Abstract:
A method of clamping elongate elements in a parallel piggybacked arrangement during subsea laying of the elements is disclosed. The method includes opposed reciprocating jaws which force together clamp segments around the elongate elements to assemble a piggybacking clamp. The piggybacking clamp applies clamping forces to the elongate elements.

Description:
This Application is a continuation application of U.S. Ser. No. 14/232,820, filed on Jan. 14, 2014, which was a 35 U.S.C. §371 national phase conversion of International Application Number PCT/GB2012/051659 filed on Jul. 12, 2012, which claims priority to Great Britain Application No. 1112131.6 filed on Jul. 14, 2011. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to systems for joining two or more parallel pipes, cables or other elongate elements during offshore operations, for example in a ‘piggyback’ arrangement during pipelaying. The invention encompasses joining devices and apparatus and methods for fitting such joining devices to and between pipes, cables or other elongate elements. 
     It is often desirable to install two or more elongate elements along the same subsea route, such as a primary larger-diameter pipe for carrying hydrocarbons and a secondary smaller-diameter pipe for carrying water, gas or chemicals used to produce hydrocarbons. 
     Whilst pipes will be used as an example in this specification, an element need not be a pipe for carrying fluids but could instead be a cable for carrying power or data. A secondary element will usually be of much smaller diameter (typically &lt;20 cm) than a primary element, but a difference in size between the elements is not essential to the invention in a broad sense. 
     Where elements such as pipes or cables are to follow the same route, it may be beneficial to install the elements simultaneously. This is commonly achieved by a piggyback technique where one or more secondary elements are attached by a succession of clamps to a primary element on a pipelay vessel, and the elements are then launched together in parallel toward the seabed. 
     Installation of a piggyback pipeline usually involves unspooling the secondary pipe on a pipelay vessel. The primary pipe may also be unspooled in a reel-lay arrangement although it could be fabricated on the pipelay vessel, for example in an S-lay operation. 
     A typical reel-lay vessel  10  shown schematically in  FIG. 1  is fitted with a storage and deployment reel  12  for deploying a primary pipe  14  and has an adjustable lay ramp  16  that is capable of deploying a range of products at varying lay angles, which may be from circa 20° to 90° to the horizontal. The inclination of the lay ramp  16  is determined by the depth of water in which the pipeline is being laid and by the characteristics of the pipeline, such as its diameter and stiffness. 
     In downstream succession from the reel  12 , the lay ramp  16  carries a guide chute  18  for guiding the primary pipe  14 ; a pipe straightener  20  for straightening the primary pipe  14 ; a track-type tensioner  22  for gripping the primary pipe  14  between articulated tracks; and a hold-off clamp  24  for clamping the primary pipe  14  whenever the tensioner  22  releases the primary pipe  14 . A travelling clamp could be used instead of a track-type tensioner  22 ; references in this specification to a tensioner should be taken to include a travelling clamp unless the context demands otherwise. 
     As  FIG. 2  shows, a piggyback reel  26  can be fitted to a vessel  10  for deploying a secondary element such as a secondary pipe  28  with the primary pipe  14  when operating in piggyback mode. In that mode, a piggyback chute  30  guides the secondary pipe  28  and the secondary pipe  28  is brought into alignment with the primary pipe  14 , such that the secondary pipe  28  lies parallel to the primary pipe  14  downstream of the tensioner  22 . The secondary pipe  28  then lies directly above the longitudinal centerline of the primary pipe  14  or, when the primary pipe  14  is vertical, directly aft of the longitudinal centerline of the primary pipe  14 . The secondary pipe  28  is then ready to be clamped to the primary pipe  14  at work platforms in a shelter  32  on the lay ramp  16  between the tensioner  22  and the hold-off clamp  24 . 
     In practice an additional straightener may be used for the secondary pipe  28  downstream of the piggyback chute  30  but this has been omitted from  FIG. 2  for clarity. Also, the secondary pipe  28  may go through an additional tensioner but such a tensioner may not be required and has also been omitted for clarity. 
     In a prior art piggybacking arrangement, it is known for a secondary pipe  28  to be diverted entirely around the tensioner  22  before being aligned with the primary pipe. This makes it difficult to align the secondary pipe  28  without overbending it or requiring additional straightening, unless there is a substantial and disadvantageous gap under the tensioner  22 . The heavy tensioner  22  should be mounted as low as possible on the lay ramp  16  to aid the stability of the vessel  10 . 
     U.S. Pat. No. 5,975,802 to Willis (Assignee: Stolt Comex Seaway Ltd.) discloses a known piggyback arrangement in detail, including the relationship between the paths of a primary pipe and a secondary pipe as they pass over their respective chutes and are brought together for clamping. In the example shown in U.S. Pat. No. 5,975,802, the primary pipe is fabricated on board the pipelay vessel and the secondary pipe is unspooled from a reel, although it will be clear to the skilled reader that both pipes could be spooled with the addition of a storage and deployment reel for the primary pipe, as in  FIG. 2 . The content of U.S. Pat. No. 5,975,802 is incorporated herein by reference, as technical background to the present invention. 
     A known piggyback clamp  34  shown in  FIG. 3  employs a tapered saddle-like block  36  of rubber or polyurethane between a primary pipe  14  and a secondary pipe  28 . The block  36  has a concave undersurface shaped to fit the cross-sectional curvature of the primary pipe  14  and a hole for encircling and retaining the secondary pipe  28 . The block  36  is in two parts that, when assembled together, define the hole and surround the secondary pipe  28 . 
