Abstract:
A method of abandoning a pipeline during subsea pipelaying from a pipelay vessel includes suspending the pipe string from a tendon element of an abandonment string and engaging further tendon elements to an upper end of the abandonment string while lowering the pipe string to a handover depth. At the handover depth, tension is applied via a wire to the top of the pipe string at an acute angle to the launch axis. This deflects the pipe string from the launch axis into axial alignment with the wire to transfer the weight load of the pipe string to the wire, whereupon the abandonment string can be decoupled from the pipe string. Recovery of the pipeline from the seabed can be effected by a reverse process.

Description:
This Application is the U.S. National Phase of International Application Number PCT/GB2014/050335 filed on Feb. 6, 2014, which claims priority to Great Britain Application No. 1302115.9 filed on Feb. 6, 2013. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     This invention relates to abandonment and recovery or ‘A&amp;R’ procedures used in marine pipelaying, in which a floating vessel such as a barge is used to lay an offshore pipeline. 
     (2) Description of Related Art 
     The invention has particular advantages when used with J-lay pipelaying equipment and it will be described in that context. 
     The J-lay technique is suitable for pipelaying in deep water. It involves welding together successive pipe sections or ‘joints’ in an upright orientation in a J-lay tower on a pipelaying vessel. The resulting pipe string is launched downwardly into the water as it is formed. The pipe string adopts a single bend as it nears the seabed to lend a J-shape to the pipe string extending between the vessel and the seabed—hence ‘J-lay’. 
     J-lay is necessary in deep water because the pipe string with attached accessories extending from the pipelaying vessel to the seabed is extremely heavy, typically weighing hundreds of tonnes. To avoid buckling, the pipe string must bear that weight in tension, suspended from a holding device on the J-lay tower. 
     An example of a J-lay pipelaying vessel is the Applicant&#39;s derrick lay barge Seven Polaris. The operation of Seven Polaris during pipelaying will now be outlined with reference to  FIGS. 1 and 2  of the drawings. It should be noted that this example is given simply to put the invention into context and so does not limit the scope of the invention. In those drawings: 
       FIG. 1  is a side view of a J-Lay tower on a barge; and 
       FIG. 2  is a perspective view of an erector arm loading a double joint into the tower of  FIG. 1 . 
     Referring to  FIGS. 1 and 2 , the J-lay tower  10  of the barge  12  is supplied with double joints  14  of pipe made onshore, which are stored horizontally on the deck  16 . As required, the double joints  14  are lifted successively in horizontal orientation from the deck  16  to a tower entry level  18  using a pipe elevator system  20  best shown in  FIG. 2 . Here, a double joint  14  is loaded into a pivoting erector arm  22 , which upends the double joint  14  into an upright orientation and passes it over to a tower handling system comprising a tensioning apparatus including a travelling clamp  24 . The double joint  14  is then lowered and aligned with the pipeline end held in a support bushing at a work station  26  on the tower  10 . 
     The double joint  14  is welded to the pipeline end at the work station  26  before the load of the pipe string is transferred from the support bushing to the travelling clamp  24  near the top of the J-lay tower  10 . The completed pipe string is then lowered down to the support bushing for the addition of the next double joint  14 . The travelling clamp  24  and the support bushing alternate to grip the pipeline end, interacting in a so-called ‘hand-over-hand’ manner. 
     Abandonment and recovery refer, respectively, to the procedures of laying down a pipeline end on the seabed and later retrieving the pipeline end from the seabed. Those procedures are necessary during normal pipelaying start-up and termination. They are also necessary whenever pipelaying must be interrupted and resumed. For example, the pipelaying vessel may suffer a critical breakdown. More commonly, pipelaying is interrupted due to deteriorating weather conditions, which may stress the pipeline and reduce its fatigue life as the pipelaying vessel rolls and pitches in a rough sea or if it has difficulty remaining in position due to winds and currents. In such cases, the pipelaying vessel may need to abandon the pipeline end and leave the work area. When the problem that caused abandonment has passed, the vessel will return later to recover the pipeline end and resume pipelaying. 
