Patent Publication Number: US-9422032-B2

Title: Line-laying operations

Description:
This Application is the U.S. National Phase of International Application Number PCT/IB2011/002574 filed on Sep. 15, 2011, which claims priority to Great Britain Application No. 1015594.3 filed on Sep. 17, 2010. 
     This invention relates to techniques for laying lines and for making connections between sequential lengths of line in offshore line-laying operations. The invention is particularly concerned with making connections between lengths of chain and lengths of wire in the installation of mooring lines for FPSOs and the like. However, the principles of the invention may have wider application in the offshore industry and the sequential lengths of line are not necessarily of different line types. 
     FPSOs and the like are typically moored on site for many years. The FPSO is held in place with a mooring system comprising several legs radiating from the FPSO. Each leg of the mooring system is defined by at least one mooring line terminating in an anchor such as a suction pile embedded in the seabed. 
     Anchors and mooring lines are typically pre-installed up to three months before the arrival of the FPSO, to allow for issues such as soil settlement. The mooring lines are deployed from an installation vessel, attached to the anchors and then abandoned on the seabed until being recovered when they are required to moor the FPSO. The mooring lines are recovered, transferred to and secured to the FPSO upon its arrival on site. 
     Each mooring line comprises, in sequence from bottom to top, a bottom or ground chain attached to the anchor, a section of wire (typically spiral strand wire or SSW) attached to the ground chain, and a top chain attached to the wire section. Various connectors make the necessary connections between these line sections. 
     The wire section constitutes most of the length of the mooring line, as—for a given tensile strength—wire is lighter, more compact to store and less expensive than chain. Chains are used instead of wire at the bottom and top of the mooring line to avoid damage to the wire at those vulnerable locations. 
     Mooring lines are typically installed by service vessels such as lay barges. Such vessels are equipped with various cranes and winches for performing subsea operations including line laying and pipe laying. As such vessels are expensive assets that are much in demand around the world, there is commercial pressure to reduce the time that they need to spend on site performing operations such as laying mooring lines. 
     The bottom chain, wire section and top chain must be deployed in sequence to create the mooring line and connectors must be placed between each of those parts of the mooring line. The wire is typically deployed from a drum of a deployment winch on the installation vessel. The chain is typically deployed from a gypsy wheel. The gypsy wheel may be driven by the same deployment winch as the drum but the deployment axes of the gypsy wheel and drum will be horizontally separate, for example spaced fore-and-aft with respect to the hull of the vessel. 
     The mooring line constitutes a heavy load that, in existing arrangements, necessitates using a main crane of the installation vessel. Existing methods for installing mooring lines employ a fixed outrigger at which connections between the line sections are made. They involve extensive use of the main crane to transfer the load repeatedly between the deployment axes of the wire drum and the gypsy wheel and the outrigger. ROV disconnection is required. Existing methods also involve considerable manhandling of moving loads. 
     The main crane of the installation vessel is in much demand for other operations such as overboarding suction piles. Hence, main crane activities tend to lie on the critical path of a mooring line installation operation. This reduces productivity and increases the length of time that the installation vessel needs to remain on site. 
     Use of the main crane also adds to the safety risks of manhandling, and introduces technical risks associated with heavy lifting in a confined and congested area. 
     Existing techniques for installing mooring lines also involve a risk of damaging the wire section by creating loops or bends in the wire or by damaging its protective coating. The wire is strong in tension but is fragile when subjected to cross-axis loads, bending and abrasion. 
     It is against this background that the present invention has been made. From one aspect, the invention resides in a line-handling apparatus for use in assembling a line being deployed from a vessel, that line comprising at least two sections in longitudinal sequence, wherein the apparatus comprises a hang-off carriage having at least one line support adapted to support the line, the line support being capable of supporting a length of line hanging from the carriage via that support, the carriage being movable around a horizontal area located beneath separate line deployment locations on the vessel spaced horizontally from each other, to align the line support with each line deployment location, and to move the supported length of line between the line deployment locations for connection of subsequent sections of the line. 
     Thus, the invention eases and simplifies the connection between sequential parts of the mooring lines. It does so by hanging the load deployed (for example, the bottom chain) and transferring the load below the next connection point (for example, the wire spooled on the drum of the deployment winch). The load is hung from a platform equipped with a chain stopper and a socket clamp sliding on rails underneath the deployment winch. 
     The invention provides a movable hang-off structure whose movement may be automated or semi-automated. The hang-off structure is used for the connection and disconnection of large diameter, heavy chains to wire sockets during various phases of mooring line deployment. 
     The moving hang-off structure of the invention allows parallel activity with the main crane and so reduces main crane lifting operations as much as possible to optimise operational time. Indeed, it is possible to reduce lifting operations by the main crane in a ratio of 1 to 5. To create a typical FPSO mooring system comprising sixteen mooring lines, only sixteen main crane handling operations will be required, excluding overboarding the suction piles. This compares with eighty such operations when using a standard fixed outrigger structure. Thus, main crane activities feature less on the critical path, shortening the deployment operation. It is anticipated that for a typical FPSO mooring deployment operation lasting circa forty days, 8-10% of the installation vessel time may be saved. This greatly increases the productivity of the vessel and its crew. 