     In use, the two parts of the block  36  are assembled around the secondary pipe  28  to retain the secondary pipe  28  in the hole. The block  36  retaining the secondary pipe  28  is then attached to the primary pipe  14  by tensioned parallel circumferential straps  38  that encircle the primary pipe  14  and the block  36 . The straps  38  keep the two parts of the block  36  together while holding the secondary pipe  28  parallel to and spaced slightly from the primary pipe  14 . 
     The service demands on the clamp  34  are high. The block  36  and the straps  38  must survive the stresses of launching the pipeline from the pipelay vessel  10  to the seabed. The block  36  and the straps  38  may also need to survive the load of pulling the secondary pipe  28  off the piggyback reel  26  if no additional tensioner is used. Thereafter the block  36  and the straps  38  must continue to retain the secondary pipe  28  on the primary pipe  14  for the life of the pipeline, typically at least twenty years, without significant relative movement between the pipes  14 ,  28 . 
     During piggyback operations on a pipelay vessel  10  such as that shown schematically in  FIG. 2  or as described in detail in U.S. Pat. No. 5,975,802, manual intervention is required close to the pipes  14 ,  28  on the lay ramp  16  at a location downstream of the tensioner  22 , to position, align and manually clamp the pipes  14 ,  28 . In particular, a succession of clamps  34  must be assembled and fitted to the pipes  14 ,  28  by workers operating in a confined space on the lay ramp  16 , which is steeply inclined and will pitch as the pipelay vessel  10  rides the waves. Considerations of safety and accuracy make it necessary to reduce the linear travel speed of the pipes  14 ,  28  with respect to the vessel  10  while the clamp installation process is carried out, or intermittently to stop the pipelay movement altogether. 
     Piggyback operations are therefore labour-intensive and inefficient, not just in labour costs but also in vessel time—which is typically worth circa US$300,000 per day. Pipelay rates in piggyback mode may be less than 500 m per hour, and possibly as little as 300-400 m per hour. This is less than half of the typical speed of reel-lay operations without piggybacking, and so approximately doubles vessel time on station and hence greatly increases vessel cost during pipelaying. It will also be apparent that if a pipelay vessel must be on station for say four days instead of two days, it is more likely to encounter weather conditions that will disrupt the pipelaying operation or force its temporary abandonment, again with a potentially great increase in time and cost. 
     If it would be possible to increase the speed of pipelaying in piggyback mode to approach the typical speed of pipelaying without piggybacking, the cost saving would be very substantial. Of course, it is essential for that saving to be achieved without compromising safety. 
     It is against this background that the present invention has been devised. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention involves securing two half-shell piggypack pads to the primary and secondary elements, clamping the two elements together. The pads are secured by barbs that engage in opposed holes in the mating pads as the pads are brought together. 
     Thus, the invention resides in apparatus for clamping elongate elements in a parallel piggybacked arrangement during subsea laying of the elements, the apparatus comprising opposed reciprocating jaws for forcing together clamp segments around the elongate elements to assemble a piggybacking clamp that applies clamping forces to the elongate elements. 
     Each jaw suitably has a cavity for accommodating a respective clamp segment. That cavity may have at least two press surfaces for applying assembly force to separate locations of a clamp segment in the cavity. Such press surfaces suitably extend substantially orthogonally with respect to a reciprocating direction of the jaws. The cavities are preferably shaped between the press surfaces to provide clearance for outward deflection of clamp segments applying clamping forces to the elongate elements. 
     The cavity advantageously has retention formations engageable with corresponding retention formations of the clamp segments, which retention formations are preferably releasable in a direction generally parallel to the elements. 
     Where the elongate elements are movable longitudinally with respect to the apparatus during clamping in a launch direction, the cavities are preferably open-sided to allow the assembled clamp to move out of the jaws with the elongate elements in the launch direction. At least one retaining pawl is suitably provided for holding a clamp segment in a cavity until that clamp segment has been assembled into a clamp. The retaining pawl may, for example, be biased into a retaining position to hold the clamp segment in the cavity and may be movable against that bias into a release position to release the clamp segment from the cavity. 
     Where each clamp segment has two or more generally parallel mutually-spaced recesses shaped to extend partially around respective ones of the elongate elements, the apparatus is advantageously arranged to apply assembly force to one side of a recess and subsequently to another side of that recess. 
     The apparatus may further comprise a tightening device downstream of the jaws to tighten the engagement of clamp segments initially assembled by the jaws. The tightening device is preferably arranged to apply tightening force between the recesses. Such a tightening device suitably comprises a constriction through which at least part of the clamp passes after leaving the jaws, which constriction may be defined by pinch wheels between which at least part of the clamp is forced after leaving the jaws. 
     Where the elongate elements are movable longitudinally with respect to the apparatus in a launch direction, the jaws are preferably supported by a carriage that is movable in the launch direction during clamping. The carriage may be movable reciprocally in an engagement stroke in the launch direction during clamping and in a return stroke opposed to the launch direction after clamping. Also, the jaws are advantageously movable in the launch direction with respect to the carriage as the carriage moves in the launch direction during clamping. In that case, the jaws may be movable toward each other on converging paths as they move in the launch direction with respect to the carriage. It is also possible for the jaws to be movable toward each other by a wedge member that is movable longitudinally relative to the carriage, or by actuators acting between the carriage and the jaws. 
     The invention encompasses a method of clamping elongate elements in a parallel piggybacked arrangement during subsea laying of the elements, the method comprising forcing together a plurality of clamp segments around the elongate elements to assemble a piggybacking clamp that applies clamping forces to the elongate elements. 
