Abstract:
A cable containing an optical fiber is used to transmit data between an underwater remotely operated vehicle (ROV) and a support vessel floating on the surface of the water. The ROV stores the cable on a spool and releases the cable into the water as the ROV dives away from the support vessel. The ROV detects the tension in the cable and the rate that the cable is released from the ROV is proportional to the detected tension in the cable. After the ROV has completed the dive and retrieved by the support vessel, the cable can be retrieved from the water and rewound onto the spool in the ROV.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 61/512,537, “Internal Winch For Self Payout And Re-Wind Of A Small Diameter Tether For Underwater Remotely Operated Vehicle,” filed Jul. 28, 2011. The entire contents of U.S. Patent Application No. 61/512,537 are hereby incorporated by reference. 
    
    
     BACKGROUND 
     With reference to  FIG. 1 , remotely operated underwater vehicles (ROV&#39;s)  101  are, widely used by industry and science for unmanned undersea work tasks. Some ROVs  101  require an electromechanical cable connection (tether)  105  to the surface for communications, power and vehicle recovery, which are typically located on a boat  109 . These cables  105  are thick and heavy because they contain the required electrical conductors to provide power to the ROV  101 . As the ROV  101  moves away from the boat  109 , the tether  105  is released from a tether storage device  111 . 
     In order to control the movement, the thrust  115  produced by the propulsion device  113  on the ROV  101  must be greater than the tension in the cable  105 . The tension on the cable  105  is generated by drag on the cable due to the movement of the cable  105  through the water. The total tension can be proportional to the wetted surface area of the cable  105 . Thus, more tension exists in the cable  105  and more thrust is required as the ROV  101  travels farther from the ship  109 . This can be problematic because cables  105  can be damaged when the tension exceeds a certain force. What is needed is an alternative system that prevents the over tensioning of the cable  105 . 
     SUMMARY OF THE INVENTION 
     The present invention is directed towards a system for preventing over tensioning of the cable tether between an ROV and a support ship. As the ROV travels away from the support ship, the ROV emits a thin optical cable. In an embodiment, the cable is pulled from the ROV by the tension from the cable or alternatively, the cable can be physically emitted from the ROV. Thus, the optical cable can be substantially stationary in the water while the ROV travels through the water. The ROV can have a tension sensor, a velocity sensor, an optical cable storage mechanism and a feeding system for releasing the optical cable from the ROV. 
     Various different types of cables can be used with the cable release mechanism. In an embodiment, the cable that only includes an optical fiber which can be used to transmit data between a battery-powered ROV and the support ship. The optical fiber can be encased in a plastic sheath that is surrounded by a high strength Kevlar sleeve. The cable can also include an abrasion resistant external coating which can be made of a high strength elastic material such as urethane. This optical fiber cable can be about 2.90 mm in diameter. In other embodiments, the optical fiber cable may only include the optical fiber without the high strength Kevlar sleeve. Although, the raw optical fiber cable can be much more fragile than the Kevlar sleeve cable, it can have a diameter of about 0.254. Thus, a spool containing a length of raw optical fiber cable will be much smaller than a spool containing an optical fiber cable having a Kevlar jacket an may be more suitable for certain types of applications. 
     If the ROV moves through the water without releasing the cable, the cable can be exposed to excess tension and the optical fiber can be damaged resulting in a communications failure. In order to prevent over tensioning the cable, the ROV can include a system which includes a cable storage unit, a cable tension sensor, a microprocessor and a cable release mechanism. The cable tension measurements can be converted into electrical signals which are transmitted from the tension sensor to the microprocessor. If the tension signal from the tension sensor exceeds a predetermined working tension of possibly 0.1-3.0 pounds, the microprocessor can cause the cable release mechanism to increase the rate at which the cable from the cable storage unit which can be a spool wrapped with the optical cable. The cable release can be reduced when the cable tension drops below a minimum tension which can be about 0.1 pounds. 
     In another embodiment, the ROV can also include one or more speed sensors which can transmit speed signals to the microprocessor. The sensors can determine how fast the ROV is traveling through the water and based upon this information, the microprocessor can cause the release mechanism to release the cable at a rate that is equal to or faster than the speed of the ROV. The cable release mechanism on the ROV can allow the ROV to travel deeper and farther away from the support ship which can greatly enhance the ability of the ROV to perform the required tasks. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a diagram of a ROV; 
