Abstract:
A traction system ( 11; 60 ) for operating lines ( 18 ), in particular mooring lines and/or riser lines, of a vessel ( 1 ), includes at least two work stations ( 26 ) distributed along at least two sides of the vessel ( 1 ) and engaged by respective operating lines ( 18 ). The traction system includes a main traction device ( 14; 15, 17; 15 A,  17 ) and a transmission device ( 36 ) having a duty line ( 28 ) connected to the main traction device and at least one guide member ( 24 ) for selectively routing the duty line ( 18 ) to each work station ( 26 ) for attachment to a respective operating line ( 18 ). A line storing space ( 22; 42, 44 ) is provided for storing parts of the duty line and/or hauled in moorings lines; the main traction device, the transmission device and the line storing space are placed together at or near a side of the floating production unit.

Description:
FIELD 
       [0001]    The present invention relates to a traction system for operating lines of a vessel. Additionally, the present invention relates to a method for such a traction system. 
       BACKGROUND OF THE INVENTION 
       [0002]    From the prior art spread mooring systems for vessels such as floating storage offloading (FSO) or floating production storage offloading (FPSO) vessels or in general, floating offshore units are known. 
         [0003]    WO2010084420 discloses a method and system for tensioning a function line in particular a mooring line of a FPSO vessel. Such a traction method for an operating line, in particular a mooring line, includes the steps of: attaching an end chain portion of the operating line to a socket of a main cable running through a sheave at a work station; reeling in the main cable, using a winch, to bring the socket of the main cable up to the sheave; locking the operating line with a chain stopper; slackening the main cable and moving the sheave closer to the chain stopper to reduce pull on the main cable; reeling in the main cable to run the socket of the main cable through the sheave,—once the socket of the main cable has run through the sheave, releasing the operating line from the chain stopper, and reeling in, by means of the winch, the main cable and the operating line connected to it. 
         [0004]    WO2004050470 discloses a mooring windlass/winch system that provides a horizontal shaft type windlass wildcat integral with a wire rope type winch mounted above and behind the windlass wildcat. The combined windlass/winch is mounted on a circular foundation that permits the windlass to rotate in the horizontal plane about the centerline of the windlass chain locker. The ability to orient the windlass and winch in the horizontal plane allows a single windlass to serve a number of mooring chains. 
         [0005]    Further as shown in  FIG. 1 , a mooring system  10  is known from the prior art that comprises a (hydraulic) power unit  12 , a chain handler  14  and a number of turn-down sheaves  16 . The turn-down sheaves  16  are arranged at a side of the vessel  1  for example an FPSO. The power unit  12  is arranged to power the chain handler  14 , that is capable of pulling or paying out the individual mooring legs or chains  18 . The mooring legs  18  run from the chain handler  14  over each of the turn-down sheaves  16 . The chain handler  14  is at a fixed position towards the centerline  2  of the FPSO and away from the turn-down sheaves  16  in such a way that each mooring leg is under a different angle β with the chain handler. Due to the different angles, pulling characteristics on each mooring leg  18  are different, which require a relatively wide power range for the chain handler  14 . 
         [0006]    It is an object of the invention to overcome the disadvantages from the prior art. 
       SUMMARY OF THE INVENTION 
       [0007]    The object is achieved by the traction system as defined in claim  1 . Advantageous embodiments are further defined by the dependent claims. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0008]    The invention will be explained in more detail below with reference to drawings in which illustrative embodiments of the invention are shown. It will be appreciated by the person skilled in the art that other alternative and equivalent embodiments of the invention can be conceived and reduced to practice without departing from the true spirit of the invention, the scope of the invention being limited only by the appended claims. 
