for diverting a pipeline to a required launch angle as it is unspooled from the reel and launched from the vessel, wherein said diverter means is so located that the pipe departs from the diverter means at a point between the bow and stern of the vessel.

The vessel is preferably equipped with dynamic positioning means.

Preferably, said reel is mounted for rotation about a vertical axis. Most preferably, the reel is centred substantially on the longitudinal axis of the vessel.

Preferably also, said diverter means is located adjacent said reel.

Preferably also, said diverter means is located so as to launch said pipeline from one side of the vessel.

Most preferably, the diverter means is located to one side of said reel and, in the preferred embodiment, extends above the uppermost part of said reel.

Preferably, the pipeline path defined by said diverter means lies in a substantially vertical plane substantially parallel to the longitudinal axis of the vessel.

Preferably also, the diverter means is configured such that the pipeline is initially diverted in an upwards direction prior to being diverted downwardly to a final, adjustable launch angle.

Preferably, the diverter means is configured such that, when the reel is arranged so that the pipeline engages the diverter means in the direction of travel of the vessel during pipelaying operations (i.e. towards the bow, if the vessel travels forward during pipelaying operations), the net vertical angle through which the pipe is diverted between an initial horizontal orientation and a final launch angle is greater than 1800 and no greater than 2700.

Alternatively, the diverter means is configured such that, when the reel is arranged so that the pipeline engages the diverter means in a direction opposite the direction of travel of the vessel during pipelaying operations (i.e. towards the stern, if the vessel travels forward during pipelaying operations), the net vertical angle through which the pipe is diverted between an initial horizontal orientation and a final launch angle is at least 2700 and less than 360".

The pipeline path may be helical and the diverter means may be configured such that the pipeline path diverts the pipeline through multiple turns.

The diverter means may be mounted for reciprocating vertical movement relative to the reel (i.e "level wind" or "fleeting" movement).

The curved pipeline path is preferably defined by means providing a continuous or discontinuous pipe bearing surface extending around said path. The pipe bearing surface may be provided by the rim of a rotatable wheel structure, or by an endless conveyor, or by a series of rollers, or by a plurality of roller track assemblies.

The curved pipeline path preferably has a constant radius of curvature.

The launch angle of the pipeline may be varied by varying the point at which the pipeline departs from said curved pipeline path, and the pipeline is preferably straightened by straightening means after departing from said curved pipeline path.

The vessel preferably also includes pipe guide means through which the pipeline passes after being straightened, and the launch angle of the pipeline may be varied by translating said pipe guide means in a fore and aft direction.

Tension may be applied to said pipeline by tensioning means located upstream of said pipeline diverter means in the direction of pipeline travel, or by tensioning means located downstream of said pipeline diverter means in the direction of pipeline travel, or by tensioning means located along the length of the path defined by the pipeline diverter means, or by a braking force applied to said pipeline as it passes around said curved pipeline path, or by any combination of these.

Preferably also, the vessel includes preliminary diverter means for diverting the pipeline in a horizontal plane into alignment with said curved pipeline path prior to engaging said curved pipeline path.

The vessel is preferably further equipped with an additional storage reel for storage of flexible pipeline and said diverter means is adapted to divert said flexible pipeline to a desired launch angle. The diverter means may be adapted to accommodate the flexible pipeline on the same pipeline path as pipeline from the main pipeline reel and/or may include a second pipeline path. The vessel may be provided with additional tensioner means located on a flexible pipeline path between said additional storage reel and said diverter means for applying tension to said flexible pipeline during flexible pipelaying operations.

In the present application, the term "roller track assembly" is used to designate an assembly of generally known type in which an endless conveyor belt or track or chain or the like is mounted around a supporting chassis including end sprockets (driven or idle) and support rollers, in the manner of a caterpillar track.

Roller track assemblies of this general type have been used in apparatus for straightening rigid pipelines, for tensioning pipelines, cables, ropes etc., and for aligning pipelines. A typical straightener employs three-point straightening as is well known in the art and is discussed in US Patents Nos.3,237,438 and 3,372,461. Examples of roller track assemblies and their use in pipeline straightening are described in US Patents Nos. 3,855,835, 4,157,023, 4,243,345 and 4,260,287. Alternative "roller-track" type assemblies are also described in US Patents Nos. 4,230,421, 4,269,540, 4,297,054, 4,340,322 and 4,345,855, and further in US Patents Nos. 3,641,778, 3,680,342, Re 30,846 and 4,723,874. In general, the roller track assemblies are arranged on opposite sides of the pipe, and means are provided for positioning the tracks to impart a reverse bending force.

