EP3573888A1

EP3573888A1 - Method and apparatus for laying subsea cable from on-board a vessel

Info

Publication number: EP3573888A1
Application number: EP18702538.2A
Authority: EP
Inventors: Michael Wilson
Original assignee: Ecosse Subsea Systems Ltd
Current assignee: Ecosse Subsea Systems Ltd
Priority date: 2017-01-26
Filing date: 2018-01-26
Publication date: 2019-12-04
Also published as: WO2018138511A1; GB201701285D0; GB2559327A

Abstract

The invention relates to a method and apparatus for laying a length of subsea cable from on-board a vessel. The method comprises the steps of releasing the length of cable from the vessel wherein the step of releasing the length of cable comprises the steps of supporting the cable on a portion of on-board cableway, extending a portion of said cableway over-board the vessel and tilting a portion of said extended cableway relative to the portion of on-board cableway. This allows a length of cable to pass over the resulting extended and tilted portion of said cableway toward the seabed whilst in supported engagement with the extended and tilted portion of said cableway. A method and apparatus of moving the location of a cable bight point along a length of cable suspended between a vessel and the seabed is also provided. A cable recovery, repair and re-laying service is also provided.

Description

Method and Apparatus for Laying Subsea Cable from On-board a Vessel

The present invention relates to a method of laying subsea cable from on-board a vessel, particularly, but not exclusively, subsea power cables. The invention also relates to apparatus for fitting to a vessel in order to perform the method and provides a method and apparatus for altering the bight point of cable suspended from a vessel.

Subsea cables that are typically laid on, or buried within, the seabed require to be repaired from time to time. This typically involves sailing a vessel into the vicinity of the damaged portion of subsea cable, lowering a grapping hook toward the seabed, sailing the vessel past the subsea cable in order to engage the grappling hook with a length of cable adjacent the damaged section of cable and then winching that portion of cable on-board the vessel by way of an upright crane or horizontal winch for subsequent repair of the subsea cable at a repair or jointing bay on-board the vessel.

However, known methods of repairing subsea cable pose a number of problems. For example, they typically require a large upright crane in order to pull the cable on-board the vessel, manipulate the cable whilst it is aboard the vessel and to overboard the cable during re-laying operations; such structures are not always available or desirable for cost and other operational reasons. Furthermore, due to the relatively precarious task of lifting such lengths of cable on-board a vessel, in often difficult weather conditions, this method requires a relatively long window of good weather and correspondingly low wave heights. This often greatly hampers the ability of a vessel to repair cable within a given time period.

Another problem with existing cable repair methods is that they pose a relatively high risk to personnel due to the height by which the loop of cable must be raised above the vessel deck during on-boarding, repair and over-boarding operations. Another problem with existing methods of cable repair is that there is a risk of further damage to the cable either upon recovery to the vessel or on deployment from the vessel. According to a first aspect of the present invention there is provided a method of laying a length of subsea cable from on-board a vessel, the method comprising the steps of releasing the length of cable from the vessel wherein the step of releasing the length of cable comprises the steps of supporting the cable on a portion of on-board cableway, extending a portion of said cableway over-board the vessel and tilting a portion of said extended cableway relative to the portion of on-board cableway in order to allow a length of cable to pass over the resulting extended and tilted portion of said cableway toward the seabed whilst in supported engagement with the extended and tilted portion of said cableway.

This allows a length of cable retained by a vessel to be deployed from the vessel whether or not the cable has been first recovered to on board the vessel for repair.

The method may also comprise first retrieving a portion of damaged cable to the vessel and repairing the damaged portion of cable prior to releasing the length of repaired cable from the vessel.

The method may also comprise providing a tiltable chute arrangement connected to the extended portion of cableway.

The step of tilting the portion of cableway may also comprise arranging the tilted portion at an angle which is between the longitudinal axis of the portion of on-board cableway and the mean longitudinal axis of a deployed cable catenary. The step of tilting the portion of cableway may comprise arranging the tilted portion at an angle which is substantially midway between the longitudinal axis of the portion of onboard cableway and the mean longitudinal axis of a deployed cable catenary

This consideration of the relative different longitudinal axes allows any bend imparted on the cable as it passes overboard the vessel to be monitored and minimised. The method may also comprise the step of cutting the length of damaged subsea cable prior to retrieval to the vessel in order to form first and second lengths of damaged cable and then retrieving the first length of damaged cable to the vessel and then retrieving the second length of damaged cable to the vessel.

The method may also comprise the step of removing or repairing any damaged portions of said first or second lengths of damaged cable in order to form first and second lengths of repaired cable. The method may also comprise the step of recording the length of any removed damaged portions.

The method may also comprise the steps of providing an intermediate section of replacement cable having a length which is substantially equal to the recorded length of removed damaged cable and attaching the first end of said intermediate section of cable to the first length of repaired cable at a first joint, and attaching the second end of said intermediate section of cable to the second end of repaired cable at a second joint in order to form a continuous length of repaired cable. The method may also comprise the steps of manoeuvring the vessel and or actuating cable actuators in order to adjust the position of the or each portion of retrieved cable in order to selectively position ends of the or each portion of retrieved cable and or intermediate cable in the vicinity of a jointing bay on board the vessel in order to facilitate joining of said sections.

The method may also comprise the step of arranging the retrieved cable and or intermediate length of cable over a horizontally arranged cable quadrant on board the vessel. The method may also comprise attaching a buoy to the first and second lengths of damaged cable in order to facilitate retrieval thereof to the vessel. The method may also comprise retrieving both ends of cable to the vessel, repairing the cable and re-joining the cable and any intermediate section at at least a joint on-board the vessel in order to form a loop of retained cable. The method may also comprise ensuring the weight of resulting suspended length of cable running from the vessel toward the seabed is selectively held by suitable holding means such as hold backs, tensioners or other holding devices in order to temporarily prevent movement of the suspended cable relative to the vessel when desired yet to selectively allow movement of the suspended cable relative to the vessel when desired.