           [0009]      FIG. 1  shows a layout of a deck portion of a FPSO vessel with a traction system according to the prior art; 
           [0010]      FIG. 2  shows a layout of a deck portion of a FPSO vessel with a traction system according to an embodiment of the invention; 
           [0011]      FIG. 3  shows a layout of a deck portion of a FPSO vessel with a traction system according to an embodiment of the invention; 
           [0012]      FIG. 4  shows a top view of a traction system according to an embodiment comprising a chain jack skid that is moveable above fairleads; 
           [0013]      FIG. 5  shows a side view cross section of a traction system according to an embodiment comprising a chain jack skid that is moveable above fairleads; 
           [0014]      FIG. 6  shows a side view cross section of a traction system according to a further embodiment; 
           [0015]      FIG. 7  shows a front view cross section of a traction system according to an embodiment comprising a module having a chain jack skid and fairleads combined; 
           [0016]      FIG. 8  shows a cross-section of an external chain locker protruding from a side of a vessel; 
           [0017]      FIG. 9  shows a side-view of horizontal acting chain jack skid system; 
           [0018]      FIG. 10  shows a traction system according to an embodiment of the invention in a perspective view; 
           [0019]      FIG. 11  shows a traction system according to an embodiment of the invention in a side view; 
           [0020]      FIG. 12  shows a traction system according to an embodiment of the invention in a cross-sectional view; 
           [0021]      FIG. 13  shows a traction system according to an embodiment of the invention in a top view; 
           [0022]      FIG. 14  shows a chain jack system for the traction system according the invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0023]    The present invention relates to a traction system for a vessel. Such a vessel may be a floating production storage offloading unit (FPSO) but can also relate to a floating offshore unit or a floating production unit (FPU), or a floating storage offloading unit (FSO). Without limitation of the scope of the invention, floating offshore unit may also comprise drilling vessels, semi-subs, SPARS, FSRU&#39;s, Otec systems, floating power plants, floating hotels. 
         [0024]    A spread moored FPSO deck space is a congested area with all kinds of processing units. Typically there is no space for winches and tensioned cables connected over the deck via sheaves to mooring lines and production lines (like risers) for hauling in or paying out operations. 
         [0025]    A floating offshore unit such as an FPSO is provided with at least one, preferably four traction systems according the invention. The FPSO is for example provided with four mooring tensioning systems (or traction systems) which are located on one of the four mooring porches of the FPSO. There will be one porch on each corner of the vessel: Port-Aft, Port-Fore, Starboard-aft, Starboard-Fore. The mooring tensioning or traction systems are subjected to the FPSO&#39;s motions and to the environmental conditions while in operation. 
         [0026]    Also, during the mooring operation of the FPSO, the traction system provides the tensioning of the mooring legs by connecting with the external mooring leg chains. Typically, each mooring leg chain is coupled to a duty line such as an installation chain that is arranged within the traction system. Each mooring leg runs from the sea level up through a fairlead to the deck level of the vessel. Next, the traction system tensions the mooring leg chain and brings it into a fixed position on the vessel&#39;s side. Subsequently, the duty line is repositioned to haul-in and tension a next mooring leg chain. 
         [0027]    Therefore the FPSO is provided with a traction system which comprises a locker space for storing parts of the duty line and/or hauled in moorings lines, a main traction or tensioning device, a transmission device which are all are placed together at or near a side of the floating production unit. 
         [0028]    It is preferred that the main traction device is placed between the locker space and the workstation to which it is connected in operation and that the main traction device and/or the guide member is moveable over a track placed along a side of the vessel. The traction system is movable over rails or a track over a workstation that comprises at least two fairleads each capable of receiving and holding a mooring line. Preferably, the locker space is an extension at the side of floating production vessel. Further the main traction device, the workstation and the locker space can be combined in one module to facilitate a quick integration of it into the floating production unit. 
         [0029]    In an embodiment, the module of the traction system is arranged to be mounted on and fixed to a vessel&#39;s deck. Advantageously, the module can be prefabricated after completion positioned and mounted on the vessel. 
         [0030]    Now with reference to the  FIGS. 3-14  components of the traction system will be discussed in detail. 
