Abstract:
An ocean bottom cable system includes: a cable adapted to be extended from a vessel at the surface of a body of water to the bottom of a body of water. The cable includes at least one electrical conductor or at least one optical fiber. A plurality of sensor units is disposed at spaced apart locations along the cable; and at least one swivel is disposed in the cable between the vessel and at least one of the sensor units. The swivel is adapted to enable relative rotation thereby relieving torsional stress between ends of the cable coupled thereto, and is adapted to transmit axial force along the cable therethrough.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     Not applicable.  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not applicable.  
       BACKGROUND OF INVENTION  
       [0003]     1. Field of the Invention  
         [0004]     The invention relates generally to the field of ocean bottom cable (OBC) seismic survey systems. More specifically, the invention is related to devices for improving the efficiency of deployment and retrieval of OBCs, and for reducing damage to OBCs during deployment and retrieval.  
         [0005]     2. Background Art  
         [0006]     Seismic surveying performed in bodies of water (marine seismic surveying), such as lakes or the ocean, includes surveying performed with ocean bottom cables.  
         [0007]     An ocean bottom cable (OBC) normally includes one or more electrical and/or optical conductors extending along the length of the cable and sensors, or sensor “units”, coupled with or disposed along the cable at spaced apart locations. The sensor units typically include one or more particle motion sensors, such as geophones or accelerometers, and at least one sensor responsive to pressure (or a sensor responsive to rate of change of pressure). Electrical and/or optical conductors in the cable conduct signals from the various sensors to a recording device typically coupled to one end of the cable.  
         [0008]     OBCs are typically deployed by unspooling the cable from a winch drum or reel located on a deployment vessel (called a “cable handling vessel”), allowing the cable to reach the bottom of the body of water. The cable handling vessel moves in a direction along which it is intended to position the OBC on the water bottom for a seismic survey. When the OBC is unspooled to an extent such that the sensor unit closest to the cable handling vessel reaches the water bottom, the cable handling vessel is typically stopped, but unspooling of the OBC continues until the portion of the cable extending from the cable handling vessel to the bottom of the body of water is substantially vertical. The portion of the cable extending from the recording system to the first sensor unit is normally referred to as the “lead in”. After unspooling is completed, a buoy or similar device may be attached to the water surface end of the OBC, such that a recording system may be coupled to the OBC for subsequent seismic data acquisition and recording. The recording system may, alternatively, be located on the cable handling vessel such that buoy connection is not required.  
         [0009]     It will be appreciated by those skilled in the art that as the “lead in” is created from the water bottom to the water surface by continued unspooling, tension that was applied to the OBC during deployment will be relieved, particularly at the end of the lead in near the water bottom. After completion of deployment, tension will be distributed along the lead in portion of the OBC in relation to the height above the water bottom of any part of the lead in.  
         [0010]     OBCs made for relatively shallow water may include a centrally disposed electrical conductor surrounded by a layer of insulation. The insulation may then be surrounded by an electrically conductive metal braid, which in combination with the central conductor serves as a coaxial cable. The exterior of the OBC is typically surrounded by a plastic jacket to exclude water and to provide electrical insulation. In such shallow water OBCs, there may be one or more reinforcement layers within the cable to provide axial strength to the OBC. Typically, in such OBCs the reinforcement layer is in the form of a woven fiber braid. Such shallow water OBCs, having only braided reinforcement devices, are substantially free of induced torque when tension on the cable is changed. Deployment of such OBCs is not typically associated with any difficulties relating to torque along the cable caused by tension. However, there is a tendency of such shallower depth OBCs to assume the shape of the winch or reel on which the OBC is wound under tension. As tension is relieved during deployment, the OBC may form loops where the OBC tries to return to its shape under tension. Such loops may not be relieved or unwound as the OBC is retrieved from the water bottom. In such cases, the loops may cause the OBC to kink when tension is reapplied as the OBC is retrieved from the water bottom. Kinking may damage the cable, thus necessitating expensive repair or replacement of the cable.  