A typical tensioner may suitably comprise two similar roller track assemblies engaging the pipe on opposite sides thereof and means, such as hydraulic motors, for driving the tracks to apply a braking force to the elongate member.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a schematic side view of a vessel forming one embodiment of the invention; Fig. 2 is a plan view of the embodiment of Fig. 1; Referring now to the drawings, there is illustrated a general purpose dynamically positioned vessel 10 converted to a pipelay vessel in accordance with the present invention. Such a vessel typically includes conventional dynamic positioning, variable-azimuth thrusters 12, superstructure 14, helipad 16, cranes 18, 20, and a moonpool 22 for use in the deployment of a remotely operated vehicle (ROV) 24 and other well-known functions.

Pipelaying apparatus in accordance with the invention is located on the main deck 25 of the vessel and comprises a main pipe storage reel 26 upon which a pipeline 28 is spooled and a pipeline diverter structure 30 for diverting the pipeline 28 to a desired launch angle during pipelaying operation of the vessel.

Optionally, as seen in Fig. 2, the pipelaying apparatus may further include an additional storage reel 31 for a flexible pipeline 32, and additional tensioning means 34 for applying tension to the flexible pipeline 32.

The storage reels 26 and 30 are preferably mounted for rotation about respective vertical axes, thereby providing a lower centre of gravity than would be the case if the reels were mounted for rotation about horizontal axes. The diverter structure 30 is adapted to divert the pipeline 28 and/or the flexible pipeline 32 to a variable launch angle, typically in the range 45" to 90" with respect to the horizontal. The lower end of this range of angles may be less than 45" if required.

Effectively, in this example where the pipeline engages the diverter structure in a direction towards the bow of the vessel, the diverter structure 30 diverts the pipeline from an initial horizontal orientation through a net vertical angle which is greater than 1800 (i.e. to a launch angle which is greater than zero degrees to the horizontal) and less than or equal to 2700 (i.e. up to a vertical launch angle). Alternatively, if the apparatus were arranged such that the pipeline were unspooled in the opposite direction from that shown in the drawings (i.e. the pipeline engages the diverter structure in a direction towards the stern of the vessel), the net vertical angle through which the pipe is diverted between an initial horizontal orientation and a final launch angle is at least 2700 (vertical launch) and less than 360" (shallow launch).

This assumes that the vessel travels forwards during pipelaying operations. If the vessel travels in reverse during pipelaying operations then the arrangements described above would also be reversed.

The pipeline diverter structure 30 defines a curved pipeline path, the pipeline 28 being plastically deformed to the radius of curvature of the path as it passes around the pipeline path (there is no plastic deformation in the case of the flexible pipeline 32; also, small bore pipes and tubes could be laid by the system without plastic deformation). The diverter structure is preferably similar in configuration to those disclosed in WO-A-95/25237 or in variants of such structures as disclosed in WO-A-96/30686. The diverter structures disclosed in these cases are particularly intended for use with very large diameter pipelines assembled on board the vessel in a "stovepiping" system. In the present case the diverter structure would be relatively much smaller but is similar in principle of operation. As in these earlier cases, the pipeline path may be defined by the rim of a rotatable wheel (as illustrated in Figs. 1 and 2) or by a single endless conveyor, or by a series of rollers, or by a series of roller track assemblies. The path might be planar or helical, and could be configured to divert the pipeline through multiple turns.

In the present embodiment, the diverter structure is located so as to launch the pipelines 28 and 32 over the side of the vessel, at a point between the bow and stern. It is desirable that the point of departure of the pipeline from the diverter structure 30 be close to the longitudinal centre of the vessel so as to minimise vessel pitching motions transferred to the pipeline and that the diverter structure 30 be located close to the main storage reel 26. In the illustrated embodiment, the diverter structure 30 is located above the main reel 26, and is cantilevered over the side of the vessel by a suitable support structure.