         [0031]      FIG. 2  shows a top view of a traction system  11  on a vessel deck  1 , which traction system comprises a chain locker space  22 , a traction device  14 , a guiding element  24 , a workstation  26  with four turn-down sheaves  16 . The chain locker space, traction device, guide sheave guiding element are grouped together at a aside of the vessel  1 , close to the workstation  26 . Each turn-down sheave  16  is configured to guide the mooring leg chain  18  from an upper opening end  40  of a fairlead  38  (not shown) to the traction device  14 . Over the turn-down sheaves  16  the mooring leg chains each can connect to a duty line  28  (an installation chain). The duty line  28  is extending along the guide sheave guiding element  24  and the traction device  14  into the chain locker space  22 . In an embodiment, the traction device  14  is a chain tensioner  15 , as will be described in more detail below. 
         [0032]    The guiding element  24  is a circular element that allows guiding the duty line  28  over each of the turn-down sheaves  16 , each time under a respective directional angle β. 
         [0033]    The traction device  14  further comprises a rail track  30  for a skidding unit  32 . The rail track  30  runs substantially parallel to the sheaves  16  in the workstation  16 . The guide sheave guiding element  24  is arranged on the skidding unit  32  and can move  34  along the track to be positioned relative to a selected turn-down sheave  16 . In this manner, the directional angle for each mooring leg chain on its respective turn-down sheave is substantially the same. 
         [0034]      FIG. 3  shows a top view of a traction system  11  according to an embodiment. The traction system comprises a chain locker space  22 , a traction device  14 , a guiding element  24 , a rail track  30 , a skidding unit  32  and a workstation  26  with for example four turn-down sheaves  16 . The chain locker space, traction device, guide sheave guiding element are grouped together at a aside of the FPSO, close to the workstation  26 . The rail track  30  runs substantially parallel to the sheaves  16  in the workstation  26 . The traction device  14  is arranged on the skidding unit  32  and can be moved  34  along the track to be positioned in a relative same position and orientation with respect to a selected turn-down sheave. For each selected turn-down sheave, the directional angle of the mooring leg chain is substantially the same. 
         [0035]    At the side of the chain locker space  22 , the traction system  11  additionally comprises a transmission device  36  for transmitting the duty line or chain  18 ;  28  to and from the chain locker space  22 . In an embodiment, the transmission device  36  may be a motorized chain wheel, which also provides the option to maintain a pre-tension on the chain  18  between the motorized chain wheel  36  and the traction device  14 . 
         [0036]      FIGS. 4 and 5  show a top view and a side view respectively of a traction system according to an embodiment. The traction system comprises a chain locker space  22 , a traction device  14 , a guiding element  24 , a rail track  30 , a skidding unit  32 , a transmission device  36  and a workstation  26  with for example six fairleads  38 . In this embodiment, the rail track  30  runs parallel to and above the upper end openings  40  of the fairleads  38 . In stead of using dedicated turn-down sheaves, the traction device  14  is an arrangement of a chain tensioner  15  and a non-driven chain wheel  17  as chain guiding element, which arrangement is mounted on the skidding unit  32  and can be moved  34  along the skidding unit&#39;s rail track  30 . The traction device  14  is configured to be positioned above a selected fairlead  38  such that the non-driven chain wheel  17  guides the chain  18  from the chain tensioner  15  into the upper end opening  40  of the selected fairlead  38 . 
         [0037]    At the side of the chain locker space  22  the traction system additionally comprises the transmission device  36  for transmitting the duty line or chain to and from the chain locker space  22 . In an embodiment, the transmission device may be a motorized chain wheel  36 , which also provides the option to maintain a pre-tension between the motorized chain wheel  36  and the traction device  14 . 
         [0038]    In this embodiment, the chain locker space  22 , the motorized chain wheel  36 , the chain tensioner  15  and the fairleads upper end openings  40  are advantageously in-line with each other which reduces lateral forces on the traction system. 
