Patent Abstract:
A control system for a subsurface safety valve references the surrounding annulus to put the operating piston in pressure balance. Depending on the configuration and which seal in the system fails, the various embodiments can differ in their failure modes. With the lower end of the piston exposed to annulus pressure all failure modes close the flapper. With the lower end of the piston exposed to tubing pressure, failure of any of the seals except one will result in flapper closure.

Full Description:
FIELD OF THE INVENTION 
   The field of this invention is control systems for operating subsurface safety valves and more particularly control systems with a piston in pressure balance to the surrounding annulus. 
   BACKGROUND OF THE INVENTION 
   Subsurface safety valves are operated from the surface normally through control lines that run outside the production tubing. These valves are typically of the flapper type where a control system, when pressurized from the surface overcomes a closure spring on a flow tube to push the flapper 90 degrees into the open position behind the shifting flow tube. Removal of pressure from the control system allows the closure spring that had previously been held in a compressed position to then push the flow tube away from the flapper so that a torsion spring can bias it back against its seat to prevent flow from the formation from going up the production string. 
   These systems have to deal with issues such as failing in a safe mode if one or more seals in the control system fail. They also have to address offsetting the hydrostatic pressure in the control line. Systems with a single control line down to the subsurface safety valve typically have a pressurized chamber at the valve preset with enough pressure for the expected depth of the valve to offset the control line hydrostatic pressure so that on removal of applied control line pressure from the surface, the closure spring that acts on the flow tube doesn&#39;t have to overcome the hydrostatic pressure from the control line. A single control line system that addresses fail safe failure modes of the various seals is U.S. Pat. No. 6,109,351. Alternatively a closure spring is provided that is strong enough to overcome the control line hydrostatic pressure particularly in shallower wells. Other systems simply cancel out control line hydrostatic pressure with a balance line from the opposite side of an operating piston than the main control line. One example of such systems is U.S. Pat. No. 6,173,785. Some two line systems also incorporate pressurized chambers such as U.S. Pat. No. 6,427,778. 
   Some of these designs employ a passage through the piston for the purpose of obtaining a fail safe closure mode if one or more of the system seals malfunction or if a control line is sheared. The prior systems typically separated tubing pressure from control line pressure and made no reference to the surrounding annulus. Typically the operating piston in the control system had to have a mechanical connection to the flow tube to move the flow tube to open the valve. That mechanical connection was exposed to tubing pressure and the operating piston featured a pair of seals in a housing so that a portion of the operating piston in the region that it connected to the flow tube was exposed to tubing pressure but remained in pressure balance from tubing pressure. 
   The present invention addresses alternative approaches to the past designs that reference the surrounding annulus. Some embodiments operate differently than others during failure modes and this will be explained in detail when the various embodiments are described in detail. Those skilled in the art will appreciate the various aspects of the invention from the description of the preferred embodiment and associated drawings that appear below with the understanding that the full scope of the invention is measured by the appended claims. 
   SUMMARY OF THE INVENTION 
   A control system for a subsurface safety valve references the surrounding annulus to put the operating piston in pressure balance. Depending on the configuration and which seal in the system fails, the various embodiments can differ in their failure modes. With the lower end of the piston exposed to annulus pressure all failure modes close the flapper. With the lower end of the piston exposed to tubing pressure, failure of any of the seals except one will result in flapper closure. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view of a single line control system with a piston pressure balanced to the annulus; 
       FIG. 2  is an alternative embodiment to  FIG. 1  and still having a pressure balanced piston to the annulus; and 
       FIG. 3  is an alternative to the embodiment in  FIG. 2  and having a piston in pressure balance to the annulus; and 
       FIG. 4  is a variation of  FIG. 1  showing an annular piston rather than a rod piston with a balance control line to the surface. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  is a schematic representation of a subsurface safety valve that those skilled in the art will appreciate can illustrate the various embodiments of the present invention. Typically, a flapper  10  is mounted on a pivot  12  that can combine a torsion spring (not shown) to urge the flapper  10  against the seat  14 . The flapper  10  is pushed to turn 90 degrees and go behind an advancing flow tube  16  that is forced to move against a return bias from closure spring  18 . Passage  20  goes through a housing that is partially shown as  22 . A string from the surface represented by arrow  24  is in flow communication with passage  20  in housing  22  in a known manner. Similarly arrow  26  represents the continuation of a tubing string to the producing zone further down in the well. 
   A single control line  28  connects into housing  22  into chamber  30  above the operating piston  32 . Chamber  34  is on the other side of piston  32  from chamber  30  and it communicates to the surrounding annulus around housing  22  through passage  36 . 
