WO2005005874A1

WO2005005874A1 - Method and associated apparatus for abandonment and recovery at sea

Info

Publication number: WO2005005874A1
Application number: PCT/IB2004/002548
Authority: WO
Inventors: Serge Camplan; Olivier Marie
Original assignee: Stolt Offshore Sa
Priority date: 2003-07-12
Filing date: 2004-07-12
Publication date: 2005-01-20
Also published as: GB0316375D0

Abstract

The invention comprises a method for abandonment or recovery of a load at sea using vessel mounted pipeline laying apparatus. In the method rigid tubular members, such as drillpipe (301), adapted to be handled as portions of pipeline, are each erected from a substantially horizontal orientation to a substantially vertical orientation and joined to a previous tubular member thus forming a string (403). The load is then suspended from a lower end of said string during said abandonment or recovery, the string being' lowered into the sea as new tubular members are joined to reach a desired depth. The invention further comprises adapted pipelaying apparatus and adapted drillpipe for carrying out the method.

Description

Method And Associated Apparatus For Abandonment And Recovery At Sea

The invention relates to a method and equipment for deployment and recovery of heavy loads in deep water and in particular to such abandonment/recovery (A/R) using equipment designed for the purposes of laying pipeline on the seabed. The invention further relates to the adaptation of drillpipe in such recovery/deployment operations

Subsea pipelines are often laid on the ocean floor using the known "J-lay" method whereby each portion of the pipeline to be connected is erected to a vertical position and then welded to the main pipeline and lowered directly into the water, with a single bend at the ocean floor. This gives the shape of a "J" and therefore the system is known as J-Laying pipe.

Pipeline is J-laid using a J lay tower. Such a tower is a purpose built apparatus comprising essentially a main mast structure, a working platform, an erector, a travelling table and a stinger. The erector raises a pipeline section from horizontal on the ship's deck to an upright position within the mast, where it is welded to the pipeline. The travelling table holds the pipeline section and lowers it (once connected) into the sea, via the stinger, which comprises internal rollers to prevent over-bending of the pipeline. Such a system is well- known for this purpose, and one example (with unique feature that allows it to be gimballed to different angles) can be found in US 6 213 686 (Baugh/Radoil) which is hereby incorporated herein by reference. Another example of a J-Lay tower system for laying welded pipe is in US 5 421 675 (Brown et al/McDermott).

To lower or raise a load between surface and seabed the most usual device is a cable or rope comiected to a winch. This applies to all kinds of handling: pipeline, manifold, skid, template piling, wreck etc. Winches have a maximum weight handling capability which vary considerably. Winches capable of handling loads of 500T or more are not common and are very expensive, costing millions of pounds. It would therefore be desirable if one could use existing equipment already fitted to a ship for abandonment/recovery operations. Furthermore, it is also desirable to have a combined device able to perform in sequence or simultaneously the handling of very large loads in deep water and the carrying of fluids between surface and deepsea locations, particularly with access to the large power sources available from surface equipment rather than R.ON.s (Remotely Operated Vehicle) or A.U.V.s (autonomous underwater vehicle). Currently, to pump a fluid between surface and seabed the most usual device is a hose, the end of which is handled by a subsea vehicle.

It is also desirable to easily and relatively cheaply have an efficient active heave compensation system for smooth landing operation of very large loads at great depth.

US 5 421 674 (Maloberti/Coflexip) proposes to use a recoverable flexible tubular conduit in place of a wire when abandoning the end of a pipe being laid. The handling conduit also provides fluid communication for flooding the laid pipeline. The use of a flexible conduit, to suspend a heavy article places limits on the load and operating depth and is an expensive solution, not compatible with typical J-Lay equipment

Drillpipe is heavy, thick-walled steel pipe used in rotary drilling to turn the drill bit and to provide a conduit for the drilling mud. Sections of drillpipe are about 12 metres long and new sections are joined to the top as the drill is lowered into the ground/seabed. These sections of drillpipe connected together form the drillstring, (sometimes taken also to include the drill collars, drill bit etc.). Such drilling equipment is well known, and used drillpipe is available at modest cost.

While both the aforementioned J-lay equipment and drilling equipment is known, there has been no attempt at using these types of equipment for purposes other than what they where designed for. Specifically no attempt has been made for using a J-lay (or any pipelaying apparatus) for either abandonment/recovery or flooding/pigging operations. Furthermore, drillpipe has not been used for these purposes, either in combination with pipelaying equipment or on its own.

