Abstract:
Disclosed are methods for replacing a used wire rope segment or for installing a new wire rope segment in a mooring line useful for mooring a floating vessel such as a floating storage and offloading (FSO) vessel. The methods utilize platforms adapted to be mounted onto anchor handling vessels (AHVs) for severing the used wire rope segment and connecting the new wire rope segment thereon.

Description:
FIELD 
       [0001]    The present disclosure relates to methods for installing or removing and replacing a segment of a mooring line, such as used to moor floating storage and offloading vessels. 
       BACKGROUND 
       [0002]    Floating storage and offloading (FSO) vessels keep station using a system of mooring lines terminating at anchoring means, such as suction piles. The mooring lines typically include multiple segments, including at least one chain segment at the anchor and, one chain segment at the FSO, and a wire rope segment there between. The wire rope is made up of multiple strands of steel and is subject to corrosion in a subsea environment over time. For instance, a wire rope having a diameter of about 4 inches may have a design life of about 7-10 years after which the wire rope must be decommissioned or removed. The entire mooring line can be replaced; however, this is extremely costly since the field production has to be shutdown to allow welding on the FSO. In order to replace the wire rope segment of the mooring line, the conventional way is to send divers down to seafloor to disconnect the old wire rope from the adjacent chain segments and connect a new wire rope to the adjacent chain segments. The divers perform the necessary subsea cuts and connections while on the seabed in poor visibility, and manually perform all related physical tasks including lifting. These are dangerous because of the weight and tension on the steel components. If weather turns severe, the divers may be trapped in the subsea environment until conditions are calmer that the boat could resume diving support. 
         [0003]    It would be desirable to have a safer method to extend the life of the mooring line by replacing the old wire rope segment which would not require divers to perform operations subsea. 
       SUMMARY 
       [0004]    In one aspect, a method is provided for removing a used segment of a mooring line located on a seabed and replacing the used segment with a new mooring line segment used for mooring a floating vessel. The method includes positioning at a first location above a first end of a mooring line segment to be removed an anchor handling vessel (AHV) having a platform for receiving a mooring line retrieved from the seabed. A lifting line which is secured on one end on the AHV is attached to the mooring line, and the mooring line is retrieved from the seabed. The mooring line is placed on the platform where the line is severed to form a first mooring line severed end and a first segment severed end. The AHV is navigated to a second location above a second end of the mooring line segment, while the mooring line segment to be removed is wound onto a used mooring line reel on the AHV. At the second location, the mooring line is severed to form a second mooring line severed end and a second segment severed end, and the second segment severed end is wound on the used mooring line reel. The method further includes on the platform at the second location, connecting a first end of a new mooring line segment wound on a new mooring line reel on the AHV to the second mooring line cut end. The AHV is then navigated to the first location, while the new mooring line segment is laid down onto the seabed. At the first location, a second end of the new mooring line segment is connected to the first mooring line cut end on the platform, and the mooring line including the new mooring line segment is lowered to the seabed. 
         [0005]    In another aspect, a method is provided for installing a new mooring line segment in a mooring line on a seabed used for mooring a floating vessel. The method includes positioning at a first location an AHV having a platform adapted to support a mooring line. A free end of a first mooring line lead is placed on the platform, and a first end of a new mooring line segment is connected to the free end of the first mooring line lead on the platform. The AHV is then navigated to a second location while the new mooring line segment is laid down onto the seabed. At the second location, a second end of the new mooring line segment is connected to a free end of a second mooring line lead on the platform. Finally, the mooring line including the new mooring line segment is lowered to the seabed. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0006]    These and other objects, features and advantages of the present invention will become better understood with reference to the following description, appended claims and accompanying drawings where: 
           [0007]      FIGS. 1A-1B  are perspective views of a platform according to one embodiment. 
           [0008]      FIG. 2  illustrates an AHV according to one embodiment. 
           [0009]      FIG. 3A  is a flow chart of method steps for replacing a mooring line segment according to one embodiment. 
           [0010]      FIG. 3B  illustrates various mooring chain and mooring wire combinations used in the methods and systems of the present disclosure. 
           [0011]      FIGS. 4A-4N  illustrate method steps for replacing a mooring line segment according to one exemplary embodiment. 