         [0039]      FIG. 5  shows the side view of a general arrangement of traction system (chain tensioning system) according to the invention. In this embodiment, the traction system comprises a chain jack skid  15 ,  32  that is moveable  34  above fairleads  38 . It also shows the position for the motorized chain wheel  36  when over-boarding chain  18  from the chain locker space  22 . 
         [0040]      FIG. 6  shows a side view of the traction system of  FIG. 6  according to a further embodiment. 
         [0041]    In this embodiment the traction system alternatively or additionally comprises a balcony  42  that is arranged at an outer end  23  of the traction system (adjacent to the chain locker  22  if present) in a direction in-line with the rail track  30 . The balcony  42  is located outside of the vessel&#39;s hull above water  50 . 
         [0042]    The rail track  30  is extended over the chain locker  22  and over at least a part of the balcony  42 . The motorized chain wheel  36  is moveably mounted on the rail track  30  to be positioned over the at least part of the balcony  42 . The balcony is provided with means  44  for attaching to an end of the chain  18  and for hanging off the attached chain in one or more loops thus configuring the balcony for storing the chain. In this manner, the chain  18  can hang-off from the balcony above or into the water. 
         [0043]    By storing the chain on the balcony, the balcony is thus equivalent to a chain locker space. In an embodiment, the chain locker space can be omitted and replaced by the balcony alone. 
         [0044]      FIG. 7  shows a cross-section of the traction system  11  with a chain  18  running from the chain tensioner  15  through one of the fairleads  38 . In an embodiment, the chain jack skid  15  and fairleads  38  are combined in a module  46  attached to the vessel&#39;s hull  48 . 
         [0045]      FIG. 8  shows a cross-section of a chain locker  22  external to the vessel&#39;s hull  48 . 
         [0046]      FIG. 9  shows a schematic side view of the arrangement of the chain tensioner  15  and the non-drive chain wheel  17  on a skidding unit  32  on the rail track  30 . 
         [0047]    The skidding unit  32  allows to position the arrangement  14 ;  15 ,  17  relative to a selected fairlead  38 . The chain tensioner  15  is arranged as an horizontally acting chain jack, which at the sea-going end is coupled to the non-driven chain wheel  17 . The chain wheel  17  guides the chain  18  into the fairlead  38  over which the chain wheel  17  is positioned. 
         [0048]      FIGS. 10-13  show a traction system according to an embodiment of the invention in a perspective view, side view, cross-sectional view and top view, respectively. 
         [0049]    In this embodiment, the traction system  60  is a modular construction that is mounted at a side of the vessel  1  over the workstation  26 . The modular construction  60  comprises a chain locker space  22 , a traction device  14 , a guiding element  24 , a rail track  30 , a skidding unit  32 , a transmission device  36  and a plurality of support beams  62 . Optionally, the chain locker  22  comprises or is replaced by a hang-off balcony  42  as described above in more detail. 
         [0050]    By means of the modular construction  60 , the traction system can be prefabricated and mounted in completed form on a vessel&#39;s deck near the workstation  26  handling the mooring leg chains  18 . Moreover, by positioning the modular construction  60  above the fairleads  38  the area on the vessel  1  taken up by the traction system can be minimized. 
         [0051]    In the modular construction  60  the plurality of support beams  62  are arranged parallel to each other separated by intermediate spaces. The rail track  30  is mounted on the parallel support beams  62  that run perpendicular to the length of the rail track. Further the support beams  62  are arranged to extend with their free ends  64  on one side of the rail track. In between the rails of the rail track  30  openings  40  are arranged for the fairleads  38  in the workstation  26 . At one end of the rail track  30  the chain locker  22  is located, with the rail track  30  extending over the top of the chain locker  22 . In case a hang-off balcony  42  is mounted on the modular construction  60 , the rail track  30  is configured to extend over at least a part of the balcony area. On the rail track  30  a motorized chain wheel  36  is positioned above either the chain locker  22  opening  23  or the balcony area  42 . Preferably, the motorized chain wheel  36  is mounted on an additional skidding unit that can be moved along the rail track  30 . 