     The invention also improves safety, by removing the requirement to manhandle moving loads during chain-to-wire connection operations. It overcomes technical problems of heavy lifting in a confined and congested area. It frees deck space on the installation vessel. It provides safe and easy access to manipulate heavy items such as H-link connectors and other subsea connector elements. It also reduces the risk of damage to the wire by holding and transferring the wire gently without imparting significant cross-axis loads or exposing the wire to abrasion. 
     The horizontal area in which the carriage is movable may extend beyond at least one of the deployment locations, for example to within the working radius of a crane for loading a connector element onto the line support. 
     Preferably, at least one of the line sections is a wire and the carriage has a wire support adapted to support the wire. It is also possible for at least one of the line sections to be a chain and for the carriage to have a chain support adapted to support the chain. In a preferred embodiment of the invention to be described below, the line comprises at least one section of chain in longitudinal sequence with at least one section of wire and the carriage comprises: a chain support adapted to support the chain; and a wire support adapted to support the wire; and the line deployment locations are chain and wire deployment locations, the carriage being movable to align the chain support with the chain deployment location and the wire support with the wire deployment location, and to move the supported length of line between said deployment locations for connection of subsequent sections of the line. 
     Advantageously, the line support is adapted to support a connector element that is attached or attachable to a line section. In this way, the carriage can be used to transport a connector element; the carriage can also be used to suspend a line section, such as a wire, from a connector element without touching the line of that section itself. For this purpose, the line support suitably comprises a socket that narrows downwardly, and preferably comprises formations that are shaped to complement different connector elements. 
     Conveniently, the carriage is mounted to an outrigger platform that is movable along the hull of the vessel. That platform is suitably mounted to the vessel by at least one rail extending generally horizontally along the hull. If the carriage is movable inboard and outboard relative to the platform, this allows the carriage to be moved in two dimensions relative to the hull. 
     The platform preferably defines a slot for accommodating hanging chain or wire, that slot extending in an outboard direction and being open to its outboard end. In that case, the carriage suitably defines a gap aligned with the slot of the platform. For example, the carriage may comprise a chassis having spaced arms that define the gap between them: the chassis may be generally U-shaped, having generally parallel arms joined by a cross-member. The carriage preferably also comprises a gate member on its outboard side that closes the gap and that can be opened to abandon the line. 
     The line support advantageously comprises portions that are separable to allow the line to be deployed. For example, the carriage may comprise a platen that carries the line support, the platen being separable into jaws that divide the line support. In that case, a junction between the jaws suitably extends along the gap in the chassis. 
     In the preferred embodiment described below, at least one line support is a chain support comprising a collar for surrounding a chain, which collar supports chain-engaging members that are co-operable to embrace and engage the chain. The chain-engaging members preferably cooperate to define a plate that is supported on top of the collar, and preferably have chain-engaging formations shaped to engage successive links of the chain. Advantageously, the collar is pivotable about the longitudinal axis of the chain to align the chain-engaging formations with the links of the chain. Where the chain support comprises portions that are separable, those portions suitably include portions of the collar and respective chain-engaging members attached to each portion of the collar. 
     More generally, the apparatus preferably has at least two line supports adapted to support different types of line. Those line supports are preferably disposed one outboard and one inboard of each other. 
     The inventive concept also embraces a line-handling method for use in assembling a line being deployed from a vessel, that line comprising at least two sections in longitudinal sequence, wherein the method comprises supporting the line to hang from a carriage movable around a horizontal area located beneath deployment locations on the vessel spaced horizontally from each other, and moving the carriage to bring the line into alignment with a deployment location for connection of subsequent sections of the line. 
     Where the line comprises at least one section of chain in longitudinal sequence with at least one section of wire, the method suitably comprises moving a supported length of the chain into alignment with a wire deployment location, and/or moving a supported length of the wire into alignment with a chain deployment location. 
     Part of the carriage may be divided to provide clearance for deployment of the chain or wire without moving the carriage, and the line may be abandoned from the carriage in an outboard direction, optionally after opening part of the carriage to permit said abandonment. 
     The inventive concept extends to a vessel fitted with the line-handling apparatus of the invention or operating in accordance with the method of the invention. 