It is now proposed to use J-lay pipelaying apparatus, with its long stroke/high capacity rams, and particularly in combination with specially modified drillpipes. as a combined abandonment/recovery system and a heavy load handling equipment with optional active heave compensation. Each of these applications can use full tower load handling capacity. It is also proposed to use the same apparatus for pumping a fluid between surface and seabed (for example: to flood or actuate a hydraulic system). The fluid can also be pumped out to de-water a line or a suction anchor, for instance, or to perform pigging and/or hydrotesting operations.

In a first aspect of the invention there is provided a method for abandonment or recovery of a load at sea using vessel mounted pipeline laying apparatus, wherein, rigid tubular members, adapted to be handled as portions of pipeline, are each erected from a substantially horizontal orientation to a substantially vertical orientation and joined to a previous tubular member thus forming a string, said load being suspended from a lower end of said string during said abandonment or recovery, said string being lowered into the sea as new tubular members are joined to reach a desired depth.

The rigid members may be drillpipe sections, applied in this case to load bearing rather than drilling. Drillpipe sections are readily available at relatively low cost, either new or previously used. The tubular members may be fitted with spacers to increase the external diameter of the pipe at points along its length, to suit handling by apparatus designed for handling larger pipes.

The spacer may be made of buoyant material such as polyurethane foam.

Said rigid tubular members may be drillpipe specially adapted as defined below in the third aspect of the invention.

Said tubular members may be connected using screwed or clamped connections, or may be screwed together. Said screwing or clamping may be done by the pipeline laying apparatus, said pipeline laying apparatus being thus modified to do so.

Said pipelaying apparatus may comprise a main mast having at least one travelling table for supporting and lowering a pipe under load, and an erector arm for receiving a new pipe section in a horizontal orientation and then while supporting the pipe section moving to a vertical orientation to transfer said pipe section into the mast to be supported by said travelling table.

In such an apparatus, the method may include joining said rigid tubular members by screwing them together, using a torquing device mounted on the erector arm.

The method may include retracting the erector arm partially from the vertical sufficiently to release the tubular member from support by the erector arm before operating said torquing device. The torquing device may be pivotally mounted to the erector arm, to compensate for the partial retraction.

The rigid tubular member may be fitted with spacer blocks to match their external diameter with that of larger pipe handled by the erector arm, a pipe stock elevator or another part of the pipe laying apparatus. The spacer blocks may be made of buoyant material such as polyurethane foam.

Said vessel mounted pipeline laying apparatus may be as defined below in the second aspect of the invention.

Said method may further comprise pumping a fluid in either direction through a channel provided in the string. When pumping is activated in a first direction, said pumping may be to flood or activate a hydraulic system and when activated in the other direction, it could be used to de-water for example, a line or suction anchor, or to perform pigging and/or hydrotesting operations.

Connection/disconnection of the string or of the load to/from the string at the sea bed may be performed by .O.V. or via a remote system.

In a second aspect of the invention there is provided pipelaying apparatus specifically adapted for abandonment/recovery operations using adapted drillpipe, said pipelaying apparatus comprising: an erector for supporting and raising each section of adapted drillpipe, a main mast having at least one travelling table and a torquing device for connection of each section of the adapted drillpipe to the previous section. The torquing device may be mounted on the erector. Alternatively it could be mounted to any of the other main components.

The erector may comprise an arm extending the length of each pipe section and pivotally mounted on at least one and so as to move from horizontal position for receiving a section of pipe to a vertical position for transmitting said pipe section into the main mast. The erector arm may be pivoted at one end near the base of the main mast in the manner of US 6 213 686, or it may be arranged with one end mounted to pivot and travel vertically up and down the mast, while the other end pivots and travels horizontally towards and away from the base of the mast. This latter arrangement is known for example from US '675 mentioned above. The torquing device may be arranged to operate when the erector arm is retracted slightly from the vertical.

The apparatus may be provided with optional heave compensation.

The heave compensation system may be an active system taking its real time heave information from a gyroscopic motion reference unit and controlling the motion of the travelling table to compensate the ship's heave in real time, such a system already exists on several other systems and in particular for winches and cranes.