The method may also comprise the steps of manoeuvring the vessel and or actuating cable actuators in order to thread the loop of retained cable relative to the vessel thereby progressing the or each joint in the repaired cable to a substantially straight section of cable residing on the seabed.

This ensures that any stresses and strains on any joints are minimised during installation and during their lifetime upon the seabed.

The step of lowering the loop of retained cable to the seabed may further comprise controlling the relative lengths of winch line paid out and retracted from an aft and fore winch connected to a cable quadrant in order to retain the cable quadrant in a

substantially upright position during re-laying of the loop of retained cable toward the seabed. The method may also comprise retracting only one of the aft or fore winch lines once the re-laid cable rests upon the seabed in order to tilt the cable quadrant and thereby release the previously retained loop of cable previously held by the cable quadrant and then retracting both of the aft and fore winch lines in order to retrieve the cable quadrant to the vessel. According to a second aspect of the present invention, there is provided subsea cable laying apparatus for laying a length of subsea cable from on-board a vessel, the apparatus comprising: - a portion of on-board cableway adapted to allow cable to rest thereon and to translate longitudinally thereover;

a portion of extendible cableway capable of being selectively extended over-board the vessel;

a tiltable arrangement capable of allowing at least a portion of the extendible cableway to be tilted relative to the portion of on-board cableway in order to allow a length of cable to pass over an extended and tilted portion of said cableway toward the seabed whilst in supported engagement with the extended and tilted portion of said cableway.

The apparatus may also comprise a tiltable arrangement having a tiltable over-boarding chute having a mouth section which is curved in the longitudinal direction in order to facilitate and support bending of any cable passing thereover when transitioning between the on-board cableway longitudinal axis and the suspended cable catenary longitudinal axis in order to minimise any structural stress imparted upon any repaired section or joint passing there over.

The tiltable over-boarding chute may comprise a mouth section which is curved in the lateral direction in order to facilitate and support bending of any cable passing there past when transitioning between the cableway longitudinal axis and the suspended cable catenary longitudinal axis in order to minimise any structural stress imparted upon any repaired section or joint passing there over.

The portion of on-board cableway may comprise a plurality of rollers along its length and which facilitate supported longitudinal movement of the cable thereupon. The tiltable over-boarding chute may comprise a plurality of rollers along its length and which facilitate supported longitudinal movement of the cable thereupon. The tiltable over-boarding chute may comprise a lattice-work space frame.

The apparatus may also comprise a ram arrangement adapted to selectively actuate the extendible cableway between a retracted configuration inboard of the vessel and an extended configuration overboard the vessel.

The ram arrangement may also comprise a pair of hydraulic rams mounted upon either side of the cableway arrangement.

The tiltable arrangement may be provided with actuation means capable of increasing or decreasing the angle of tilt relative to the portion of on-board cableway in order to provide a resultant cable deployment path having minimal angles of deviation there along and whose cable deployment path may be altered by adjustment of said angle of tilt depending upon laying conditions.

The actuating means may comprise a wire and winch arrangement connected to the tiltable arrangement.

The apparatus may also comprise a cable reel arrangement comprising a length of intermediate cable for connection between first and second lengths of damaged cable at first and second joints.

The apparatus may also comprise a pair of cable guide posts in order to guide the first or second end of a length of retrieved cable into alignment with the respective cableways.

The cableway arrangement may be telescopically extendible.

The apparatus may also comprise a cable quadrant adapted to retain a loop of cable thereover during repair of the cable.

The apparatus may also comprise a jointing bay for facilitating repair and jointing of the damaged section of cable. According to a third aspect of the present invention there is provided a method of moving the location of a cable bight point along a length of cable suspended between a vessel and the seabed, the method comprising: - providing a bight point support on the vessel, wherein the bight point support is adapted to support the cable at or toward a first bight point such that the cable extends from the cable toward the seabed and wherein at or toward the seabed a second bight point is created; and

raising or lowering the bight point support in order to move the longitudinal location of the or each first or second bight points along the length of cable.

The method may also comprise the step of monitoring the bight point position adjacent the seabed by way of a cable mounted monitoring device.

The method may also comprise the step of assessing the stresses and or strain which may have been placed upon the cable whilst said first and or second bight points are at a particular longitudinal location along the length of cable and, depending upon the outcome of that assessment either maintaining, lowering or raising the vertical position of the bight point support in order to move the longitudinal locations of said the or each bight point.

The method may also comprise the step of correlating the vertical position of the bight point relative to tidal cycles.

The method may also comprise maintaining a constant tension on a winch wire connected to the or each bight point support in order to maintain a constant tension on any cable suspended from the vessel by the apparatus.

According to a fourth aspect of the present invention, there is provided apparatus for moving the location of a cable bight point along a length of cable suspended between a vessel and the seabed, the apparatus comprising: - bight point support means on the vessel, the bight point support means being adapted to support the cable at or toward a first bight point such that the cable extends from the cable toward the seabed and wherein at or toward the seabed a second bight point is created, and the bight point support being adjustably mounted to the vessel such that the height of the bight point support may be altered whilst supporting the cable in order to move the longitudinal location of the or each first or second bight points along the cable.

The apparatus may also comprise at least a cable mounted monitoring device adapted to monitor the location of the cable bight point adjacent the seabed. The cable mounted monitoring device may also comprise a visual cable monitoring device adapted to obtain and transmit visual information on the cable bight point adjacent the seabed.

The bight point support means may also comprise an aft-ward profiled deflector adapted to act as a fulcrum over which the cable suspended from the vessel rests upon during transition of the cable from a substantially horizontal configuration along the vessel deck to a catenary configuration suspended from the vessel to the seabed.

The apparatus may also comprise a fore-ward profiled deflector positioned to guide the length of cable over the aft-ward profiled deflector and to retain the forward portion of cable toward the deck of the vessel.

According to a fifth aspect of the present invention, there is provided a cable recovery, repair and re-laying service comprising: - positioning a plurality of vessels having equipment according to claim 18 installed thereon at different geographical locations and;

raising a service fee to subscribers for the purpose of allowing said subscribers to have their cables repaired by an appropriately located vessel when required.

