Patent Publication Number: US-7591317-B2

Title: Tubing pressure insensitive control system

Description:
FIELD OF THE INVENTION 
   The field of this invention is control systems for downhole valves and, more particularly, for subsurface safety valves where the system is tubing pressure insensitive. 
   BACKGROUND OF THE INVENTION 
   Subsurface safety valves are used in wells to close them off in the event of an uncontrolled condition to ensure the safety of surface personnel and prevent property damage and pollution. Typically these valves comprise a flapper, which is the closure element and is pivotally mounted to rotate 90 degrees between an open and a closed position. A hollow tube called a flow tube is actuated downwardly against the flapper to rotate it to a position behind the tube and off its seat. This is described as the open position. When the flow tube is retracted the flapper is urged by a spring mounted to its pivot rod to rotate to the closed position against a similarly shaped seat. 
   The flow tube is operated by a hydraulic control system that includes a control line from the surface to one side of a piston. Increasing pressure in the control line moves the piston in one direction and shifts the flow tube with it. This movement occurs against a closure spring that is generally sized to offset the hydrostatic pressure in the control line, friction losses on the piston seals and the weight of the components to be moved in an opposite direction to shift the flow tube up and away from the flapper so that the flapper can swing shut. 
   Normally, it is desirable to have the flapper go to a closed position in the event of failure modes in the hydraulic control system and during normal operation on loss or removal of control line pressure. The need to meet normal and failure mode requirements in a tubing pressure insensitive control system, particularly in a deep set safety valve application, has presented a challenge in the past. The results represent a variety of approaches that have added complexity to the design by including features to ensure the fail safe position is obtained regardless of which seals or connections fail. Some of these systems have overlays of pilot pistons and several pressurized gas reservoirs while others require multiple control lines from the surface in part to offset the pressure from control line hydrostatic pressure. Some recent examples of these efforts can be seen in U.S. Pat. No. 6,427,778 and 6,109,351. 
   Despite these efforts a tubing pressure insensitive control system for deep set safety valves that had greater simplicity, enhanced reliability and lower production cost remained a goal to be accomplished. The present invention offers a system that features a single control line that acts on a piston that extends through spaced blocks so that it is substantially in pressure balance from tubing pressure. Each block has a tubing pressure seal while the piston carries a control line pressure seal in the upper block. A passage between the seals in the upper block extends preferably through the piston to a reservoir holding a compressible gas preferably near atmospheric pressure. The movement of the piston compresses the fluid in the reservoir and compresses a closure spring acting on the flow tube. Optionally, a spring or/and an equivalent can act on the piston directly to move the flow tube to close the valve. A redundant system can be provided so that when the primary system fails and is pressure equalized because of such failure, access into a redundant system from the same or separate control line can be obtained for continued operation of the valve. 
   Those skilled in the art will better appreciate the details of the invention from the description of the preferred embodiment and the drawings that appear below while recognizing that the full scope of the invention is indicated by the claims. 
   SUMMARY OF THE INVENTION 
   A control system can be used with a single control line to a subsurface safety valve. The operating piston is exposed to the flow tube between two blocks with near identical seals to make the piston insensitive to tubing pressure. A control system seal is carried by the piston in the upper block and a passage between the control system seal and the tubing pressure seal in the upper block communicates to a compressible fluid reservoir in the lower block that is also isolated from tubing pressure by a tubing pressure seal. Movement of the piston compresses the fluid in the reservoir. The reservoir can also include a spring to return the piston and the flow tube to a position to close the valve. A redundant system can be actuated if the primary system fails. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic system diagram of the proposed control system. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows a control system for downhole equipment and preferably a subsurface safety valve (SSSV). A single control line  10  extends to a first connection  12  in upper block  14  that is part of the SSSV housing (not shown). A piston  16  carries a seal  18  to define a variable volume  20  that is in part defined by interior surface  22  in upper block  14 . Surface  22  defines a seal bore  24  in which a seal  26  is located. Seal  26  bridges the gap  28  from surface  22  to piston  16 . Piston  16  has a shoulder  30  to abut flow tube  32  to push it down against a closure device, typically a spring and shown schematically in one location as arrow  34 . Flow tube  32  is intended to generically refer to an operating mechanism in a downhole tool and to a flow tube in a specific embodiment of a SSSV. Those skilled in the art will know that when flow tube  32  is pushed down, a flapper (not shown) is pushed open on the SSSV. If the closure spring  34  is bearing directly on the flow tube  32 , then a single shoulder  30  on the piston  16  is sufficient to shift the flow tube  32  down under pressure applied from control line  10  and to shift the flow tube  32  back up on removal of pressure at control line  10  so that the closure spring or equivalent, pushes directly up on flow tube  32  to allow the flapper to close. 