In this particular case, the travelling table hydraulics may be directly controlled by the motion reference unit output when the system is in active heave compensation mode.

The drillpipe may be adapted for example by the provision of collars suitable for engagement by the pipelaying apparatus to support the weight of the string and load.

Said travelling table may be adapted to handle single collars or double collars. A double collar allows easier transfer (handover) of the suspended load from a travelling table to a fixed table, but systems are known which can do this with a single collar (see for example US '675 mentioned above). The drillpipe may be adapted for example by the provision of spacers to match the diameter of the drillpipe to that of expected by the pipelaying apparatus.

In a third aspect of the invention there is provided a rigid tubular element specifically adapted for use in abandonment or recovery operations using vessel mounted pipeline laying apparatus, the element comprising at least one drillpipe having squared-off shoulders for mounting at least one collar at one end to allow manipulation by said pipeline laying apparatus, and detachable attachment means to enable a plurality of said tubular elements to be joined together to form a load handling string.

Said element may comprise two drillpipes preassembled together, either directly screwed, connected with a screwing or clamping element, or welded. Said apparatus may further comprise two spacers, one towards either end of said apparatus.

Said element preferably has a chamiel rumiing through to allow the flow of fluids. Said element, including any connections, may be able to withstand a fluid pressure of 1000 bars from inside or outside.

Said collars may be provided in two halves and attached together around the drillpipe. This allows the fitting of two collars, one at either end.

Said element may be further modified by the addition of a connector at the bottom end for retaining a load. Said connector may have openings to allow the connection of conduits such as flexible pipes to perform operations such as pipeline filling or emptying, pumping, dewatering or gas injection operations through the bore of the string.

Said collars may be fitted to the drillpipe without the shoulders being squared off but by another method which allows heavy loads to be managed. Accordingly, the invention in its first and second aspects is not limited to the use of squared-off drillpipe of the type set forth in the third aspect. Each aspect of the invention is independent of the others in scope. 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:

Fig. 1 shows known J-lay equipment mounted on a vessel for laying pipeline which can be adapted for use according to the present invention;

Fig. 2 is a perspective view of the J-lay equipment of Fig. 1 in more detail;

Figs. 3a to 3c show in elevation and two cut-away plan views J-lay equipment modified for connecting drillpipes in abandonment and recovery (A/R) operations;

Figs. 4a to 4e show a torquing device of the modified equipment in five views;

Fig. 5 shows in partial cross-section a drillstring element adapted for use in A/R operations using pipelaying apparatus;

Fig. 6 is a detail view of a collar arrangement in the string element of Fig.5;

Figs. 7a and 7b are two detail view of a spacer in string element of Fig.5;

Figs. 8a to 8c show two elevations and cut-away plan view in partial cross section a bottom end connector for further modifying the string element of Fig.5. for the attaching of a load to the string; and

Figs. 9a to 9c show three stages of a method of abandonment/recovery using pipelaying equipment and modified drillpipes.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring now to Fig. 1, this shows part of a vessel fitted with a known J-lay tower 10, used conventionally for pipelaying operations. The drawing taken from US 621 3686, (and Fig. 2) are and corresponds broadly to the applicant's vessel, Seaway Polaris. It is shown on a floating vessel 12 in a body of water 14 with a pipeline 16 extending below the J-lay tower 10 around a bend 18 and onto the ocean floor 20. The J-lay tower 10 is shown with a mast 21, a working platform 28; and a skid 30. A new pipeline section 32 is shown on the erector 34 with cable 36 attached for pulling the erector up to the mast sections. As can be noted, the tower is inclined at an angle convenient to the laying of the pipeline. Jack assemblies, 42 assist in the changing of the tower angle as required. Stinger 44 provides internal rollers whose inner diameter provide a curvature to prevent over-bending of the pipeline.

Fig. 2 shows a perspective view of the J-lay apparatus of Fig. 1 with the erector 34 partially raised toward the mast 21. Main cylinders 80, with cables 82 over drums 84 are used to lift the travelling table 86 to power the system. Outriggers 88 are used to distribute the weight of the system over a large deck area. Notch 90 in the top of the mast 21 allows longer sections of new pipe to be added to the pipeline by having intermediate J-lay collars in place. Bushings 92 are provided for the support of the pipeline at the working platform 28.