Further features and advantages of the present invention will become apparent from the claims and the following description. Embodiments of the present invention will now be described by way of example only, with reference to the following diagrams, in which: - Fig. 1 is a plan view illustration of a typical vessel that is not equipped with the apparatus of the present invention and where certain components of the apparatus of the present invention are figuratively illustrated adjacent the vessel; Fig. IB is a schematic illustration of an extendible cable way arrangement and associated over-boarding chute in a fully downward tilted / free hanging position; Fig. 1C is a more detailed illustration of rollers which are provided on the extendible cableway of Fig. IB and which provide the upper cable support surface of the cableway;

Fig. ID is a more detailed illustration of a hinged attachment between the extendible cableway arrangement and over-boarding chute of Fig. IB;

Fig. IE is a plan view of the over-boarding chute of Fig. IB;

Fig. IF is a transverse illustration of the over-boarding chute and extendible cableway arrangement of Fig. IB where a chute tilting mechanism is illustrated; Fig. 1G is a schematic illustration of an extendible cableway arrangement and an alternative associated over-boarding chute in a partially downward tilted position; Fig. 1H is a more detailed illustration of rollers provided on the extendible cableway of Fig. 1G;

Fig. II is a more detailed illustration of a hinged attachment between the extendible cableway arrangement and over-boarding chute of Fig. 1G;

Fig. 1J is a plan view of the over-boarding chute of Fig. IB;

Fig. IK is a front view of the over-boarding chute of Fig. 1G in the partially downward tilted position;

Fig. 1L is a transverse illustration of the over-boarding chute and extendible cableway arrangement of Fig. 1G;

Fig. 2 is a plan view schematic illustration of the vessel where the apparatus of the invention is installed thereon;

Fig. 3 is a transverse schematic illustration of the vessel of Fig. 2; Figs. 4 and 5 are respective transverse and plan illustrations of the vessel trailing a cutting hook whilst approaching a length of cable requiring repair in accordance with a "cut-then-lift" method;

Figs. 6 and 7 are respective transverse and plan illustrations of the vessel of Figs. 4 and 5 having sailed further toward the length of cable requiring repair;

Figs. 8 and 9 are respective transverse and plan illustrations of the vessel of Figs. 6 and 7 where the cutting hook is engaged with a length of cable at or adjacent the length of cable requiring repair;

Fig. 10 is a plan illustration of the vessel of Fig. 9 where the cutting hook has been pulled through and hence has cut the cable;

Fig. 11 is a transverse illustration of the vessel of Fig. 10, where a buoy is shown during the process of being attached to an end of the cut cable and where a Remotely Operated Vehicle (ROV) is provided to facilitate attachment of the buoy thereto;

Fig. 12 is a transverse illustration of the Fig. 11 arrangement whereby the buoy has been attached to the cable;

Fig. 13 is a plan illustration of the arrangement shown in Fig. 12 where a buoy has been attached to either end of the cut cable;

Fig. 14 is a plan view illustration of one end of the cut cable being recovered to onboard the vessel;

Fig. 15 is a plan view illustration of an intermediate section of new cable shown during attachment to an end of the recovered cable at a first joint following removal or repair of a damaged section of the recovered cable;

Figs. 16 to 20 are plan views of the intermediate section of cable being paid out in order to overboard the first joint between the existing cable and intermediate section of cable and hence deploying the first joint and attached intermediate section of cable toward the seabed;

Figs. 21 and 22 are plan view illustrations of the second end of cut cable being recovered onto the deck of the vessel;

Fig. 23 is a plan view illustration of the second end of cable pulled adjacent the end of the intermediate section of cable ready for connection thereto; Fig. 24 is a plan view illustration of the second end of cable connected to the other end of the intermediate section of cable at a second joint;

Figs. 25 and 26 are respective transverse and plan illustration of a vessel recovering the damaged cable by way of an alternative "no-cut-and-lift" method, whereby a grappling, rather than cutting, hook engages the cable and where slack in the cable is taken up as the cable is progressed towards the surface;

Figs. 27 and 28 are respective transverse and plan illustrations of the vessel of Figs. 25 and 26 where the vessel has further retrieved the cable toward the vessel; Figs. 2 and 30 are respective transverse and plan illustrations of the vessel of Figs. 27 and 28 where the cable has been winched onto the deck of the vessel and where the cable is presented for engagement with a cable quadrant on-board the vessel;

Fig. 31 is a plan view illustration of the retrieved cable retained by the cable quadrant for repair in a jointing bay;

Fig. 32 A is a plan view illustration of a repaired joint located along the length of cable on-board the vessel and where the repaired joint is located at a jointing bay of the vessel following repair of the cable at that section;

Fig. 32B is a transverse illustration of the arrangement shown in Fig. 32 A;

Fig. 33A is a plan view illustration of the repaired joint where the vessel has been manoeuvred along the cable lay-line and hence the repaired joint has moved away from the jointing bay;

Fig. 33B is a transverse illustration of the arrangement shown in Fig. 33 A;

Fig. 34A is plan view illustration of the vessel where a starboard cableway has been extended to support the repaired section of cable during deployment from the vessel;

Fig. 34B is a transverse illustration of the arrangement shown in Fig. 34A;

Fig. 39 is a plan view illustration of the vessel of Fig. 34A and B where the starboard cableway has been tilted to facilitate support of the repaired joint during deployment from the vessel;

Fig. 40A is a plan view illustration of the vessel where the repaired section of cable has been deployed from the rear of the extendible cableway toward the seabed; Fig. 40B is a transverse illustration of the arrangement shown in Fig. 40A; Figs. 41 A and 41 B are transverse illustrations of the arrangement shown in Fig. 40B progressively where increasing amounts of cable have been paid out and hence the repaired joint has descended by progressively increasing amounts towards the seabed;

Fig. 42 is a plan view illustration of the vessel whereby the vessel has been manoeuvred laterally along the cable lay-line in order to rest the repaired joint on the seabed;

Figs. 43 and 44 are respective plan and transverse view illustrations of the vessel where the cableway is retracted and both forward and aft winch arrangements are attached to the cable quadrant for deployment of the remaining cable length from on-board the vessel;

Figs. 45 and 46 are respective plan and transverse view illustrations of the Fig. 43 and 44 arrangement where the vessel has manoeuvred forward and the cable quadrant and attached loop of cable is progressed to the aft of the vessel;

Fig. 47 is a transverse view of the cable quadrant prior to over-boarding from the vessel;

Figs. 48 and 49 are respective plan and transverse view illustrations of the cable quadrant and associated cable loop supported thereby being over-boarded from the vessel;

Figs. 50 and 51 are transverse view illustrations of the cable quadrant and associated cable loop supported thereby being progressively lowered to the seabed;

Fig. 52 is a transverse view illustration of the cable quadrant having been decoupled from the cable loop and being recovered to the vessel after having deposited the cable on the seabed;

Fig. 53 is a plan view of the vessel having deposited the repaired cable on the seabed;

Fig. 54 is a transverse schematic view of the vessel and suspended retrieved cable retrieved by way of either the no-cut-and-lift or by way of the cut-then-lift method and where an aft bight point deflector is located in a lowered configuration;

Fig. 55 is a transverse schematic view of the vessel and suspended retrieved cable where an aft bight point deflector is located in a raised configuration; Fig. 56 is a more detailed transverse illustration of the lowered bight point tensioning means of Fig. 54; and

Fig. 57 is a more detailed transverse illustration of the raised bight point tensioning means of Fig. 55.

In the present application the term "seabed" means any bed submerged below a body of water and includes for example an ocean bed, an estuary bed or a fresh water bed.

In the present application the term "cable" means any elongate cable-like member of suitable properties and may include for example a power cable, a telecommunications cable or any solid or hollow tubular member. Furthermore, the term "subsea" refers to any sub-aqua environment and includes for example fresh water sub-aqua environments.

Referring to Figs. 1 to 2 the apparatus requires to be installed on a suitable vessel V prior to use. Such a vessel will typically be provided with a variety of standard cable lay and cable handling equipment which may be useful in performing the method of the invention. These include for example, snatch blocks, tensioners, hold backs, winches, cranes etc.

Cable recovery and repair apparatus, generally designated 10, comprises a first extendible cableway arrangement 12 and a second fixed cableway arrangement 14, a cable loop retaining arrangement in the form of a cable quadrant 16, a reel of intermediate cable 17, a transverse roller 18, a pair of guide members 20, a pair of aft deflectors 21 for attachment to either side of the vessel and a number of winch arrangements such as those illustrated as 22. A pair of fore deflectors 23 (Figs. 3, 56 and 57) are also provided for attachment to either side of the vessel.

Although shown as a semi-circular / arched arrangement in the present embodiment, the cable quadrant 16 may be take any form or geometry capable of retaining a loop of cable thereon.

With particular reference to Fig. IB, in one embodiment the cableway arrangement 12 comprises a series of rollers 21 which are each rotationally mounted upon a base frame 23 in order to provide a rolling bed upon which cable may rest and be longitudinally transported therealong. At the end of the cableway 12 a lattice-framed over-boarding chute 25 is attached by retaining lugs 29A and pivot bar 29B. A pair of actuating rams 31 are also provided adjacent the over-boarding chute 25 such that the over-boarding chute 25 may be retracted / deployed upon lengthening / shortening of the rams 31 as required.

The lattice-frame of the chute 25 maximises the strength of the arrangement whilst minimising material and weight. The chute 25 is provided with a curved mouth section 33 which is curved at 35 (Fig. IF) in order to provide a gradual transition between the chute longitudinal axis and the catenary of any suspended cable running toward the seabed. The chute 25 is also provided with a curved and widened mouth section 37 in Fig. IE in order to similarly account for any lateral movement in the catenary of cable relative to the vessel. As illustrated in Fig. IF, the chute 25 is also provided with a tilting mechanism comprising a pair of masts 35, a winch wire 37 and a pair of sheaves 39 which together allow the chute 25 to be tilted around the pivot bar 29B relative to the vessel to any desired angle as will be described subsequently. As illustrated in Figs. 1H to 1L in an alternative embodiment, the lattice work chute 25 may instead comprise a solid-walled chute 25 provided with rollers 21 therealong.

When it is desired to fit-out the vessel V with cable recovery and repair capabilities the components of the apparatus 10 may be lifted onto the vessel by a quayside crane or other suitable lifting mechanism. As illustrated in Fig. 2, the vessel is also equipped with a jointing bay 24 which provides an enclosed environment that is conducive to the repair of cable in often adverse weather conditions or darkness. The jointing bay 24 is typically a large closed container having slots in either end thereof which accept the cable therein during cable repair operations.

With reference to Figs. 4 to 7, in accordance with a "cut-then-lift method", once the components of the apparatus 10 are installed on the vessel V it may be sailed towards the vicinity of a cable 26 requiring repair. As the vessel V approaches the cable 26 a cutting hook 28 is trailed on a winch line 30 along the seabed 32. The winch line 30 is attached at its upper end to one of the winches 22 on-board the vessel V. As illustrated in Figs. 8 and 9, the vessel is manoeuvred further forward and, when the cutting hook 28 reaches the cable 26, it engages therewith and then cuts through the cable to form a first end of severed cable 26A and a second end of severed cable 26B as illustrated in Fig. 10. Alternatively, an ROV or diver operated cutting tool may be provided in order to cut through the cable 26. The damaged portion of cable may be located on either or both of the ends 26A, 26B meaning that portions of damaged cable may require subsequent removal or repair on either or both ends 26A, 26B as described subsequently.

As illustrated in Figs. 11, 12 and 13, an ROV 23 is utilised in order to attach first and second buoys 38A, 38B to the respective first end second severed ends of cable 26A; 26B by an attachment line 40 extending between the buoy and the cable.

With reference to Fig. 14, the first buoy is then grappled, and the first end of cable 26A is connected to a winch line 42 which is in turn connected to winch 22. The winch 22 is then retracted in order to pull the first end of cable 26A around the guide post 20 and into approximate alignment with the starboard cable way 12 until the end of the first end of cable 26A is located within the jointing bay 24 as illustrated in Fig. 15. Starboard hold backs, tensioners or other retaining means may be engaged with the first end of cable 26A at this point if required.