    
    
     
       In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings in which: 
         FIG. 1  is a layout diagram of a site for a FPSO that has yet to be positioned, showing pre-installed mooring piles and lines for the FPSO; 
         FIG. 2 a    is a side view of a mooring line extending from the now-positioned FPSO to a suction pile, with  FIG. 2 b    being an enlarged detailed view of the ground chain end of the mooring line and the pile; 
         FIG. 3  is a perspective view of a lay barge with a cargo barge alongside; 
         FIG. 4  is an enlarged perspective view showing a detail of the lay barge shown in  FIG. 3 , including a hang-off platform in accordance with the invention; 
         FIG. 5  is a partial plan view of the lay barge of  FIG. 3  as chain is deployed; 
         FIG. 6  is a partial side view of the lay barge of  FIG. 3 ; 
         FIG. 7  is a cross-sectional view of the lay barge of  FIG. 3  as wire is being trans-spooled; 
         FIGS. 8 and 9  are perspective views of the hang-off platform showing its mounting to the lay barge of  FIG. 3 ; 
         FIGS. 10 and 11  are perspective views of a carriage being part of the hang-off platform, respectively in outboard and inboard positions; 
         FIGS. 12 and 13  are perspective views of the carriage of  FIGS. 10 and 11 , with halves of the carriage separated to expose a slot in the hang-off platform, the outboard end of the slot being respectively closed and open; 
         FIGS. 14 and 15  are enlarged perspective views of the carriage of  FIGS. 10 and 11 , showing a detail in the form of flaps respectively in closed and open states; 
         FIGS. 16 and 17  are enlarged perspective views of the carriage of  FIGS. 10 and 11 , showing a collar carrying the flaps pivoted respectively anticlockwise and clockwise with respect to the remainder of the carriage; 
         FIG. 18  is a flow diagram summarising method steps involved in installing and abandoning a mooring line using a pre-installed anchor such as a suction pile; 
         FIG. 19  is a perspective view of the upper ground chain being routed around a gypsy wheel of a deployment winch; 
         FIG. 20  is a perspective view of the upper ground chain being affixed to a subsea connector element supported by a connector socket in the carriage of the hang-off platform; 
         FIG. 21  is a perspective view of the carriage halves separated to free the subsea connector element and the upper ground chain for deployment by the winch; 
         FIG. 22  is a perspective view of the carriage halves closed again to seat the upper end of the upper ground chain in the closed jaws of the carriage; 
         FIG. 23  is a perspective view of the carriage moved to attach a connector to the upper end of the upper ground chain; 
         FIG. 24  is a perspective view of the carriage moved back to attach a lower end of the spiral strand wire (SSW) to the connector at the upper end of the upper ground chain; 
         FIG. 25  is a perspective view of the carriage halves separated to free the connector and the spiral strand wire for deployment by the winch; 
         FIG. 26 a    is a side view of the lay barge deploying the upper section of ground chain in readiness for subsea connection to the lower section of ground chain, with  FIG. 26 b    being an enlarged detailed view of the connection operation itself; 
         FIG. 27  is a side view of a spiral strand wire laying operation; 
         FIG. 28  is a perspective view of the carriage halves closed again to support a connector element at the upper end of the spiral strand wire in the connector socket of the carriage; 
         FIG. 29  is a perspective view of a connector element being attached to the upper end of the spiral strand wire; 
         FIG. 30  is a perspective view of the top chain being attached to the connector element at the upper end of the spiral strand wire; 
         FIG. 31  is a perspective view of the carriage halves separated to free the connector and the top chain for deployment; 
         FIG. 32  is a perspective view of the carriage halves closed again to seat the upper end of the top chain in the closed jaws of the carriage; 
         FIG. 33  is a perspective view of the carriage halves separated to free the top chain for abandonment by outboard movement through the open-ended slot of the hang-off platform; and 
         FIG. 34 a    is a side view of completion of the abandonment operation, with  FIG. 34 b    being an enlarged detailed view of the top chain end of the mooring line. 
     
    
    
     Referring firstly to  FIG. 1  of the drawings, this shows the layout of the seabed  10  around a site for a Floating Production Storage and Offloading vessel (FPSO). Seabed contour lines and other details of the site are shown. The FPSO has yet to be positioned and so is not shown in  FIG. 1 . 
     The FPSO will be held in place with a passive spread mooring system comprising sixteen semi-taut legs arranged in a 4×4 spread-type pattern. In other words, the FPSO will be moored with a total of sixteen mooring lines  12  in four groups of four, one group at each corner of the FPSO. Each mooring line  12  is anchored by a respective suction pile  14  embedded in the seabed  10 . A mooring line  12  and the associated suction pile  14  together define one leg of the mooring system. The suction piles  14  may be pre-installed some time before attachment of the mooring lines  12  or may be installed during installation of the mooring lines  12 . 
     Each mooring line  12  comprises, in sequence from bottom to top, a bottom or ground chain  16  attached to the associated suction pile  14 , a length of spiral strand wire (SSW)  18  attached to the ground chain  16  that constitutes most of the length of the mooring line  12 , and a top chain  20  attached to the SSW section  18 . Different connectors such as H-Links, a twisted H-Link, Y-links and Balltec subsea connectors make the connection between different line components as will be explained. Balltec subsea connectors are supplied by Balltec Ltd of Lancashire, UK and have ball-and-roller engagement mechanisms. 
     The SSW  18  will generally be of coated steel but recent developments suggest that it may be possible to use a synthetic plastics material: references to ‘wire’ in this specification are not intended to limit the meaning only to wires of metallic materials. 