         [0011]     As OBCs are made to be used in deeper bodies of water, it has proven necessary to use cable structures that have various forms of wound wire armor, in order that the cable will have sufficient axial strength to support its own weight when suspended in the body of water. For example, in a typical OBC used for water depths of 3,000 meters, the cable may include three, concentrically placed, helically wound layers of armor wires surrounding the center conductor and shield layer. When helically wound armor wires are subjected to axial stress, they impart a torque to the cable as they tend to unwind. While typical armored electrical cables include a plurality of contrahelically-wound layers of armor wires (meaning that successive layers are wound with opposing helical lay direction), it is impracticable to create a completely torque balanced, wound wire armored cable. Torque balanced in this context means that there is substantially no torque along the cable within a specific range of cable loads. In the foregoing example of a deeper water OBC, as the lead in is created, substantially all of the axial stress is relieved at the water bottom position of the lead in. Such stress relief generates substantial torque along the cable near the water bottom. Frequently, such torque will result in cable loops being formed. While such loops are by themselves not harmful, they can cause the cable to kink when the cable is retrieved from the water bottom.  
         [0012]     In multiple-cable OBC surveys, a plurality of OBCs are typically deployed on the water bottom substantially parallel to each other along a selected direction. Each OBC in the multiple-cable survey includes a lead in made substantially as described above for a single cable OBC survey. In a multiple cable OBC survey, however, the lead in for each of the cables is typically terminated at a common location at the water surface. During a multiple cable survey, a recording vessel is connected to the water surface ends of all the OBCs. During the survey, a laterally endmost one of the OBCs is disconnected from the surface location, and the recording vessel is moved laterally while still connected to several of the remaining OBCs. The disconnected OBC is retrieved by the deployment vessel and may be moved to a location along the opposed lateral end of the “spread” of OBCs on the water bottom. Lateral movement of the recording vessel imparts lateral tension along the connected OBCs and causes the cable to ‘roll’ along the water bottom. Such lateral movement is another source of torque which may result in loops in the OBCs. Just as in the case of the single OBC survey operation, when an OBC having loops therein is retrieved, the rapid application of axial stress may result in kinks in the cable as the torque along the loop cannot be quickly relieved.  
         [0013]     It is desirable to have a system for OBC surveying which reduces the possibility of looping and consequent kinking in the cable.  
       SUMMARY OF INVENTION  
       [0014]     One aspect of the invention is an ocean bottom cable system. A system according to this aspect of the invention includes a cable adapted to be extended from a vessel at the surface of a body of water to the bottom of a body of water. The cable includes at least one electrical conductor or at least one optical fiber. A plurality of sensor units is disposed at spaced apart locations along the cable; and at least one swivel is disposed in the cable between the vessel and at least one of the sensor units. The swivel is adapted to enable relative rotation between ends of the cable coupled thereto, and is adapted to transmit axial force along the cable therethrough. The swivel is also adapted to maintain electrical or optical contact between the at least one electrical conductor or the optical fiber in ends of the cable connected to the swivel.  
         [0015]     Other aspects and advantages of the invention will be apparent from the following description and the appended claims.  
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0016]      FIG. 1  shows a typically cable handling vessel deploying an ocean bottom cable (OBC) and system according to one embodiment of the invention.  
         [0017]      FIG. 2  shows a cross sectional view of a sensor section of the OBC system of  FIG. 1 .  
         [0018]      FIG. 3  shows a cross-sectional view of a swivel of the OBC system of  FIG. 1 .  
         [0019]      FIG. 4  shows an oblique view of the swivel shown in  FIG. 3 .  
         [0020]      FIG. 5  shows a segment of OBC including a swivel as shown in  FIG. 3 .  
         [0021]      FIG. 6  shows an end view of cable that may be used with a system as shown in  FIG. 1 . 
