Abstract:
The invention provides a tether connection apparatus for remotely interconnecting respective free ends of two portions of a mooring tether, such as that used to moor a floating production vessel to the seabed. The apparatus comprises: pull-in means that are operable to pull-in to said apparatus the free end of one of the portions of mooring tether, subsequent to deployment of the apparatus. Said apparatus also comprises means for releasably mounting a connector, said connector being designed to interconnect the respective free ends of two portions of the mooring tether, once the pull-in means has pulled in to said apparatus the free end of a first of said portions of mooring tether, said means being operable to release the connector subsequent to interconnection.

Description:
[0001]    This invention relates to methods of and apparatus for connecting lines to articles on the seabed. The primary purpose is to provide a mooring apparatus and a method of mooring suitable for use with large offshore production vessels, such as drilling vessels and production buoys. However, the invention can be applied anywhere where it is desired to attach a heavy tether to an article already deployed on the seabed, for anchoring another article, or for lifting the seabed article itself. The term “tether” will be used to refer to a mooring leg, but also includes a hoisting cable or chain, unless the context requires otherwise. 
         [0002]    Offshore production vessels need to be held within a strict geographical area at all times no matter the weather and tidal flow to ensure safe and continuous production and flow of hydrocarbons or gases. A typical mooring system for a production vessel comprises a loading buoy (otherwise known as a catenary anchor leg mooring (CALM) buoy) having a number of anchors deployed about it to hold the buoy in position via connecting lines. A typical arrangement for the deployment of these anchors is in a circle centred on the buoy. This type of arrangement means that the loads and forces that will be exerted on the buoy by the sea and climatic conditions can be resisted no matter the direction from which they originate. Normally any moored vessel will be able to rotate (or “weathervane”) in relation to the buoy. In alternative installations, the vessel may be directly anchored to the seabed without use of a loading buoy. The present invention may be used to set an array of anchors in the seabed about the buoy/vessel. The anchors may comprise simple weights, suction anchors or drag-embedment anchors. 
         [0003]    In conventional arrangements the buoy or production vessel is held in place by a series of anchors buried in the seabed. This normally means between 6 and 12 anchors equidistantly spaced (or spaced in groups) about the circumference of a circle formed with the buoy at its centre so that the buoy is held in place. Each anchor is at the end of a mooring leg extending from the vessel or buoy to the seabed. The terms “mooring leg” and “main tether” are used to encompass a variety of specific forms of tether, of which heavy chain is the most typical example. Other forms of mooring leg are of course possible, and often comprise sections of chain and sections of cable, connected together according to the properties required at each point along their extent. 
         [0004]    Needless to say, the greater the depth of water in which the buoy is deployed the longer the chain and the greater the weight that has to be deployed. In order to deploy and embed the anchor conventional methods involve locating and deploying the anchor with it connected to a mooring leg of the buoy. The anchor is therefore deployed with the full weight of the chain, which forms a part of the mooring leg and connects the anchor to the loading buoy. The sheer weight even with the apparent lightening effect of the water becomes burdensome and difficult to operate with. In addition, the buoy or vessel to be moored has to be on site when the operation is being carried out, which could be a number of days. Logistically this is inefficient as the buoy/vessel is in effect out of service and the operation is more difficult to timetable. 
         [0005]    Methods are known to deploy the anchor, either by itself or with a portion of the mooring leg chain or wire attached, before connection to the main mooring leg. One such method is disclosed in WO 02/062653. In this case connection is made by use of a Remotely Operated Vehicle (ROV) manipulator and vessel crane only, which is difficult to control properly and therefore slow to do, requiring particularly skilled ROV piloting. Other systems make use of purpose made mooring line connectors, such as disclosed U.S. Pat. No. 6,158,093, and as such increase the cost and complexity of the mooring line system. 
         [0006]    It would be desirable, therefore, to be able to pull-in and connect subsea mooring line connections remotely without assistance from divers or purpose made mooring line connectors. 