At this point any damaged portion of the first end of cable 26A may be repaired or removed as desired.

With reference to Fig. 15, a length of intermediate new cable 27 is then reeled off from the cable reel 17 and joined at the jointing bay 24 to the first end of cable 26A at a first joint 43. With reference to Fig. 16, the first joint 43 is then progressed out of the jointing bay 24 and along the starboard cableway 12 by appropriate manoeuvring of the vessel V and / or paying out by the tensioners or hold backs and the cable reel 17. With reference to Fig. 17, as the first joint 43 moves along the cableway 12, an extendible table 46 is extended from the starboard cableway 12; this is achieved by actuating the rams 3 (Fig IB).

With reference to Fig. IF the chute 25 is then tilted downward until it reaches a desired tilt angle. The desired tilting angle will typically be large / free hanging (as shown in Fig. IF) if re-laying cable in deep water operations and small (as shown in Fig. 1L) if re-laying cable in shallower water operations. This alteration in tilt angle is achieved by controlling the amount of wire 37 paid out on the tilting actuation mechanism. This ability to adjust the angle of the chute 25 ensures that any repair joint passing thereover will be subjected to the minimum amount of bending stresses.

Continued port manoeuvring of the vessel V and continued paying out of the intermediate cable section 27 from the cable reel 17 results in continued progression of the first joint 43 into the water towards the seabed 32 as illustrated in Figs. 19 and 20 until the first joint 43 rests on or adjacent the seabed 32. This step of manoeuvring the vessel and the cable between the configurations illustrated in Fig. 19 and Fig. 20 is carefully controlled and monitored so as the ensure that neither the first end of severed cable 26A, its attached first joint 43 or intermediate cable section 27 are bent through a bend radius which is a less than the Minimum Bend Radius (MBR) thereof. Furthermore, the cable will often be laid on the seabed with a greater degree of bight / bend than that illustrated in Fig. 20; however, in order to reduce any bending forces imparted on the first joint 43 the joint 43 is deliberately positioned on the seabed in such a way (as illustrated in Fig. 20) that the first joint 43 does not reside at a bight / bend in the first end of cut cable 26A but instead resides at a relatively straight section thereof.

At this point the extendible table 46 of the cableway is retracted by shortening the rams as illustrated in Fig. 20. With reference to Figs. 21 and 22 a similar retrieval process is then performed on the second end of cable 26B in order to also pull the second end of cable 26B over the port cableway 14, and then around a cable quadrant 16 as illustrated in Fig. 23. At the same time the intermediate cable 27 is cut from the cable reel 17 such that the join between the end of the intermediate cable 27 and the end of the second cable 26B is located in the jointing bay 24.

At this point any damaged portion of the second end of cable 26B may be repaired or removed as desired.

A second joint 44 is then created between the end of the intermediate cable 27 and the second end of the severed existing cable 26B at the jointing bay 24. Alternatively, if the length of cable damaged is relatively short, only a single joint may be required without any intermediate portion of cable being required. The repaired cable joint and

intermediate cable (if provided) are now ready to be deployed to the seabed.

With reference to Figs. 25 to 34 and Figs. 39 to 42, an alternative "no-cut-and-lift" method of recovering cable to a vessel for repair, where a degree of slack is present in the existing cable, will now be described.

As illustrated in Fig. 25, the vessel V is sailed towards the vicinity of a cable 26 requiring repair. As the vessel V approaches the cable 26 a grappling hook 28 (or alternatively a pair of grappling hooks mounted on an I-beam arrangement) is trailed on a winch line 30 along the seabed 32. The winch line 30 is attached at its upper end to one of the winches 22 on-board the vessel V. As the vessel is manoeuvred further forward and, when the grapping hook 28 reaches the cable 26, it engages therewith and the vessel is stopped.

As illustrated in Figs. 26 to 29, the winch 22 is then retracted in order to pick the cable 26 up from the seabed 32. Whilst the winch 22 is being retracted the vessel V is slowly manoeuvred astern. Winching in of the winch line 30 is continued until the cable 26 is adjacent the stern of the vessel V as illustrated in Fig. 29 at which point continued winching in of the winch line 30 pulls the loop of cable 26 inboard and onto the vessel as illustrated in Fig. 30. It will be noted that, as the loop of cable 26 is pulled inboard from the outboard position indicated in Fig. 29 to the inboard position indicated in Fig. 30, the guide posts 20 will guide the loop of cable 26 into approximate alignment with the first and second cable ways 12, 14. In addition, the operator ensures that the vertical position of the cable lengths 26 are such that they are threaded above the aft deflectors 21 and below the fore deflectors 23 at either side of the vessel.

The winch 22 is then retracted further in order to pull the loop of cable 26 over the cable quadrant 16 into the configuration illustrated in Fig. 31. The winch 22 can then be disconnected from the cable 26 and a portion of damaged cable at 34 can be removed adjacent the jointing bay 24. The vessel may be manoeuvred along the cable lay-line in order to ensure that the portion of damaged cable is positioned within the jointing bay 24.

Removal of the section of damaged cable 34 may involve removing a single long length of damaged cable or multiple consecutive smaller lengths of damaged cable. In either scenario, the total length of damaged cable is recorded such that the correct length of repaired intermediate cable can be replaced in subsequent steps. In this way, the total length of the repaired cable will be substantially the same as the original cable.

With reference to Figs. 31 to 34 and Figs. 39 to 42 a first step of laying the repaired section of cable will now be described. As illustrated in Fig. 32A and 32B, the repaired section of cable is illustrated as a section of cable having a single repaired joint 44;

however, the skilled reader will appreciate that, where a larger section of intermediate cable is required in order to repair the damaged cable first and second joints may be provided, each having a section of intermediate new cable extending therebetween. With reference to Figs. 33A and 33B, in order to progress the joint 44 out of the jointing bay 24 and along the starboard cableway 12, the vessel V is manoeuvred port in the direction indicated by arrow A. At the same time the guide rollers 20 may be rotated to facilitate movement of the cable 26 there past.

With reference to Fig. 34A and 34B, as the repaired section 44 moves along the cableway 12, an extendible table 46 is extended from the starboard cableway 12 by actuating the rams 31 as previously discussed.

With reference to Fig. 39, the extended table 46 supporting the repaired joint 44 is then tilted downward (by paying out cable on the tilting mechanism as generally described above) in order to introduce the repaired joint 44 into the water without placing any significant bending stresses thereon. Continued port movement of the vessel V in the direction A results in continued progression of the repaired joint 44 into the water towards the seabed 32 as illustrated in Figs. 40A, 40B, ,41A and 41B until the repaired joint 44 rests on the seabed 32 in line with the remainder of the cable 26 as illustrated in Fig. 42. At this point the tiltable over-boarding chute 25 and extendible table 46 are retracted by winching in on the wire 37 of the tilting mechanism and retracting the rams 31.

Lateral movement of the vessel may be continued if desired in order to thread the loop of cable further past / through the apparatus on the vessel. This ensures that the joint 44 is positioned at a straight or substantially straight section of cable residing on the seabed. With this arrangement, the loop of cable retained by the vessel at this point in the method of deployment may be a loop of original unrepaired cable. A second re-laying step will now be described with reference to Figs. 43 to 53. This second re-laying step may be used to deploy cable which has either been recovered to the vessel by way of the previously described "cut-then-lift" methodology or by the "no-cut- and-lift" methodology. As illustrated in Figs. 43 and 44, a forward winch line 54 is connected at one end to the fore of cable quadrant 16 and at the other end to starboard winch 56 after passing starboard snatch blocks 58. This allows the starboard winch 56 to provide a retaining or hold-back function. In addition, an aft winch line 60 is connected at one end to the aft of cable quadrant 16 (by a suitable bridle arrangement 62) and at the other end to port winch 64 after passing over the aft transverse roller 18 and port snatch blocks 66.

Actuation of the port winch 64 is not typically required in order to release the loop of cable 26 from the vessel since the tension (and/or forward movement of the vessel) alone on the cable will normally be sufficient to do this once any hold-backs or tensioners are released; however, the port winch is connected at this stage regardless in preparation for the final stages of the operation as will be discussed subsequently. With reference to Figs. 45 and 46, the vessel V is now manoeuvred forward whilst the starboard winch 56 pays out the forward winch line 54 and the port winch 64 pulls in the aft winch line 60 until the cable quadrant 16 resides at the aft of the vessel V ready for over-boarding thereof. With reference to Fig. 47 over-boarding of the cable quadrant 16 is initiated by moving the vessel V forward and simultaneously paying out the forward winch line 54 whilst pulling in the aft winch line 60 around the aft transverse roller 18. These actions complement one another in order to tip the cable quadrant 16 over the over-boarding chute 25.

With reference to Figs. 48 and 49, continued deployment of the cable quadrant 16 is achieved by continuing to move the vessel forward, paying out both the forward winch line 54 and the aft winch line 60. At around this point the weight of the cable quadrant 16 will gradually be transferred to the forward winch line 54 whilst the aft winch line 60 will slacken for deployment of the cable quadrant 16 and associated loop of cable 26 toward the seabed 32 as seen in Fig. 50. During descent of the quadrant 16 toward the seabed, the aft winch line 60 may be paid out or pulled in order to control the orientation of the cable quadrant 16. This action is continued until the cable quadrant 16 is adjacent the seabed 32 in an upright position as illustrated in Fig. 51. With reference to Fig. 52, the aft winch line 60 is now paid out slightly (in order to allow even tipping of the cable quadrant 16) before then being pulled in order to tip the cable quadrant 16 and release the loop of cable previously held thereby to the sea bed 32. With reference to Fig. 53, once the cable quadrant 16 has been recovered to the vessel the operation is complete and the vessel may be sailed away for further operations. It will be appreciated that the previously described steps of laterally manoeuvring the vessel whilst a loop of cable was retained thereby has resulted in the joint 44 being located on a straight or substantially straight section of cable 26 and that the bend / bight in the loop of cable is therefore advantageously distant from the location of the joint 44.

The reader will appreciate that the above described apparatus and method does not place the repaired joint 44 (or either of the first and second joints 43; 44 if provided) under any undue stress or strain and this greatly reduces the risk of damage thereto during re-laying of the cable on the seabed.

Additionally, neutral buoyancy techniques may be used on the cable joints in order to minimise any stress placed thereon during submergence and during the operational lifetime of the joint.

With reference to Figs. 54 to 57, a method of refreshing the bight point on a cable suspended between a vessel and the seabed will now be described.

The components of the bight refreshing mechanism are illustrated schematically in Figs. 54 and 55 and in greater detail in Figs. 56 and 57.

First, second and third bight points Bl, B2 and B3 exist along the length of a cable 26. Depending upon the depth, geometry and other conditions, more or fewer bight points may in fact exist along the length of cable 26. If left unchecked, the bend in the cable 26 at these bight points risks weakening or damaging the structure of the cable 26 during the (sometimes lengthy) repair process. Furthermore, although the exact location of the bight points may change in tidal areas in certain circumstances, the bight points may remain at substantially the same position for long periods of time which could cause undue risks of damage to the cable. In order to address this, the operator may periodically raise the aft deflector 21 on the guide post 20 and / or adjust the angle / position of the fore deflector 23 in order to move the previous position of the bight points Bl, B2, B3 along the cable 26 to new bight points (illustrated as B4, B5, B6 in Fig. 23). Raising of the aft deflector 21from the lowered configuration illustrated in Fig. 56 to the raised configuration illustrated in Fig. 57 is achieved by heaving in a winch wire 27 on the bight winch 29.

In this regard, it should be noted that the old bight points Bl, B2 and B3 illustrated in Fig. 54 are in fact no longer bight points as such; however, these are illustrated in order indicate the relative position of the previous bight points relative to the new bight points B4, B5, B6. This also illustrates how the method and apparatus allows the longitudinal location of the bight points along the cable 26 to be altered in order to thereby reduce the risk of damage to the cable during recovery, repair and re-laying operations.

The operator may choose to manually or automatically raise and lower the aft deflector 21 and / or fore deflector 23 periodically during recovery, repair and / or relaying operations in order to account for factors such as weather, tidal conditions, depth, cable type etc. Additionally/alternatively, an imaging device may be used to visually monitor the location at which the cable meets the seabed and alter this accordingly. Examples of suitable devices for monitoring this include an ROV or cable mounted camera device which is able to reciprocate along the length of the cable, retrieve images of the cable and transmit such images and associated information to a controller for further processing.

The bight winch 2 is also set up to automatically heave in / pay out wire 27 in order to retain a constant tension on the wire 27 and hence retain a constant tension in the deployed cable deflected by the deflectors. In this way, the system acts like a spring in order to mechanically compensate for vertical movement of the vessel. This has the further advantage of allowing operators to measure the load and ensure that the cable is not overstressed during re-laying operations. Although particular embodiments of the invention have been disclosed herein in detail, this has been done by way of example and for the purposes of illustration only. The aforementioned embodiments are not intended to be limiting with respect to the scope of the appended claims.

It is contemplated by the inventors that various substitutions, alterations, and

modifications may be made to the invention without departing from the spirit and scope of the invention as defined by the claims. Examples of these include the following: -

In the described embodiments, the vessel is illustrated as a powered shipping vessel; however, the skilled reader will appreciate that the method and apparatus of the invention are also well suited to use on any suitable vessel such as e.g. a barge or jack-up arrangement.

Although in the described embodiments, the starboard cableway is extendible and the port cableway is not, these could be oppositely arranged such that the port cableway is extendible and the starboard cableway is not. Alternatively, both the port and starboard cableways could be extendible.

Although the method and apparatus described in is described with reference to recovery, repair and re-laying of a single cable, the apparatus may alternatively be used in order to recover, repair and re-lay several, substantially parallel, cables simultaneously.

Claims

1. A method of laying a length of subsea cable from on-board a vessel, the method comprising the steps of releasing the length of cable from the vessel wherein the step of releasing the length of cable comprises the steps of supporting the cable on a portion of on-board cableway, extending a portion of said cableway over-board the vessel and tilting a portion of said extended cableway relative to the portion of on-board cableway in order to allow a length of cable to pass over the resulting extended and tilted portion of said cableway toward the seabed whilst in supported engagement with the extended and tilted portion of said cableway.

2. A method according to claim 1, further comprising first retrieving a portion of damaged cable to the vessel and repairing the damaged portion of cable prior to releasing the length of repaired cable from the vessel.

3. A method according to either of claims 1 or 2, further comprising providing a tiltable chute arrangement connected to the extended portion of cableway.

4. A method according to any of claims 1 to 3, wherein the step of tilting the portion of cableway comprises arranging the tilted portion at an angle which is between the longitudinal axis of the portion of on-board cableway and the mean longitudinal axis of a deployed cable catenary.

5. A method according to claim 4, wherein the step of tilting the portion of cableway comprises arranging the tilted portion at an angle which is substantially midway between the longitudinal axis of the portion of on-board cableway and the mean longitudinal axis of a deployed cable catenary

6. A method according to any of claims 1 to 5, further comprising the step of cutting the length of damaged subsea cable prior to retrieval to the vessel in order to form first and second lengths of damaged cable and then retrieving the first length of damaged cable to the vessel and then retrieving the second length of damaged cable to the vessel.

7. A method according to claim 6, further comprising the step of removing or repairing any damaged portions of said first or second lengths of damaged cable in order to form first and second lengths of repaired cable.

8. A method according to claim 7, further comprising the step of recording the length of any removed damaged portions.

9. A method according to any of claims 6 to 8, further comprising the steps of providing an intermediate section of replacement cable having a length which is substantially equal to the recorded length of removed damaged cable and attaching the first end of said intermediate section of cable to the first length of repaired cable at a first joint, and attaching the second end of said intermediate section of cable to the second end of repaired cable at a second joint in order to form a continuous length of repaired cable.

10. A method according to any preceding claim, further comprising the steps of manoeuvring the vessel and or actuating cable actuators in order to adjust the position of the or each portion of retrieved cable in order to selectively position ends of the or each portion of retrieved cable and or intermediate cable in the vicinity of a jointing bay on board the vessel in order to facilitate joining of said sections.

11. A method according to any preceding claim comprising the step of arranging the retrieved cable and or intermediate length of cable over a horizontally arranged cable quadrant on board the vessel.

12. A method according to any of claims 6 to 11 further comprising attaching a buoy to the first and second lengths of damaged cable in order to facilitate retrieval thereof to the vessel.

13. A method according to claim 12, comprising retrieving both ends of cable to the vessel and, repairing the cable and re-joining the cable and any intermediate section at at least a joint on-board the vessel in order to form a loop of retained cable.

14. A method according to any preceding claim, wherein the weight of resulting suspended length of cable running from the vessel toward the seabed is selectively held by suitable holding means such as hold backs, tensioners or other holding devices in order to temporarily prevent movement of the suspended cable relative to the vessel when desired yet to selectively allow movement of the suspended cable relative to the vessel when desired.

15. A method according to either of claims 13 and 14 further comprising the steps of manoeuvring the vessel and or actuating cable actuators in order to thread the loop of retained cable relative to the vessel thereby progressing the or each joint in the repaired cable to a substantially straight section of cable residing on the seabed.

16. A method according to claim 15, wherein the step of lowering the loop of retained cable to the seabed further comprises controlling the relative lengths of winch line paid out and retracted from an aft and fore winch connected to a cable quadrant in order to retain the cable quadrant in a substantially upright position during re-laying of the loop of retained cable toward the seabed.

17. A method according to claim 16, further comprising retracting only one of the aft or fore winch lines once the re-laid cable rests upon the seabed in order to tilt the cable quadrant and thereby release the previously retained loop of cable previously held by the cable quadrant and then retracting both of the aft and fore winch lines in order to retrieve the cable quadrant to the vessel.