     Abandonment involves attaching a cap to the pipeline end, which may be a pipeline end termination or PLET. A shackle is attached to a hook on the PLET to secure a wire running through a winch on the pipelaying vessel, and tension is transferred from the travelling clamp of the J-lay tower to the winch. The winch then lowers the PLET into the sea until the pipeline and PLET rest on the seabed. The wire is then detached from the hook of the PLET, for example using a remote-controlled linkage or an ROV, and is retracted by the vessel for storage. 
     A recovery buoy is attached to the PLET during abandonment, enabling the PLET and the pipeline end to be located and retrieved during a subsequent recovery procedure. In essence, the recovery procedure is the reverse of abandonment as the wire is reattached to the PLET, typically using an ROV, and the PLET with the attached pipeline end is winched back up to the pipelaying vessel for pipelaying to resume. 
     It may be possible in some situations for the pipelaying vessel to remain on station above the abandoned pipeline, for example while riding out a period of bad weather. In that case, the wire may be kept attached to the PLET resting on the seabed until the bad weather has passed. This eases the recovery procedure considerably. 
     Traction (capstan) winches are generally used in A&amp;R procedures to handle the high top tensions characteristic of deep-water pipelaying. Such winches require a continuous length of wire, generally of steel. That wire must be of large diameter to support the great weight of the pipe string that extends above the seabed as the pipeline end nears the surface. Obviously, the wire must also be very long: generally several thousand meters long. Consequently, the wire itself may weigh in excess of 300 tonnes, and it takes up a great deal of space on the vessel when not in use. 
     Until recently, it was not possible to manufacture continuous lengths of wire of the necessary diameter. Indeed, A&amp;R wires remain a high-cost item. They are also susceptible to damage, particularly in the corrosive marine environment, and so have a limited life. If damaged, the whole wire may have to be down-rated or rejected; this makes it advisable for the pipelaying vessel to carry a spare wire but this, of course, doubles the problems of high cost and storage space. 
     It has been proposed to use complete single lengths of synthetic rope with traction winches for A&amp;R purposes but that technology is not yet fully proven. It is also noted that any damage to any part of a continuous rope may lead to the entire rope being down-rated or rejected, like a wire. 
     Of course, a heavy pipe string also requires a large, powerful and hence expensive winch. Exploitation of oil and gas reserves in ever-deeper water and the use of intrinsically heavier pipes such as pipe-in-pipe (PiP) systems could involve a top tension of as much as 1400 tonnes, by way of example. This is significantly above the load that can be handled by conventional J-Lay A&amp;R systems as it considerably exceeds the capacity of a typical A&amp;R winch. Unfortunately, limitations of cost and vessel space militate against merely scaling up an A&amp;R winch in accordance with the higher load. 
     Multiple winch and wire systems have been proposed in an effort to mitigate these problems. An example is disclosed in U.S. Pat. No. 7,507,055 to Subsea 7. This recognises that A&amp;R operations do not always take place at extremes of depth and that those operations can be handled more conveniently, where possible, with a smaller-capacity wire and winch than with a larger-capacity wire and winch. Consequently, pipelaying vessels are often equipped with both larger-capacity and smaller-capacity wires and winches. 
     These differently-rated wires and winches may be used together or successively. For example, when abandoning a pipeline, the larger-capacity wire and winch may be used to lower the pipeline end to an intermediate depth at which the top tension reduces to an extent that the load can be transferred to the smaller-capacity wire and winch. The higher-capacity wire can then be disconnected from the pipeline end and retracted to the pipelaying vessel. The smaller-capacity wire and winch then takes over to lower the pipeline end the rest of the way to the seabed. This means that a shorter length of larger-diameter wire is required on the drum of the higher-capacity winch, reducing space requirements and potentially also cost. However each wire remains vulnerable to damage and if spares are kept on board for both wires, the pipelaying vessel must accommodate four wires and not just two. This consumes space and reduces any cost advantage. 