     
    
     DETAILED DESCRIPTION  
       [0022]     One embodiment of an ocean bottom cable (OBC) system according to the invention is shown in  FIG. 1  as it would be deployed in a body of water  10 . A cable handling vessel  12 , which may in some embodiments include seismic data recording equipment  15  of any type known in the art, moves in a selected direction along the surface  11  of the water  10 . A winch, reel or similar spooling device, shown generally at  14  is disposed on the cable handling vessel  12  such that an OBC  18  can be deployed from the cable handling vessel  12 , typically from its aft end. The winch  14  can be any type known in the art for deployment of marine seismic sensor cables, has a selected length of OBC  18  spooled thereon. The winch  14  extends the OBC  18  into the water  10  as the cable handling vessel  12  moves along the selected direction. The rate of unspooling and the speed of the cable handling vessel  12  are selected such that the OBC  18  eventually rests on the water bottom  16  in a substantially straight line along the direction of motion of the cable handling vessel  12 .  
         [0023]     The OBC  18  in the present embodiment includes a plurality of selected length cable segments  20 A, which may be formed from armored coaxial cable, as will be further explained with reference to  FIG. 6 . Each end of each cable segment  20 A is preferably terminated in an electrical connector (explained below with reference to  FIG. 2 ) which can couple to either axial end of a swivel  22 , a sensor unit  24  or a swivel cable section  22 A (shown in more detail in  FIG. 5 ). The OBC  18  can include at its distal end a weight  26  to urge the OBC  18  to rest on the water bottom  16  during deployment. A cable segment  20 A will typically be 25 to 50 meters in length. The lead in portion  20  of the OBC, extending from the sensor unit closest to the vessel  12  to the recording equipment  15  may typically comprise cable sections that are longer, such as 900 meters in length, but otherwise may be similar in structure to the cable segments  20 A.  
         [0024]     As the vessel  12  moves, and the OBC  18  is extended from the winch  14 , the OBC  18  comes to rest on the water bottom  16 . After the last sensor unit  24  is deployed so as to be proximate or on the water bottom  16 , the vessel  12  stops moving. The winch  14  continues to extend the lead in cable  20  such that it is substantially vertical from the water bottom  16  to the vessel  12 . In some embodiments, the vessel end of the lead in cable  20  can be coupled to a buoy (not shown) or other flotation device such that a recording vessel (not shown) may electrically couple to the lead in cable  20  at the water surface for power and data communication to the various sensor units  24  along the OBC  18 . In the present embodiment, the recording system  15  is on the deployment vessel  12  and thus no such buoy (not shown) is used. The surface termination and connection of the OBC used in any embodiment is not intended to limit the scope of the invention.  
         [0025]     The embodiment shown in  FIG. 1  includes only one OBC  18 , primarily for clarity of illustrating the principle of the invention. It is to be clearly understood, however, that the arrangement of the OBC  18  in  FIG. 1  is only an example of OBC systems within the scope of the present invention, and that the number of OBCs used in any implementation is not a limit on the scope of the invention. Moreover, the arrangement of sensor units  24  and swivels  22  in  FIG. 1  is only an example of such arrangements, and is not intended to limit the scope of the invention. For purposes of defining the scope of the invention, it is only necessary to have one such swivel  22 , preferably included within a swivel cable section  22 A in a position along the OBC  18  most susceptible to looping as tension (and resulting torque) on the OBC  18  changes.  