Figs 3a to 3c. show the equipment of Figs 1 and 2 as modified for the novel purpose of handling drillpipes in A/R applications. The main modification is the addition of a hydraulic torquing device 150 on to the erector 34 near its "bottom" end. Also shown are the existing pipe holding clamps 51 on the erector and elevators 152 for raising pipe sections from deck to erector. After laying pipe with a 16" diameter, for example, these elevators would require modification to cope with drillpipe having a diameter of 4.5". This can be avoided by using spacers as described later. String element 403 (see below) is held by the travelling table 86, in position to be lowered onto the string.

The torquing device 150 provides the turning action to screw a new drillpipe section to the main string during abandonment operations, and to unscrew drillpipe sections during recovery. Figs. 4a to 4e show the torquing device 150 in detail in front elevation in two planes (4a, 4b), side elevation (4c) and plan view in two planes (4d, 4e). Two girder sections 153 provide a frame for the device, which can then be mounted as a unit onto the erector 34 as shown in Fig. 3a. This torquing device is in fact a standard hydraulic drill pipe which has been used conventionally in drill ships for several years. It is being used without any real modification and in the same way as it is used in drilling operations, but mounted and applied in a novel situation. The torquing device 150 is mounted to project generally at right angles to the axis of the erector 34, but can be adjusted five degrees away from vertical by two hydraulic rams 154 (Fig. 4a). The reason for this will be described later with reference to operation of the apparatus. The tong can be opened to allow fitting around the pipe through mouths 156a and 156b whereupon the new pipe section is engaged and driven rotationally by a motor and gearing system indicated generally at 158. This happens in the upper section 150a of the device, while the top of the suspended drill string is held against rotation in the lower section 150b.

As an alternative and emergency back up in case of drill tong damage, it is also envisaged to use standard drilling ship type manual tongs operated with tugger winches (not shown in the drawing). This is a technique which have been used by drillers since the beginning of the use of drill pipe, and details can be found in standard drilling texts. It has not been mounted to J-lay apparatus before, however.

Fig. 5 shows a drillpipe string section 403 (broken onto three parts to fit page) modified for use with the J-lay apparatus. It consists of two screwed tubular pieces of drillpipes 301, approximately 12 m length each, and pre-assembled and locked together in pairs with a double collar arrangement 307 and 2 sets of spacers 305. There is a bore 314 running through the length of the drillpipe to enable the flow of fluids in either direction. The two drillpipes are screwed together by connection 303. The screwed connection includes a mechanical seal which is designed to take a pressure of up to 1000 bars from inside or outside.

The double collar arrangement 307 at one end of each section enables handling in the J-lay tower by the travelling table. These collars are designed to match claws of the said travelling table and fixed bushing. This allows the drillpipe string section to be held and lowered such that new sections can be added to the string as the load is lowered into the sea. Fig 6 shows the double collar arrangement 307 in more detail. The shoulders 401 of the drillpipes have been squared off to allow for the use of suitable J-Lay collars, without the collars slipping, such that they are able to withstand a 500T load. Normally drillpipes are adapted to be used with a self-locking wedge device which are not practical for use with the J-lay equipment, in that they are too long (1 metre) effectively shortening the pipe, and would also require substantial travelling table redesign.

The standard conical shoulder pattern of the standard drill pipe is not suitable for supporting the J-lay collar inserts at a 500T load when provided in two halves. A J-lay collar insert in one piece, could theoretically take the 500T on the standard conical shoulder. However it would be very heavily loaded and it would be very difficult to install on the drill pipe which has shoulders at both ends.

In the case of Seaway Polaris, the elevators can be set up differently to handle pipes in three ranges: 4" to 10", 10" to 24", and 24" to 36", but it is inconvenient to change this set-up in the middle of a job. The spacers 305 in this embodiment consist of polyurethane (PU) foam blocks and bulk out the diameter of the drillpipe sections to 12" to enable handling by the elevators 152 which may be set up to handle diameters in the range of 10" to 24" without the need for the elevators to be modified. Figs 7a and 7b show half sections of these blocks in more detail from side and end views. If the elevators are already set up to handle 4" to 10", of course such spacers may be unnecessary. However, they do have the further advantage in that they add buoyancy to the string, thus decreasing weight when submerged. On another vessel, the holding clamps 151 of the erector 34 may also be limited in their capacity, and spacers provided at these points also.

The bottommost drillpipe section is further modified by the addition of a screwed connector 600 which allows suspension of any type of load (pipeline, subsea module, etc.) for lowering/lifting from/to the seabed. Such a comiector is shown in figs 8a to 8c. This consists of a main body 601, with a bevel at either end, to which there is connected an anchor shackle 603 by nut and bolt 605. At the top of the main body 601 is a threaded boring for a drilling stem 607, shown attached on fig 7a. In addition, bores drilled in the connector allow connection of flexible pipe to perform pipeline filling or emptying, pumping, de watering or gas injection operations through the bore of the string using pumps or gas generators/compressors, such as a HP/high flow pump or inert gas generator (Max. pressure up to 1000 bars) located on board the ship. For these operations, the end connector would be provided with a port (not shown), adapted to receive a flexible jumper pipe. The jumper pipe provides fluid communication between the drill string bore and the pipe or other article being deployed. This avoids the need for specialist contractors/vessels to be employed for commissioning operations such as flooding and pigging.

Each element of this abandon and recovery string therefore reproduces the outline of a standard J-lay 12" OD pipeline double joint and are therefore handled as such through all the existing pipe handling devices of the J-lay system. Also the safe working load of each A/R string element is 500 T which is the same as the maximum safe working load of the J- Lay system.

It is important to note that the string depicted here has many particular modifications which result from a particular J-lay apparatus, and consequently modified drillpipe string sections can depart from this actual design without departure from the scope of the invention. In particular they may be different lengths (e.g three joined together, for different laying equipment strokes, or may have different styles of collar, (such as a single collar arrangement). The techniques and accessories described can be adapted for operation with different vessels and different designs of pipe-laying apparatus, towers, erectors, elevators and so forth.

To make substantial savings on investment and on environmental costs, the drillpipes used need not be new, but instead may have already been used for their intended purpose and since decommissioned. Such "used" drillpipes can be obtained from drilling companies for little or no cost. Even new drillpipe costs much less than the special flexible pipe of US 5 421 674 mentioned above, and is easier to store and handle.

Fig 9a to 9c shows a close-up of the working deck area of Fig. 3 during stages of the operation. Showing is the lower potion of the mast 21 with working platform 28 attached at a lower end. A potion of the erector 34 is shown, hinged to the mast 21 /platform 28 at hinge 902. Mounted to the lower end (when vertical) of the erector 34 is the torquing device 150. Also shown is the centraliser 406 and bushing 408.

Prior to use, during A/R string deployment, the pre assembled string elements are stored on deck like normal pipeline. The new 24 m element is first elevated to the erector 34 by elevators 52, when the erector is in its horizontal position (Fig 9a with string element not shown). The erector is then raised with the string element in place, until said string element 403 is in a vertical position and is presented on the J-lay mast 21 above the previous element. The string element 403 is the positioned resting on the J-lay lower bushing 408 using the existing J-lay handling and centring devices, (shown in Fig 9b). The erector is then moved from the vertical where it has positioned the string element in the travelling table to a position about five degrees away from the vertical mast 21, so that the erector's holding clamps 151 (Fig. 3a) are retracted from the pipe sufficiently for the travelling table to move. The torquing device 150 located on the erector 34 is then tilted to remain in its operative position around the pipe by rams 154 (Fig. 3a) and the string element is screwed to the previous element with the proper torque by the torquing device 150 (Fig. 9c). Each section is preferably connected to the previous one via a screwed or clamped connector capable to withstand high pressure and suitable for fast coupling/fast disconnecting by automated mechanical means.

The connection/disconnection of the string or of the load to/from the string at the sea bed is performed by R.O.V. or via a remote system.

The methods and apparatuses as disclosed have may fields of application which include: All heavy pipelaying abandon, recovery and transfers using Polaris or similar J-Lay; all subsea heavy handling including lifts requiring active heave compensation; all subsea fluid pumping when Polaris or a similar vessel is involved..

There are many significant advantages in such a proposal over what is disclosed in US 5421 674 for example. These include the following: 1) The cost of drillpipes is much less than that of special flexible conduit, as they are more or less a "commodity" product.

