Abstract:
An operating control line is in communication with an operating piston for the safety valve as well as an equalizing piston such that pressure in the operating control line opens the safety valve and holds the equalizer valve closed. A balance chamber receives fluid from an operating piston in the safety valve when the valve opens to displace a floating piston to the open position. Operating control line pressure reduction allows valve closure and opposite floating piston movement to the closed position. If the floating piston is forced by a tubing seal leak against the open position travel stop, pressure in a balance control line against the equalizing valve member moves it from a seat to then equalize pressure on opposed ends of the floating piston allowing a bias force to move the floating piston off the open position stop so the safety valve can open despite the tubing leak.

Description:
FIELD OF THE INVENTION 
       [0001]    The field of the invention is hydraulic control systems for borehole tools and more particularly systems that employ a control line and a balance line to the surface with a floating piston isolating a balance chamber. In the event leakage of tubing pressure prevents downward movement of the floating piston on safety valve closure, a pressure equalization enabled by applied pressure on the balance line allows reset of the floating piston to allow continued operation of the safety valve despite the tubing pressure leak. 
       BACKGROUND OF THE INVENTION 
       [0002]    Subsurface safety valves are typically hydraulically controlled from a remote location using one or two control lines. An advantage of a two control line system is that hydrostatic pressure in each line is canceled out so that a closure spring for a flow tube does not need to resist hydrostatic pressure as is the case with single control line systems. In two line control systems pressure on top of an operating piston moves a flow tube against a flapper to open the valve. Removal of such pressure from the main control line allows a closure spring to reverse movement of the flow tube to allow the flapper to rotate  90  degrees to closed position of the safety valve. In the past operators have wanted or regulations required a barrier in the second or balance control line so that if tubing pressure leaks into the hydraulic system there would be a barrier to keep hydrocarbons from reaching a surface location through the balance line. 
         [0003]    The floating piston in the balance line served this purpose as a barrier. In normal valve operations pressure applied in the main control line to the top of a piston whose movement shifted the flow tube would result in hydraulic fluid displacement to the underside of the floating piston. Conversely, as pressure was removed from the main control line and the closure spring pushed up the flow tube hydraulic fluid would be drawn into the safety valve from under the floating piston to enable the safety valve to close. The floating piston would just move up when the safety valve open and reverse its motion when the safety valve closed, each time displacing an equal volume of hydraulic fluid as movement of the operating piston had displaced. The floating piston was sometimes biased toward the down position to put it in the ready position for safety valve opening. 
         [0004]    Sometimes, seals could leak in such safety valve hydraulic systems such that the much higher tubing pressure could leak into the balance control line and against the underside of the floating piston. This could happen slowly taking months or even years to reach an extreme condition where the floating piston would be up against an upper travel stop with tubing pressure under it. As a result the safety would not be functional to open since the operating piston in the safety valve could not displace hydraulic fluid because the floating piston could not move because it was forced against an upward travel stop due to tubing pressure leaking past a seal. When this happened in the past the safety valve would need to be removed, which caused very expensive downtime. 
         [0005]    The present invention is a reconfiguration of the two control line system that incorporates the floating piston working normally the same way as it worked in the past. What is different is the addition of an operable one way valve that can be opened with pressure applied to the balance line such that when such equalizing valve was forced open from the balance line applied pressure, the pressure on opposed sides of the floating piston could equalize and the position of the floating piston could change. The floating piston, now placed in pressure balance on its opposed ends could be biased away from its upper travel stop. Doing this would again make the safety valve operable to open as the hydraulic system would no longer be liquid locked by virtue of the floating piston sitting against its upper travel stop under tubing pressure. In essence the balance line pressure would be raised to the level of the tubing pressure or less depending on seal geometries to get the equalizer valve to open to allow a return spring acting on the floating piston to bias it back to a lower travel stop to allow reopening of the valve without well shutdown and safety valve removal. Many times the seal leakage is so slow that the ability to reposition the floating piston can allow many more years of service for the safety valve. These and other aspects of the present invention will be more readily apparent to those skilled in the art from a review of the description of the preferred embodiment and the associated drawings while recognizing that the full scope of the invention is to be determined from the appended claims. The following references are illustrative of control systems used in the past for safety valves in a borehole application: U.S. Pat. No. 5,906,220; U.S. Pat. No. 7,743,833; U.S. Pat. No. 8,534,317 and US 2008/0314599. 