         [0026]      FIG. 2  shows a cross section of a typical sensor unit  24  and electrical connectors  27  used to terminate the ends of the cable segments ( 20 A in  FIG. 1 ). Each connector  27  includes a pressure resistant housing  27 E adapted to exclude fluid under pressure from entering an interior space thereof, and adapted to transfer axial stress or tension from the cable segment ( 20 A in  FIG. 1 ) to the housing  27 E, and then transfer the axial stress or tension to a mating housing  24 B of the sensor unit  24  to which the connector  27  is coupled. The interior of the connector housing  27 E includes a centrally disposed electrical contact  27 C coupled to a central electrical conductor (see  FIG. 6 ) in the cable segment ( 20 A in  FIG. 1 ). The central contact  27 C couples to a corresponding contact  24 A in the sensor unit  24 . A laterally displaced, outer electrical contact  27 D electrically connects a shield (see  FIG. 6 ) in a cable segment  20 A to a corresponding outer contact  24 F in the sensor unit  24 . The electrical conductor arrangement in the cable segment and connector  27  are only one example of connections than may be made between cable segments and a sensor unit. Other embodiments may include three or more electrical conductors in cable segments and a corresponding number of electrical contacts in the connector  27 . Still other embodiments may include one or more optical fibers in addition to or in substitution of the electrical conductors in the cable, and appropriate optical couplings may be included in such embodiments of the connector  27 . Accordingly, the electrical and/or optical configuration of the connector  27  is not intended to limit the scope of the invention.  
         [0027]     The connector  27  includes an external sealing surface  27 AA for engagement to a corresponding, sealing interior surface  27 G of the sensor unit housing  24 B. Sealing to exclude fluid entry can be effected by an o-ring  27 A or similar sealing element. A threaded coupling  27 B on the connector  27  engages a corresponding coupling  27 C on the interior surface of the sensor unit housing  24 B to effect the coupling of the connector  27  and the housing  24 B, and to effect transfer of axial stress therebetween.  
         [0028]     When a connector  27  configured as shown in  FIG. 2  is engaged to each axial end of the sensor unit housing  24 B, electrical contact is made between circuits  24 D disposed inside the sensor unit housing  24  and the electrical conductors (see  FIG. 6 ) in the cable segment ( 20 A in  FIG. 1 ), and axial stress is transmitted from the cable segment ( 20 A in  FIG. 1 ) through the sensor unit housing  24 B. As importantly, fluid is excluded from entering the sensor unit housing  24 B by the sealing engagement of the connectors  27  to the sensor unit housing  24 B.  
         [0029]     The circuits  24 D disposed in the sensor unit housing  24 B can include conventional seismic sensors such as particle motion sensors (shown as geophones  24 E) coupled to suitable signal amplification, processing, and telemetering circuitry (shown collectively, but not individually at  24 D) for communicating signals from the sensors  24 E to the recording system (such as  15  in  FIG. 1 ). The sensors  24 E may also include one or more hydrophones (not shown separately) or other sensor responsive to pressure and/or rate of change in pressure. Although the present embodiment includes geophones, as is known in the art, any other type of sensor responsive to motion, such as accelerometers, may be used in other implementations of a sensor unit.  
         [0030]     It should also be understood that the embodiment of sensor unit as shown in  FIG. 2 , which is intended to be coupled between cable segments, is only one implementation of a system according to the invention. The implementation as shown in  FIG. 2  is particularly suited to OBCs used in deeper water depths, e.g., up to about 3,000 meters depth. Implementation intended for shallower depth water may include sensor units coupled to the exterior of the cable segments, and the cable segments  20 A would connect directly to each other by connection means known to those of ordinary skill in the art.  
         [0031]     In the present embodiment, the cable segments ( 20 A in  FIG. 1 ) can be about 25 meters or 50 meters in length, thus the sensor units  24  are typically separated by about 25 meters or 50 meters.  
         [0032]      FIG. 3  shows a cross sectional view of one of a swivel  22 . The swivel  22  includes a first connector housing  30  sealingly, rotatably engaged to a second connector housing  31 . Sealing engagement in the present embodiment can be effected by o-rings  33  or similar sealing devices disposed on a seal extension  33 A forming part of the second connector housing  31 . The seal extension  33 A fits inside a corresponding receptacle in the first connector housing  30 . Each of the connector housings  30 ,  31  has disposed centrally therein an electrical connector  34  adapted to mate electrically and mechanically with the contact ( 27 C in  FIG. 2 ) in one of the cable segment connectors ( 27  in  FIG. 2 ). The seal extension  33 A is rotatably supported inside a receptacle in the first housing  30  by bearings  32 . Rotatable electrical contact can be obtained by a slip ring  35  or similar device. Interior surfaces of the axial outer ends of the housings  30 ,  31  are adapted to threadedly receive the threaded couplings ( 27 B in  FIG. 2 ) on a connector, such as connector  27 , shown in  FIG. 2 . In combination, the first housing  30 , second housing  31 , and connectors  27  define an apparatus that maintains electrical continuity between two connectors  27  coupled to each end of the swivel  22 , that maintains electrical insulation between conductors within each connector  27 , and enables relative rotation between the connectors  27  coupled to each end of the swivel  22 .  
