Abstract:
A system for abandonment or recovery of a load, e.g., a pipeline or sub-sea structure, includes: at least two cables, each with a length adapted for a certain water depth and with a first end and a second end; a winch system for the at least two cables, the first ends of each cable being connected to the winch system; and a connection device for connecting the second ends of each cable to the load to be abandoned or recovered.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to deepwater pipeline laying and more particularly to an abandonment and recovery (A&amp;R) system and method, and an A&amp;R cable connector, in particular for deep water applications. 
     2. Background of the Related Art 
     The J-lay method is commonly used for deepwater pipelaying. With this method each portion of the pipeline to be laid is erected to a vertical position, welded to the main pipe line and lowered directly into the water with a single bend, giving the pipeline which is being laid the form of a “J”. 
     Also the S-lay technique is becoming more and more popular for deepwater and ultra-deepwater pipelaying. With this method each portion of the pipeline to be connected is welded to the main pipeline and moved in a horizontal position over the stinger into the water, with a double bend giving the pipeline which is being laid the form of an “S”. The S-lay technique has the advantage that it allows higher lay rates. 
     When a load, such as a pipeline or a sub-sea structure must be abandoned from a vessel, a typical method consists of welding an abandonment and recovery (A&amp;R) head with a lifting eye to the end of the pipeline or sub-sea structure. A coupling shackle connected to a cable coupled to a winch on the vessel is connected to the lifting eye of the A&amp;R head and the pipeline or sub-sea structure is lowered to the seabed. 
     J-lay and S-lay for deepwater applications entail not only the use of heavy tension equipment, but also require extra winch capacity and in particular cables with a very high tensile strength to support the high load resulting from the long length of pipeline, which initially extends from the vessel to the seabed. However, the diameter of the cable is typically limited, so that choosing to use a thicker cable at these lengths cannot solve the extra capacity needed. Also winches capable of handling loads of 500 T or more are not common and expensive. 
     Another problem of the existing A&amp;R systems and methods relates to the control of the movement of the cables, and especially the rotation thereof during abandonment or recovery. 
     International patent application WO 01/48410 discloses a method of abandoning a pipeline being laid by a vessel having an A&amp;R system, wherein a sealine is initially held by a pipe laying and tensioning arrangement. The method includes a step of reducing the tension at sea level by connecting one or more light buoyant elongate members to the end of the pipeline. This method has the disadvantage that special buoyant elongate members are needed and that the connection thereof may be time consuming. Moreover this technique is not well suited for S-lay because the buoyant elongate members have to pass over the stinger. 
     Hence, it would be desirable to provide an A&amp;R system and A&amp;R cable connector that solves at least partly the problems discussed above. 
     SUMMARY OF THE INVENTION 
     According to a first aspect, the present invention provides a system for abandonment or recovery of a load, typically a pipeline, at sea, and in particular in deep and ultra-deep water. In order to be able to handle heavy loads the A&amp;R system according to the invention comprises at least two cables, each with a length adapted for a certain water depth, and in particular for deep water. These two cables are connected with first ends to a storage of a winch system for providing a suitable cable length. Connecting means for coupling the second ends of each cable to the load to be abandoned or recovered, are also provided. By using more than one cable the tension weight is divided over the different cables, so that thinner cables can be used. The connecting means and the winch system ensure that the cables do not get intermingled during abandonment or recovery of a load. 
     Ultra-deep water applications preferably use at least three cables, and most preferably four cables. 
     According to a further aspect of the invention the A&amp;R system preferably comprises an even number of cables. This has the advantage that the tendency of the cables to untwist under load can be compensated by using the same number of steel cables with a right-hand lay as with a left-hand lay. 
     According to a preferred embodiment of the invention the A&amp;R system comprises further traction regulating means for adjusting the traction in each of the cables such that the rotation of the multi-cable assembly, especially at the end that is held together by the connecting means, is controllable. The traction regulating means can for example consist of at least one traction winch for each cable. 
     The A&amp;R systems of the prior art had only one cable and did not allow the rotation of the connecting means to be controlled in such a simple yet very effective manner. 
     This rotation control can be further improved by providing a rotation-measuring device for measuring the rotation of the connecting means. This measurement can be fed back to a control device for controlling the traction regulating means. In that way a very precise control of the rotation is possible. 