     It has also been proposed in the prior art to effect abandonment and recovery by adding elements to, or removing elements from, an end of a pipe string using the same pipe-laying and tensioning apparatus that is used for laying the pipeline. To enable this, the elements may be of much the same general dimensions in terms of length and diameter as the ordinary lengths of pipe that make up the pipeline. 
     For example, US 2003/0099515 to Saipem proposes reducing the top tension of a sealine being laid by the vessel, this sealine being defined as pipeline laid on the seabed together with any pipeline or other elongate members extending upwardly from the pipeline. Tension is reduced by connecting one or more elongate members to the end of the pipeline and lowering the elongate members into the sea. These elongate members are lighter, length-for-length, when submerged in water, than the weight in water of the pipeline to which they are connected. Consequently, the effect is to reduce the apparent weight of the overall sealine. 
     WO 2005/005874 to Stolt Offshore proposes the use of rigid tubes such as drill pipe sections to lower and raise a pipeline during abandonment and recovery, the tubes being handled and connected by a J-lay system. The successive tubular sections may be screwed together by cooperating threads. 
     WO 2011/083340 to Subsea 7 proposes the use of elongate sling sections that are cooperable with like sling sections to form a sling for use in abandonment or recovery of a pipeline. Each sling section comprises complementary connector formations at opposite ends, such that each connector formation is cooperable with a complementary connector formation of a neighbouring sling section. Each sling section further comprises a tensile load-bearing sling element extending between the ends and a sleeve around the sling element that provides rigidity for the sling section and protection for the sling element. 
     GB 2488767 to Technip describes another way to abandon a pipeline, in that case using a continuous flexible pipe as a sling. Flexible pipe is an expensive solution and as a storage reel is necessary, flexible pipe may be difficult to store on a vessel that is configured for laying rigid pipe. Additionally, the length of a flexible pipe makes it difficult to manage and its length cannot easily be modified: it can only be shortened. 
     Also, flexible pipe tends to be fragile: its structure and particularly its important tensile layer has failure modes such as unlocking or disbondment. The outer sheath of the pipe is also fragile and has to be handled with care. Like a wire, failure of part of the pipe may condemn the whole pipe. There is also a tendency to twist because of unbalanced armour layers in torsion. 
     WO 2012/168702 to Flexlife discloses a contact damage protector for a flexible marine riser. The protector is made up of sleeve elements connected together to form a sleeve. Each sleeve element has a male section at one end and a female section at the other end, such that the male section of one sleeve element is received within a female section of the next sleeve element and so on. However, the sleeve elements are not suitable for use as sling sections for abandonment or recovery. 
     It is against this background that the present invention has been devised. 
     BRIEF SUMMARY OF THE INVENTION 
     In one sense, the invention resides in methods of abandonment and recovery during subsea pipelaying using a pipelay vessel. 
     A method of abandoning a pipeline in accordance with the invention comprises: coupling the top of a pipe string directly or indirectly to a lower tendon element of an abandonment string; suspending the pipe string from the lower tendon element; releasably engaging further tendon elements in succession to an upper end of the abandonment string while lowering the pipe string into the sea, such that the pipe string and the abandonment string are in end-to-end alignment under tension along a launch axis under the weight load of the pipe string; lowering the top of the pipe string to a handover depth; at the handover depth, applying tensile force via a wire from the vessel to the top of the pipe string, the wire being at an acute angle to the launch axis; by application of tensile force via the wire, deflecting the pipe string from the launch axis toward end-to-end axial alignment with the wire to transfer the weight load of the pipe string to the wire; and decoupling the abandonment string from the pipe string. 
     Conveniently, the wire is attached to the top of the pipe string at the surface and is lowered with the pipe string to the handover depth. 
     It is preferred that the abandonment string bears substantially the whole weight load of the pipe string during lowering to the handover depth. 