     Assembly force may be applied locally to the clamp segments at different locations of the clamp segments at different times. For example, where each clamp segment has two or more generally parallel mutually-spaced recesses shaped to extend partially around respective ones of the elongate elements, the method suitably comprises applying assembly force to one side of a recess and subsequently to another side of that recess. Assembly force may be applied outboard of the recesses to push together ends of the clamp segments while allowing the clamp segments to bow centrally upon clamping the elongate elements, and subsequently applying force between the recesses to push together central regions of the clamp segments to tighten clamping of the elongate elements. 
     The clamp segments may be allowed to move with the elongate elements in a launch direction while forcing them together around the elongate elements. 
    
    
     
       DETAILED DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       To describe the state of the art, reference has already been made to  FIGS. 1 to 3  of the accompanying drawings, in which: 
         FIG. 1  is a schematic side view of a typical reel-lay vessel; 
         FIG. 2  is a schematic side view of a reel-lay vessel adapted for piggyback pipe laying; and 
         FIG. 3  is a perspective view of primary and secondary pipes joined by a block and straps in accordance with the prior art. 
       In order that the invention may readily be understood, reference will now be made, by way of example, to the remaining drawings in which: 
         FIG. 4  is a perspective view of a pair of segments of a piggybacking block in accordance with a first embodiment of the invention; 
         FIG. 5  is a side view of one of the segments of  FIG. 4 ; 
         FIG. 6  is a front view of the segment of  FIG. 5 ; 
         FIG. 7  is a perspective view of a pair of segments of a piggybacking block in accordance with a second embodiment of the invention; 
         FIG. 8  is a perspective view of piggybacking blocks in accordance with the first embodiment of the invention being assembled and in use on piggybacked pipes; 
         FIG. 9  is an end view of one of the piggybacking blocks of  FIG. 8  in use on the piggybacked pipes; 
         FIGS. 10 to 14  are side views of barb variants that may be used in the segments shown in  FIGS. 4 to 9 ; 
         FIGS. 15 to 17  are perspective views of test clamping operations involving prototype piggybacking blocks of the Invention; 
         FIGS. 18 a  to 18 d    are partial schematic side views of an apparatus in accordance with the invention for applying piggybacking blocks of the invention to primary and secondary pipes, showing an operational sequence of the apparatus; 
         FIGS. 19 and 20  are schematic cross-sectional views showing two operational steps of the apparatus shown in  FIGS. 18 a    to  18   d;    
         FIG. 21  is a schematic side view of an alternative apparatus in accordance with the invention for applying piggybacking blocks of the invention to primary and secondary pipes; and 
         FIGS. 22 and 23  are schematic perspective views of alternative apparatuses in accordance with the invention for applying piggybacking blocks of the invention to primary and secondary pipes. 
       Reference will also be made to the  FIG. 24 , which sets out push-in and pull-out loads for a variety of barb profiles under testing with an interference fit in holes provided in test ‘pucks’ of Nylon 6-6. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring firstly to  FIG. 4  of the drawings, a piggybacking block  40  in accordance with a first embodiment of the invention comprises pads in the form of two identical segments  42 . The segments  42  are brought together in face-to-face relation about a central bisecting longitudinal plane of symmetry. This forms a block  40  with a figure-eight cross-section that surrounds and locates primary and secondary elements such as pipes, as will be explained. 
     Referring now also to  FIGS. 5 and 6  which show one of the segments  42 , the inner side of a segment  42  comprises two approximately half-cylindrical recesses whose axes of curvature are parallel to each other, namely a larger primary recess  44  and a smaller secondary recess  46 . The recesses  44 ,  46  are separated by a central generally oblong face  48  that lies substantially on the central bisecting longitudinal plane. Two further oblong faces  50 ,  52  lie substantially in the same plane at opposite ends of the segment  42 , a lower face  50  being outboard of the primary recess  44  and an upper face  52  being outboard of the secondary recess  46 . The axes of curvature of the recesses  44 ,  46  are parallel to and lie slightly beyond the central bisecting longitudinal plane. 
     As  FIG. 9  will show, the radii of curvature of the primary and secondary recesses  44 ,  46  are selected to correspond to the outer radii of the primary and secondary pipes  14 ,  28 . When selecting the radii of curvature, allowance may be made for flexing of the segment  42  during assembly of the block  40  as the walls of the recesses  44 ,  46  bear resiliently against the primary and secondary pipes  14 ,  28  to apply clamping loads to them. 
     Each face  48 ,  50 ,  52  of the segment  42  has a longitudinally-offset barb  54  that projects orthogonally from the face  48 ,  50 ,  52 . The barb  54  is spaced longitudinally from a through-hole  56  set into the face  48 ,  50 ,  52 . The hole  56  and the barb  54  are disposed symmetrically about the longitudinal centre of the face  48 ,  50 ,  52 . The arrangement of the barbs  54  and the holes  56  is such that when two segments  42  are aligned face-to-face for assembly into the block  40 , the barbs  54  of each segment  42  align with the holes  56  of the opposite segment  42 . The barbs  54  thus enter the opposed holes  56  when the segments  42  are pressed together around primary and secondary pipes  14 ,  28  or other elements, to form a block  40  with a figure-eight cross-section. 