2) This proposal works with various J-lay equipment and methods such as the collar system of US 6 213 686 or Bouygues/Saipem moving friction clamps. The system of US '674 only works with track tensioners.

3) Drillstring is much lighter than flexible conduit if used for the same 500 tonnes SWL While this is not so relevant for pipe abandon/recovery operations where the weight decreases as the load is lowered, it is a big advantage for handling of constant heavy loads all the way to the bottom as the total weight of the flexible will severely limit the maximum load that the US '674 system can handle.

4) The maximum fluid flow which can be passed through the drillpipe is much greater than the flow rate which can be passed through the flexible conduit. This is because the flexible conduit needs to be made so thick and strong to withstand the squeeze of the tensioners and consequently the bore through the flexible conduit is much smaller than the bore through the drillstring.

5) The drillpipe sections can be stored on deck in manageable lengths and added to the string as required. The flexible conduit has a very high minimum bending radius (over 6m) and it is therefore impossible to store a reasonable length on a drum or reel. Consequently this abandon flexible needs to be stored on a good size carousel. This requires a vessel fitted with such a carousel which is a serious limitation.

6) When abandoning pipeline using the US '674 disclosure, a steel wire sling or piece of chain needs to be used between the flexible conduit and the pipe as the flexible is in fact stiff which would render the disconnecting operation on the bottom is very difficult. This is a problem as with this system the maximum length of chain or wire which can be installed is quite small (about 12 to 15 m maximum). While the present proposal also employs chain or wire between the drill string and the pipe, there is no limitation in length and consequently disconnecting operations are much easier and faster.

The skilled person will recognise that the above examples are intended for illustration only, and many variations are possible without departing from the spirit and scope of the invention.

Claims

1. A method for abandonment or recovery of a load at sea using vessel mounted pipeline laying apparatus, wherein, rigid tubular members, adapted to be handled as portions of pipeline, are each erected from a substantially horizontal orientation to a substantially vertical orientation and joined to a previous tubular member thus forming a string, said load being suspended from a lower end of said string during said abandonment or recovery, said string being lowered into the sea as new tubular members are joined to reach a desired depth.

2. A method as claimed in claim 1, wherein the rigid tubular members are drillpipe sections, applied in this case to load bearing rather than drilling.

3. A method as claimed in claims 1 or 2, wherein the rigid tubular members are fitted with spacers to increase the external diameter of the pipe at points along its length, to suit handling by apparatus designed for handling larger pipes.

4. A method as claimed in claim 3, wherein the spacer is made of buoyant material such as polyurethane foam.

5. A method as claimed in any preceding claim, wherein said rigid tubular members comprise drillpipe specially adapted as claimed in any of claims 31 to 42.

6. A method as claimed in any preceding claim, wherein said rigid tubular members are connected using screwed or clamped connections.

7. A method as claimed in any of claims 1 to 5, wherein said rigid tubular members are connected by being screwed together.

8. A method as claimed in claims 6 or 7, wherein said screwing or clamping is performed by the pipeline laying apparatus, said pipeline laying apparatus being thus modified to do so.

9. A method as claimed in claim 8, wherein said pipelaying apparatus comprises a main mast having at least one travelling table for supporting and lowering a pipe under load, and an erector arm for receiving a new pipe section in a horizontal orientation and then while supporting the pipe section moving to a vertical orientation to transfer said pipe section into the mast to be supported by said travelling table.

10. A method as claimed in claim 9, further comprising the step of joining said rigid tubular members by screwing them together, using a torquing device mounted on the erector arm.

11. A method as claimed in claims 9 or 10, further comprising the step of retracting the erector arm partially from the vertical sufficiently to release the tubular member from support by the erector arm before operating said torquing device.

12. A method as claimed in claim 11, wherein the torquing device is pivotally mounted to the erector arm, to compensate for the partial retraction.

13. A method as claimed in any of claims 9 to 12, wherein the rigid tubular member is fitted with spacer blocks to match their external diameter with that of larger pipe handled by the erector arm, a pipe stock elevator or another part of the pipe laying apparatus.

14. A method as claimed in claim 13, wherein the spacer blocks are made of buoyant material.

15. A method as claimed in any preceding claim, wherein said vessel mounted pipeline laying apparatus is as claimed in claims 18 to 30.

16. A method as claimed in any preceding claim, further comprising pumping a fluid in either direction through a channel provided in the string.