     A link element may be coupled to the top of the pipe string, in which case the pipe string may be suspended from a lower tendon element of the abandonment string via the link element. This allows the pipe string, the link element and the abandonment string to lie in end-to-end alignment under tension along the launch axis under the weight load of the pipe string. Angular movement of the link element with respect to the abandonment string and the pipe string may then be accommodated by deflection of the pipe string from the launch axis toward alignment with the wire. For example, the link element may move angularly with respect to the abandonment string and the pipe string by pivoting about upper and lower couplings of the link element, or by flexing. 
     The abandonment method of the invention is suitably followed by raising and dissembling the abandonment string by removing tendon elements in succession from the top of the abandonment string on the pipelay vessel. 
     Within the inventive concept, a corresponding method of recovering a pipeline in accordance with the invention comprises: using a wire from the vessel to raise the top of the pipe string to a handover depth while the wire and the pipe string are in end to end alignment on a lift axis; releasably engaging tendon elements in succession to an upper end of a recovery string of such elements to lower the recovery string to the handover depth; at the handover depth, coupling the recovery string to the top of the pipe string; applying tensile force via the recovery string at an acute angle to the lift axis to deflect the pipe string from the lift axis toward end-to-end axial alignment with the recovery string to transfer the weight load of the pipe string to the recovery string; and raising the pipe string from the handover depth toward the surface by raising the recovery string while dissembling tendon elements from the recovery string. 
     In the context of recovery, it is convenient for the wire to be raised with the pipe string from the handover depth and to be detached from the top of the pipe string at the surface. Again, the recovery string suitably bears substantially the whole weight load of the pipe string during raising from the handover depth. 
     A link element may be attached to the top of the pipe string so as to accommodate deflection of the pipe string from the lift axis toward alignment with the recovery string by angular movement of the link element with respect to the recovery string and the pipe string. The pipe string may be suspended from a lower tendon element of the recovery string via the link element such that the pipe string, the link element and the recovery string are in end-to-end axial alignment under tension under the weight load of the pipe string. Again, the link element may pivot about the upper and lower couplings to achieve this, or it may flex. 
     The use of tendon elements in a discontinuous abandonment or recovery string in accordance with the invention is advantageous for various reasons. For example, the individual elements are easy to handle and to store on board a pipelaying vessel, where they may be stored horizontally in racks that are routinely provided on a pipelaying vessel configured for laying rigid pipe. The tendon elements do not give rise to twisting and they are less fragile than flexible pipe, with more predictable failure modes; even if one of the elements is damaged, it can be swapped out without discarding the other elements. Also, the length of the string may easily be adjusted by adding or subtracting tendon elements from the string. 
     Inventive content also resides in a tendon element cooperable with other tendon elements to form an abandonment or recovery string, the tendon element being elongate to define opposed ends, and comprising: complementary connector formations at respective ends, each connector formation being cooperable, in use when forming a string, with a complementary connector formation of a neighbouring tendon element in the string; a tensile load-bearing member extending between the connector formations; and at least one sleeve removably mounted around the load-bearing member. 
     Advantageously, the sleeve is arranged to be handled by pipe-joint handling apparatus associated with a J-lay tower of a pipelay vessel. The sleeve may be comprised of sleeve sections of generally part-circular cross-section that can be assembled together around the load-bearing member. 
     The load-bearing member preferably extends along a central longitudinal axis of the tendon element. For example, the sleeve may be coaxial with the load-bearing member in cross-section. Nevertheless, the load-bearing member may have an external hang-off formation shaped to engage with a hang-off clamp of a pipelay vessel; such a formation may be offset axially with respect to the sleeve. 
     The load-bearing member is suitably shaped to define at least one shoulder that locates the sleeve axially with respect to the load-bearing member. For example, the shoulder may be defined by an end of a recess in the load-bearing member that accommodates the sleeve. 