     The segments  42  are of cast or injection-moulded plastics material such as polyamide or polyurethane and the barbs  54  are of steel, although other materials are possible. A segment  42  may be moulded around the barbs  54  in an insert or outsert moulding process or the barbs  54  may be engaged in mounting holes  58  provided in a pre-moulded segment  42 . There may, for example, be a threaded engagement between the barbs  54  and the mounting holes  58 . Alternatively, there may be an interference fit between the barbs  54  and the mounting holes  58 , whose strength may be increased by ribbing, threading or otherwise texturing a root portion of a barb  54  to be received in a mounting hole  58 . 
     As best appreciated in  FIG. 4 , in this embodiment of the invention, the outer side of each segment  42  has integral longitudinally-spaced ribs  60  that lie in parallel planes. The smooth, plain surfaces of the primary and secondary recesses  44 ,  46  spread the clamping load on the products to be coupled by the block  40 , and maximise the contact area between the segments  42  and the products to ensure even contact pressure distribution. 
       FIGS. 4 and 5  best show that the outer side of each segment  42  comprises a first convex part-cylindrical formation  62  being the outer side of the primary recess  44 . The radius of curvature of the first formation  62  is centred on the same axis of curvature as the primary recess  44 . The first formation  62  terminates at its lower end behind the lower face  50  in longitudinally-spaced bulk regions  64  that respectively contain a hole  56  and a barb  54  set into a parallel mounting hole  58 . The ribs  60  extend from over the first formation  62  to between the bulk regions  64 . 
     A second convex part-cylindrical formation  66  is on the outer side of the secondary recess  46 . The radius of curvature of the second formation  66  is centred on the same axis of curvature as the secondary recess  46 . Longitudinally-spaced bulk regions  68  each extend from behind the central face  48  to behind the upper face  52 . One of those bulk regions  68  contains two holes  56 ; the other contains two barbs  54  set into parallel mounting holes  58 . The ribs  60  extend over the second formation  62  between the bulk regions  68 . 
     The ribs  60  stiffen the segments  42  with minimum material usage, while retaining some helpful compliance. They also resist post-moulding distortion of the segments  42 . The bulk regions  64 ,  68  add strength at the key interface between the segments  42  via the barbs  54  and the holes  56 . The bulk regions  64 ,  68  ensure there is sufficient material surrounding the barb holes  56 ; they also provide flat outer surfaces parallel to the central longitudinal plane of the block  40 , suitable for the application of inward load to the segments  42  during assembly of the block  40 . 
     Longitudinal grooves  70  are disposed on the upper and lower sides  72 ,  74  of each segment  42 , each extending parallel to and spaced slightly from the lower face  50  and the upper face  52 . The grooves  70  are retention features for holding the segments  42  in an assembly machine before the segments  42  are pressed together around primary and secondary pipes  14 ,  28  or other elements to assemble the block  40 . 
     Chamfers and radii are employed on edges and corners of the segments  42  to minimise stress concentrations, and also to ensure suitable lead-ins for automated handling, for example in hoppers and assembly rollers of assembly machines. 
     In a second embodiment of the invention shown in  FIG. 7 , a piggybacking block  76  comprises two identical segments  78  that each have longitudinally-spaced parallel ribs  80  disposed within a primary recess  82  and a secondary recess  84 . A first convex part-cylindrical formation  86  on the outer side of the primary recess  82  and a second convex part-cylindrical formation  88  on the outer side of the secondary recess  84  are substantially smooth. This variant has the benefit that the internal ribs  80  improve grip on the products coupled by the block  76 ; they enhance friction by increasing clamping pressure per unit area, and create a mechanical interface by locally keying into the coatings of the products. 
     The segments  78  of the second embodiment also have pocket-like indents  90  between bulk regions in a central face  92  and a lower face  94 , to reduce material usage without significantly reducing strength. A similar indent  96  is disposed between the bulk regions on the outer side of the lower face  94 . 
     Other features of the second embodiment such as the barbs  54  and the grooves  70  correspond in function to those of the first embodiment; like numerals are used for like features. 
       FIG. 8  of the drawings shows segments  42  of the first embodiment being pressed together in face-to-face relation around primary and secondary pipes  14 ,  28  to assemble a block  40  that connects and separates the pipes  14 ,  28  in a piggyback arrangement. Segments  74  of the second embodiment will work in the same way. The pipes  14 ,  28  may move continuously or may intermittently be held stationary during assembly of the block  40 . 
     Distal ends of the barbs  54  on each face initially locate in the holes  56  in the counterpart faces of the opposed segments  42 . Inward pressure applied to the flat outer surfaces of the bulk regions  64 ,  68  at the arrows P shown in  FIG. 8  then forces the segments  42  together as the barbs  54  are urged deeper into the holes  56 . 
     As best shown in the cross-sectional view of the assembled block in  FIG. 9 , the semi-cylindrical primary recesses  44  of the opposed segments  42  form a substantially circular enclosure for the primary pipe  14  and the semi-cylindrical secondary recesses  46  of the opposed segments  42  form a substantially circular enclosure for the secondary pipe  28 . The secondary pipe  28  is spaced from the primary pipe  14  by the height of the central face  48 . 
     When the segments  42  are fully pressed together, contact between the faces  48 ,  50 ,  52  and their counterparts of the opposed segment  42  is not essential. Indeed, it is advantageous for at least one of the faces  48 ,  50 ,  52  to remain slightly apart upon assembly because if the faces  48 ,  50 ,  52  on both sides of a clamped pipe  14 ,  28  come together, no additional clamping force will be applied to that pipe  14 ,  28  clamped between the segments  42 . 