         [0007]    In a first aspect of the invention there is provided tether connection apparatus for remotely interconnecting respective free ends of first and second portions of a mooring tether, said apparatus comprising:
       pull-in means operable to pull-in to said apparatus the free end of one of said portions of mooring tether, subsequent to deployment of the apparatus;   means for releasably mounting a connector, said means being designed to mount a connector that is operable to interconnect the respective free ends of said first and second portions of the mooring tether, once said pull-in means has pulled in to said apparatus the free end of a first of said portions of mooring tether, said means being operable to release the connector subsequent to interconnection.       
 
         [0010]    Said tether connection apparatus may be operable to have said connector mounted to said means for releasably mounting a connector, wherein said connector is attached to the free end of a second of said portions of mooring tether, prior to deployment of the apparatus Said tether connection apparatus may be operable such that said first of said portions of mooring tether is attached to an anchor on the seabed, while the second of said portions of mooring tether is attached to an object to be moored. 
         [0011]    Said means for releasably mounting a connector may be designed specifically to enable mounting of standard connectors, already commonly used for connecting lines, or portions of lines together. Such connectors may be any of the type commonly used to connect together, for example, chain with chain, chain with wire or wire with wire, as appropriate. Such connectors may include, a shackle with associated pin, spelter sockets, chain links or triplates. This means that expensive and complicated purpose made mooring line connectors can be dispensed with. Said means for releasably mounting a connector may be adapted to be able to mount two or more types of connectors. 
         [0012]    Said apparatus may further comprise a suitable connector, pre-installed on said means for releasably mounting a connector. 
         [0013]    Said apparatus may further comprise alignment means for aligning one portion of the mooring tether with the other prior to their being interconnected. Said alignment means may comprise a channel Said channel may be arranged to accommodate the free end of said portions of mooring tether to correctly locate and align said free end, for subsequent interconnection. Said channel may have a relatively wide opening, tapering inwards, thus forming a funnel shape. Said channel may include a restricted section for locating and orienting one of said free ends of portion of mooring tether. Said restricted section may include one or more locating pins. Said one or more locating pins may be designed to cooperate with and therefore properly locate and orientate a link of mooring chain. Said one or more locating pins may be adaptable, either by themselves or by way of the addition of an adaptor, so to be able to properly position wire, as well as chain. Said channel may be located on carriage allowing movement of said channel, and therefore said portion of said mooring leg located therein, relative to said first portion of the mooring tether and/or said connecting means. Said carriage may allow movement along the axis of the connection. The carriage may also allow for lateral movement. 
         [0014]    Said pull-in means may comprise a winch. 
         [0015]    In a further aspect of the invention there is provided a method of mooring an object to the seabed using the tether connection apparatus of the first aspect of the invention, said method comprising:
       installing a connector on said tether connection apparatus;   installing at least one anchor on the seabed, said anchor having a first portion of mooring tether attached thereto.   installing said tether connection apparatus to the free end of a second portion of mooring tether, the other end of which being attached to the object to be moored;   connecting the free end of a second portion of mooring tether to said connector;   deploying said free end of the second portion of mooring tether with said tether connection apparatus attached thereto;   using said pull-in means to pull in the free end of said first portion of mooring tether to said tether connection apparatus;   connecting the free end of said first portion of mooring tether to said connector, thus interconnecting the first and second portions of mooring tether; and   releasing the connector from said means for releasably mounting a connector.       
 
         [0024]    Although some of the above steps rely on other steps being completed, it should be apparent to the skilled person that not all the steps necessarily need be done in the above order, and that some steps may be done simultaneously with others. 
         [0025]    The step of installing said connector on said tether connection apparatus may be done as an initial step such that the connector is pre-installed on said tether connection apparatus. In this case said connecting of the free end of a second portion of mooring tether to said connector may be done prior to the deployment step, for example when installing said tether connection apparatus to the free end of a second portion of mooring tether. Some or all of these steps may be done on an installation vessel. Equally, the free end of a second portion of mooring tether may be connected to said tether connection apparatus before installation of the connector. 
         [0026]    Alternatively the step of installing said connector on said tether connection apparatus may be done after deployment of the tether connection apparatus. In this case the connecting of the free end of a second portion of mooring tether to said connector will also be done after deployment, once said connector has been installed on said tether connection apparatus. 