     The suction piles  14  and mooring lines  12  are typically pre-installed up to three months before the arrival of the FPSO. The mooring lines  12  are deployed from an installation vessel, attached to the suction piles  14  and then abandoned on the seabed  10  until being recovered when they are required to moor the FPSO. As can be seen in  FIG. 1 , the mooring lines  12  of each group are laid in a generally parallel arrangement with their SSW sections  16  curved while observing a minimum permissible bend radius. The mooring lines  12  will be recovered and transferred to the FPSO upon its arrival, whereupon their top chains  20  will be secured to the FPSO as now shown in  FIG. 2   a.    
     Moving on then to  FIGS. 2 a  and 2 b   , these show one of the mooring lines  12  having been recovered and now in use mooring an FPSO  22 . It will be apparent from the detail view of  FIG. 2 b    that each ground chain  16  is in two sections, namely a lower section  24  and an upper section  26 . In sequence from the bottom to the top of the mooring line  12 :
         the lower section  24  of the ground chain  16  is connected to a buried lower side wall of the suction pile  14  by a twisted H-Link connector  28 ;   the upper section  26  of the ground chain  16  is connected to the lower section  24  of the ground chain  16  by a Balltec subsea connector  30 , with a female receptacle element of the subsea connector  30  being connected to the lower section  24  by a Y-link and a cooperating male element of the subsea connector  30  being connected to the upper section  26  by a further Y-link;   the SSW  18  is connected to the upper section  26  of the ground chain  16  by a H-link connector  32 ; and   the top chain  20  is connected to the SSW  18  by a H-link connector  34 .       

     As will be described more fully later, the lower section  24  of the ground chain  16  is attached to the associated suction pile  14  before the suction pile  14  is overboarded and lowered to penetrate the seabed  10 . When the FPSO is moored, the lower section  24  of the ground chain  16  and the connector  30  are buried under the seabed  10  and the upper section  26  of the ground chain  16  extends from there above the seabed  10  to the SSW  18  via connector  32 . 
     In a non-limiting example: the length of the top chain  20  is between 165 m and 200 m; the lower section  24  of the ground chain  16  is 23 m long; the upper section  26  of the ground chain  16  is 182 m long; and the SSW section  18  is 1285 m long. 
     Referring now to  FIGS. 3 to 7  of the drawings, these give an overview of a lay barge  36  exemplified here by the applicant&#39;s vessel Acergy Polaris. Among many other capabilities, the lay barge  36  is adapted in accordance with the invention to lay mooring lines  12 . 
     The lay barge  36  is shown in  FIG. 3  with a cargo barge  38  tied alongside to supply and provide temporary storage for supplies including baskets  40  of chain  16 ,  20 , reels  42  of SSW  18  and suction piles  14 . An offloading chute  44  extending outboard from the port side of the lay barge  36  lies on the deck of the cargo barge  38  to facilitate transfer of chain  16 ,  20  from the cargo barge  38  to the lay barge  36  as required. 
     As  FIG. 3  also shows, the lay barge  36  comprises a J-lay tower  46 . The J-lay tower  46  is not relevant to the present invention and so will not be discussed further, but an abandonment and recovery (A&amp;R) winch associated with the J-lay function may be used by the invention as will be described. The lay barge  36  further comprises a Clyde-type main crane  48 . The main crane  48  is used in certain major lifting operations in accordance with the invention, such as overboarding the suction piles  14 . However, by virtue of the invention, use of the main crane  48  is advantageously minimised. An auxiliary crane  50  on the lay barge  36  is used to transfer loads such as reels  42  from the cargo barge  38  to the lay barge  36 . 
     As best shown in  FIGS. 4 to 7 , the lay barge  36  further comprises deployment apparatus for deploying mooring lines  12 , which apparatus is largely disposed on the starboard side of the lay barge  36  directly opposite the offloading chute  44 . The deployment apparatus comprises a deployment winch  52  having a drum  54  for deployment of wire and a gypsy wheel  56  for deployment of chain. The deployment winch  52  will typically have a capacity of 150 tonnes. 
     The invention adds a sliding hang-off platform (SHOP)  58  disposed generally beneath the deployment winch  52 , outboard of the hull  60  of the lay barge  36 . The hang-off platform  58  slides longitudinally on parallel upper and lower rails  62  attached to the outer side of the hull  60 , extending fore-and-aft. Those rails  62  and the overall structure of the hang-off platform  58  are best appreciated in the detail views of  FIGS. 8 and 9 . In principle, the platform  58  is an automatic outrigger able to move the load below the deployment winch  52  where that load needs to be connected, thus avoiding using the crane  48  for connecting the mooring leg parts. Details of the platform  58  will be described after this continued overview of the deployment apparatus with reference to  FIGS. 5 to 7 . 
       FIG. 5  shows the chain  16 ,  20  from the cargo barge  38  sliding inboard onto the port side of the lay barge  36  over the chute  44 . The end of the chain  16 ,  20  has been pulled from there to the starboard side of the lay barge  36  along gutters  64  extending across the lay barge  36 . There, the chain  16 ,  20  has been engaged with the gypsy wheel  56  of the deployment winch  52  for deployment into the sea over the starboard side of the lay barge  36 . 