     Resilience of the segments  42  helps to ensure a snug fit around the primary and secondary pipes  14 ,  28  and continuous application of clamping force to the pipes  14 ,  28 . This helps to avoid movement of the block  40  with respect to the pipes  14 ,  28  for the working life of the piggybacked pipeline, whether axially along the pipes  14 ,  28  or circumferentially around the pipes  14 ,  28 . It also helps to avoid relative movement between the pipes  14 ,  28 , such as separation beyond the spacing predetermined by the block  40 . 
     Insertion force and insertion movement may easily be measured to infer that there will be sufficient resistance to separation of the segments  42 , which could otherwise cause loosening or unintended disassembly of the block  40  due to withdrawal of the barbs  54  from the holes  56 . Test results such as those discussed below may be used to develop targets for insertion force and insertion movement that will ensure sufficient resistance to separation of the segments  42 . 
     After assembly, a block  40  is carried downstream by the overboarding or launching movement of the pipes  14 ,  28  from right to left as shown in  FIG. 8 , allowing the next block to be assembled from further segments  42  upstream of the preceding block  40 . 
     The block of the invention is apt to be assembled in a largely automated process, to the benefit of speed, clamping strength and safety. Advantageously, there is no need to encircle the primary and secondary pipes with straps, hence avoiding an awkward and time-consuming operation that is difficult to automate and that gives unpredictable clamping strength. Instead, the segments are brought together as two halves from opposite sides of the pipes and assembled robustly in a simple press-fit operation with predictable and easily-verifiable results. 
     Moving on now to the barb variants in  FIGS. 10 to 14  of the drawings, these show some examples of the many profiles that may be adopted to tailor insertion and withdrawal forces. 
     Each barb variant  54 A to  54 D in  FIGS. 10 to 13  has three portions: a root portion  98  at a proximal end; a narrowed alignment portion  100  at a distal end; and a shank portion  102  disposed between the root portion  98  and the alignment portion  100 . The barb variant  54 E in  FIG. 14  has just a root portion  98  at a proximal end and a shank portion  102  at a distal end, although the distal end of the shank portion  102  is tapered slightly to aid alignment with a hole  56  of a segment  42 ,  74 . 
     The root portion  98  of each barb  54 A to  54 E is adapted for engagement within a mounting hole  54  of a segment  42 ,  74 . As mentioned previously, the root portion  98  may be threaded or otherwise textured; see for example the ribbed root portion  98  of the barb  54 E in  FIG. 14 . It is also possible for a segment  42 ,  74  to be moulded around the root portion  98  with the remainder of the barb  54 A to  54 E protruding from the moulding. 
     The narrowed alignment portion  100  at the distal end of each barb  54 A to  54 D and the tapered distal end of the barb  54 E help to locate and align the barbs  54 A to  54 E in the holes  56  in the counterpart faces of the opposed segments  42 ,  74 , before inward pressure forces together the segments  42 ,  74  by urging the barbs  54 A to  54 E deeper into the holes  56 . 
     The barbs  54 A to  54 E differ by the profiles of their shank portions  102 , which are used to determine insertion and withdrawal forces when engaged in the holes  56  of the opposed segments  42 ,  74 . 
     The shank portion  102  of the barb  54 A of  FIG. 10  has a plain cylindrical surface for an interference fit within a hole  56 . The shank portions  102  of the barbs  54 B to  54 E of  FIGS. 11 to 14  are shaped or textured to strengthen the interference fit within a hole  56 . Testing has shown that such shaping or texturing is advantageous and may be necessary to achieve acceptable pull-out loads. 
     The shank portions  102  of the barbs  54 B and  54 C of  FIGS. 11 and 12  respectively each have a ribbed or ridged surface comprising circumferential, radially-projecting ridges or ribs  104  equi-spaced along the shank portion  102 . Each rib  104  has a distally-facing frusto-conical ramp surface  106  and a proximally-facing shoulder  108  orthogonal to the otherwise cylindrical surface of the shank portion  102 . The ramp surface  106  is at an angle of nominally 30° to the longitudinal axis of the barb  54 B.  54 C, and the height of each rib  104  is about 0.5 mm as part of an overall shank diameter of nominally 12 mm. Advantageously, the directionality imparted by the ramp surfaces  106  and shoulders  108  increases pull-out loads without increasing push-in loads to the same extent. 
     The barbs  54 B and  54 C differ in the pitch of the ribs  104 , the ribs  104  of the barb  54 B of  FIG. 11  being more widely spaced than those of the barb  54 C of  FIG. 12 . For example, the pitch of the ribs  104  of the barb  54 B may be 5 mm and the pitch of the ribs  104  of the barb  54 C may be 3 mm. 
     The shank portion  102  of the barb  54 D of  FIG. 13  is an example of a threaded profile, in this case with an American buttress thread  110  of, for example twelve, sixteen or twenty threads per inch (25.4 mm). Other threads and pitches are possible, such as M12×1.75. A threaded shank portion  102  is not used for threaded engagement with a hole  56  but simply as an easy-to-manufacture high-grip texture to increase the strength of the push-fit between the barb  54 D and the hole  56 . 
     The barb  54 E of  FIG. 14  has a similar ribbed profile on its shank portion  102  as the barbs  54 B and  50 C of  FIGS. 11 and 12 , in this instance with a 3 mm pitch between ribs  104  like that of the barb  54 C. The root portion  98  of the barb  54 E also has a ribbed profile with the same pitch between ribs  104  as the shank portion  102  but with the ramp surfaces  106  and shoulders  108  of the ribs  104  reversed in direction. The barb  54 E is therefore symmetrical about a transverse plane at its longitudinal mid-point. 
     Possible alternative profiles for the shank portion  102  include a ring shank profile, a rebar profile with a spiraled or twisted form, and a knurled finish. However rebar and knurling have been found to have an undesirable combination of high push-in loads and lower pull-out loads. 
     The profiles of barbs  54 A to  54 E illustrated in  FIGS. 10 to 14  and the alternative profiles mentioned above have been tested by being pushed into and pulled out holes in cylindrical circular-section puck-like test pieces of Nylon 6-6, representing the moulded body of a segment  42 ,  74 . The hole extends axially through the puck and so is disposed centrally on a circular face of the puck. Pucks of 30 mm diameter and 60 mm diameter across the circular face were used in testing to replicate different amounts of plastics material around the barb at different regions of a segment  42 ,  74 . The pucks of 30 mm diameter were 50 mm thick and the pucks of 60 mm diameter were 60 mm thick. 
     The barbs  54 A to  54 E were pushed in to the pucks until their shank portions  102  were fully engaged, with the proximal root portions  98  protruding from the pucks. The peak push-in load was recorded in each case. The barbs  54 A to  54 E were then pulled out of the pucks by tensile loads applied via their protruding root portions  98 . The peak pull-out load was recorded in each case. 
     The results of these tests are set out in the appended  FIG. 24 . The barb profiles that performed best were the ribbed barbs  54 C and  54 E with 3 mm pitch between the ribs  104  as shown in  FIGS. 12 and 14  and a threaded barb  54 D with an American buttress thread  110  of twenty threads per inch (25.4 mm) as shown in  FIG. 13 . The ribbed barb  54 C of  FIG. 12  gave better results than the threaded barb  54 D of  FIG. 13  but the ribbed barb  54 C has the disadvantage of being a non-standard profile that may cost more to manufacture than a standard thread profile. 
     Various alternatives to Nylon 6-6 were tested, including Aquanyl (a copolymer of Nylon 6 and Nylon 12) supplied by Nylacast Ltd and LUCPREEN-DT 75D (a polyurethane product) supplied by LUC Group. All trade marks are acknowledged. These are merely examples of materials that have achieved encouraging results in testing; other materials are possible. Key considerations for material choice are: cost; weight; sufficient bulk material at the fixing locations; sensitivity to tolerance; ease of manufacture; interaction with an assembly machine; and interaction with the pipes or other elongate products being clamped in a piggyback arrangement. 
     Moving on now to  FIGS. 15 to 17  of the drawings, these show test clamping procedures involving prototype segments  112  of the invention. The prototype segments  112  are milled from Nylon 6-6 rather than moulded and they lack the stiffening ribs  60 ,  80  of the preceding embodiments. Also, the primary and secondary pipes  14 ,  28  are disposed side-by-side for test purposes whereas, as noted in the introduction, the secondary pipe  28  will generally be directly above and/or aft of the primary pipe  14  in field operations. 
     In  FIGS. 15 to 17 , the primary and secondary pipes  14 ,  28  extend in parallel through an encircling rigid frame  114 . A lower segment  112  lies face-up, supported at each end by load-bearing spacers  116  at the bottom of the frame  114 . The mutually-spaced pipes  14 ,  28  are received within respective primary and secondary recesses  44 ,  46  of the lower segment  112 . 
     An upper segment  112  is disposed face-down above the lower segment  112 . The primary and secondary recesses  44 ,  46  of the upper segment  112  lie atop the primary and secondary pipes  14 ,  28  respectively. The barbs  54  of each segment  112  are received within the opposed holes  56  of the other segment  112 . 
     A pair of hydraulic jacks  118 , each or nominally 10 Te capacity, acting against the underside of a cross-member  120  of the frame  114  apply load to the upper segment  112  via steel plates  122 . This forces the upper segment  112  into closer engagement with the lower segment  112  as the barbs  54  advance into the holes  56 , eventually clamping the pipes  14 ,  28  between the segments  112 . The jacks  118  and plates  122  may be moved laterally along the underside of the cross-member  120  to apply localised forces to different parts of the upper segment  112 . 
       FIG. 15  shows one of the jacks  118  applying force locally to an end of the upper segment  112 , outboard of the secondary recess  46  of the upper segment  112 . This applies compressive load in alignment with the opposed upper faces  52  of the segments  112 . The other Jack  118  simultaneously applies force locally to the other end of the upper segment  112 , outboard of the primary recess  44  of the upper segment  112 . This applies compressive load in alignment with the opposed lower faces  50  of the segments  112 . 
     In contrast,  FIG. 16  shows the first-mentioned jack  118  and its plate  122  moved inboard to apply force locally to a central part of the upper segment  112 , inboard of its secondary recess  46 . This applies compressive load in alignment with the opposed central faces  48  of the segments  112 , between their primary and secondary recesses  44 ,  46 . 
       FIG. 17  shows that it is also possible to apply compressive load simultaneously at all three opposed pairs of faces of the segments  112 , namely the central, lower and upper faces  48 ,  50 ,  52 . This is achieved by using a wider plate  124  under one of the jacks  118  to bridge the secondary recess  46  of the upper segment  112  and hence to apportion load from that jack  118  between the central and upper faces  48 ,  52 . Again, the other jack  118  simultaneously applies force locally to the other end of the upper segment  112 , outboard of the primary recess  44  of the upper segment  112 . This applies compressive load in alignment with the opposed lower faces  50  of the segments  112 . 