     Another aspect of the invention relates to the winch system. The winch system preferably comprises at least a storage winch and a traction winch for each cable. 
     According to a preferred embodiment the winch system comprises: 
     at least a storage winch and a traction winch for each cable, placed at one end of the vessel; 
     at least a return sheave for each cable placed at the other end of the vessel; and 
     a number of sheaves for guiding the cables from the storage winches via the traction winches to the return sheaves. 
     Such a system leads to longer cable lengths and has the advantage that a more structural damping is obtained and that the system is dynamically improved. 
     According to the preferred embodiment of the invention the connecting means comprise a cable connector which is provided with at least one hook, in which cable connector the second ends of the at least two cables are grouped. 
     According to a further aspect of the invention there is provided an abandonment and recovery cable connector. The cable connector comprises an elongate body with a first end and a second end. At said first end at least one hook is provided, which hook can be coupled to the load to be recovered or abandoned. The connector further comprises connection members for the at least two cables, and optionally also a rotation measuring device for measuring the rotation of said body. 
     According to a preferred embodiment the A&amp;R cable connector comprises one central hook capable of recovering a load and two side release hooks capable of abandoning a load via an intermediate sling. A first end of this intermediate sling is coupled to a first side hook, while the sling passes through a lifting eye connected with the load, and is coupled at its second end to the second side hook. Such a construction has the advantage that abandonment can take place very rapidly. Also by providing two side hooks, a certain redundancy is introduced, whereby even in the event that one side hook is not working properly the A&amp;R system can still be used. Immediately after placement of the load on the seabed, the release of one of the side release hooks is opened. The cables are pulled in whereby the sling being still connected to the other side release hook is pulled out of the shackle of the opened release hook. Further pulling the cables in by means of the winches on the vessel will lift the cable connector from the sea bed. 
     According to yet another aspect of the invention there is provided a method for abandoning or recovering a load, typically a pipeline or sub-sea structure, at sea. The method uses a multi-cable assembly with at least two cables for lowering, respectively raising a load during abandonment, respectively recovery of a load. Preferably the traction in each of said at least two cables is adjusted in function of the desired rotation of the load end of the multi-cable assembly. The rotation control has the advantage that the rotation of the multi-cable assembly can be kept small, and hence that the tension and torsion in the cables can be kept limited. Also the rotation control improves the positioning accuracy of the connecting means, especially during recovery of the load, so that the recovery time can be reduced. 
     According to a further improvement of the method of the invention the rotation at the load end of the multi-cable assembly is measured and the traction in each of said at least two cables is adjusted on the basis of the measured rotation. In that way a certain feed-back is introduced, further increasing the accuracy of the rotation adjustment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top view illustrating schematically a part of a vessel provided with a A&amp;R system according to a preferred embodiment of the invention; 
         FIG. 2  is a perspective view of the wiring system with cable connector according to the preferred embodiment of the invention; 
         FIG. 3  is a detailed perspective view of the hook end of the cable connector according to the preferred embodiment of the invention; 
         FIG. 4A  is a side view of the cable connector according to the preferred embodiment of the invention; 
         FIG. 4B  is an axial section of the cable connector according to the preferred embodiment of the invention; 
         FIG. 4C  is a cross section along line C-C of  FIG. 4A ; 
         FIGS. 5A and 5B  illustrate the abandonment procedure, and in particular the removal of the cable connector after placement of a pipeline on the seabed; 
         FIGS. 6A-C  illustrate the recovery procedure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows schematically a top view of a vessel which is provided with an A&amp;R system according to a preferred embodiment of the invention. The winch room  17  is situated at one end of the vessel  1 . In the winch room  17  four storage winches  2  and four traction winches  6  are provided. From each storage winch  2  a cable  3  is guided via travelling sheaves  4 ,  5  to traction winch  6 . Each cable  3  is brought in the axial direction of the vessel via sheaves  7 ,  8 ,  9 ,  10 , respectively. The four cables  3  leave the winch room in the longitudinal direction of the vessel, from where they are guided to the other end of the vessel. At this other end return sheaves  11 ,  12  direct the cables back into the opposite direction. Each cable  3  is connected with its first end to a storage winch  2 , and with its second end to a cable connector  13 . The cable connector will be discussed in more detail when referring to  FIGS. 3 and 4 . A perspective view of this winch system with four cables and cable connector is shown in  FIG. 2 . As can be seen in  FIG. 2  a number of additional guide rolls  20 ,  21  may be provided. 
       FIG. 3  shows a detailed perspective view of one end part of the cable connector  13 . This end part is provided with one central hook  30  with safety latch  34 . Said hook  30  is used for recovery of a load. During a recovery operation the safety latch is opened and the so-called “fishing” operation takes place, wherein the operator has to place a sling coupled to the A&amp;R head in the hook, as will described in detail when referring to  FIGS. 6A-C . 
     The end part of the A&amp;R cable connector is further provided with two release side hooks  32 . These release side hooks are provided with opening means which are operated when a sling has to be removed from a side hook during abandonment. This will be further elucidated with respect to  FIG. 5 . The operation of the opening means of the release side hooks can for example be controlled with acoustics signals, or mechanically, using a robot. The cable connector further comprises four connection members  35  which are provided with load and torsion sensors. 
       FIGS. 4A  and B show a side view and an axial section of the cable connector, respectively. The main body of the cable connector consists of a number of half shells  36  which are bolted together. By using this modular structure for the cable connector, it can be given a suitable length and weight by adding a sufficient number of shells  36 . The minimum length of the cable connector will typically be a function of the construction of the ship and stinger. The weight of the cable connector will typically be chosen in function of the cable geometry.  FIG. 4C  shows a cross section of the cable connector in which the arrangement of the four cables  3  is visible. 
     The A&amp;R cable connector may comprise other electronic means e.g. for controlling the operation of the central and side release hooks. According to the preferred embodiment the A&amp;R cable connector is further provided with means for measuring the rotation of the cable connector. The means for measuring the rotation can for example be a number of gyroscopes. A signal based on the measured rotation is transferred to a control system that is coupled with traction regulating means for regulating the traction in the cables. The transfer of measurement data can for example take place by means of an acoustic modem. In the embodiment of the figures the traction regulating means comprise the traction winches  6 . By providing one pair of traction winches  6  for each cable the traction in each cable can be adjusted accurately. Hence the rotation of the cable connector  13  can be controlled in an improved manner using the rotation feedback signal that is sent to the control system of the traction regulating means. 
       FIGS. 5A  and B illustrate the abandonment procedure after placement of a load, here a pipeline, on the seabed. In  FIG. 5A  the pipeline is coupled to the A&amp;R cable connector  13  via an A&amp;R head  52  that is welded to the pipeline. The A&amp;R head  52  is provided with a lifting eye  54 . An intermediate sling  53  passes through this lifting eye  54  and is coupled with its first and second end to a first and second side hook  32 , respectively. To abandon the load on the seabed, the vessel first sails backward without paying in the cables to reduce the tension in the wires. Then one of the release side hooks  32  is opened, and the intermediate sling  53  is pulled out of the shackle by pulling in the A&amp;R cables. Finally the cables are further pulled in to lift the cable connector from the seabed. 
     Now the recovery procedure will be described with reference to  FIGS. 6A-C . The cable connector  13  is guided along the stinger in the direction of the sea bottom. During this lowering of the cable connector the rotation of the cables may be controlled by adjusting the traction in the cables as described above. When the cable connector has almost reached the bottom, the vessel is positioned such that the cable connector is in the vicinity of the recovery sling. Now the so called fishing operation can start. To increase the “fishing” chances a buoyancy assembly  60  is provided. This assembly comprises a sling  61  coupled to the A&amp;R head  52  and a buoyancy element  62  connected to the sling  61 . First the rotation of the hook  30  is checked and next the sling is guided into the hook  30 , e.g. with the help of a robot  63 , whereupon the safety latch  34  is closed. Next the vessel is moved over a certain distance so that the A&amp;R cables  3  are brought in an inclined position as shown in  FIG. 6C , wherein the tension in the cables is increased. The load in the cables  3  is verified and if necessary adjusted by regulating the traction in the cables and then the actual recovery the pipeline can start. 
     Although illustrative embodiments have been shown and described, a wide range of modification, change and substitution is contemplated in the foregoing disclosure and in some instances some features of the embodiments may be employed without a corresponding use of other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the embodiments disclosed herein.