     A common inventive concept extends to an abandonment or recovery string comprising a plurality of tendon elements of the invention. The inventive concept also embraces a method of abandoning or recovering a pipeline during subsea pipelaying, comprising assembling an abandonment or recovery string from a plurality of tendon elements of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Reference has already been made to  FIGS. 1 and 2  of the accompanying drawings to put the invention into context. In order that the invention may be more readily understood, reference will now be made, by way of example, to the remaining drawings in which: 
         FIG. 1  is a side view of a J-Lay tower on a barge; 
         FIG. 2  is a perspective view of an erector arm loading a double joint into the tower of  FIG. 1 ; 
         FIGS. 3 to 7  are side views of a pipelay vessel showing steps involved in an abandonment procedure in accordance with the invention; 
         FIG. 8  is a side view of an abandonment head, grommet and A&amp;R connection at a lower part of a tendon element in accordance with the invention, including an inset enlarged perspective view of a top coupling detail; 
         FIG. 9  is a side view of a tendon element in accordance with the invention; 
         FIG. 10  is a fragmented side view of the tendon element shown in  FIG. 9 ; 
         FIG. 11  is a perspective view of a gripping sleeve forming part of the tendon element shown in  FIGS. 9 and 10 ; 
         FIG. 12  is a perspective view of a pipe sleeve pup piece forming part of the tendon element shown in  FIGS. 9 and 10 ; 
         FIG. 13  is a side view of a male connector stub forming part of the tendon element shown in  FIGS. 9 and 10 ; and 
         FIG. 14  is a side view in central longitudinal section of a female connector of a tendon element as shown in  FIGS. 9 and 10 , engaged with a male connector of a similar tendon element connected in axial succession. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 3 to 7  show a vessel  28  during an abandonment procedure. In this example, the vessel  28  is the Applicant&#39;s pipelay/heavy lift vessel Seven Borealis. Seven Borealis is fitted with a top tension J-lay system including a J-lay tower  30  that can gimbal up to 15° either side of vertical; the J-lay tower  30  is shown in  FIGS. 3 to 6  inclined accordingly. Seven Borealis also has S-lay facilities, including a traction winch  32  of 600-tonne rating mounted on its deck  34  several meters from the J-lay tower  30 . 
     Elongate tendon elements  36 , which will be described later with reference to  FIGS. 8 to 14  of the drawings, are stored horizontally on the deck  34  together with double joints of pipe made on shore. 
     During a J-lay pipelaying operation as described already with reference to  FIGS. 1 and 2  of the drawings, double joints of pipe are lifted successively from the deck  34  and upended into the J-lay tower  30  to be aligned with and welded to the pipeline end supported at the base of the tower  30 . The resulting pipe string  38  is shown in  FIGS. 3 to 7  in the process of abandonment. 
       FIG. 3  shows an early stage of the abandonment procedure. Here, a tendon element  36  has been lifted from the deck  34  to be supported by a tensioner  40  on the J-lay tower  30 . A flexible synthetic grommet  42  of plastics rope has been coupled by a bottom coupling  44  to the top end of the pipe string  38  and by a top coupling  46  to the bottom end of the tendon element  36 . The grommet  42  is under tension here to transmit the full weight load of the pipe string  38  to the tendon element  36  via the couplings  44 ,  46 . Consequently, the tendon element  36 , the grommet  42  and the upper portion of the pipe string  38  are in axial alignment at this stage. 
     An A&amp;R wire  48  extends from the winch  32  over the side of the vessel  28  to join the grommet  42  at its bottom end, at or adjacent to the bottom coupling  44 . The arrangement of the grommet  42 , the A&amp;R wire  48  and the couplings  44 ,  46  will be described in more detail later with reference to  FIG. 8  of the drawings. 
       FIG. 4  shows the abandonment procedure underway, with an additional tendon element  36  having been lifted from the deck  34  to the J-lay tower  30  to be connected endwise to the top of the first, lowermost tendon element  36  already coupled to the grommet  42 . 