17. A method as claimed in any preceding claim, wherein connection/disconnection of the string or of the load to/from the string at the sea bed is performed by R.ON. or via a remote system.

18. Pipelaying apparatus specifically adapted for abandonment/recovery operations using adapted drillpipe, said pipelaying apparatus comprising: an erector for supporting and raising each section of adapted drillpipe, - a main mast having at least one travelling table and - a torquing device for connection of each section of the adapted drillpipe to the previous section.

19. Pipelaying apparatus as claimed in claim 18, wherein the torquing device may be mounted on the erector.

20. Pipelaying apparatus as claimed in claim 18, wherein the torquing device is mounted to any of the other main components.

21. Pipelaying apparatus as claimed in any of claims 18 to 20, wherein the erector comprises an arm extending the length of each pipe section and pivotally mounted on at least one and so as to move from horizontal position for receiving a section of pipe to a vertical position for transmitting said pipe section into the main mast.

22. Pipelaying apparatus as claimed in claim 21, wherein the erector arm is pivoted at one end near the base of the main mast.

23. Pipelaying apparatus as claimed in claims 21 or 22, wherein the erector arm is arranged with one end mounted to pivot and travels vertically up and down the mast, while the other end pivots and travels horizontally towards and away from the base of the mast.

24. Pipelaying apparatus as claimed in any of claims 21 to 23, wherein the torquing device is arranged to operate when the erector arm is retracted slightly from the vertical.

25. Pipelaying apparatus as claimed in any of claims 18 to 24, further comprising a heave compensation system.

26. Pipelaying apparatus as claimed in claim 25, wherein the heave compensation system is an active system taking its real time heave information from a gyroscopic motion reference unit and controlling the motion of the travelling table to compensate the ship's heave in real time.

27. Pipelaying apparatus as claimed in claim 26, wherein the travelling table hydraulics are directly controlled by the motion reference unit output when the system is in active heave compensation mode.

28. Pipelaying apparatus as claimed in any of claims 18 to 27, wherein the drillpipe is adapted, by the provision of collars suitable for engagement by the pipelaying apparatus to support the weight of the string and load.

29. Pipelaying apparatus as claimed in any of claims 18 to 28, wherein said travelling table is adapted to handle single collars or double collars.

30. Pipelaying apparatus as claimed in any of claims 18 to 29, wherein the drillpipe is adapted, by the provision of spacers to match the diameter of the drillpipe to that of expected by the pipelaying apparatus.

31. A rigid tubular element specifically adapted for use in abandonment or recovery operations using vessel mounted pipeline laying apparatus, the element comprising at least one drillpipe having squared-off shoulders for mounting at least one collar at one end to allow manipulation by said pipeline laying apparatus, and detachable attachment means to enable a plurality of said tubular elements to be joined together to form a load handling string.

32. A rigid tubular element as claimed in claim 31, wherein said element comprises two drillpipes preassembled together.

33. A rigid tubular element as claimed in claim 32, wherein said drillpipes are preassembled by being directly screwed.

34. A rigid tubular element as claimed in claim 32, wherein said drillpipes are preassembled by being connected with a screwing or clamping element.

35. A rigid tubular element as claimed in claim 32, wherein said drillpipes are preassembled by being welded.

36. A rigid tubular element as claimed in any of claims 32 to 35, further comprising two spacers, one towards either end of said pipeline laying apparatus.

37. A rigid tubular element as claimed in any of claims 31 to 36, further comprising a channel rumiing through to allow the flow of fluids.

38. A rigid tubular element as claimed in any of claims 31 to 37, which, including any connections, is able to withstand a fluid pressure of 1000 bars from inside or outside.

39. A rigid tubular element as claimed in any of claims 31 to 38, wherein each said at least one collar is provided in two halves which are attached together around the drillpipe.

40. A rigid tubular element as claimed in any of claims 31 to 39, further comprising a connector at the bottom end for retaining a load.

41. A rigid tubular element as claimed in claim 40, wherein said connector further comprises openings to allow the connection of conduits such as flexible pipes to perform operations such as pipeline filling or emptying, pumping, dewatering or gas injection operations through the bore of the string.

42. A rigid tubular element as claimed in any of claims 31 to 41, wherein said collars are fitted to the drillpipe without the shoulders being squared off.