           [0012]      FIGS. 5A-5B  are aerial views of a platform according to one embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    A floating vessel can be moored by a mooring line connected to an anchor according to conventional practice. One end of the mooring line is commonly attached to the floating vessel, and the other end of the mooring line is attached to an anchor such as a pile driven into the seabed. The mooring line can be attached to the floating vessel using any convenient arrangement, e.g., at a turret at the bow of the floating vessel. The mooring line includes three segments, a first chain segment attached to the floating vessel, a wire rope segment attached to the first chain segment and a second chain segment attached to the wire rope segment and the anchor. The wire rope segment is substantially on the seabed. Over time, the wire rope segment is subject to corrosion and aging and must be replaced or decommissioned. 
         [0014]    The present disclosure will describe methods and apparatus for replacing a used wire rope segment of a mooring line, as well as methods and apparatus for installing a new wire rope segment. 
         [0015]      FIGS. 1A-1B  are perspective views of embodiments of an apparatus also referred to as a platform  100  useful for carrying out the methods described herein. The platform  100  which can be formed of carbon steel is configured to support and receive a mooring line (not shown) along its length. The platform  100  includes a frame  114  and has a first end, also referred to as the chain end, having a chain chute  110  therein for supporting a length of chain hanging off the first end of the platform. The platform has a second end, also referred to as the wire rope end, which is movable relative to the first end having a wire rope chute  116  therein for supporting a length of wire rope hanging off the second end of the platform. In one embodiment, the wire rope chute  116  has rollers  112  mounted on the wire rope chute  116  adapted to contact a mooring line segment, e.g., a segment of wire rope, passing over the wire rope chute.  FIG. 1A  illustrates pairs of rollers  112  having a vertical axis of rotation, which the wire rope can pass between.  FIG. 1B  illustrates horizontally oriented rollers  112  which the wire rope can pass over. In either embodiment, the rollers  112  mounted in the wire rope chute  116  of the platform  100  serve to protect a mooring line segment e.g., a segment of wire rope, during the winding up of a mooring line segment from the seabed or the laydown, also referred to as the payout, of a mooring line segment on to the seabed. 
         [0016]    A chain stopper  118  movable between a chain locking position and a released chain position is provided to enable locking the chain in a position securely on the platform  100 . In the chain locking position, the chain stopper  118  protrudes from the surface of the chain chute  110  and engages a chain link. In the released chain position, the chain stopper  118  is receded below the surface of the chain chute. The position of the chain stopper relative to the surface of the chain chute is controlled by a control mechanism such as a hydraulic piston (not shown). 
         [0017]    Similarly, a wire socket stopper  120  movable between a wire rope locking position and a released wire rope position is provided to enable locking the wire rope in a position securely on the platform  100 . In a protruded locking position, the wire socket stopper  120  can engage a fixed element along the wire rope such as a connector also referred to as a union, e.g. a wire socket, at the end of a wire rope. The position of the wire socket stopper  120 , either in the protruded locking position or the receded released position, is controlled by a control mechanism such as a hydraulic piston (not shown). 
         [0018]    The platform  100  utilizes a positional control mechanism for controlling the position of the chain end relative to the wire rope end. The positional control mechanism can be any appropriate means as would be apparent to one skilled in the art, such as a hydraulic piston (not shown), in which case the stroke distance of the piston determines the range of separation between the chain end and the wire rope end. When the chain stopper  118  and the wire socket stopper  120  are engaged to lock a chain segment and a wire rope segment of a mooring line in position on the platform  100 , the piston can be used to control the amount of tension in the mooring line. Hydraulic pistons if present can be connected to a hydraulic power unit on the AHV on which the platform  100  is mounted as would be apparent to one skilled in the art. 
         [0019]    The rollers  112  on the wire rope chute protect the wire rope as it passes over the platform  100 . This protects a new wire rope segment as it is being laid down from abrasion and damage caused by contact with sharp edges and the like. The rollers  112  also prevent the wire rope from kinking and bending excessively. In addition to protecting the wire rope, the rollers also facilitate smooth movement of the wire rope as a used wire rope segment is being retrieved from the seabed or a new wire rope segment is being laid down, and as the AHV is moving from one location to another. 