The pipelaying apparatus will include additional components and features as follow: 1. Horizontal pipeline diverter means.

As the pipeline 28 leaves the main reel 26, means (not shown) are required to divert the pipeline in the horizontal plane into alignment with the pipeline path of the diverter structure 30. Such means suitably comprise a "shoe" having an arcuate pipe bearing surface, of the general type known, for example, from the above referenced US-A-3680342. The horizontal diverter might be mounted for reciprocating, vertical level wind movement parallel to the axis of the reel 26. As is known in the art, it is desirable for the horizontal diverter to bend the pipe to a fixed, known radius of curvature, in order to simplify subsequent straightening. This is also true of the main diverter structure 30, which may also be mounted for reciprocating, vertical level wind movement parallel to the axis of the reel 26. The location of the reel 26 may be selected to help balance the weight of the diverter structure 30 located at the side of the vessel.

2. Pipeline straightening means.

As the pipeline 28 leaves the diverter structure 30, any plastic deformation must be removed so as to straighten the pipe. This may be done by means of "three-point" or "reverse bending" straightener mechanisms (not shown) of various types which are well known in the art, examples of which are described in the various prior publications referenced herein. In the present case, the pipeline 28 is bent in both the horizontal and vertical planes. Accordingly, straightening forces must be applied to straighten the pipeline 28 in both planes. This might be done by means of first and second sets of straightener rolls oriented mutually at right angles to one another, or by a single straightener oriented to match the net, resultant plane of bending of the pipeline 2B.

The straightening means will be mounted such that its position and orientation may be varied relative to the diverter structure 30 to suit the required pipe launch angle, and is suitably mounted on the diverter structure 30 for movement about the centre of curvature of the pipeline path defined by the diverter structure 30.

3. Pipe guide means.

A pipe guide (not shown) will preferably be located downstream of the straightener means in the pipelay direction of pipeline travel. Guides of this type are well known in the art, typically comprising pairs of orthogonally mounted rollers between which the pipeline passes, and may include means for monitoring the angle of orientation of the pipe and other parameters for controlling the pipelay operation. The pipe guide might suitably be mounted on the side of the vessel for fore and aft movement relative to the diverter structure, to suit the required launch angle.

4. Pipeline tensioning means.

As is well known in the art, tension must be applied to the pipeline during pipelay operations to control the catenary of the pipespan between the vessel and the touchdown point on the seabed, so as to prevent damage to the pipe. The required tension is set in combination with the pipe launch angle as a function of the water depth and the characteristics of the pipeline. In the present case, the primary pipeline tension may be provided, at least in part, by a braking force or "capstan effect" via the diverter structure.

This might be supplemented or replaced by additional tensioning mechanisms upstream and/or downstream of the diverter structure 30, and/or arranged around the path defined by the diverter structure. Flexible pipeline generally requires higher tension than conventional pipe, for which purpose the tensioning unit 34 is provided.

5. Abandonment & recovery (A & R) systems and pipeline clamps The vessel will also include means (not shown) for the abandonment and recovery of pipelines during pipelay operations, comprising systems of cables and winches as are well known in the art. In the present case the A & R cable would be arranged to pass around the diverter structure 30. Suitable pipeline clamps will be provided, particularly between the straightener mechanism and pipe guide, for securing the pipelines during the course of pipelay operations. Clamps of this type are also well known in the art.

The invention thus provides a reel pipelay vessel having a number of advantages over known pipelay systems, which is well suited to the temporary conversion of vessels for pipelay operations, and which is suited to a variety of types of pipelay operations involving conventional and/or flexible pipelines in a wide range of water depths.

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

Claims

1. A reel pipelaying vessel having a pipe storage reel and pipeline diverter means defining a curved pipeline path for diverting a pipeline to a required launch angle as it is unspooled from the reel and launched from the vessel, wherein said diverter means is so located that the pipe departs from the diverter means at a point between the bow and stern of the vessel.

2. A vessel as claimed in Claim 1, further equipped with dynamic positioning means.

3. A vessel as claimed in Claim 1 or Claim 2, wherein said reel is mounted for rotation about a vertical axis.

4. A vessel as claimed in Claim 3, wherein the reel is centred substantially on the longitudinal axis of the vessel.

5. A vessel as claimed in any preceding Claim, wherein said diverter means is located adjacent said reel.

6. A vessel as claimed in any preceding Claim, wherein said diverter means is located so as to launch said pipeline from one side of the vessel.

7. A vessel as claimed in Claim 6, wherein the diverter means is located to one side of said reel.

8. A vessel as claimed in Claim 7, wherein the diverter means extends above the uppermost part of said reel.

9. A vessel as claimed in any preceding Claim, wherein the pipeline path defined by said diverter means lies in a substantially vertical plane substantially parallel to the longitudinal axis of the vessel.

10. A vessel as claimed in any preceding Claim, wherein the diverter means is configured such that the pipeline is initially diverted in an upwards direction prior to being diverted downwardly to a final, adjustable launch angle.

11. A vessel as claimed in any preceding Claim, wherein the diverter means is configured such that, when the reel is arranged so that the pipeline engages the diverter means in the direction of travel of the vessel during pipelaying operations, the net vertical angle through which the pipe is diverted between an initial horizontal orientation and a final launch angle is greater than 180C and no greater than 270 .

12. A vessel as claimed in any one of Claims 1 to 11, wherein the diverter means is configured such that, when the reel is arranged so that the pipeline engages the diverter means in a direction opposite the direction of travel of the vessel during pipelaying operations, the net vertical angle through which the pipe is diverted between an initial horizontal orientation and a final launch angle is at least 2700 and less than 3600.

13. A vessel as claimed in any preceding Claim, wherein the pipeline path defined by the diverter means is helical.

14. A vessel as claimed in Claim 13, wherein the diverter means is configured such that the pipeline path diverts the pipeline through multiple turns.

15. A vessel as claimed in any preceding Claim, wherein the diverter means is mounted for reciprocating vertical movement relative to the reel.

16. A vessel as claimed in any preceding Claim, wherein the curved pipeline path is defined by means providing a continuous or discontinuous pipe bearing surface extending around said path.

17. A vessel as claimed in Claim 16, wherein the pipe bearing surface is provided by the rim of a rotatable wheel structure, or by an endless conveyor, or by a series of rollers, or by a plurality of roller track assemblies.

18. A vessel as claimed in Claim 16 or Claim 17, wherein the curved pipeline path has a constant radius of curvature.

19. A vessel as claimed in any preceding Claim, including means whereby the launch angle of the pipeline may be varied by varying the point at which the pipeline departs from said curved pipeline path.

20. A vessel as claimed in any preceding Claim, wherein the pipeline is straightened during pipelaying operations by straightening means after departing from said curved pipeline path.

21. A vessel as claimed in Claim 20, further includinc pipe guide means through which the pipeline passes after being straightened.

22. A vessel as claimed in Claim 21, wherein the launch angle of the pipeline may be varied by translating said pipe guide means in a fore and aft direction.

23. A vessel as claimed in any preceding Claim, wherein tension is applied to said pipeline during pipelaying operations by tensioning means located upstream of said pipeline diverter means in the direction of pipeline travel, or by tensioning means located downstream of said pipeline diverter means in the direction of pipeline travel, or by tensioning means located along the length of the path defined by the pipeline diverter means, or by a braking force applied to said pipeline as it passes around said curved pipeline path, or by any combination of these.

24. A vessel as claimed in any preceding Claim, further including preliminary diverter means for diverting the pipeline in a horizontal plane into alignment with said curved pipeline path prior to engaging said curved pipeline path.

25. A vessel as claimed in any preceding Claim, further equipped with an additional storage reel for storage of flexible pipeline.

26. A vessel as claimed in Claim 25, wherein said diverter means is also adapted to divert said flexible pipeline to a desired launch angle.

27. A vessel as claimed in Claim 26, wherein the diverter means is adapted to accommodate the flexible pipeline on the same pipeline path as pipeline from the main pipeline reel.

28. A vessel as claimed in Claim 26 or Claim 27 wherein said diverter means is adapted to provide a second pipeline path.

29. A vessel as claimed in any one of Claims 25 to 28, further provided with tensioner means located on a flexible pipeline path between said additional storage reel and said diverter means for applying tension to said flexible pipeline during flexible pipelaying operations.