   Piston  16  extends into a lower block  36  that defines a chamber  38  having a wall  40  in which a seal  42  is located in seal bore  44  to span the gap  46 . A passage  48  from gap  28  between seals  18  and  26  extends to chamber  38 . Preferably, this passage goes through piston  16  but it can go through the valve body tubing, or some other alternate path to connect gap  28  and chamber  38 . 
   The size of seals  26  and  42  is preferably nearly identical so that pressure effects from tubing pressure in area  50  have little to no effect on moving the piston  16  in either direction. In this context, the term “nearly identical” can be defined as the fact that a difference in tubing seal diameters is not enough to produce a detrimental increase in opening or closing pressure of more than 25%. Because of passage  48  seals  26  and  42  see a fairly high differential of tubing pressure  50  minus the pressure in chamber  38  which is preferably far lower. The pressure differential helps the sealing function in gaps  28  and  46 . 
   Since seal  18  moves with piston  16  closer to seal  26  when shifting the flow tube  32  down to open the valve, the presence of passage  48  leading to chamber  38  allows this movement to happen because passage  48  and chamber  38  preferably contain, at least in part, a compressible fluid and preferably at fairly low pressures compared to tubing pressure  50  which can easily exceed 20,000 PSI. Apart from seal resistance to movement of piston  16  the force needed in the control line  10  to move piston  16  is principally to overcome the closure device  34  that directly acts on the flow tube, as one option. Alternatively, the closure can be accomplished with a spring or equivalent  52  located inside chamber  38  and acting directly on piston  16  instead of spring or equivalent  34  acting on the flow tube  32 . In yet another option both locations can have springs or equivalent devices so that closure forces act on flow tube  32  and piston  16 . A wave spring is preferred for spring  52  but equivalent energy storing devices can also be used. The preferred pressure in chamber  38  is atmospheric or a pressure close to it, but such a pressure can be higher and high enough to act as a partial or total closing force on the piston  16 . This is a trade off as it is also desirable to have larger pressure differentials across seals  26  and  42  as possible to enhance sealing performance across gaps  28  and  46 . To the extent any closure force for flow tube  32  comes from chamber  38  another shoulder  54  can be used for pushing the flow tube  32  up to allow the valve to close. 
   Normal operation is nothing more than applying pressure to control line  10  to move the piston  16  against a closure force, be it  34  or  52  or both or pressure from within chamber  38 . Movement of piston  16  simply reduces the volume of chamber  38  and compresses the fluid inside it. To close the valve normally, the pressure is simply reduced in control line  10  and the closure device(s) take over and reverse the movement of the piston  16  and the flow tube  32 . 
   Failure of seal  26  or  42  puts tubing pressure in chamber  38  to oppose control line pressure in control line  10 . The control line pressure in applications with very high tubing pressure  50  will generally be no match in chamber  20  and the piston will move up under the greater force from chamber  38  or from simply the closure force from spring  34  or  52 . Once equalized about piston  16  due to a seal failure of seal  26  or  42  further application of control line pressure will not reopen the valve. If seal  18  fails, the control line  10  pressure equalizes between chambers  20  and  38  and the valve closes by virtue of spring  34  or  52  and cannot be reopened. 
   In the event of a seal failure of the types described above, it is advantageous to have a redundant system shown schematically as  56  that is preferably identical to the system illustrated and works the same way. System  56  can be connected to control line  10  or through an independent control line through a rupture disc  58  that is set higher than the normal pressures expected for operation of the previously described control system. A filter  60  can be optionally used to contain any rupture disc parts after it is broken by elevating the pressure in the control line  10 . Accordingly, if the main control system fails in the manners described above, the rupture disc  58  can be broken and system  56  will take over after the initial system is disabled. No amount of pressure to the initial operating system will actually move piston  16  due to the equalization that had already occurred to reach the point of having to break rupture disc  58  to be able to keep operating the valve. Alternatively, rupture disc  58  and filter  60  can be eliminated and the redundant systems can operate at all times in tandem from a single control line  10  that branches to service the redundant unit(s). Alternatively, another option can be to run a second, separate control line from the surface to rupture disc  58 , to filter  60  and redundant operating system  56 . If one system fails, as described above and becomes inoperative, the other system(s) can be activated and can continue operating in the normal manner. 
   Those skilled in the art will appreciate that the system is simple and features a piston insensitive to tubing pressures  50 . While being insensitive to tubing pressures, it features a compressible fluid reservoir in a simple design with just  3  seals. It further provides an option to have a closure device acting right on the piston  16  rather than the flow tube  32  making the design more compact and possibly allowing a larger bore in the valve despite pressure ratings that can go above 20,000 PSI. The compactness of the design leaves room for a redundant system that can be selectively deployed if the initial system has a seal failure. 
   The above description is illustrative of the preferred embodiment and various alternatives and is not intended to embody the broadest scope of the invention, which is determined from the claims appended below, and properly given their full scope literally and equivalently.