         [0020]    In order to monitor the wire rope passing through the wire rope chute of the platform, the mooring chain passing through the chain chute of the platform, or the connecting and severing operations on the platform, one or more video cameras  115  may be mounted on the platform  100  or a nearby structure as would be apparent to one skilled in the art. The video camera can be part of a closed-circuit television system which can be monitored by persons remotely. This can reduce the need persons on the platform, thus enhancing the safety of operations. The video camera can also be used to monitor the lowering of a mooring line once a new wire rope has been installed. 
         [0021]    In one embodiment, a curved deflector plate  104  is mounted on a support  102  adjacent the platform  100 . The deflector plate  104  can be used to direct or guide a used mooring line  24  from the wire rope chute  116  of the platform towards a used mooring line reel  52   b  located on the AHV  50  (as shown in  FIG. 2 ), or to direct or guide a new mooring line  23  from a new mooring line reel  52   a  located on the AHV  50 . The deflector plate  104  serves to change the direction of the line passing over it with a sufficient radius to protect the line, e.g., a new wire rope segment, and to prevent kinking and excessive bending. The diameter of the curved deflector plate  104  should be sufficiently large to prevent any excessive bending in the wire rope. For example, the diameter of the curved deflector plate  104  can be 20 times or more greater than the diameter of the wire rope. 
         [0022]      FIG. 5A  is an aerial view of platform  100  as mounted on the stern of AHV  50 . An end sheave  111  can be mounted adjacent the platform at one or at both ends of the platform, to hold a wire rope, e.g. new wire rope segment  23 , and a mooring chain, e.g. proximal mooring chain segment  21 , guide the wire rope and chain. The position of the end sheave  111  can be controlled in multiple directions using any convenient positional control mechanism is would be apparent to one skilled in the art. For example, a hydraulic cylinder (not shown) can be used to control the outboard distance  97  of the end sheave  111 , or the distance of the end sheave  111  to the AHV  50 . The angle of the end sheave  111  with respect to the horizontal can also be adjusted by rotating end sheave  111  with respect to axis of rotation  99 , so that the line coming from the end sheave  111  is properly aligned with the chain chute on the platform. Optionally, a second end sheave (not shown) can also be used on the AHV for guiding the mooring line (mooring chain) at the second end of the platform. 
         [0023]      FIG. 5A  illustrates a mooring line segment placed on the platform, including a portion of wire rope in the wire rope chute  116 , and a portion of chain in the chain chute  110 . As can be seen, the wire rope and the chain are in contact with rollers  112  in their respective chutes. Wire socket stopper  120  is engaged so that the wire rope is locked in place, and chain stopper  118  is engaged so that the chain is locked in place.  FIG. 5B  is an aerial view illustrating the platform  100  when the wire socket stopper  120  is moved towards the chain stopper  118 , thereby placing slack in the portion of the mooring line between 120 and 118. 
         [0024]      FIG. 2  illustrates an exemplary AHV  50  floating in water  1  with the platform  100  mounted thereon, useful for replacing and/or installing a wire rope segment of a mooring line. The platform  100  is advantageously mounted on the stern of the AHV  50 . 
         [0025]    The AHV  50  supports one or more reels  52   a ,  52   b  and  52   c  for storing and transporting new wire rope  23  to be installed, temporary working wire  25  to facilitate the disclosed methods and used wire rope  24  to be retrieved from the seabed, respectively. 
         [0026]    The AHV  50  can include an A-frame structure  108  (shown in  FIG. 4A ; for simplicity not shown in  FIG. 2 ) mounted at the stern of the AHV as shown in  FIG. 4A  to support gantry  106 . The gantry  106  also referred to herein as the lifting line, can include a block and tackle. The A-frame  108  can be detachable from the deck  54  of the AHV  50 . 