     This starts the creation of an abandonment string comprising a plurality of tendon elements  36  connected successively end to end, such that the abandonment string lengthens with the addition of each successive tendon element  36  to the top. The abandonment string may be regarded as a tensile string made up of tensile elements. The tendon elements  36  of the abandonment string are handled and launched by the pipelay equipment of the J-lay tower  30  in much the same way as the pipe joints that make up the pipe string  38 . 
     In this way, the top end of the pipe string  38  is lowered deeper into the sea with the addition of each successive tendon element  36  to the abandonment string. As it does so, the weight of the pipe string  38  hanging in the water column between the top end and the seabed progressively reduces. The tendon element  36 , the grommet  42  and the upper portion of the pipe string  38  remain in axial alignment at this stage. 
     The A&amp;R wire  48  is paid out from the winch  32  and hangs as a catenary as shown in  FIGS. 3 and 4  until the top end of the pipe string  38  reaches a suitable handover depth below the sea surface but still significantly above the seabed. In one example, this depth may be in the region of 280 m although this depends upon the overall water depth and the pipeline characteristics. Here, the weight of the pipe string  38  hanging in the water column has reduced enough for the winch  32  to bear that weight load via the A&amp;R wire  48 . 
     When the top end of the pipe string  38  reaches the handover depth, the A&amp;R wire  48  is tautened either by being wound onto the drum of the winch  32  or by continued downward movement of the pipe string  38  while the drum of the winch  32  is held stationary. 
     Initially, on tautening, the A&amp;R wire  48  adopts an acute angle with respect to the common axis of the tendon element  36 , the grommet  42  and the upper portion of the pipe string  38  as shown in  FIG. 5 .  FIG. 6  shows the next step, in which the winch  32  pulls in the A&amp;R wire  48  to pull the pipe string  38  toward axial alignment with the A&amp;R wire  48 . 
     It will be apparent from a comparison between  FIGS. 5 and 6  that the grommet  42  serves as an articulated link between the abandonment string and the pipe string  38  to facilitate this lateral angular movement of the pipeline  38 . This avoids bending the abandonment string, whose tendon elements  36  are substantially rigid and are connected inflexibly. The linkage function of the grommet  42  is enabled by its ability to pivot about the top and bottom couplings and/or by its intrinsic flexibility. 
     When the pipe string  38  and the A&amp;R wire  48  are substantially in axial alignment, the load path extends from the pipe string  38  directly into the A&amp;R wire  48 , hence transferring the load of the pipe string  38  from the abandonment string to the A&amp;R wire  48 . It is then possible to decouple the bottom coupling  44  by remote actuation or by an ROV—if the handover depth is too deep for diver intervention to be practical—so as to detach the grommet  42  from the pipeline  38 . 
     Once the grommet  42  has been detached from the pipeline  38  as shown in  FIG. 7 , the J-lay process is reversed to pull in the abandonment string with the grommet  42  hanging from the bottom of the lowermost tendon element  36 . The J-lay tower  30  is shown here returned to the vertical. As the abandonment string is pulled in, the tendon elements  36  are detached successively from the top of the abandonment string in the J-lay tower  30  and lowered back onto the deck  34  of the vessel  28  to store them for re-use in a future abandonment procedure. 
     Recovery may involve a reverse process in which the A&amp;R wire  48  hanging from the winch  32  lifts the top end of the pipe string  38  to a handover depth to meet a recovery string of tendon elements  36  assembled by, and lowered from, the J-lay tower  30 . It will be evident that the recovery string is the same as the abandonment string: it is simply used for the opposite purpose. 
     A grommet  42  hanging from the end of the recovery string may be coupled by an ROV or other means to the top end of the pipe string  38 . The recovery string may then be raised as described above, firstly to transfer the load of the pipe string  38  from the A&amp;R wire  48  to the J-lay tower  30  via the recovery string and then to lift the top end of the pipe string  38  back to the surface, with increasing top tension that eventually exceeds the load capacity of the winch  32 . 
     The A&amp;R wire  48  may remain attached to the top of the pipe string  38  all the way to the surface although it can merely hang passively after the load of the pipe string  38  has been transferred to the recovery string at the handover depth. 
       FIG. 8  shows an exemplary arrangement of the grommet  42 , the A&amp;R wire  48  and the couplings  44 ,  46  in detail. The grommet  42  has looped terminations at its top and bottom ends, defining a top eye  50  and a bottom eye  52 . The top eye  50  of the grommet  42  is shown in an enlarged inset view in  FIG. 8  fixed permanently to a shackle  54  at the bottom end of the lowermost tendon element  36  to define the top coupling  46 . 
     The bottom eye  52  of the grommet  42  is fixed releasably to a hook  56  at the top of the pipe string  38  to define the bottom coupling  44 . A latch closure  58  prevents accidental release of the bottom eye  52  from the hook  56 . The A&amp;R wire  48  is fixed to the hook  56  by a releasable connector  60 . By connecting three parts of the system in this way, the hook  56  may be described as a ‘delta plate’. 
     Moving on now to  FIGS. 9 and 10 , these show one of the tendon elements  36  in accordance with the invention. The tendon element  36  is an elongate tensile member that is typically  24  meters in length although other lengths are possible. At one end, the tendon element  36  terminates in a male connector  62 , shown externally in  FIG. 12  and in cross-sectional detail in  FIG. 14 . At the other end, the tendon element  36  terminates in a complementary female connector  64 , also shown in cross-sectional detail in  FIG. 14 , for receiving the male connector  62  of an identical tendon element  36  in end-on engagement. 
     The male and female connectors  62 ,  64  cooperate with a quick action, in a manner best appreciated with reference to the later description of  FIG. 14 . They operate in a manner akin to the subsea connectors known for many years in the offshore industry and supplied, for example, by the GE business VetcoGray. 
     The connectors  62 ,  64  can be engaged quickly to assemble the abandonment string from successive tendon elements  36  and can be disengaged similarly quickly to disassemble the abandonment string. This minimises handling time and speeds the abandonment and recovery procedures, noting that time represents a great deal of money in offshore operations. In any event, it will be appreciated that abandonment should be performed as quickly as possible in the event that bad weather is approaching, whereas quick recovery allows the vessel  28  to resume pipelaying as quickly as possible. The result is to save significant critical path vessel time. 
     Much of the length of the tendon element  36  comprises a plain pipe section  66  welded to the female connector  64 . Conversely, two longitudinally-spaced gripping sleeves  68  are offset toward the male connector  62 , namely an outboard sleeve  68  adjoining the male connector  62  and an inboard sleeve  68  nearer the centre of the tendon element  36 . Each gripping sleeve  68  comprises longitudinally-divided half-shells  70  of semi-circular cross-section bolted together at intervals along their length. One such half-shell  70  is shown in isolation in  FIG. 11  of the drawings. 
     The gripping sleeves  68  provide an area for tensioner friction clamps to grip the tendon elements  36  as they pass through the J-lay tower  30 . The gripping sleeves  68  are matched to the outer diameter of the project pipeline and so are project-specific; their replaceability allows the tendon elements  36  to be tailored for different projects and also to be maintained, as the half-shells  70  can easily be swapped out in the event of wear. By matching the outer diameter of the project pipeline, the gripping sleeves  68  ensure that there need be no downtime associated with tooling change-overs to reconfigure the J-lay tower  30  for A&amp;R operations. 
     The outboard gripping sleeve  68  adjoining the male connector  62  surrounds a male connector stub  72  shown in isolation in  FIG. 12  of the drawings. The male connector stub  72  is a unitary tubular component through which axial load is transmitted when the tendon element  36  is in use. One end of the male connector stub  72  is shaped to define the male connector  62  as will be described. A recess  74  extends inboard from there along the male connector stub  72  to provide positive axial location for the two half-shells  70  of the outboard gripping sleeve  68 . 