       FIG. 6  shows how an ancillary crane  66  is positioned close to the deployment winch  52  to insert the chain  16 ,  20  into the gypsy wheel  56  (a process shown in  FIG. 19 , to be discussed below) and also to transfer elements of the various connectors to the hang-off platform  58  as will also be described in detail later. 
       FIG. 7  shows a trans-spooling operation in progress. A reel  42  of SSW  18  has been lifted from the cargo barge  38  by the auxiliary crane  50  of  FIG. 3  and placed on a dolly base  68  beside the offloading chute  44  on the port side of the lay barge  36 . From there, a transporter arrangement feeds the SSW  18  across to the starboard side of the lay barge  36 , where it is then wound onto the drum  54  of the deployment winch  52 . A service wire of the deployment winch  52  pulls the end of the SSW  18  onto the drum  54 . 
     The SSW  18  is pre-fitted with padeye connector elements at both ends for connection to complementary connector elements of the bottom and top chains  16 ,  20  in due course. The padeye connector element at the leading end of the SSW  18  provides a convenient connection point for the transporter arrangement and for the service wire of the deployment winch  52 . 
     A tensioner  70  maintains back-tension in the SSW  18  during trans-spooling and is mounted on a rack that oscillates fore-and-aft with respect to the lay barge  36  to work as a spooler during that operation. Typically the tensioner  70  will have a capacity of 20 tonnes and will apply 15 tonnes of back-tension to the SSW  18 . 
     Moving on now to  FIGS. 8 and 9 , these show further details of the hang-off platform  58 . The platform  58  comprises a framework of members that move together fore-and-aft along the rails  62 . The members comprise four sets of uprights  72 , outriggers  74  and struts  76 . 
     The generally vertical uprights  72  on the inboard side of the platform  58  are attached to the rails  62  via bearings enabling relative fore-and-aft movement. In each of the four sets of members, the generally horizontal outrigger  74  extends in an outboard direction from the top of the upright  72  and the inclined strut  76  extends from the bottom of the upright  72  to the outboard end of the outrigger  74 . Consequently, the port-to-starboard cross-section of the framework through the upright  72 , outrigger  74  and strut  76  is generally that of an inverted right-angled triangle in which the strut  76  is the hypotenuse. 
     The sets of uprights  72 , outriggers  74  and struts  76  are arranged in two pairs, each pair being joined by a respective longitudinal beam  78 . Each beam  78  extends between the junctions of the outriggers  74  and struts  76  of the associated pair. A gap between the beams  78  of the respective pairs defines a slot  80  between the innermost two of the outriggers  74 , that slot  80  being open at its outboard end. 
     The outriggers  74  support a carriage  82  in alignment with the slot  80 . The carriage  82  will now be described in detail with reference to  FIGS. 10 to 17  of the drawings. 
       FIGS. 10 and 11  show how the carriage  82  comprises a chassis  84  surmounted by a platen  86 . The platen  86  is penetrated by a socket  88  and also carries a clamp  90  aligned with and outboard of the socket  88 . 
     As will be explained, the socket  88  is adapted to receive and engage elements of the connectors  30 ,  32 ,  34  used to connect the SSW section  18  of a mooring line  12  being deployed. As will also be explained, the clamp  90  is adapted to receive and engage chain  16 ,  20  used at the bottom and top of the mooring line  12 . Via either the socket  88  or the clamp  90  as appropriate, the carriage  82  and the platform  58  are capable of supporting and transferring the load of the mooring line  12  to minimise intervention from cranes or winches on the lay barge  36 . 
     The clamp  90  comprises a circular-section tubular collar  92  upstanding from the platen  86 , which surrounds a hole penetrating the platen  86 . Flaps  94  are pivotably mounted to the top of the collar  92 . The flaps  94  are shown in a closed operational position in  FIGS. 10 and 11 , in which they cooperate to form a plate for receiving, engaging and supporting the chain  16 ,  20 . 
     The features and operation of the platen  86 , socket  88  and clamp  90  will be described in more detail below. Meanwhile, it should be noted from  FIGS. 10 and 11  that the carriage  82  is movable inboard and outboard with respect to the platform  58  above the slot  80  defined between two outriggers  74 . The carriage  82  is mounted to the platform  58  for sliding movement in a transverse direction perpendicular to the rails  62  on which the platform  58  itself moves fore and aft. That movement of the carriage  82  with respect to the platform  58  is driven by parallel rams  96 , one on each outrigger  74  to each side of the slot  80 . 
     By virtue of fore-and-aft longitudinal movement of the platform  58  with respect to the lay barge  36  and inboard-and-outboard lateral movement of the carriage  82  with respect to the platform  56 , the carriage  82  may be moved horizontally to any location in a rectangular area situated outboard of the hull  60  of the lay barge  36  and generally below the deployment winch  52 . That area encompasses, and extends beyond, the region below the deployment winch  52 . Thus, the carriage  82  may be moved for the socket  88  to receive SSW  18  deployed by the drum  54  of the deployment winch  52  and for the clamp  90  to receive chain  16 ,  20  deployed by the gypsy wheel  56  of the deployment winch  52 . The carriage  82  may also be moved for the socket  88  to receive elements of the connectors  30 ,  32 ,  34  used in the mooring line  12 , before carrying those elements back to below the location where the SSW  18  emerges from the drum  54  of the deployment winch  52 . It will be noted that due to spooling of the SSW  18  on the drum  54 , that location will vary as the SSW  18  is deployed. 