         [0027]      FIG. 3A  is a flowchart listing steps in a method  300  for replacing a used mooring line segment according to one embodiment. The AHV is initially positioned at a desired location above a first end, also referred to as the proximal end, of a mooring line segment to be removed. The mooring line segment to be removed can be a used wire rope segment  24  between a proximal mooring chain  20  and a distal mooring chain  21 , as shown in  FIG. 3B . The used wire rope segment  24  is also referred to herein as the old wire segment  24 . Each end of the used wire rope segment  24  has a connector also referred to as a union,  22   a  and  22   b , for securely connecting the wire rope segment  24  to the mooring chains  20 ,  21  on each end. The unions  22   a - 22   f  illustrated are for illustration purposes only. Any suitable connection means for securely connecting a wire rope segment to a mooring chain segment can be used. For example, shackles or H-links may be used as the unions, as would be apparent to one skilled in the art. 
         [0028]    In one embodiment, in step  310 , illustrated in  FIG. 4A , a winch wire  109  which is secured on one end on the AHV  50  is attached to the proximal mooring chain  20  by snaring a mooring chain link, so that the mooring chain can be retrieved from the seabed  3 . As illustrated, the proximal mooring chain  20  can be attached to a turret  32  on a FSO  30 . The winch wire  109  is attached to the mooring line by a remotely operated vehicle  28 . 
         [0029]    Again assisted by the remotely operated vehicle  28 , in step  312 , as illustrated in  FIG. 4B , the gantry  106  suspended from the A-frame  108  is mated to chain links of mooring chain  20 . Note, any other suitable crane and rigging can be used as would be apparent to one skilled in the art. The winch wire  109  then releases the mooring chain  20 . As illustrated in  FIG. 4B , the mooring chain  20  is then lifted from the seabed  3 . 
         [0030]    Step  316  is illustrated in  FIGS. 4C and 4D  (including a cutaway view of platform  100 ). The mooring line at the juncture of the mooring chain  20  and the old wire rope  24  is lifted up over the platform  100 . The union  22   a  is at the juncture of the mooring chain  20  and the old wire rope  24 . 
         [0031]    The mooring line is then placed on the platform  100  (step  318 ), as illustrated in  FIGS. 4E and 4F .  FIG. 4F  includes a cutaway view of platform  100 , showing the used wire rope  24  placed in the wire rope chute, and the proximal mooring chain  20  placed in the chain chute. In step  320 , the old wire rope  24  and proximal mooring chain  20  are locked securely in place in their respective chutes. The old wire rope  24  is locked in place by engaging the wire socket stopper  120 . The mooring chain  20  is locked in place by engaging the chain stopper  118 .  FIG. 4F  illustrates one embodiment for locking the wire rope and chain, although other locking means could be used as would be apparent to one skilled in the art. 
         [0032]    As shown in  FIG. 4F , an end sheave  111  mounted on the deck of the AHV  50  can be used to hold a guide wire which can be attached to gantry  106  for holding the position and guiding the placement of the mooring line on the platform  100 . The combined proximal mooring chain segment, including chain links  31 , and wire rope segment, including union  22   a , between the chain stopper  118  and the wire socket stopper  120  is referred to as the combined mooring line segment  127 . As initially placed and locked on the platform, if the line is taut, the tension in the combined mooring line segment  127  may be too high for safely cutting the combined mooring line segment  127 . The tension in the combined mooring line segment  127  is relieved in step  322  by moving the wire socket stopper  120  and/or the chain stopper  118  relative to one another in order to decrease the distance there between. Slack is introduced into segment  127 , as can be seen in  FIG. 4G . In the embodiment shown, wire socket stopper  120  is moved towards chain stopper  118  using hydraulic piston  123 . At this point, the gantry  106  can be released (step  324 ). 
         [0033]    As shown in  FIG. 4H , a cutting mechanism  132  such as an oxyacetylene torch can be used to cut one or more links of the proximal mooring chain  20  (step  326 ), thus forming a proximal mooring chain cut end, also referred to as a first mooring line cut end, and a first segment cut end. The first mooring line cut end can be a chain link. The first segment cut end can be union  22   a  at the end of used wire rope  24 . Any other suitable cutting mechanism can be used as well. 
         [0034]    In one embodiment, as shown in  FIG. 4I , a temporary work wire  25  is attached to the proximal mooring chain  20  on the platform  100  (step  328 ). The temporary work wire  25  can be unwound from a temporary work wire reel located on the deck of AHV  50 , and guided through end sheave  111  which can hold the temporary work wire  25  in place while the attachment is made to the terminal chain-link using union  22   c.    