         [0052]    In this embodiment, the traction device  14  can be either the arrangement of a horizontally acting chain tensioner  15  and the non-driven chain wheel  17  as chain guiding element as described above with reference to  FIGS. 4-6 . Alternatively, the traction device  14  comprises a combination of a vertically acting chain tensioner  15 A and non-driven chain wheel  17  as shown here in  FIGS. 10-13 . 
         [0053]    In a further embodiment, the traction system comprises an auxiliary winch  66  at the end of the rail track  30  opposite the motorized chain wheel  36 . The functions of the auxiliary winch  66  are described in more detail below. 
         [0054]      FIGS. 12 and 13  show a cross-sectional view and top view of the traction system  60 , in which the free ends  64  of the support beams  62  are mounted to a vessel&#39;s deck  1 , for example by welding. The rail track  30  holding the traction device  14  and chain wheels  17 ,  36 , and the chain locker  22  and/or hang-off balcony  42  are extending outside the vessel&#39;s hull  48  above water  50 . 
         [0055]      FIG. 14  shows a side view of a chain jack skid system  15 . The chain jack skid system  15  comprises a chain jack base frame  151  with a lower ratchet  152  and a lifting frame  153  with an upper ratchet  154 . By means of parallel hydraulic actuators  155  that couple the base frame  151  with the lifting frame  153 , the lifting frame can move relative to the base frame. By the movement and coordinated opening and closing of the lower and upper ratchets  152 ,  154  a chain (not shown) can be pulled or paid out by the chain jack skid system. 
         [0056]    Below, a further description of components of the traction system is given. 
       Components 
       [0057]    Hydraulic Power Units 
         [0058]    Provided on the FPSO are also two electric powered hydraulic power units (HPU&#39;s) to operate the equipment of the complete tensioning systems. After the installation period, when the tensioning systems are not operated any more, the HPU&#39;s can be used to supply power to other equipment like hose reels. Therefore, these operations are not simultaneous. 
         [0059]    During the installation period, the power consumers comprise of the 4 mooring tensioning systems located Aft &amp; Fore, PS &amp; SB side on the vessel, that are controlled by their own local control panels. 
         [0060]    One HPU will be located forward of the FPSO for operation of forward PS and SB systems, and the other HPU will be located aft of the FPSO for operation of aft PS and—SB systems. 
         [0061]    The maximum distance between HPU and consumers is 90 m. The consumers within each mooring tensioning system are (as a minimum): chain tensioner, motorized chain wheel, skidding system and auxiliary winch. 
         [0062]    Three separate mooring tensioning operating cases are distinguished for each HPU. Each HPU operates 1 out of 2 connected chain tensioner systems at a time. All motions are enabled to be operated at their maximum specified loads and speeds:
       Chain tensioning operations: dumping chain into the chain locker, operating the chain tensioner and the motorized chain wheel and the skidding unit (while positioning the motorized chain wheel for chain spreading)   Relocation operations: operating the skidding unit to relocate the (non-operating) chain tensioner.   Chain overboarding from the locker: operating the auxiliary winch, the motorized chain wheel and the skidding unit for moving the motorized chain wheel.       
 
         [0066]    Chain Tensioner 
         [0067]    The chain tensioner (traction) system is designed to handle and tension the mooring chains. 
         [0068]    The tensioner stroke is preferable horizontal but can also be vertical. Via a non-driven chain wheel (integrated in the chain tensioner skid), the chain will be lowered vertical. 
         [0069]    The chain tensioner skid is used during the initial mooring chain installation and (re-)tensioning operations. The chain tensioner is capable of pulling-in and paying-out a loaded chain up to the stall pull rating. The chain tensioner lifts two chain links per cylinder cycle. The upper and lower ratchets hold only the links that are in a “horizontal” orientation, the vertical links can slide through. 
         [0070]    The chain tensioner components are mounted onto a skid that runs over a rail. The rail is (preferably) welded onto the foundation top flange. 