       FIGS. 12 and 13  show that the platen  86  of the carriage  82  is divided along a vertical outboard-extending plane that bisects the socket  88  and the clamp  90 . The platen  86  thus defines opposed jaws  86 ′ that may be moved horizontally apart and together again with respect to the chassis  84  by means of rams  98  acting between the chassis  84  and the jaws  86 ′. A latch lever  100 , when closed, confirms that the jaws  86 ′ are locked together and must be released as shown in  FIGS. 12 and 13  when the jaws  86 ′ are to be moved apart. The latch lever  100  is shown in the closed position in  FIGS. 10 and 11 . 
     The socket  88  and the clamp  90  also divide as the platen  86  divides. A respective semi-circular half  88 ′ of the socket  88  moves with each jaw  86 ′ such that the circular socket  88  is completed when the jaws  86 ′ come together. Similarly, a respective semi-circular half  92 ′ of the collar  92  moves with each jaw  86 ′ such that the semi-circular collar  92  is completed when the jaws  86 ′ come together. Also, each half of the collar  92  carries a respective one of the flaps  94 . 
       FIGS. 12 and 13  also show how the chassis  84  is generally U-shaped in plan view. More specifically, the chassis  84  comprises an inboard cross-member  102  parallel to the hull  60  of the lay barge  36  and two outboard-extending arms  104 , the arms  104  defining a gap between them aligned with the slot  80 . The gap has an open outboard end but may be closed by a hinging gate member  106  shown closed in  FIG. 12  and open in  FIG. 13 . When in its normal closed state, the gate member  106  adds to the structural integrity of the carriage  82 . As will be described, the gate member  106  need be opened only to allow the mooring line  102  to be abandoned, at which stage the carriage  82  no longer bears the load of the mooring line  102 . 
     As mentioned above, the flaps  94  are pivotably mounted to the top of the collar  92 . Each flap  94  is hinged about a respective generally horizontal axis, the axes being parallel to each other.  FIG. 15  shows how the flaps  94  may thereby be raised into an open clearance position by means of rams  108  acting between the collar  92  and the flaps  94 . 
     When lowered into the closed operational position shown in  FIG. 14 , the flaps  94  cooperate to form a square plate for receiving, engaging and supporting the chain  16 ,  20  as noted above. That plate has chain-engaging formations comprising spaced holes  110  for accommodating a lower chain link and an elongate ellipsoidal cup  112  extending transversely between the holes for seating an upper chain link. The holes  110  and the cup  112  are bisected by the division between the flaps  94 ; the free edge of each flap  94  therefore has semi-circular cut-outs defining half  110 ′ of each hole  110  and a recess  112 ′ defining half of the cup  112 , as  FIG. 15  makes clear. 
       FIGS. 16 and 17  show how the collar  92  is pivotable with respect to the platen  86  by virtue of opposed rams  114  acting between the collar and the platen  86 . By extending and contracting in synchronisation, the rams  114  can turn the collar  92 —and hence the chain-engaging formations  110 ,  112  defined by the flaps  94 —to match the orientation of chain links hanging from the gypsy wheel  56  of the deployment winch  52  or the SSW socket hanging from the drum  54  of the deployment winch  52 . The range of pivotal movement is approximately 90°, being approximately ±45° about a central orientation in which the pivot axes of the flaps  94  are parallel to the plane on which the socket  88  and the collar  92  are centred. 
     A final detail of the carriage  82  is also best shown in  FIGS. 16 and 17 , namely that the socket  88  has a stepped cross-section that narrows with increasing depth. That cross-section defines formations such as shoulders  116  shaped to co-operate, engage with and support various types of connector elements. 
     Having now described the lay barge  36  and details of the platform  58  and carriage  82 , their operation when installing a mooring line  12  will now be described. The general process is summarised in  FIG. 18  and the first two steps—namely transferring the reel  42  of SSW  18  to the dolly  68  and then trans-spooling the SSW  18  from the reel  42  to the drum  54  of the deployment winch  52 —have already been described. 
       FIG. 19  shows the ancillary crane  66  being used to insert the upper section  26  of the ground chain  16  into the gypsy wheel  56 . In this respect, it will be recalled that the lower section  24  of the ground chain  16  is pre-attached to the associated suction pile  14  before the suction pile  14  is overboarded and lowered to penetrate the seabed  10 . The main crane  48  of the lay barge  36  shown in  FIG. 3  is used to overboard the suction pile  14 , whose load is then transferred to an A&amp;R winch that lowers the suction pile  14  from approximately 30 m underwater to the seabed  10  and so frees the main crane  48  for other duties. However it would be possible in principle to use the main crane  48  to lower the suction pile  14  all the way to the seabed  10 . 