         [0035]    In step  330 , as illustrated in  FIG. 4J , the AHV  50  is navigated through the water away from the turret  32  of the FSO  30  and towards the mooring pile  17  to a second location at a second end of the old wire rope segment  24 , at the juncture of the old wire rope segment  24  and the distal mooring chain  21 . As the AHV  50  moves from the first location to the second location, the old wire rope segment  24  is wound onto a used wire rope reel  52   b  on the deck of the AHV  50  and the temporary work wire  25  is paid out or laid down from the temporary work wire reel  52   c  onto the seabed  3 . A full new wire rope reel  52   a  is also on board the deck of the AHV  50 . 
         [0036]    In one embodiment, as the old wire rope  24  is taken up and wound onto the used wire rope reel  52   b , the old wire rope  24  is passed over the curved deflector plate  104  to assist in winding up the old wire rope  24 . Optionally, the curved deflector plate  104  includes guides (not shown) over which the wire rope can pass. 
         [0037]    In one embodiment, as the old wire rope  24  is taken up and wound onto the used wire rope reel  52   b , the old wire rope  24  is passed over rollers  112  in the wire rope chute  116  to protect and facilitate the winding up of the old wire rope  24 . 
         [0038]    At the second location, a combined mooring line segment including a portion of distal mooring chain  21 , temporary work wire  25  and union  22   d  is positioned on platform  100  with the chain  21  in the chain chute  110  and the temporary work wire  25  in the wire rope chute  116 . The distal mooring chain  21  is locked in place using the chain stopper  118 , and the temporary working wire  25  is locked in place using the wire socket stopper  120  (step  332 ). The tension in the combined mooring line segment including the portion of distal mooring chain  21 , temporary work wire  25  and union  22   d  is relieved in step  334  as in step  322  by moving the wire socket stopper  120  and/or the chain stopper  118  relative to one another in order to decrease the distance there between. 
         [0039]    In step  336 , as shown in  FIG. 4K , one or more links of the distal mooring chain  21  are cut to form a distal mooring chain cut end, also referred to as a second mooring line cut end, and a second segment cut end. The second mooring line cut end can be a chain link. The second segment cut end can be union  22   b  at the end of used wire rope  24 . At this point, used wire rope  24  is freed completely from the mooring line, and the end of used wire rope  24  can be completely wound onto the used wire rope reel  52  on the AHV  50 . 
         [0040]    In step  336 , as shown in  FIG. 4L , one end of the new wire rope  23  wound on a new wire rope reel  52  is connected to the distal mooring chain  21  on the platform  100 . The connection is made by way of union  22   f . As shown in  FIG. 4L , end sheave  111  holds the temporary working wire  25  during this step. 
         [0041]    In step  340 , as illustrated in  FIG. 4M , the AHV  50  is navigated through the water away from the mooring pile  17 , returning towards the turret  32  of the FSO  30  to the first location at a first end of the temporary working wire  25 , at the juncture of the temporary working wire  25  and the proximal mooring chain  20 . As the AHV  50  moves from the second location to the first location, the temporary working wire  25  is wound onto a temporary working wire reel  52   c  on the deck of the AHV  50  and the new wire rope  23  is paid out or laid down from the new wire rope reel  52   a  onto the seabed  3 . At this point, used wire rope reel  52   b  is full with used wire rope  24 . 
         [0042]    In one embodiment, as the new wire rope  23  is unwound from the new wire rope reel  52   a , it is passed over the curved deflector plate  104  to assist in unwinding the new wire rope  23 . Optionally, the curved deflector plate  104  includes guides (not shown) over which the wire rope can pass. 
         [0043]    In one embodiment, as the new wire rope  23  is unwound from the new wire rope reel  52   a , it passes over rollers  112  in the wire rope chute  116  to protect and facilitate the winding up of the new wire rope  23 . 
         [0044]    At the first location, a combined mooring line segment including a portion of temporary working wire  25 , proximal mooring chain  20  and union  22   c  (previously connected in step  328 ) is positioned on platform  100  with the chain  20  in the chain chute  110  and the temporary working wire  25  in the wire rope chute  116 . The proximal mooring chain  20  is locked in place using the chain stopper  118 , and the temporary working wire  25  is locked in place using the wire socket stopper  120 . The tension in the combined mooring line segment is relieved again by moving the wire socket stopper  120  and/or the chain stopper  118  relative to one another in order to decrease the distance there between. 