         [0027]    Said first and second portions of mooring tether may consist of chain or wire. In one embodiment said first portion consists of chain and said second portion consists of wire. 
         [0028]    One or more of the steps including and subsequent to the deployment step may be done by a Remotely Operated Vehicle (ROV). 
         [0029]    Said method preferably comprises the final step of recovering said tether connection apparatus. This may be done by an ROV. 
         [0030]    Said method may further comprise the step of aligning one portion of the mooring tether with the other prior to their being interconnected. In one embodiment it is the free end of first portion of the mooring tether that is aligned relative to the free end of said second portion of the mooring tether and connector. Said free end of first portion of the mooring tether may be pulled into a channel comprised in said tether connection apparatus to correctly locate and align said free end, for subsequent interconnection. Said channel, and therefore said portion of said mooring leg located therein, may be moved relative to second portion of the mooring tether and said connecting means to aid alignment. 
         [0031]    Said connector may comprise a shackle with associated pin, spelter sockets, chain links or triplates. 
         [0032]    Said first portion of mooring tether may be pre-routed on the seabed prior to it being pulled-in to said tether connection apparatus. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    Embodiments of the invention will now be described, by way of example only, by reference to the accompanying drawings, in which: 
           [0034]      FIG. 1  shows (a) a plan of a production vessel mooring arrangement, and (b) a side view of part of the arrangement; 
           [0035]      FIG. 2  shows a pull-in and connection device in accordance with an embodiment of the invention, and 
           [0036]      FIGS. 3   a  to  3   k  depict a method of installing a mooring leg using the device according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0037]      FIG. 1  of the accompanying drawings shows a typical schematic representation of an installation of a floating production vessel P moored in place via buoy B such that it can “weathervane” around the buoy B. The buoy B is held in place using 8 (eight) anchors  1  fixed to the buoy B by chains C of respective mooring legs and buried in the seabed so that they are equally spaced around the circumference of a circle centred on the vessel P. 
         [0038]      FIG. 2  shows a device  100  suitable for undertaking chain and wire pull-in and connection of mooring legs subsea. The mooring legs in this example are Chain-Wire, but could be Chain-Chain, Wire-Wire. or Wire-Chain. The device  100  comprises a frame  110 , an alignment system  120 , a connector  140  and shackle/socket pin  160 , preinstalled on said frame  110 , and a pull in winch  180 . Also shown are the first chain section  200   a  of the mooring leg  200  and the second wire section  200   b  of the mooring leg  200 . 
         [0039]    Considering the alignment system  120  of this embodiment in more detail, this comprises a carriage  220  movable along the frame  110  on tracks  240 . On the carriage is a funnel  260  leading to a narrow channel  270  with locating pins  280  therein. 
         [0040]    In use, the device  100  is pre-installed onto one end of one of the portions  200   b  of the mooring leg  200  to be connected. The device  100  has remote operated functions, controlled by, and powered from, a Remotely Operated Vehicle (ROV), for pull-in, alignment and locking of the other portion of the mooring leg, installation and securing of shackle/socket pin and release of connection from device  100 . Pull-in and connection operations can be performed on seabed or midwater. 