     Once the upper section  26  of the ground chain  16  has been inserted into the gypsy wheel  56  as shown in  FIG. 19 , the gypsy wheel  56  is turned to lower the end of the upper section  26  to just above the level of the hang-off platform  58 . Meanwhile the carriage  82  of the hang-off platform  58  is moved to present its connector socket  88  to the ancillary crane  66 . The ancillary crane  66  then lifts elements of connector  30  into the socket  88 , those elements being a Y-link  118  atop a male element  120  of a Balltec subsea connector. Then, the carriage  82  moves into alignment with the gypsy wheel  56  to align the Y-link  118  with the end of the upper section  26 , whereupon the end of the chain is fixed to the Y-link  118  as shown in  FIG. 20 . 
     The upper section  26  of the ground chain  16  is now ready for deployment overboard into the sea. To enable this, the jaws  86 ′ of the platen  86  of the carriage  82  are separated to split the socket  88  as shown in  FIG. 21 . This disengages the male connector element  120  from the socket  88  and provides clearance for the upper section  26  of the ground chain  16  to pass between the jaws  86 ′ as the chain is deployed by rotation of the gypsy wheel  56 . 
       FIG. 22  shows the carriage  82  repositioned and reconfigured after the full length of the upper section  26  of the ground chain  16  has been deployed. The main crane  48  has lifted the upper end of the chain clear of the gypsy wheel  56  and then lowered the chain such that the chain link  122  at the upper end is just above the collar  92  of the carriage  82 . As the jaws  86 ′ of the carriage  82  were still open during that movement, the collar  92  was split whereby the collar  92  could readily embrace the chain. For this purpose, the carriage  82  has been moved inboard relative to the platform  58  to align the collar  92  with the chain. 
     The jaws  86 ′ have then been brought together to complete the collar  92  around the chain. The collar  92  may then be pivoted if necessary to align the chain-engaging formations  110 ,  112  of the flaps  94  with the orientation of the chain links, whereupon the flaps  94  are lowered to engage the chain links. The link  122  at the upper end of the chain is received by the cup  112  and the penultimate link  124  of the chain is received by the holes  110 . The horizontal plate defined by the cooperating lowered flaps  94  now bears the full load of the upper section  26  of the ground chain  16 , meaning that the main crane  48  is free for other duties. 
       FIG. 23  shows the next operation, which is to use the ancillary crane  66  to lift an H-link connector  126  into engagement with the exposed chain link  122  supported by the flaps  94  of the carriage  82 . For this purpose, the carriage  82  is moved toward the ancillary crane  66  by moving the platform  58  forward along the rails  62  with respect to the hull  60  of the lay barge  36 . Next, the carriage is moved into alignment with the drum  54  of the deployment winch  52  by moving the platform  58  aft along the rails  62 . This is shown in  FIG. 24 , which also shows how a padeye connector element  128  pre-installed on the bottom end of the SSW  18  is lowered by the deployment winch  52  into engagement with the aligned H-link connector  126 . 
     Via the SSW  18 , the deployment winch  52  may now take the load of the deployed upper section  26  of the ground chain  16 . This enables the jaws  86 ′ of the carriage  82  to be opened once more as shown in  FIG. 25 , freeing the chain from the flaps  94  and allowing the SSW  18  to be deployed from the drum  54  of the deployment winch  52 . The upper section  26  of the ground chain  16  hangs from the SSW  18  until it reaches the seabed  10  as will now be described. 
     Fore-and-aft unspooling movement of the SSW  18  with respect to the deployment winch  52  may be accommodated by moving the platform  58  fore-and-aft to keep the carriage  82  in alignment with the SSW  18  throughout deployment. 
       FIG. 26 a    shows how the upper section  26  of the ground chain  16  is handled underwater during the deployment process. For better control, an ROV  130  connects an auxiliary winch wire  132  to the upper section  26  of the ground chain  16 , near its lower end. This leaves a short free end portion  134  of the upper section  26  hanging from the auxiliary winch wire  132 . The remainder of the upper section  26  hangs as a catenary between the auxiliary winch wire  132  and the deployment winch  52  via the SSW  18 . The auxiliary winch wire  132  is paid out as the deployment winch  52  lowers the free end portion  134  of the upper section  26  toward the suction pile  14 . 
       FIG. 26 b    best shows how the lower section  24  of the ground chain  16  was attached to the suction pile  14  before the suction pile  14  was installed. The lower section  24  extends upwardly along the side wall of the suction pile  14  from the connector  28  positioned at a low level on that side wall. At its upper end, the lower section  24  of the ground chain  16  terminates in a female subsea connector socket that remains fixed temporarily to the upper exposed end of the suction pile  14  during installation of the pile  14 . 
     To complete the ground chain  16 , the male connector element  120  (shown in  FIG. 20 ) at the bottom end of the upper section  26  of the ground chain  16  is guided by an ROV  130  into the socket at the top end of the lower section  24  of the ground chain  16 . There, the male connector element  120  engages with the socket to complete the subsea connector  30  as shown in  FIG. 26   b.    