         [0071]    The chain tensioner is movable by a skidding unit, typically to be relocated from one mooring leg to the next one. Due to the preferred horizontal orientation of the chain tensioner, sufficient guides shall be integrated in the chain tensioner, in line with all ratchet entries, to properly guide the chain into the ratchets. 
         [0072]    The chain is guided from vertical to horizontal direction via the non-driven chain wheel. 
         [0073]      FIG. 14  shows the jacking cylinders, the lifting frame and chain ratchets. The required minimum effective cylinder stroke of the chain tensioner system is for example two chain links. The jack&#39;s stroke includes an additional to ensure that the opening chain ratchets are able to close and engage on the proper link. The hydraulic system distributes the load equally between the two jacking cylinders. 
         [0074]    Since the jacking cylinders are preferably positioned horizontally, they are designed to take lateral (vertical) loadings from dead weights of chain catenary and upper ratchet, also when induced at full extension of the cylinders. The chain tensioner contains a flow control device to be used by the operator to control the chain pull-in speed and the direction of the cylinder stroke. 
         [0075]    A lifting frame is mounted at the ends of the cylinder rods and is designed to support the upper ratchet. It is possible to take the chain out of the lifting frame (upwards) without cutting the chain. 
         [0076]    The horizontal chain tensioner system houses two chain ratchets, “upper” and “lower”. The ratchets have both the same orientation, to allow the chain to be lifted and taken out of the system in upward direction. 
         [0077]    The ratchets are designed for the same operational loads. The function of the chain ratchets is to act in tandem with the reciprocating chain jacks as a hand-over-hand pulling system. A device is installed to hold the chain in position while the jacking cylinders are retracted (downward) for the next stroke upward (normal tensioning mode). Same applies for releasing of chain, however vice versa. 
         [0078]    The chain ratchets are machined to match the profile of the chain and are activated by the double acting hydraulic cylinders. The ratchets automatically open and closes when pulling or lowering chain. 
         [0079]    Each retract cylinder are provided with a flow control device on the chain jack to control the opening/closing of the ratchets. The hydraulic power supply for operating the upper and lower set of ratchets is obtained from the main HPU supply. 
         [0080]    Counterbalance valve(s) or other safety systems are provided on the chain jack to prevent the jacking cylinders from lowering the load, or to open the ratchets, due to a loss of hydraulic pressure. 
         [0081]    When pulling or lowering a chain towards or from the locker, the chain will enter the chain tensioner in horizontal position. In order to guarantee proper chain alignment in the ratchets, the chain is guided by a supporting device. Also inside the chain tensioner, in between the two ratchets, the chain has to be supported to guarantee the chains remain correctly positioned. There is pretension to be generated by the motorized chain wheel in order to guarantee proper pulling and lowering of the chain. 
         [0082]    The frame of the chain tensioner systems functions as a base for the jacking cylinders, the lower chain ratchet and the chain wheel, in order to skid over the track, and to equally distribute the loads over the track foundation. Skidding pads are mounted below the frame 
         [0083]    When the chain tensioner is located in its correct position on the skid track, it is locked in position by application of locking pins on the frame, to be lowered into the rail profile. 
         [0084]    Skidding System 
         [0085]    The skidding system consist of an automatic hand over hand push and pull unit. It is equipped with 2 sets of hydraulic rail grippers (one set clamped while the other set is moving) and sliding pads to suit the skid rail. It has (manually) pinned connections at both ends to connect to the motorized chain wheel and to the chain tensioner system. The skid system can actuate 2 motions:
       For the chain tensioner: It enables the chain tensioner skid frame to be skidded from one mooring chain position to the next over the complete length of the track (except above the chain locker). The push/pull unit only needs to transport the chain tensioner when the chain is unloaded.   For the Motorized Chain Wheel: It enables the motorized chain wheel frame to be skidded lengthwise above the chain locker, to be able to distribute the chain in the locker. Maximum horizontal load during skidding is for example 15 tons.       