         FIG. 2  illustrates a diagram of a ROV with a cable tension control mechanism; 
         FIG. 3  illustrates a view of an optical cable; 
         FIG. 4  illustrates a view of an optical cable spool; 
         FIG. 5  illustrates block diagram of the optical cable tension control mechanism components; and 
         FIG. 6  illustrates an embodiment of a cable release mechanism. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention is directed towards a system for storing and releasing an optical fiber cable that extends between an ROV and a support ship. ROVs typically require a tether cable to connect the undersea vehicle with the surface supplied electrical power so have power conductors in their tether cable together with a communications link which are typically fiber optic cables and steel wires to allow recovery. Alternatively, ROVs can be battery-powered, typically using rechargeable lithium battery packs. Such ROVs are able to use very small diameter armored fiber optic cable which can be about 1-3 mm in diameter for high bandwidth two way communications for command and control as well as to transmit sensor data such as HD video signals from vehicle to the surface. In an embodiment, the optical fiber cables can be about 2.9 mm in diameter or any other diameters less than about 5.0 mm. 
     Typically the ROV cable is held on a spool or a winch on the surface ship. The cable can be released from the surface ship into the water where it is dragged by the ROV through the water to the depth and distance required. Such dragging of the cable through water causes both skin friction and form drag which on long lengths of cable can generate sufficient force to overwhelm the maximum thrust capability of the ROV propulsion system. Hence, surface deployment of the tether creates drag forces on the tether and limits the practical depth and/or range that ROVs can be used. 
     The present invention includes ROVs that use small diameter armored optical fiber cables that are stored aboard and released from the ROV rather than from the surface ship. The ROV cable release system can include a tension sensor which can be coupled to drive motor which cause the cable to pay out from a storage spool as the vehicle moves through the water. Thus, the cable is not dragged but can be effectively stationary in the water where the ROV has travelled through. Since the cable is only pulled with a minimum tension through the water and the optical fiber cable generates almost no drag on the ROV. Thus, the ROV is freed from cable drag forces and it can move much more freely through the water. Because the cable drag has been effectively eliminated, the ROV is able to travel to greater depth and for longer ranges with less propulsion power required. 
     In an embodiment, the present inventive system can use substantially neutrally buoyant armored fiber optic cables with battery-powered ROVs. The neutral buoyancy causes the released cable to be substantially stationary in the water after it is released from the ROV. The neutral buoyancy also prevents the cable from floating or sinking quickly. This up or down cable movement would result in unwanted cable tension on the ROV. Because the cable drag has been eliminated, the ROV using the inventive system can be designed to operate at depths that exceed 7,000 meters and ranges that exceed 20,000 meters. These depths and ranges are many times greater than what has been proven possible with ROVs using surface-deployed cables which are released from the support ship and dragged through the water by the ROV. 
     With reference to  FIG. 2  a simplified drawing of an ROV  201  is illustrated. The cable  205  can be stored on a cable release mechanism  221  on the ROV  201 . As the ROV  201  travels away from the support ship  209 , the tension sensor  229  detects tension in the cable and the cable  205  can be released from a spool  553  in the release mechanism  221 . The cable  205  may not be stored on the support ship  209  or possibly a short length of the cable  205  can be stored on the support ship  209  and released into the water when the ROV  201  initially placed into the water and before travels away from the ship  209 . 
     During normal operations, the cable  205  stored on the spool  553  in the ROV  201  can be released as the ROV  201  moves through the water. The ROV  201  can move in any direction in the water by using horizontal and vertical thrust  215  to move the ROV  201  through the water. As the ROV  201  travels away from the support vessel  209 , the cable  105  is released and therefore it is not pulled through the water. Thus, the propulsion system  213  only needs to produce enough thrust  215  to overcome the hydrodynamic drag forces on the ROV  201  and possibly a small amount of tension from the cable  205  which can be less than about 3 pounds of force. Since the cable  205  is not being pulled through the water, the propulsion system  213  of the ROV  201  does not have any significant added drag forces which would be present if the cable  205  was being pulled through the water. The cable  205  can maintain an optical transmission path between the ROV  201  and a controller or communications device  558  on the support vessel  209 . 
     In order to minimize the tension on the cable  205 , the system can release the cable  205  at a rate that is greater than or equal to the speed of the ROV  201  through the water. In an embodiment, the ROV  201  may also include a speed sensor  227  which is coupled to the cable release mechanism  221 . As the ROV  201  moves, the speed sensor  227  can detect the movement and the cable release mechanism  221  can begin to release cable  205  at a speed equal to or even slightly greater than the velocity of the ROV  201  through the water. The ROV  201  can move in any path while leaving the cable  205  in the water. Because the system will normally keep the tension in the cable  205  to a nominal level, the cable tension should always be well below the maximum working tension. If the cable  205  is not released at a rate close to the speed of the ROV  201 , the cable  205  can be pulled by the ROV  201  creating tension in the cable  205 . The path of the ROV should also be controlled to prevent loops or over lapping routes that may cause the released cable  205  to become tangled. 
     After the ROV  201  travels a significant distance from the support ship  209 , there can be a significant amount of exposed cable  205  in the water. There can be some isolated areas of tension in the cable  205  that has been released into the water due to movement of the support ship or variations in water current at different depths or due to traversing “tide lines.” The amount of tension in the cable  205  can vary along the exposed length of cable  205 . However, since these non-uniform current movements can be minimal, the expected cable  205  tension caused by the support ship  209  and water movement should be nominal and well below the safe maximum operating tension of the cable  205  which might be about 50-75 pounds. This cable tension may be isolated to certain regions of the cable  209 . However, the cable  205  tension can also be transmitted to the ROV where it can be detected by the tension sensor  229  in the ROV  201 . When cable tension is detected, the system will release additional cable  205  from the ROV  201  to reduce the tension. 
     With reference to  FIG. 3 , an embodiment of a typical fiber optic cable  205  is illustrated. The cable  205  can have a center single-mode optical fiber  301  encased in a plastic sheath  303  surrounded by a high strength jacket member  305  which can be made of a high strength composite fiber such as Kevlar. The high strength jacket member  305  can be surrounded by an abrasion resistant external coating or layer  307  which can be made of urethane or other similar materials. Such cables  205  are becoming standard with outside diameter (OD) that is approximately 2.9 mm. Although the Kevlar-strengthened small diameter cable  205  illustrated in  FIG. 3 , may not mechanically break until being loaded to 400 lbs or more, the optical fiber core  301  can be damaged if a tension above 70 lbs is applied to the cable  205 . Thus, a small cable  205  may have a maximum safe working tension of about 50 lbs or less. In other embodiments, the cable  205  OD can be between about 1.0 and 5.0 mm and the strengths of these cables may have a maximum working strength that is less than or greater than 50 lbs. These cables  205  can be used with subsea remote vehicles where the high strength, toughness and abrasion resistance are needed to survive harsh environments. 
     In other embodiments, the optical cable used with the inventive cable deployment system may not have the protective structures described in  FIG. 3 . In this embodiment, the same long and deep dives can be achieved using a raw optical cable which can have an outer diameter of about 0.254 mm. The optical fiber can be stored aboard the vehicle and released through the inventive release system as described. This thinner raw cable can occupy substantially less space than the armored optical cable but will also have a much lower maximum working tensile strength. However, since the cable is exposed to a minimal amount of tension, the raw thin cable can operate in the described matter without the extra strength provided by the Kevlar jacket. The raw optical cable is further described in U.S. patent application Ser. No. 12/795,971, “Ocean Deployable Biodegradable Optical Fiber Cable” which is hereby incorporated by reference. 
     With reference to  FIG. 4 , this small diameter fiber optic cable  205  that can be wound on a drum inside the ROV  201 . For example, a 20,000 ft long 2.9 mm cable  205  can be wound on a spool  553 , a drum or other structure in the ROV. In an exemplary embodiment, the spool  553  can have an outer diameter (D 1 ) that is about 10 cm and a width (W) of about 200 cm wide. The cable  205  can be wrapped on the spool  553  in a repeating spiral pattern onto the spool  553  so that there are about 42 layers. The equation for estimating the length of cable  205  stored on the spool  553  can be calculated by the question.
 
Cable length=(average circumference)×(number of layers)×(number wraps/layer)
 
 D 2 =D 1+2×number of layers×OD of cable
 
Average circumference=π×( D 1 +D 2)/2
 
Number of wraps per layer= W /(OD of cable)
 
     First Exemplary Embodiment 
     OD of cable=0.29 cm 
     Number of layers of cable on spool=42 
     Outer diameter of spool (D 1 )=10 cm 
     Width of spool (W)=200 cm
 
Outer diameter of spool and cable ( D 2)=10 cm+2×42×0.29 cm=34.36 cm
 
Average circumference of cable on spool=π×(10 cm+34.36 cm)/2=69.68 cm
 
Number of wraps per layer=200 cm/0.29 cm=690
 
Length of cable stored on spool=(70 cm)×42×690=2,028,600 cm=20,286 meters
 
     Second Exemplary Embodiment 
     OD of cable=0.0254 cm 
     Number of layers of cable on spool=30 
     Outer diameter of spool (D 1 )=10 cm 
     Width of spool (W)=50 cm
 
Outer diameter of spool and cable ( D 2)=10 cm+2×30×0.0254 cm=11.52 cm
 
Average circumference of cable on spool=π×(10 cm+11.52 cm)/2=33.80 cm
 
Number of wraps per layer=50 cm/0.0254 cm=1,968
 
Length of cable stored on spool=(33.02 cm)×30×1,968=1,949,501 cm=19,495 meters
 
     With reference to  FIG. 5 , a block diagram of the cable release system components is illustrated. In order to eliminate cable tension, the ROV  201  can detect the cable tension through a cable tension sensor  229  which detects the cable tension as it exits the ROV. The tension sensor  229  can transmit a tension signal to the controller/microprocessor  226 . If the controller/microprocessor  226  detects that the tension has exceeded a predetermined value such as a nominal working tension, the controller/microprocessor  226  can transmit a signal to the cable release mechanism  221  to release the stored cable at a first rate or speed. As discussed, the cable release mechanism can include a spool that the cable is stored on. To release the cable, the cable release mechanism can include a motor which causes the spool to rotate and release the cable from the ROV. The speed at which the controller/microprocessor  226  causes the cable release mechanism  221  to release the cable can be proportional to the tension in the cable detected by the cable tension sensor  229 . In an embodiment, the cable tension should decrease as the cable is released from the cable release mechanism. However, if the tension sensor  229  continues to detect cable tension, the controller/microprocessor  226  can transmit signals to the cable release mechanism  221  to increase the rate that the cable is released. Once the tension drops to a normal working level the controller/microprocessor  226  can transmit signals to the cable release mechanism  221  to reduce the rate at which the cable is released. By monitoring the cable tension and releasing the cable at a speed that is proportional to the tension, the cable tension can be kept to a nominal level and the ROV can travel without any significant drag due to cable tension. 
     In an embodiment, the cable release mechanism  221  can be configured to maintain the cable tension at any predetermined tension. For example, if the cable release mechanism  221  is set to a predetermined tension of 1 lb. of force or a normal working tension of 0.8 to 1.2 pounds, the cable tension sensor  229  can monitor the tension of the cable and transmit the cable tension data to the controller/microprocessor  226 . The controller/microprocessor  226  can control the cable release mechanism  221  to release cable at a steady first rate, for example 10 cm per second. If the tension rises above 1.2 lb force, the controller/microprocessor  226  can increase the cable output rate above 10 cm per second until the cable tension is reduced to 1 lb force. For example, cable output may be increased to 16 cm per second at which speed the cable tension drops to 1.0 pounds. The controller/microprocessor  226  can maintain this higher cable output if the cable tension is held within the predetermined range. Conversely, if the cable tension decreases below 0.8 pound force, the controller/microprocessor  226  can decrease the output rate of the cable until the cable tension increases to the normal working range. By constantly monitoring and adjusting the output speed, the cable can be maintained in its normal working tension. 
     In an embodiment, the ROV may also detect the speed of the ROV with a ROV speed sensor  227 . The speed signals from the ROV speed sensor  227  can also be transmitted to the controller/microprocessor  226  which can then control the cable release mechanism to release the cable at a rate that matches or is slightly greater than the speed of the ROV. As the ROV accelerates through the water, the speed sensor  227  will detect the increased speed and transmit this information to the controller/microprocessor  226  which can increase the cable release rate from the cable release mechanism. Conversely, if the ROV slows down, the slower speed can be transmitted to the controller/microprocessor  226  which can reduce the cable release rate from the cable release mechanism  221 . In an embodiment, the speed sensors  227  may only detect the speed of the ROV in a single direction. Thus, the ROV may require multiple speed sensors  227  which are aligned in different directions. For example, a first speed sensor  227  may only detect vertical velocity while a second speed sensor may only detect horizontal velocity. The controller/microprocessor  226  may need to calculate a cumulative ROV speed based upon the multiple speed signals. The cumulative velocity may be represented by the formula V cumulative   2 =V vertical   2 +V horizontal   2 . The controller/microprocessor  226  can then control the output rate from the cable release mechanism  221  to match or be slightly faster than the ROV speed. 
     In an embodiment, the controller/microprocessor  226  can use a combination of speed detection from the speed sensor  227  and cable tension from the tension sensor  229  to control the cable output speed from the cable release mechanism  221 . In this embodiment, the controller/microprocessor  226  can start emitting cable at a speed that approximately matches the ROV speed from the speed sensor  227 . If additional tension is detected above the normal working range from the tension sensor  229 , the controller/microprocessor  226  cause the cable release mechanism to increase the rate at which the cable is released. If the tension drops to the normal working range, the controller/microprocessor  226  can resume releasing the cable at or slightly above the speed of the ROV. If the tension drops below the normal working range, the controller/microprocessor  226  can either maintain releasing the cable at the ROV speed or decrease the speed that the cable is released. 
     The cable  205  can continue to be released from the ROV until the stored cable  205  is depleted. However, this may problematic because the lack of cable  205  on the ROV can prevent the ROV from traveling any further without inducing tension into the cable. In an embodiment, the cable release mechanism  551  can transmit signals to indicate the quantity of cable  205  remaining on the spool  553 . The cable release mechanism  551  may be able to determine the length of cable  205  released by counting the number of rotations of the spool  553  when the cable  205  is released. Thus, if necessary, an operator of the ROV  201  can transmit a signal from the support vessel to cut the mission of the ROV  201  short or have the ROV  201  surface for retrieval. These steps can prevent the ROV  201  from running out of or damaging the cable  205 . 
     With reference to  FIG. 6 , an embodiment of the cable release mechanism  221  is illustrated. In this embodiment, a spool  501  can be wrapped with a long continuous length of optical cable  510 . The spool  501  can be coupled to a drive shaft  502  which allows the spool  501  to rotate. One end of the optical cable  510  can be coupled to a rotary optical joint  503  to allow the spool  501  and optical cable  510  to rotate and maintain an optical communications path through the cable  510 . An optical cable  515  can be coupled to the opposite side of the rotating optical joint  503  which is connected the controller on the ROV. 
     The spool  501  rotates to release the cable  510  which can be fed through a cable tension sensor  507  which detects the tension of the cable  510  as it leaves the ROV. The cable can also be fed through a guide  508  which can be a smooth bell mouth which has a curved guide surface having a radius that prevents the cable  510  from being bent at a sharp angle that may damage the cable  510  as it exits the cable release mechanism  221 . Because the surfaces of the guide  508  are smooth, the sliding of the cable  510  against the guide  508  does not produce any significant friction. 
     The guide  508  can be coupled to a reversing lead screw  505  which is part of a level winding system  504 . The reversing lead screw  505  can be can be coupled to a belt driven lead screw pulley  506  which is coupled to a spool pulley  513  attached to the drive shaft  502 . The drive motor  509  can rotate a pinion gear which can be connected to the spool pulley  513  with a drive belt. When the controller  511  causes the drive motor  509  to rotate, the pinion gear  514  rotation causes the spool pulley  513  to rotate which spins the spool  501  to release the cable  510 . The spool pulley  513  also rotates the lead screw pulley  506  which causes the guide  508  to move back and forth across the width of the spool  501  to match the position of the cable  510  being removed from the spool  501 . In an embodiment, the level wind system  504  can use a reversing lead screw  505  with a belt drive that rotates at about ¼ the winch drum speed. The lead screw pulley  506  may have interchangeable belt drive sprockets so that the winding angle can be changed to suit different diameter cables. In the preferred embodiment, the winding pitch of the cable is an open “universal” wind where the cable pitch is ¼ of the reversing lead screw pitch. 
     The cable  510  can be released from the cable release mechanism  221  in various different modes of operation. In an embodiment, the payout method is to monitor the tether tension from the tension sensor  507  at the ROV as the ROV moves through the water. The tension sensor  507  can transmit tension signals to the controller  511  and microprocessor  512 . If the tension is too high, the controller  511  and microprocessor  512  can control the drive motor  509  to unwind or payout the cable  510  to maintain a constant, small tension that is within the predetermined normal operating tension range. For example, when the tether  510  tension tugs gently on the ROV of about 1 lb of force, the controller  511 , microprocessor  512  and drive motor  509  function together to release the tether  510  as required to automatically maintain the 1 lb±0.5 lb. force tension. As also discussed, the controller  511  and microprocessor  512  can be in communication with speed sensors to control the drive motor  509  to release the cable  510  at a rate that is greater than or equal to the speed of the ROV through the water. 
     In addition to releasing the cable, the cable release mechanism  221  can be used to retrieve the cable  510  that has been released from the ROV. When the ROV is retrieved by the support vessel, the cable  510  can still extend from the ROV. The cable release mechanism  221  can be reversed to retrieve the cable  510  to the spool  501 . The same cable tension control system can be used when the cable  510  is being re-wound onto the spool  501 . The motor  509  can be powered in a reverse direction by the controller  511  that is controlled by the microprocessor  512  based upon tension feedback from the tension sensor  507  in order to automatically control the cable  510  tension. The drive motor  509  can cause the spool  501  to rotate and wrap the cable  510  back onto the spool  501 . If the tension rises above a predetermined retrieval force the motor  509  can slow the rotation of the spool  501 . In the cable retrieval mode, the cable  510  is being dragged through the water, so drag forces on the cable  510  can be proportional to the length of the cable  510  in the water. If the cable tension exceeds the normal retrieval working tension range, the motor  509  can slow to reduce the retrieval speed and reduce the tension. Conversely, if the tension is below the normal retrieval tension range, the motor  509  can be controlled to increase the rate of rotation of the spool  501 . 
     The level wind system  504  can cause the guide  508  to move back and forth across the spool  501  so that the cable  510  is wound evenly onto the spool  501 . The wind angle of the cable  510  on the spool  501  should be sufficient to prevent top layers of cable  510  from burying into underlying cable  510  wraps. Although, the rewind system is illustrated as a motor drive system, in other embodiments, it is possible to have a mechanical lever attached to the drive shaft  502  so that the cable  510  can be retrieved by manually turning the drive shaft  502 . 
     It will be understood that the inventive system has been described with reference to particular embodiments, however additions, deletions and changes could be made to these embodiments without departing from the scope of the inventive system. Although the systems that have been described include various components, it is well understood that these components and the described configuration can be modified and rearranged in various other configurations.