     These test procedures have shown some benefits in moving the location of force application along the segments  112  during the clamping process. There is an advantage in pressing together the end regions of the segments  112  first as shown in  FIG. 15  to locate the segments  112  relative to one another; thereafter, further pressure achieves light clamping that helps to locate the segments  112  relative to the pipes  14 ,  28 . This causes the segments  112  to bend along their length, bowing slightly as the barbs  54  of their central faces  48  resist insertion into the opposed holes  56 . Subsequent application of force in alignment with the central faces  48  as shown in  FIG. 16  presses together the middle of the segments  112 , straightening the bend, and tightens the clamping load on the pipes  14 ,  28 . 
       FIGS. 18 a  to 18 d   ,  19  and  20  illustrate an apparatus  126  for holding and dispensing segments  42  and for assembling blocks  40  from such segments  42  around primary and secondary pipes  14 ,  28 .  FIGS. 18 a  to 18 d    show only half of the apparatus  126  whereas  FIGS. 19 and 20  show the whole apparatus  126 . The pipes  14 ,  28  are shown in vertical orientation in  FIGS. 18 a  to 18 d    although their path may be inclined at other angles as explained previously.  FIGS. 19 and 20  are horizontal cross-sections at upstream and downstream parts of the apparatus  126 . 
     The apparatus  126  comprises opposed reciprocating jaws  128 , each having a cavity  130  shaped to accommodate a segment  42  with its recesses  44 ,  46  facing out of the cavity  130  toward the segment  42  in the cavity  130  of the opposed jaw  128 . The apparatus  126  further comprises pinch wheels  132  downstream of the jaws  128 , aligned with the central faces  48  of the segments  42 . The pinch wheels  132  contra-rotate about parallel axes in a plane orthogonal to the pipes  14 ,  28 . As will be explained, this arrangement having pinch wheels  132  downstream of the jaws  128  achieves the two-step engagement operation found to be advantageous during testing as illustrated in  FIGS. 15 to 17 , with application of compressive loads to different parts of the segments  42  in successive steps. 
     Opposing reciprocating movement of the jaws  128  is driven by double-acting hydraulic actuators  134 . The actuators  134  extend to push the jaws  128  toward one another in an assembly stroke, which forces the segments  42  together to form a block  40  around the pipes  14 ,  28 . When the actuators  134  retract in a return stroke, they pull the jaws  128  away from the assembled block  40  and the block  40  is then carried downstream by overboarding or launching movement of the pipes  14 ,  28 . The jaws  128  are then loaded with fresh segments  42  from a stack  136  in a jaw loading step and the assembly stroke begins again, to assemble a further block  40  at a location spaced a suitable distance upstream of the preceding block  40 . 
     As  FIG. 19  shows, the segments  42  are held in the jaws  128  by latch formations in the form of ridges  138  in the ends of the cavities  130  that engage the grooves  70  in the ends of the segments  42 . The resilience of the segments  42  allows the grooves  70  to disengage from the ridges  138  to snap out of the cavities  130  upon assembly of a block  40  but to be held by the jaws  128  until that point. The direction of the grooves  70  and ridges  138  allows the segments  42  to start sliding out of the jaws  128  when the segments  42  grip the pipes  14 ,  28  during an assembly stroke, so that the pipes  14 ,  28  can move continuously as blocks  40  are applied to them. Also, the direction of the grooves  70  and ridges  138  allows a supply of segments  42  to be retained in the stack  136  as shown in  FIGS. 18 a  to 18 d    and for the retained segments  42  in the stack  136  to slide under gravity or to be driven down into engagement with an associated jaw  128  in a jaw loading step. 
       FIG. 19  also shows that each cavity  130  fits closely against the associated segment  42  at locations aligned with the faces  48 ,  50 ,  52 . This applies compressive loads locally where barbs  54  are to be driven into opposed holes  56  in those faces  48 ,  50 ,  52 . Clearance is provided around the part-cylindrical formations  62 ,  66  corresponding to the primary and secondary recesses  44 ,  46 , to allow for deflection of the segments  42  under load when the segments  42  apply clamping forces to the pipes  14 ,  28 . 
     The cavities  130  are shaped to apply pressure preferentially to the end regions of the segments  42 , which firstly locates the opposed segments  42  relative to one another and then applies light clamping pressure to the pipes  14 ,  28 . This helps to locate the opposed segments  42  relative to the pipes  14 ,  28  for further operations on the resulting block  40 . In this case, the cavities  130  are shaped to accommodate slight bowing of the segments  42  as the barbs  54  of their central faces  48  resist insertion into the opposed holes  56 . Consequently, the segments  42  are not fully engaged to each other when a block  40  exits the jaws  128  and is carried downstream with the pipes  14 ,  28 . Instead, engagement of the segments  42  is completed by squeezing the segments  42  between the pinch wheels  132  located downstream of the jaws  128 . 
     Blocks  40  with partially-engaged segments  42  may be driven between the pinch wheels  132  by virtue of movement of the pipes  14 ,  28  to which they are clamped, in which case the pinch wheels  132  may simply idle and freewheel. Alternatively one or both of the pinch wheels  132  may be driven to drive the blocks  40  between them. The pinch wheels  132  press together the middle of the segments  42  in alignment with their central faces  48  and tighten the clamping load on the pipes  14 ,  28 . The pipes  14 ,  28  and the attached blocks  40  are now ready for launching into the sea. 