     The inboard gripping sleeve  68  surrounds a pipe sleeve pup piece  76  as shown in isolation in  FIG. 13  of the drawings. This, too, has a recess  78  extending along its length between enlarged ends to provide positive axial location for the two half-shells  70  of the outboard gripping sleeve  68 . The enlarged inboard end of the pipe sleeve pup piece  76  incorporates a hang-off collar  80 . 
     Turning finally to  FIG. 14 —and as  FIG. 12  also shows—the male connector  62  comprises a stud  82  of circular cross section extending along the central longitudinal axis of the tendon element  36 . That stud  82  is encircled by spaced circumferential ridges  84  lying in planes perpendicular to the central longitudinal axis. 
       FIG. 14  also shows that the female connector  64  has a recess that lies on the central longitudinal axis of the tendon element  36  and that is complementary to the stud  82  of the male connector  62 . Three locking dogs  86  with ridged formations complementary to the ridges of the stud  82  are spaced equi-angularly around the recess. The locking dogs  86  are movable radially in response to rotary circumferential movement of a locking ring  88 . The locking ring  88  holds the locking dogs  86  captive in the assembly and resists radial loads. Rotation of the locking ring  88  in opposite angular directions engages and disengages the locking dogs  86 . The assembly can be locked by securing the locking ring  88  to resist rotation. 
     Specifically, the locking ring  88  is turned around the female connector  64  in one direction to interact in a cam-like manner with the locking dogs  86  to lock the coupling as required. A quarter turn of the locking ring  88  is sufficient to lock together the male and female connectors  62 ,  64  in the way. Retracting links couple the locking dogs  86  to the locking ring  88  to retract the dogs  86  so as to release the connectors  62 ,  64  when the locking ring  88  is turned in the opposite direction. 
     A body component  90  of the female connector  64  is welded to the pipe section  66  of the tendon element  36 . The body component  90  supports moving parts including the locking dogs  86  and locking collar  88  and defines the recess that embraces the machined stud  82  of the male connector  62 . In use, the body component  90  transmits axial loads from the locking dogs  86  to the pipe  66  of the tendon element  36 . 
     Upper and lower centralising rings  92 ,  94  sandwich the locking ring  88  to centralise it axially and to support its weight. It will be seen that the lower centralising ring  94  is externally tapered with a frusto-conical surface to help the connector assembly to roll on pipe conveyors and through roller boxes in pipe handling equipment of the vessel  28 . An upper fairing ring  96  is similarly but oppositely tapered for the same purpose. 
     The many benefits of the connector arrangement described above include:
         ease of engagement, as the male connector simply stabs into the female connector;   ease of locking and unlocking once the connectors have engaged;   a compact design having no sharp or protruding edges, making it suitable for, and friendly to, rigid and flex-lay equipment;   easily scalable to any size or shape depending on the application;   an extremely strong design that can easily be scaled up or down;   based on a successful drill connector, namely the H4 connector known since 1964; and   much lower cost than comparable connectors.       

     The connector arrangement requires only a small force to be applied to lock or unlock it. If performed underwater, this is suitable for diver operation and is also within the capabilities of standard ROVs. If required, however, the force required to operate the connector arrangement could be reduced by adding a torque device such as a rack and pinion design to make the connector more ROV-friendly. 
     Another advantage of the invention is that if a tendon element is damaged, only that one tendon element need be rejected and replaced with another tendon element at reduced cost. The cost of emergency spares carried on board the pipelaying vessel is also reduced. 
     Many other variations are possible within the inventive concept. For example, an abandonment string made up of the tendon elements of the invention could be used to abandon and recover a pipeline all the way to and from the seabed. 
     The solutions of the invention could easily be scaled to produce a whole series of connectors suitable for alternative solutions. With a range of differing capacity connectors, connectors may be combined and selected to suit reducing capacity as top tension decreases. For example, it may only require four or five high-capacity connectors before top tensions and water depths are such that that it would be possible to switch to smaller, lower capacity variants. With potential top tensions of around 1400 tonnes already being discussed, the huge cost of A&amp;R winches with such capacity makes the tendon element concept of the invention an attractive alternative.