 
         [0088]    The chain tensioning system&#39;s components can be skidded over the skidding track. This track is to form one continuous skid track for all tensioning system&#39;s components: chain tensioner, skidding unit, and motorized chain wheel. There are 2 rails for each track (in total 8 rails). 
         [0089]    The length of total skid track is as for example 35 m and material of rail is to be weldable steel. Mating pad materials and designs can be selected to suit the rail. 
         [0090]    Purpose of rails is to facilitate skidding of chain tensioner from one mooring leg to the next. The chain tensioner is to be (manually) pin-locked above its mooring leg positions. 
         [0091]    The rails also facilitate skidding of the motorized chain wheel above the chain locker in order to evenly distribute the chain in the locker. Both the motorized chain wheel skid and the skidding system itself are then “clamped” to the track only by grippers of the skidding system. 
         [0092]    The rails also facilitate skidding of the motorized chain wheel ‘towards the edge’ of the porch in order to lower the chain to a supply boat. The motorized chain wheel is then to be (manually) pin-locked in this position. The rails are used for vertical support but also for horizontal guiding of the skids and for the skidding unit. 
         [0093]    The skidding system is an easily manually pin-connected/disconnected at both ends to the chain tensioner skid and the motorized chain wheel skid. 
         [0094]    When the chain tensioner skid and the motorized chain wheel skid are located in their correct working positions or in their sea-fastening positions, all skids are able to be manually pin-connected to the rail track, in order to withstand the vessel motions. 
         [0095]    The main functions of motorized chain wheel are:
       A Lift the chain from and into the locker. In order to properly distribute the chain into the chain locker, the motorized chain wheel is skidded above the locker by means of the skidding unit.
 
Skidding of the motorized wheel will as a base case occur while the motorized wheel is keeping the chain (catenary) running towards the chain tensioner under tension. The motorized chain wheel will also be used for lifting the chain dead weight from the locker, combined with “digging out” of buried chain lengths, and feeding this chain towards the chain tensioner.
   B Provide pre-tension. The motorized chain wheel is used to guide the chain from the chain-locker in the direction of the chain tensioner and vice versa, when the chain is lifted or lowered via the chain tensioner system. The system is able to maintain the required pretension level. regardless of:   C Hand over the excess chain to a tugboat. The system shall be able to lift a length of excess chain (typically 60-80 m) from the chain locker and lower it towards a tugboat.       
 
         [0099]    Auxiliary Winch 
         [0100]    An auxiliary winch can mounted at the end of the skid track. The main functions are:
   1 Chain handling towards the chain tensioner skid. The installation chain can lifted from the chain locker with the aid of an auxiliary winch, and pulled over the Retractable chain-support tables and through the chain tensioner, so that it can be spooled over the non-driven chain wheel. The installation chain will then be connected (at deck level) to a messenger chain, that is pre-installed through the I-tubes. Load on the auxiliary winch rope is created when pulling the chains, which are supported by the retractable chain-supports to avoid sagging. Total estimated pull load can be 15 ton.   2 To handover the excess chain to a tugboat. The motorized chain wheel will therefore be positioned at the end of the track. Pieces of 60-80 m length of excess chain will be lowered towards the tugboat. When nearly all chain is outboard, the auxiliary winch helps by pulling the chain when it leaves the motorized wheel. The auxiliary rope will then run from the auxiliary winch, via the motorized wheel, to the sea level, to be pulled in by the tugboat&#39;s winch. Onboard the tug, the rope will be released.   
 
         [0103]    Local Control Panel 
         [0104]    Each chain tensioner system may have a local (portable) control console: one console on each porch. Each control panel is required to control the chain tensioner including ratchets, the skidding system, the motorized wheel, the auxiliary winch and the appertaining HPU (one HPU for two clusters). 
         [0105]    It will be apparent to the person skilled in the art that other embodiments of the invention can be conceived and reduced to practice without departing form the true spirit of the invention, the scope of the invention being limited only by the appended claims as finally granted. The description is not intended to limit the invention.