         [0041]      FIGS. 3   a  to  3   k  depict a method of installing a mooring leg  200  using the device  100  described above. To begin, the loading buoy that is to be moored is suspended by a crane, temporary suspended midwater and held in place by a deadman anchor on the seabed or held in position on the surface. The mooring anchor  1  is also installed on the seabed with its portion of the mooring leg  200   a  (in this case a chain, although it may be wire, rope or of other suitable construction) attached, this chain being pre-routed on the seabed such that its free end is in the vicinity of the buoy;
         FIG. 3   a  shows one end of the other portion of the mooring leg  200   b,  which had been previously abandoned on the seabed, being picked up by a Remotely Operated Vehicle (ROV)  310  and attached to the crane wire  315  of the installation vessel. (The other end of this portion of the mooring leg  200   b  is attached to the loading buoy).   In  FIG. 3   b , this end of the portion of the mooring leg  200   b  (which in this case is a wire, but may again be of other suitable construction) is lifted to the work platform  315  of the installation vessel, where it is attached by line  320  to a tugger  325 . In place on the installation vessel is device  100 . At this point buoyancy is removed from the portion of mooring leg  200   b.      The end of the mooring leg  200   b  is lifted further by the crane wire  315 , and with the catenary weight taken by holdback slings  330  and with the help of the tugger  325 , the end of the mooring leg  200   b  is lowered toward the device  100 .     FIG. 3   c  shows the end of the mooring leg  200   b  having been lowered into place on device  100 . The socket  335  at the end of the mooring leg  200   b  is then placed in the clevis of the device  100  and the socket  335  body is strapped down to the device  100 .     FIG. 3   d  shows the device  100  effectively installed to the end of the mooring leg wire portion  200   b.  On the work platform  315  of the installation vessel, a shackle and pin  160  with locking plate is installed to the socket  335  at the end of the mooring leg  200   b,  and an ROV releasable locking beam is used to secure the fixing to the device  100 .     FIG. 3   e  shows the tugger line  320  being disconnected, this being done after disconnection of the holdback slings and attachment of the device  100  to the crane wire  315 . The wire portion  200   b  of the mooring leg  200 , with the device  100  attached can now be deployed;     FIG. 3   f  shows the device having been lowered to about 1 metre above the seabed, and within about 20 metres of the end of the chain portion  200   a  to which it is to be connected. The pull-in wire/tether  340  from the winch  180  on the device  100  is attached to the free end of the chain mooring leg  200   a  (pre-routed on seabed, the other end being attached to the anchor);     FIG. 3   g  shows the ROV  310  using its torque tool to operate the winch  180  so that the pre-routed chain mooring leg  200   a  is pulled in to the device  100 , until the chain hits stops on the device  100 ; In doing so it pulls the chain  200   a  into a funnel or channel  260 . This funnel or channel  260  aligns the chain connector, in this case the final chain link  130 , laterally with the wire connector  140 . while the pins  280  locate in another chain link  250 , thus holding that link  250 , and as a result the connector  130 , vertical.   The ROV then uses its torque tool to lock and align the chain  200   a  in the device  100 . The winch wire  340  is then removed, while a stop is placed between chain links  1  and  3  to hold the chain in place.     FIG. 3   h  shows the ROV using its torque tool to move the alignment carriage  220  toward the shackle  160 . Precise longitudinal alignment can be made moving the carriage along the frame until the end of the chain  100   a  is aligned properly with the shackle  160 . This carriage may also be movable laterally to aid alignment.   Once aligned, each of the connectors are locked relative to each other to prevent misalignment during engagement of shackle/socket pin  160 ;     FIG. 3   i  shows the ROV using a pin insertion tool  370  to engage and secure the socket pin  160   b  into the shackle  160   a  and chain link  130  A C-plate can then secure the socket pin, making a permanent connection. Once made the surface winch line  350  can be attached to the connection, and the locking bar and straps attaching the connection to the device  100  can be released     FIG. 3   j  shows the connection being lifted by the winch clear of the device  100 . Device  100  is then recovered to the installation vessel.     FIG. 3   k  shows the ROV releasing the shackle connected to the socket, the connection now being complete.         
         [0056]    The above steps can then be repeated for the next mooring leg connection. 
         [0057]    The above embodiments are provided for illustration only, and other embodiments and variations are envisaged without departing from the spirit and scope of the invention. For example, the connector (a shackle and socket pin in the above example, although use of any suitable connector is envisaged) does not need to be pre-installed, and instead could be installed after deployment using an ROV. Furthermore, the above example is shown in relation to mooring a loading buoy, but other applications for this apparatus and method include, but are not limited to, the connections for the mooring lines of FPSOs (Floating, Production, Storage and Offloading vessels), rigs and riser systems. 
         [0058]    The alignment system shown, in particular, is subject to many variations and embodiments. For example, the pins  280  of the alignment system may be adapted should it be a wire portion of a mooring leg that requires pulling in and aligning. Said pins  280  may be adaptable, either by themselves or by way of the addition of an adaptor, so to be able to properly position both chain and wire.