     The subsea connector  30  may then be detached by an ROV  130  from the suction pile  14  so that the full length of the ground chain  16  can extend freely from the suction pile  14  while its bottom end remains anchored by the connector  28  to the buried side wall of the suction pile  14 .  FIG. 27  shows the lower section  24  of the ground chain  16  inclined away from the suction pile  14  as the lay barge  36  deploys the SSW  18  and moves relative to the seabed  10  to follow the desired lay path of the mooring line  12  shown in  FIG. 1 . The touchdown point is continuously monitored by an ROV  130 . 
     Deployment of the SSW  18  continues until all of the SSW  18  has been paid out from the drum of the deployment winch  52 . At that stage, as shown in  FIG. 28 , the SSW  18  hangs from the service wire  136  of the deployment winch  52 , which remains joined to a padeye connector element  128  pre-installed at the top end of the SSW  18 . Now, the carriage  82  has again repositioned and reconfigured, having first moved outboard to align the socket  88  with the connector element  128  while the jaws  86 ′ of the carriage  82  were still open. Thus, the socket  88  was split whereby the socket  88  could readily embrace the SSW  18  hanging below the connector element  128 . 
     Now, the jaws  86 ′ have been brought together to complete the socket  88  around the SSW  18 , whereupon the connector element  128  has been lowered into and engaged with the socket  88  as shown in  FIG. 28 . The socket  88  now bears the full load of the upper section  26  of the ground chain  16  and the SSW  18  hanging between the connector element  128  and the seabed  10 . The service wire  136  may now be disconnected from the connector element  128 , ready for the engagement of an H-link connector  126  lifted by the ancillary crane  66  as shown in  FIG. 29 . To do so, the platform  58  is moved forward along the rails  62  to move the carriage  82  out of alignment with the drum  54  of the deployment winch  52  to within the working radius of the ancillary crane  66 . 
     When the H-link connector  126  has been engaged with the connector element  128 , the platform  58  is moved aft along the rails  62  to move the carriage  82  back into alignment with the gypsy wheel  56  of the deployment winch  52 . Here, as shown in  FIG. 30 , the top chain  20  inserted previously into the gypsy wheel  56  by the ancillary crane  66  is lowered into engagement with the H-link connector  126 . Now, the deployment winch  52  may again take the load of the upper section  26  of the ground chain  16  and the SSW  18  hanging above the seabed  10 . The jaws  86 ′ of the carriage separate again as shown in  FIG. 31  to split the socket  88  and hence disengage the connector element  128  from the socket  88 , freeing the top chain  10  to be deployed into the sea. 
       FIG. 32  shows the carriage  82  repositioned and reconfigured after the full length of the top chain  20  has been deployed. The main crane  48  has lifted the upper end of the chain  20  clear of the gypsy wheel  56  and then lowered the chain  20  such that four chain links  138  at the upper end are just above the collar  92  of the carriage  82 . As the jaws  86 ′ of the carriage  82  were still open during that movement, the collar  92  was split whereby the collar  92  could readily embrace the chain  20 . For this purpose, the carriage  82  has again been moved inboard relative to the platform  58  to align the collar  92  with the chain  20 . 
     The jaws  86 ′ have then been brought together to complete the collar  92  around the chain  20 . As before, the collar  92  may be pivoted if necessary to align the chain-engaging formations  110 ,  112  of the flaps  94  (as shown in  FIGS. 16 and 17 ) with the orientation of the chain links  138 , whereupon the flaps  94  are lowered to engage the chain links  138 . The fourth link  138  in from the upper end of the chain  20  is received by the cup  112  and the link  138  below that is received by the holes  110 . The horizontal plate defined by the cooperating lowered flaps  94  now bears the full load of the top chain  20 , the SSW  18  and the bottom chain  16  extending from the carriage  82  to the seabed  10 . 
     As  FIG. 33  shows, a sling  140  from the main crane  48  may then be reconnected to the top chain  20  a few links down from the end. The end link of the top chain  20  is then fitted with a loop  142  to facilitate transferring the load underwater to the A&amp;R winch of the lay barge  36 . However it is also possible for the main crane  48  to abandon the top chain  20  all the way to the seabed  10 . It will be noted in this respect that the hinging gate member  106  of the carriage  82  has opened in  FIG. 33  to allow the top chain  20  to be pulled by the main crane  48  in an outboard direction out of and away from the carriage  82  before being abandoned. The jaws  86 ′ of the carriage  82  have opened to disengage the links  138  of the top chain  20  from the flaps  94  and to provide clearance for outboard movement of the top chain  20  out of the carriage  82 . 
     Referring finally to  FIGS. 34 a  and 34 b    of the drawings, these show the mooring line  12  being finally abandoned on the seabed  10  by the A&amp;R winch  144  of the lay barge  36 . An ROV  130  has disconnected the loop  142  from the top chain  20 . A float arrangement  146  near the end of the mooring line assists a subsequent recovery procedure. It will be noted that an anti-twist arm  148  projects perpendicularly with respect to the wire  150  of the A&amp;R winch  144  to ease control of the mooring line  12  during abandonment.