       FIGS. 18 a  to 18 d    show a retaining pawl  140  that holds a segment  42  in a cavity  130  of a jaw  128  until the segment  42  has been engaged to an opposed segment  42  to assemble a block  40  around the pipes  14 ,  28 . The retaining pawl  140  comprises a flexible flap attached to the jaw  128  that lies flat and horizontal by virtue of its resilience before the assembly stroke as shown in  FIG. 18 a   , supporting the segment  42  in the cavity  130  of the jaw  128  and the stack  136  of segments  42  stored above.  FIG. 18 b    shows the assembly stroke where the segment  42  has been advanced by the jaw  128  to engage the opposed segment  42  (not shown in this view) and hence to grip the pipes  14 ,  28 . Now, the segment  42  must move with the pipes  14 ,  28  and so exits the cavity  130  of the jaw  128 . The retaining pawl  140  flexes downwardly to allow the segment  42  to pass as shown in  FIGS. 18 b  and 18 c    before snapping back resiliently to the horizontal as shown in  FIG. 18 d   , as the block  40  just assembled encounters the pinch wheels  132  to complete the engagement of its segments  42 . 
     The apparatus of the invention may take other forms; three further examples of such apparatus are shown in  FIGS. 21 to 23  of the drawings. In each case, opposed jaws  142  move orthogonally on connecting rods  144  with respect to the direction of movement of the pipes  14 ,  28  to drive together opposed segments  42  to form a block  40  around the pipes  14 ,  28 . The jaws  142  are supported by a reciprocating carriage frame  146  surrounding the pipes  14 ,  28 , which allows the segments  42  to be engaged as the pipes  14 ,  28  continue moving in an overboarding or launching direction. 
     In an engagement stroke, the carriage frame  146  moves downwardly from a start position in the direction of movement of the pipes  14 ,  28  while the jaws  142  move together to engage the segments  42 . Once the segments  42  are engaged to form a block  40  at the bottom of the engagement stroke, the jaws  142  separate to free the block  40  and the carriage frame  146  moves in a return stroke against the direction of movement of the pipes  14 ,  28  back to the start position. 
     The carriage frame  146  may move in the engagement stroke passively as a result of the segments  42  held by the jaws  142  gripping the moving pipes  14 ,  28 . Alternatively, movement of the carriage frame  146  in the engagement stroke may be driven by a drive means such as a downwardly-acting hydraulic actuator, which is not shown. Movement of the carriage frame  146  in the return stroke is driven or aided by springs  148  acting in compression under the carriage frame  146 : other drive means such as a hydraulic actuator are of course possible. 
     The jaws  142  may be arranged to engage the segments  42  fully to complete a block  40  or a further tightening apparatus is possible downstream of the carriage frame  146 , for example having a pair of pinch wheels like those described in the apparatus  126  described above. Such further tightening apparatus has been omitted from  FIGS. 21 to 23  for clarity. Similarly a retaining pawl like that shown in  FIGS. 18 a  to 18 d    may be applied to a jaw  142  to hold a segment  42  in a cavity of the jaw  142  until opposed segments  42  have been engaged to each other to form a block  40  around the pipes  14 ,  28 . 
     The examples shown in  FIGS. 21 to 23  differ in how the jaws  142  are driven to move relative to the carriage frame  146 . 
     The apparatus  150  shown in  FIG. 21  employs opposed wedge surfaces  152 ,  156  to drive the jaws  142  together. Specifically, outer faces of the jaws  142  have wedge surfaces  152  that taper inwardly and upwardly, and the carriage frame  146  carries wedge blocks  154  with complementary wedge surfaces  156  that taper outwardly and downwardly. The wedge blocks  154  are driven downwardly with respect to the carriage frame  146  by one or more hydraulic actuators  158  to force the jaws  142  together by a sliding cam action of the wedge surfaces  152 ,  156 . 
     Springs or other drive means (not shown) may be used to push the jaws  142  apart at the end of the engagement stroke, or there may be a mechanical link between the wedge blocks  154  and the jaws  142  to pull the jaws  142  apart as a wedge block  154  is pulled upwardly by the actuator  158  relative to the carriage frame  146 . 
     The apparatus  160  shown in  FIG. 22  mounts the jaws  142  on converging ramp rods  162  fixed to the carriage frame  146  that are disposed in parallel pairs on each jaw  142 , the ramp rods  162  of each pair being inclined inwardly and downwardly. A hydraulic actuator  158  drives the jaws  142  downwardly relative to the carriage frame  146  along the ramp rods  162  to force the jaws  142  together during the engagement stroke. The actuator  158  is suitably double-acting to pull the jaws  142  back up along the ramp rods  162  during the return stroke, separating the jaws  142  ready for the insertion of further segments  142 . 
     The apparatus  160  of  FIG. 22  has the benefit that the jaws  142  can move further during the engagement stroke, which maximises the pipelaying speed. This is because the jaws  142  move relative to the carriage frame  146  in the direction of movement of the pipes  14 ,  28  as the carriage frame  146  itself moves in the direction of movement of the pipes  14 ,  28 . 
       FIG. 23  shows an apparatus  164  in which the jaws  142  are simply mounted for reciprocal movement with respect to the carriage frame  146  in directions orthogonal to the direction of movement of the pipes  14 ,  28 . The reciprocal movement of the jaws  142  is driven by respective double-acting hydraulic actuators  158 . It would be possible also to mount the jaws  142  and actuators  158  to the carriage frame  146  via a subframe (not shown) permitting longitudinal movement of the jaws  142  and actuators  158  with respect to the carriage frame  146 , to maximise movement of the jaws  142  in the direction of movement of the pipes  14 ,  28  during the engagement stroke.