         [0045]    One or more links of the proximal mooring chain  20  are cut to form a proximal mooring chain cut end and a temporary working wire segment cut end. The temporary working wire segment cut end can be union  22   c . At this point, temporary working wire  23  is freed completely from the mooring line, and the end of temporary working wire  23  can be completely wound onto the temporary working wire reel  52  on the AHV  50 . 
         [0046]    In step  342 , the new wire rope  23  is connected to the proximal mooring chain  20  on the platform  100 . The connection is made by way of union  22   e . At this point, the old wire rope segment  24  of the mooring line has been replaced with the new wire rope segment  23 . To achieve a desired tension in the mooring line, lengths of mooring line, e.g., chain links, can be added or removed. 
         [0047]    The new wire rope is then released from the wire rope chute  116  and the proximal mooring chain  20  is released from the chain chute  110 , and the gantry  106  is attached to the mooring line. Using the gantry  106 , the mooring line can be lifted above the platform  100 , and lowered to the seabed  3 . The ROV  28  can assist with disconnecting the gantry  106  from the mooring line.  FIG. 4N  illustrates the resulting mooring line including the new wire rope segment  23  on the seabed  3 . 
         [0048]    Although in the process embodiment  300  described above, the proximal mooring chain  20  is described and illustrated as the mooring chain segment closest to the FSO  30  and the distal mooring chain  21  is described and illustrated as the mooring chain segment closest to the pile  17 , it should be understood that the proximal and distal mooring chains could be reversed, in which case the replacement of the old wire rope segment  24  would begin at the end of the old wire rope segment closest to the pile  17 . 
         [0049]    In another embodiment, a process is provided for installing a new mooring line where there was not previously an existing mooring line. In this embodiment, one end of a proximal mooring chain  20  can be attached to an FSO  30 , e.g. at the turret  32  of an FSO, The other end can be secured in the chain chute  110  of the platform  100  and locked in place using the chain stopper  118 . An end of a new wire rope  23  wound on a new wire rope reel  52  on the AHV  50  can be secured in the wire rope chute  112  and locked in place using the wire socket stopper  120 . The end of the new wire rope  23  can be attached to the proximal mooring chain  20  on the platform  100  using a union  22   e.    
         [0050]    The AHV  50  can then be navigated through the water away from the FSO and towards the intended anchor location (pile  17 ) while the new wire rope  23  is laid down onto the seabed  3 . As described before, in one embodiment, as the new wire rope  23  is unwound from the new wire rope reel  52   a , the new wire rope  23  passes over a curved deflector plates  104  having optional guides thereon. As described before, in one embodiment, as the new wire rope  23  is unwound from the new wire rope reel  52   a , the new wire rope  23  passes over rollers within the wire rope chute  112 . At the distal end of the new wire rope segment  23 , the wire rope is secured in the wire rope chute  112 , and the distal mooring chain  21  is placed in the chain chute  110  and secured with the chain stopper  118 . The distal end of the new wire rope segment is connected to the distal mooring chain  21  using a union  22   f . The mooring line can then be lifted by the gantry  106  and lowered to the seabed  3 . The distal end of the distal mooring chain  21  can then be attached to pile  17 , thus completing the installation of the mooring line anchoring the FSO  30 . 
         [0051]    Where permitted, all publications, patents and patent applications cited in this application are herein incorporated by reference in their entirety, to the extent such disclosure is not inconsistent with the present invention. 
         [0052]    Unless otherwise specified, the recitation of a genus of elements, materials or other components, from which an individual component or mixture of components can be selected, is intended to include all possible sub-generic combinations of the listed components and mixtures thereof. Also, “comprise,” “include” and its variants, are intended to be non-limiting, such that recitation of items in a list is not to the exclusion of other like items that may also be useful in the materials, compositions, methods and systems of this invention. 
         [0053]    From the above description, those skilled in the art will perceive improvements, changes and modifications, which are intended to be covered by the appended claims.