       SUMMARY OF THE INVENTION 
       [0006]    An operating control line is in communication with an operating piston for the safety valve as well as an equalizing piston such that pressure in the operating control line opens the safety valve and holds the equalizer valve closed. A balance chamber receives fluid from an operating piston in the safety valve when the valve opens to displace a floating piston to the open position. Operating control line pressure reduction allows valve closure and opposite floating piston movement to the closed position. If the floating piston is forced by a tubing seal leak against the open position travel stop, pressure in a balance control line against the equalizing valve member moves it from a seat to then equalize pressure on opposed ends of the floating piston allowing a bias force to move the floating piston off the open position stop so the safety valve can open despite the tubing leak. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic of the present invention showing the safety valve closed or pressure reduced in the balance chamber; 
           [0008]      FIG. 2  is the view of  FIG. 1  with the safety valve open or the balance chamber gaining pressure; 
           [0009]      FIG. 3  shows pressure applied into the balance line opening the equalizing valve and allowing the bias on the floating piston to reposition the floating piston such that the safety valve can be opened. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0010]    Referring to  FIG. 1 , the normal operation of the control system  10  will be described. An operating control line  12  extends from a remote location to a subsurface safety valve  14  located in a borehole or conduits associated with a borehole that are not shown. The safety valve  14  is a type well known in the art and generally has a hydraulic piston moving a flow tube to rotate a flapper to open the valve when pressure is applied to the operating control line  12 . When pressure is removed from operating control line  12  a closure spring is able to push the flow tube away from the flapper to let the flapper rotate  90  degrees to a closed position against a flapper seat. Moving the flow tube requires delivery of hydraulic fluid against an operating piston in the safety valve  14 . Movement of such a piston displaces fluid out of the safety valve body to a balance chamber  16  that is directly below the floating piston  18 . Floating piston  18  is biased by spring  20  pushing from support  22  against shoulder  24  on the floating piston  18 . Taper  26  represents a lower travel stop for the floating piston  18 . Support  22  surrounds the floating piston  18  and guides its movement up to upper stop  28 . The piston  18  does not necessarily have to reach the stop  28  as its upper movement can be limited by fully compressing spring  20  between shoulder  24  and support  22  or limited by maximum fluid displacement from valve  14 . 
         [0011]    Operating control line  12  branches into lines  30  and  32 . Line  32  goes to the top of the operating piston inside the safety valve  14  and line  30  goes to the underside of equalizing valve  34  at inlet  36  below the valve member  38  that has a seal  40  to hold the pressure in the operating control line  12 . Coming out of the safety valve  14  from below the operating piston of the safety valve  14  is line  44  that branches into lines  46  and  48 . Line  46  goes into an annular space where spring  42  is located. Spring  42  pushes up on valve member  38  to hold head  50  against seat  52 . Pressure in line  46  acts below head  50  also acts in the same direction as spring  42 . Note that the seal area at seat  52  is larger than the seal  40  so that pressure in line  46  creates a net force on head  50  against seat  52 . Stop  54  limits the movement of head  50  away from seat  52 . Lines  56  and  58  join to become the balance line  60  that goes to a remote surface location. As previously stated the purpose of line  60  is to offset the hydrostatic pressure in operating control line  12  but it has another purpose as will be described. 
         [0012]    Valve member  38  does not move during normal operation of the safety valve  14 . Floating piston  18  is in a lower position shown in  FIG. 1  when the safety valve  14  is closed. To open the safety valve  14  the pressure in operating control line is raised. This opens the safety valve as described above and displaces hydraulic fluid into lines  44  and  48  causing the floating piston  18  to move up as shown in  FIG. 2 . Note that the volume of chamber  16  has increased in  FIG. 2  as compared to  FIG. 1 . When this happens there is no flow in line  46  because the head  50  is against seat  52 . Upward movement of the floating piston  18  displaces fluid into lines  58  and  60 . There is no flow in line  56  as the path of least resistance is into the balance line  60 . This is because when the pressure is raised in operating control line  12  it is also applied at  36  to push up on the equalizing valve member  38  and displaced fluid from valve  14  through lines  44  and  46  adds to the force to hold the head  50  against the seat  52 . 
         [0013]    As  FIG. 1  shows the floating piston  18  needs to be in the down position so that the valve  14  can go from closed as shown in  FIG. 1  to open as shown in  FIG. 2 . This is because the movement of the operating piston in the valve  14  displaces hydraulic fluid into lines  44  and  48  in response to raised pressure in line  12  that is used to open the valve  14 . If for any reason the floating piston  18  is in the  FIG. 2  position when the valve  14  is trying to open, then the valve  14  will be liquid locked as the floating piston  18  cannot be displaced toward stop  28  because it is already there. One way this situation can happen is when tubing pressure inside valve  14  from the tubing string that is not shown and to which it is connected finds a leak path around a seal for the hydraulic system. The tubing pressure can often times be substantially higher than the operating hydraulic pressure. The hydraulic pressure at valve  14  typically reflects the hydrostatic at the location of valve  14  and the pressure needed to overcome seal friction and the force of the closure spring when the valve is in the open position. Tubing pressure can be significantly higher. Since the seals in the valve  14  hydraulic system are fairly small it is possible that leakage around such seals can be at such a slow rate that it could take months or even years to get the floating piston  18  displaced to the  FIG. 2  position with such leaked tubing pressure such that the valve  14  can only be closed if it was open but cannot thereafter be reopened. 