18. Subsea cable laying apparatus for laying a length of subsea cable from on-board a vessel, the apparatus comprising: - a portion of on-board cableway adapted to allow cable to rest thereon and to translate longitudinally thereover;

19. Subsea cable laying apparatus according to claim 18, wherein the tiltable arrangement comprises a tiltable over-boarding chute having a mouth section which is curved in the longitudinal direction in order to facilitate and support bending of any cable passing thereover when transitioning between the on-board cableway longitudinal axis and the suspended cable catenary longitudinal axis in order to minimise any structural stress imparted upon any repaired section or joint passing there over.

20. Subsea cable repair apparatus according to claim 19, wherein the tiltable over- boarding chute comprises a mouth section which is curved in the lateral direction in order to facilitate and support bending of any cable passing there past when transitioning between the cableway longitudinal axis and the suspended cable catenary longitudinal axis in order to minimise any structural stress imparted upon any repaired section or joint passing there over.

21. Subsea cable repair apparatus according to any of claims 18 to 20, wherein the portion of on-board cableway comprises a plurality of rollers along its length and which facilitate supported longitudinal movement of the cable thereupon.

22. Subsea cable repair apparatus according to any of claims 18 to 21, wherein the tiltable over-boarding chute comprises a plurality of rollers along its length and which facilitate supported longitudinal movement of the cable thereupon.

23. Subsea cable repair apparatus according to any of claims 19 to 22, wherein the tiltable over-boarding chute comprises a lattice-work space frame.

24. Subsea cable repair apparatus according to any of claims 18 to 23 further comprising a ram arrangement adapted to selectively actuate the extendible cableway between a retracted configuration inboard of the vessel and an extended configuration overboard the vessel.

25. Subsea cable repair apparatus according to claim 24, wherein the ram arrangement comprises a pair of hydraulic rams mounted upon either side of the cableway

arrangement.

26. Subsea cable repair apparatus according to any of claims 18 to 25, wherein the tiltable arrangement is provided with actuation means capable of increasing or decreasing the angle of tilt relative to the portion of on-board cableway in order to provide a resultant cable deployment path having minimal angles of deviation there along and whose cable deployment path may be altered by adjustment of said angle of tilt depending upon laying conditions.

27. Subsea cable repair apparatus according to claim 26, wherein the actuating means comprises a wire and winch arrangement connected to the tiltable arrangement.

28. Subsea cable repair apparatus according to any of claims 18 to 27, further comprising a cable reel arrangement comprising a length of intermediate cable for connection between first and second lengths of damaged cable at first and second joints.

29. Subsea cable repair apparatus according to any of claims 18 to 28, wherein a pair of cable guide posts are provided in order to guide the first or second end of a length of retrieved cable into alignment with the respective cableways.

30. Subsea cable repair apparatus according to any of claims 18 to 29, wherein the cableway arrangement is telescopically extendible.

31. Subsea cable repair apparatus according to any of claims 18 to 30, further comprising a cable quadrant adapted to retain a loop of cable thereover during repair of the cable.

32. Subsea cable repair apparatus according to any of claims 18 to 31, further comprising a jointing bay for facilitating repair and jointing of the damaged section of cable.

33. A method of installing cable repair facilities on a vessel, the method comprising installing the apparatus of claim 18 on the vessel.

34. A method of removing cable repair facilities from a vessel, the method comprising removing the apparatus of claim 18 from the vessel.

35. A vessel comprising the equipment of claim 18.

36. A method of moving the location of a cable bight point along a length of cable suspended between a vessel and the seabed, the method comprising: - providing a bight point support on the vessel, wherein the bight point support is adapted to support the cable at or toward a first bight point such that the cable extends from the cable toward the seabed and wherein at or toward the seabed a second bight point is created; and

37. A method according to claim 36, further comprising the step of monitoring the bight point position adjacent the seabed by way of a cable mounted monitoring device.

38. A method according to either of claims 36 or 37, further comprising the step of assessing the stresses and or strain which may have been placed upon the cable whilst said first and or second bight points are at a particular longitudinal location along the length of cable and, depending upon the outcome of that assessment either maintaining, lowering or raising the vertical position of the bight point support in order to move the longitudinal locations of said the or each bight point.

3 . A method according to any of claims 36 to 38, comprising correlating the vertical position of the bight point relative to tidal cycles.

40. A method according to any of claims 36 to 39, comprising maintaining a constant tension on a winch wire connected to the or each bight point support in order to maintain a constant tension on any cable suspended from the vessel by the apparatus.

41. Apparatus for moving the location of a cable bight point along a length of cable suspended between a vessel and the seabed, the apparatus comprising: - bight point support means on the vessel, the bight point support means being adapted to support the cable at or toward a first bight point such that the cable extends from the cable toward the seabed and wherein at or toward the seabed a second bight point is created, and the bight point support being adjustably mounted to the vessel such that the height of the bight point support may be altered whilst supporting the cable in order to move the longitudinal location of the or each first or second bight points along the cable.

42. Apparatus according to claim 41, further comprising at least a cable mounted monitoring device adapted to monitor the location of the cable bight point adjacent the seabed.

43. Apparatus according to claim 42, wherein the cable mounted monitoring device comprises a visual cable monitoring device adapted to obtain and transmit visual information on the cable bight point adjacent the seabed.

44. Apparatus according to either of claims 42 or 43, wherein the bight point support means comprises an aft-ward profiled deflector adapted to act as a fulcrum over which the cable suspended from the vessel rests upon during transition of the cable from a substantially horizontal configuration along the vessel deck to a catenary configuration suspended from the vessel to the seabed.

45. Apparatus according to claim 44, further comprising a fore-ward profiled deflector positioned to guide the length of cable over the aft-ward profiled deflector and to retain the forward portion of cable toward the deck of the vessel.

46. A cable recovery, repair and re-laying service comprising: - positioning a plurality of vessels having equipment according to claim 18 installed thereon at different geographical locations and; raising a service fee to subscribers for the purpose of allowing said subscribers to have their cables repaired by an appropriately located vessel when required.