Abstract:
There is provided subsea repair apparatus for performing repair of a subsea cable located beneath the sea, said apparatus comprising: an environment capsule capable of providing a substantially water-free environment within the capsule; and repair equipment located within the environment capsule arranged to repair said subsea cable without the need for a person to be located within the environment capsule.

Description:
FIELD OF THE INVENTION 
     The invention relates to the repair of subsea cables. 
     BACKGROUND OF THE INVENTION 
     Direct Electrical Heating (DEH) is a method for preventing wax and hydrates forming in subsea production pipelines of oil and gas. DEH is based on the fact that an electric alternating current (AC) in a metallic conductor generates heat in a single phase circuit, and DEH may be performed as follows. One cable is connected to the first end of the pipeline and a single core cable is piggybacked on (ie supported by) the pipeline and connected to the far end of the pipeline. The two cables together with the pipeline form a single phase electrical circuit. The single core piggyback cable is either strapped directly to the pipeline or located inside a mechanical protection profile which is strapped to the pipeline. 
     A traditional method for cable repair is to cut the cable subsea at the fault location, pull one end of the piggyback cable to the surface on a vessel and join the piggyback cable with an excess cable length stored on the vessel. The excess cable length is approximately 2.5 to 3 times water depth. The other end of the damaged cable is then pulled to the surface and dry spliced with the other end of the excess cable length. The piggyback cable is then re-installed on the pipeline with the excess cable length loop installed perpendicular to the pipeline. After electrical testing of the cable, the excess cable loop is rock dumped. 
     The following patent publications describe this existing technology: 
     EP1381117B1 (U.S. patent, filed 1982 Sep. 13, US NAVY) 
     U.S. Pat. No. 4,479,690 (European patent, filed 2003 Jul. 8, NEXANS) 
     There are some problems with the existing technology. With the current technology an excess length of 2.5 to 3 times the water depth is installed perpendicular to the pipeline and needs to be rock dumped. The pipeline also needs to be rock dumped in this area in order to avoid pipeline buckling. This is a time consuming operation with high cost. This operation requires a typical weather window of H s &lt;3 m in 24 hours. 
     For ultra deep water the excess cable length is up to 9 km which in some cases is longer than the length of the piggyback cable and pipeline. The total cost for a repair using existing technology can therefore be very high in ultra deep water. 
     In addition, the existing technology requires that the piggyback cable is able to carry its own weight at relevant water depth. This is not a challenge at 300-400 m water depth but for water depth in the area of 1000 m and deeper the copper conductor is not able to carry its own weight. This makes a repair scenario using existing technology very challenging. 
     The following documents also describe methods for repairing subsea cables. Chinese utility model CN 200949707Y (Shengli) describes a working cabin to allow maintenance of underwater cables without a surface boat. JP 4067711 (Hitachi) describes a capsule within which people may work underwater to cut a submarine cable. RU 2,336,196 (Uchrezhdenie) describes a compartment which allows personnel to work underwater. JP 10-145955 describes a container  102  filled with an insulating liquid  101  whose specific gravity is higher than water. Underwater cutting and connecting of cables  103  is performed in the liquid  101 . 
     SUMMARY OF THE INVENTION 
     The invention provides a method and apparatus as set out in the accompanying claims. 
     Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a perspective view showing an environment capsule deployed to repair a subsea cable; an 
         FIG. 2  is a perspective view of the interior of the environment capsule. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     We describe a method of performing cable joint activity by remote control subsea in a seawater free environment. 
       FIG. 1  shows a pipeline  2  located on a seabed  3  and provided with a DEH cable  4 . The DEH cable  4  is also known as a piggyback cable because in use it is secured along (ie piggybacked on) the pipeline  2 .  FIG. 1  shows a portion of the DEH cable  4  which has been released from the pipeline  2  in order to carry out a repair of the DEH/piggyback cable  4 . A capsule  6  is lowered to the pipeline  2  by means of a support cable  8 , which may be wound around a suitable winch (not shown) on a surface vessel (not shown). Alternatively the capsule  6  may be lowered to the seabed  3  in a container or basket  9 , in which case an umbilical cord (not shown) may be provided between the basket  9  and the capsule  6  in order to provide electrical and/or hydraulic power and/or control signals to the capsule  6 . The capsule  6  may be provided with continuous tracks ( 40 ) or other suitable means for allowing the capsule  6  to move around the seabed  3 . 