         [0014]      FIG. 3  illustrates a workaround for this situation while still providing a seal in the balance line  60  against hydrocarbons getting to a surface location and the dangers that can ensue if that happens. Thus, when raising pressure at operating control line  12  fails to open the valve  14  because the floating piston  18  is forced by leaking tubing pressure into line  48  and balance chamber  16 , the pressure in operating control line  12  is turned off. Instead the pressure is applied in the balance line  60  in the direction of arrow  62 . It should be noted that during normal operation no pressure is applied to balance line  60 . However, when valve  14  refuses to open with pressure in operating control line  12 , then the extraordinary measure of pressurizing balance line  60  in the direction of arrow  62  needs to be implemented. 
         [0015]    The pressure under the equalizing valve  34  at inlet  36  is at this time equal to the hydrostatic pressure in operating control line  12  because no pressure is being applied to operating control line  12 . This pressure tends to push the valve member  38  and the head  50  toward seat  52 . Opposing this force is the pressure in balance line  60  communicating with head  50  through line  56 . Since the area of the head  50  is larger than seal there is a net force developed in the direction of moving the head  50  away from seat  52 . As the pressure in balance line  60  in the direction of arrow  62  increases so does the net force on the valve member  38  until the force of spring  42  is overcome and the  FIG. 3  position for the valve member  38  is assumed. When this happens, the pressure in lines  60 ,  58  and  56  equalizes with lines  46  and  48  with the result that there is no longer a net force acting on the floating piston  18  so that spring  20  can move the floating piston  18  from the  FIG. 2  to the  FIG. 3  position. After that happens the valve  14  will no longer be liquid locked in the hydraulic system and the operating piston inside the valve  14  can once again move to allow the valve  14  to open. Removal of pressure in balance line  60  will then allow spring  42  to move head  50  back to seat  52  and, if the tubing pressure leak is small enough, the valve  14  can be operated normally for some time until enough leakage reoccurs to again pin the floating piston  18  in the  FIG. 2  position so that the valve  14  again fails to open. The above described procedure can then be repeated in the hope of getting some additional service life for valve  14  without having to pull it out of the hole. In essence the equalizer valve  34  is a bypass passage around the floating piston  18  that can be selectively opened from a remote location by pressurizing balance line  60  in the direction of arrow  62  that opens the equalizer valve  34  to allow the spring  20  to then reposition the floating piston  18  to give it room to move up from the  FIG. 3  position to facilitate another opening of the valve  14  for further production. 
         [0016]    If the balance chamber  16  loses pressure/volume, the floating piston  18  will move to compensate for that volume loss. If the floating piston reaches its downward stop  26 , it will not be able to compensate for any additional fluid loss from the balance chamber  16 . If the balance chamber continues to lose pressure, a pressure differential will be created across the equalizer piston  38  causing an opening force on the equalizing piston  38 . This opening force is created by hydrostatic pressures from the balance line  60  and control line  12  acting on the area differential between the larger seal on the head  50  of the equalizing piston  38  and the smaller seal  40  on the equalizing piston  38 . These pressures are normally counter-acted by the pressure of the balance chamber  16  in the annular area around the equalizing piston  38  but differential pressures are formed across the head  50  and seal  40  of the equalizing piston  38  when pressure decreases in the balance chamber  16 . When the balance chamber  16  has lost sufficient pressure to create a sufficient pressure differential to overcome the closing force of the equalizing spring  42  the equalizing piston will shift open and pressure/volume from line  60  will travel through line  56  and refill the lost pressure/volume from the balance chamber  16 . 
         [0017]    Those skilled in the art will appreciate that the equalizer valve  34  is piped up to be in parallel with the end connections on the floating piston  18  such that its opening, however achieved, puts the floating piston in pressure balance in the balance line  60 . At that point the bias of spring  20  repositions the floating piston  18  closer to valve  14  as shown in  FIG. 3  so that valve  14  can move to the open position because its operating piston can displace fluid by again moving balance piston  18  against the bias of spring  20 . Connecting the operating control line  12  to under the equalizer piston  38  helps insure contact of head  50  on seat  52  during normal operations. Any applied pressure in operating control line  12  is removed prior to trying to open the equalizer valve  34  using pressure in balance line  60  in the direction of arrow  62 . It should be noted that line  44  is part of the balance line  60  with lines  56  and  46  forming one parallel branch for the equalizer valve  34  and lines  48  and  58  providing a parallel branch for the floating piston  18 . 
         [0018]    The above description is illustrative of the preferred embodiment and many modifications may be made by those skilled in the art without departing from the invention whose scope is to be determined from the literal and equivalent scope of the claims below: