Abstract:
A control system for a downhole tool, such as a subsurface safety valve, features an operating piston that is insensitive to tubing pressure in the valve. The hydrostatic forces from the single control line from the surface are significantly reduced with a branch line to a piston bottom that is slightly smaller than the piston top. A variable volume between piston seals is connected to a low pressure compressible fluid reservoir to permit piston movement. The piston can be modular to facilitate assembly or bore offsets in the valve body. Failsafe closure upon seal failures is contemplated.

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. That is 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. insure the fail safe position is obtained regardless of which seals leak. 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. Nos. 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 provides for a tubing pressure insensitive operating piston. It neutralizes the hydrostatic forces in the control line to a significant extent while running a single control line to the surface. It provides a low pressure compressed gas volume to allow the piston to move when such movement reduces the volume of a cavity between piston seals. These and other features of the present invention will become more apparent to those skilled in the art from a review of the description of the preferred embodiment and the associated drawing of the control system, while recognizing that the full scope of the invention is to be found in the claims. 
   SUMMARY OF THE INVENTION 
   A control system for a downhole tool, such as a subsurface safety valve, features an operating piston that is insensitive to tubing pressure in the valve. The hydrostatic forces from the single control line from the surface are significantly reduced with a branch line to a piston bottom that is slightly smaller than the piston top. A variable volume between piston seals is connected to a low pressure compressible fluid reservoir to permit piston movement. The piston can be modular to facilitate assembly or bore offsets in the valve body. Failsafe closure upon seal failures is contemplated. 

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

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The present invention can be used as a control system for a subsurface safety valve (SSSV) or for that matter other types of downhole tools that are hydraulically operated from the surface, generally via a control line  10 . In a SSSV application the end component is a flapper  12  that is pushed open by a flow tube  14  that moves against the bias of a power spring  16 . Since the present invention has applications beyond SSSVs any reference to flow tube is intended to generically refer to a part of a tool that is actuated by a piston assembly  18  of a control system. Since those skilled in the art are well aware of common components of SSSVs, they are omitted from the drawing to allow greater clarity in understanding the operation of the control system. For example, it is well known that the flapper  12  in the position shown in  FIG. 1  is in the closed position against a seat that surrounds a passage in a valve housing. That passage is exposed to internal tubing pressure while being isolated from pressure in the control line  10 . The flow tube  14  and parts of the piston assembly  18  are similarly exposed to tubing pressure in the passage. Only a portion of the valve housing adjacent the piston assembly  18  is shown for clarity. 
   With that as an introduction, it can be seen that an upper housing  20  is juxtaposed opposite a lower housing  22 . They may be in one piece or two pieces that are connected. There are opposed spaced bores  24  and  26  that accept the piston assembly  18 . Preferably, the bores  24  and  26  are aligned but some offset can be accommodated with a modular design of the piston assembly  18 . A connector  28  can be used to connect upper piston  30  to lower piston  32 . Due to the channels at the ends of connector  28  the upper piston  30  can be connected to the lower piston  32  with a centerline offset. Although a rod piston design is preferred, other piston shapes are contemplated. 
   Lower piston  32  has a seal  34  to define a third variable volume chamber  36 . Control line  10  has a branch  38  connected at connection  40  to chamber  36  and a branch  39  connected to connection  46 . They form a junction  41  in close proximity to upper housing  20 . Options exist as to how to route branch  38 . It can be routed so that connection  40  is exposed to tubing pressure that affects the flow tube  14  and the flapper  12 , for example. Optionally, branch  38  can be routed outside the valve housing in the surrounding annular space. Depending on what choice is made there will be different considerations regarding how the system responds if a component fails, as will be explained below. The preferred embodiment is to run branch  38  to connection  40  along a route that has exposure to either tubing pressure or annulus pressure with annulus pressure preferred to assure desired failure modes in the event of leakage. 
   Pressure applied to the control line  10  goes through branch  38  to chamber  36  where it will exert an uphole force on lower piston  32 . Upper piston  30  has a seal  42  that is a larger diameter than seal  34 . Upper piston  30  has another seal  44  that is preferably the same or very close to the same size as seal  34 . Since both seals  44  and  34  are on the piston assembly  18  and are exposed on one side to the same tubing pressure, the piston assembly  18  experiences no net force from exposure to tubing pressure and can be referred to as tubing pressure insensitive for that reason. However, seal  42  is made larger than seal  34  by design and both are exposed to pressure in control line  10  and its branch  38 . While there is but a single control line  10  that runs from the surface that terminates at connections  40  and  46 , it can be seen that hydrostatic pressure in control line  10  is substantially offset by this arrangement. There is a net force from hydrostatic pressure in control line  10  on the piston assembly  18  in a downhole direction equal to the pressure near the connections  40  and  46 , which should be identical, divided by the area difference of seal  34  subtracted from the area of seal  42 . Of course, on application of pressure to control line  10  the net downhole force on piston assembly  18  increases to overcome the power spring  16  to shift the piston assembly  18  until shoulder  48  on the lower piston  32  engages shoulder  50  on flow tube  14  to rotate the flapper  12  to the open position. 
   In between seals  42  and  44  is a first variable volume chamber  52  that gets smaller as the piston assembly  18  is displaced against spring  16 . In order to allow the piston assembly  18  to move in that direction without getting bound, connection  54  has a line  56  leading to a reservoir  58  which is preferably at least  4  times the volume of chamber  52 . Line  56  continues to a valve  60  that is normally closed and whose purpose will be later explained. Beyond valve  60  line  56  ties into control line  10 . Reservoir  58  is preferably at atmospheric pressure or slightly higher and contains a compressible fluid. In normal operation, movement of the piston assembly  18  against spring  16  slightly raises the pressure in reservoir  58  to a degree related to the volume ratios between chamber  52  and reservoir  58  but in no way measurably impeding the movement of piston assembly  18 . 
   If there is a seal failure of seal  34  high tubing pressure can get into chamber  36  and from there through connection  40  and branch  38  to connection  46  and into chamber  62 . Since the pressure is now the same in third chamber  36  and second chamber  62  (i.e. tubing pressure)there would be a net opening force on piston assembly  18  due to the diameter of seal  42  being larger than the diameter of seal  34  (that has now failed). Without valve  60  in the system, the flapper  12  could be held open upon failure of seal  34  or, for that matter, failure of connections  40  and  46 . Valve  60  senses a pressure buildup in line  56  that occurs due to failure of seal  34  and tubing pressure migrating that far through branch  38 . Valve  60  can be a rupture disc or a piston held by a pin that shears or any other equivalent device that goes open at a predetermined pressure. When valve  60  opens the pressure at connections  46  and  54  equalizes removing any influence of tubing pressure on the piston assembly  18  that occurred due to failure of seal  34 . At that point the spring  16  pushes the piston assembly  18  to the valve closed position shown in  FIG. 1 . From that point the piston assembly  18  can no longer be operated from control line  10  and flapper  12  is in its fail safe closed position. 
   Those skilled in the art will appreciate that the present invention illustrates a downhole tool control system that can run off a single control line from the surface  10  and that is further configured to address opposing ends of a piston assembly in a way that minimizes the effect of control line hydrostatic pressure. This reduction of the net effect of hydrostatic pressure despite use of a single control line to the surface allows the use of a lower pressure to move the piston assembly  18 . Differing diameters of the opposed ends of the piston assembly allow a sufficient net opening force to be applied to move the piston assembly  18  against the spring  16 . The piston assembly is insensitive to tubing pressure which dramatically lowers the required opening pressure as compared to conventional subsurface safety valves. The movement of the piston assembly  18  reduces the volume of a chamber  52  but with the addition of a reservoir of fairly large volume the resistance to movement from the compression effect of volume reduction in chamber  52  is made insignificant by the presence of large reservoir  58  which operates at an initial pressure that is close to atmospheric. With very high tubing pressures in the order of 20,000 PSI or more seals  44  and  34  see fairly large pressure differentials to help them seal more effectively. Failure of seal  34 , connection  40 , or connection  46  opens valve  60  to equalize pressure across seal  42  to let the spring  16  urge the flapper  12  to the fail safe closed position. Piston bores  24  and  26  may have a misalignment that can be compensated for by making the piston assembly  18  modular using a connector  28  that tolerates offset between the upper piston  30  and the lower piston  32 . 
   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.