       FIG. 2  shows the interior of the capsule  6 , which provides a seawater free environment within which repair of the DEH cable  4  may be carried out. Within the capsule  6  there is provided a manipulator arm  10 , which may be a marinised robot or manipulator arm  10  or a remotely operated vehicle (ROV) arm or similar, supported by a support rail  12  which is fixed to opposite sides of the capsule  6 . More than one such arm  10  may be provided if necessary. 
     In this embodiment the capsule  6  is substantially rectangular in shape, and has four side walls  14  (three of which are visible in  FIG. 2 ) arranged as two orthogonal pairs. Two opposite side walls  14  are provided with shaped openings  16  which are arranged to accommodate the cable  4  when the cable  4  is lifted by ROV arm  10  into the water-free environment within the capsule  6 . Each opening  16  is formed by a skirt portion  18 , wherein the skirt portion  18  is formed from a portion of its respective side wall  14 , and wherein the bottom edge of the skirt portion  18  follows a line which is at substantially the same height as the bottom of the side walls  14 , thus ensuring that water does not enter the interior of the capsule  6  via the openings  16 . 
     Two support arms  20  are each pivotally mounted, each about a vertical axis, to respective side walls  14  by hinges  22  (one of which is visible in  FIG. 2 ). Each support arm  20  is provided, at the opposite end to hinge  22 , with a cable holder  24 , into which the cable  4  can be placed by the remotely controlled arm  10 . Each cable holder  24  is generally cup-shaped, or of generally semi-cylindrical shape, so that it can receive and hold the cable  4 . 
     The environment within the capsule  6  is open at the bottom part (floor), but keeps the water out by filling the environment with gas or liquid which equalizes the water pressure as the environment is lowered from a surface vessel (not shown) to the seabed. The gas or liquid preferably has lower electrical conductivity than seawater, and preferably has an electrical conductivity of less than 0.1 Siemens per meter (0.1 S/m) at 20 degrees centigrade. Alternatively the gas or liquid may have an electrical conductivity of less than 0.2 Siemens per meter or less than 0.05 Siemens per meter. All the cable cutting, cable end preparation and cable splicing activities are performed by the remotely controlled ROV arm  10  in the seawater free environment subsea. 
     Typical steps in the method of repairing a DEH/piggyback cable  4  are as follows: 
     Damage to piggyback cable  4  is located by traditional test equipment and/or by a ROV. The water-free environment within capsule  6 , with all equipment including cable joint, is lowered from a vessel (not shown) to a position close to the pipeline  2  where damage to piggyback cable  4  is located. 
     A cable drum  26  with approximately 50 to 100 m of repair cable  28  is lowered close to the environment capsule  6 . Each end of the repair cable  28  can be prepared for jointing on the surface vessel prior to being lowered to the seabed  3 . The end of the repair cable  28  is provided with a heat shrink cap  29  to protect against water ingress. In  FIG. 1  a winch  30  is provided for pulling the repair cable  28  along the pipeline  2  by means of a wire  32  which passes through a running block  34 . The running block  34  is attached to the pipeline  2  by means of a clamp  36 . Although it can be convenient to pull the repair cable  28  along the pipeline  2  in this manner, the repair cable  28  must then be positioned in a gap  38  (described below) in the DEH cable  4  in order to effect the repair of the DEH cable. 
     Straps (not shown) which fix the piggyback cable  4  to the pipeline  2  are cut by a separate ROV at a distance of up to 50 m on each side of the piggyback cable damage. As an alternative, the remotely controlled arm  10  within the capsule  6  may be used. This allows a portion of the piggyback cable  4  to be separated from the pipeline  2  as shown in  FIG. 1 . 
     The piggyback cable  4  is cut at a fault location and relocated parallel to the pipeline  2 , as shown in  FIG. 1 . 
     The environment capsule  6  is located above the piggyback cable  4  and the piggyback cable  4  is picked up by remotely operated manipulator arm  10  and fastened in holders  24  in the seawater-free area of the environment, as shown in  FIG. 2 . 
     The first end of the repair cable  28  from the drum  26  is guided into the seawater free area of the environment capsule  6 . 
     The piggyback cable  4  is cut approximately 10 to 50 m from the fault location in order to remove a length of cable  4  where water may be trapped inside the conductor. This creates a gap  38  in the piggyback cable  4 . 
     The cable&#39;s outer sheath is thoroughly cleaned in order to avoid any contaminations. A cable joint body  26  is threaded onto the piggyback cable prior to cable preparation and protected against contaminants. Cable end preparations start by removing outer sheath and preparing insulation system according to cable joint requirements. Several video cameras (not shown) installed inside the environment capsule  6  continuously monitor the cable preparation work in order to make sure the work is done according to requirements. 
     A connector/sleeve (not shown) located inside the joint body  26  is installed/clamped on the cable conductors thereby mechanically and electrically connecting the conductors. 
     The cable ends at the joint area are thoroughly cleaned for any contaminations before a joint body is guided onto the cables splice area at the correct position. A protection sheath (not shown) is installed over the joint body and cable sheath for sealing purposes. 
     Once the repair is complete the gap  38  in the DEH/piggyback cable  4  is filled by a portion of the repair cable  28  which replaces the removed portion of the DEH/piggyback cable  4 . The capsule  6  and winch  26  can then be removed. A loop may be left in the repair cable  28  to ensure that very little tension is applied to the cable splices during operation, and particularly in the case of any expansion of the pipeline  2 . 
     The embodiment described allows cable repair in a seawater free pressurized environment located on the seabed. Cable cutting, preparation and joining are performed by remote control in a substantially water-free environment located on the seabed. Various types of cable can be repaired, including umbilical cables. 
     Some advantages of the described system are listed below: No excess cable length installed perpendicular to pipeline. 
     No need to have long spare cable lengths stored for repair scenario. 
     No rock dumping of excess cable loop is required. 
     No water depth limitations as is the case for standard copper cable (which is required to carry its own weight as described above). For water depths greater than about 1000 m traditional repair technology is not considered feasible. 
     Cost and time efficient cable repair. 
     Repair operation is less sensitive to weather/sea conditions. 
     Excess cable length of 2.5 to 3 times water depth is not needed. 
     A feasible method for cable repair in ultra deep water is provided (not limited by mechanical characteristics of the copper conductor). 
     Cable joint operations are performed in a pressurized environment with the advantage that the splice is exposed to minimal differential pressure from preparation mode to operation mode. 
     It will be appreciated that repair of cable  4  is carried out without the need for a person to be present within the capsule  6 . The arm  10  may be remotely controlled by an operator who is located outside of capsule  6 , for example on a surface vessel (not shown). However, other embodiments are possible in which the arm  10  is automated or at least partly automated so that not all of the repair steps need to be controlled by the remote operator. 
     The capsule  6  may be provided with continuous tracks ( 40 ) or other driving means, for moving the capsule around on the seabed  3 , either autonomously or under the control of a remote operator. 
     The capsule  6  may be provided with one or more umbilical cords, either from the basket/container  9  or directly from a surface vessel (as in the case of cord  8  in  FIG. 1 ). The umbilical cord or cords may provide electrical and/or hydraulic power to the capsule  6 , control signals for the repair apparatus within the capsule  6  such as the arm  10 , and/or low conductivity gas or liquid for filling the capsule  6 . After the repair the low conductivity gas or liquid may be removed from the capsule  6  via an umbilical cord.