         [0033]     An oblique view of the swivel  22  having protective caps  36  on each end for shipment is shown in  FIG. 4 . Preferably the exterior shape of the first housing  30  and second housing  31  is cylindrical to reduce the chance of rotational sticking during use of the swivel.  
         [0034]     In particular implementations of a swivel, the interior chamber of the swivel may be filled with dielectric liquid (not shown), such as oil. In some embodiments of a swivel, the dielectric liquid may be subjected to external hydrostatic pressure such as by means of a pressure compensating device (not shown), such as a piston or bladder of any type well known in the art, for such pressure compensation.  
         [0035]     While the swivel  22  shown in  FIG. 3  includes only one electrical conductor in the slip ring  35 , multiple conductor slip rings are known in the art and may be used in other embodiments of an OBC system in which there is more than one insulated electrical conductor forming part of the cable thereof. It is also know in the art to provide optical slip rings, to obtain a continuous, rotatable optical connection between two optical fibers. Other implementations of the swivel  22  may include one or more optical slip ring channels. As used in the context of this invention, therefore, the term “swivel” is intended to mean any device that maintains an electrical and/or optical contact between two members, while enabling relative rotation between the two members.  
         [0036]     In a preferred embodiment of an OBC system according to the invention, one or more of the cable segments, such as shown at  20 A in  FIG. 1 , may be substituted by a swivel cable section  22 A, such as shown in  FIG. 5 . A swivel cable section  22 A may include two, shorter cable segments  20 AA, with each end of each segment being terminated with a connector  27 , such as explained above with reference to  FIG. 2 . One connector  27  from each cable segment  20 AA is coupled to a swivel  22 , such as explained above with reference to  FIG. 3 . The other end of each of the shorter cable segments  20 AA is coupled to a sensor unit  24 . A swivel  22  may also be coupled between two cable segments  22 A in lieu of a sensor unit  24 , however, such placement of a swivel  22  would alter the regularity of the spacing of the sensor units  24 . Typically, a swivel  22  will be coupled directly between segments of the lead in portion of the cable, because no sensor units are used in the lead in portion of the OBC.  
         [0037]     A typical cable that may be used in various embodiments of a system according to the invention, such as for the lead in cable ( 20  in  FIG. 1 ), cable segments ( 20 A in  FIG. 1 ) or swivel cable segments ( 20 AA in  FIG. 5 ) is shown in end view in  FIG. 6 . The cable may include a central conductor core  40  consisting of a nylon monofilament strength member  42  surrounded by copper strands  41 . The strands  41  may be helically wound around the strength member  42 . The conductor core  40  may be surrounded by an insulation layer  43 , such as high density polyethylene (HDPE). The insulation layer  43  may be surrounded by a shield conductor layer  44 , which may include copper strands and supporting tape. An insulator  45  may surround the shield layer  44 . The cable is armor reinforced, in the present embodiment, by three, contrahelically wound layers  46 A,  46 B,  46 C of steel wires (which may be galvanized) to form armor  46 . While the foregoing embodiment of a cable includes only a single, centrally located electrical conductor (core  40 ), other embodiments may include a plurality of such electrical conductors surrounded by steel wire armor. See, for example, part no. A305338, Rochester Corporation, Culpeper, Va. 22701, which includes seven insulated electrical conductors in its core.  
         [0038]     While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.