   Piston  32  is preferably a rod piston with seals  40 , a lower seal, and seal  42  an upper seal. There is a through passage  44  going from lower end  46  to upper end  48  of piston  32 . Above upper end  48  is a chamber  50  in housing  22  that gets tubing pressure communicated to it through the passage  44  from inlet  52 . Link  53  connects piston  32  to flow tube  16 . 
   In operation, applied pressure from control line  28  raises the pressure in chamber  30  to the point that spring  18  is compressed and the flapper  10  goes open. Removal of pressure from the control line  28  allows the spring  18  to overcome the net difference between hydrostatic pressure in line  28  and the surrounding annulus pressure. The spring  18  is sized to overcome the net pressure on piston  32  between control line hydrostatic and annulus pressure apart from seal friction at seals  40  and  42  when piston  32  moves. Piston  32  is mechanically coupled to flow tube  16  below seal  40  which is exposed to tubing pressure on one side and annulus pressure on the other side. Seal  39 , the piston seal, separates chambers  30  and  34 . Seal  42  is on one side of piston seal  39  and seal  40  is on the opposite side of seal  39  from seal  40 . In most cases a net closing force acts on piston  32  from tubing pressure pushing up on seal  40  and annulus pressure pushing down on seal  42 . 
   If seal  40  fails, the pressure in the tubing will communicate to the surrounding annulus and pressurize chamber  34  forcing the piston  32  up and the flapper  10  will go closed. If seal  39  fails in any illustrated embodiment, there cannot be a pressure differential across the piston  32  from control line  28  and the closure spring  18  will make the flapper  10  close. However if seal  42  fails then tubing pressure will get into chamber  30  and prevent spring  18  from closing the flapper  10  since spring  18  is not sized for overcoming tubing pressure because the flow tube  16  is in pressure balance to tubing pressure. Hence in this embodiment, failure of seal  42  makes the valve stay open. 
     FIG. 2  is a modified design of  FIG. 1 . The difference is that a second lower seal  38  is added and the lower  46 ′ end of piston  32 ′ is now exposed to annulus pressure rather than tubing pressure. Annulus pressure also goes through inlet  52 ′ to chamber  50 ′. The piston  32 ′ is in pressure balance from annulus pressure acting up on lower seal  38  and down on upper seal  42 ′ through chamber  50 ′. Piston  32 ′ is also in pressure balance from tubing pressure pushing up at seal  40 ′ and down at seal  38  because those seals straddle the link  53 ′ that connects the piston  32 ′ to the flow tube  16 ′. 
   If seal  40 ′ fails tubing pressure enters chamber  34 ′ and the annulus through passage  36 ′ pushing the piston  32 ′ up and the flapper  10 ′ will close. If seal  38  fails tubing pressure will leak into the annulus and get into chamber  34 ′ and again the flapper  10 ′ will close. If seal  42 ′ breaks pressure in the control line  28 ′ will pass into the annulus through chamber  50 ′ and passage  44 ′ and the closure spring  18 ′ will be able to close the flapper  10 ′. The design of  FIG. 2  fails closed if any seal  38 ,  40 ′ and  42 ′ fails. 
     FIG. 3  is virtually the same as  FIG. 2  with the difference being that piston  32 ″ is solid and the passage through it has been eliminated. However, a connection  60  to the annulus has been added to chamber  50 ″ so that the top  48 ″ of the piston  32 ″ is again in communication with the annulus despite there being no passage through piston  32 ″. Inlet  52 ″ exposes the lower end  46 ″ of piston  32 ″ to annulus pressure present in chamber  62 . In all other respects, the  FIG. 3  design functions and fails the same way as the  FIG. 2  design. 
     FIG. 4  is similar to  FIG. 1  except the piston has an annular shape rather than a rod shape as illustrated in  FIG. 1  and is pressure balanced with a balance line that runs to the surface. The flow tube  100  has a piston  102  integrated into it with a seal  104  to separate compartments  106  and  108 . Tubing pressure is in passage  110 . Downward movement of the flow tube  100  rotates the flapper  112  and compresses the spring  114 . Compartment  106  is connected to a first control line represented schematically by arrow  116  and compartment  108  is connected to another control line running back to the surface and schematically represented by arrow  118 . Seals  120  and  122  are preferably the same size so that piston  102  is in pressure balance from the equal hydrostatic pressure in lines  116  and  118  when no pressure is being applied to either line from the surface. Seals  120  and  122  have tubing pressure in passage  110  acting on one side and control line pressure  116  acting on the other side of seal  120  and balance line pressure  118  acting on the other side of seal  122 . 
   In operation, the flapper  112  is opened with pressure applied in line  116  that compresses spring  114  and drives the flow tube  100  down against the flapper  112 . Removal of pressure on line  116  allows the spring  114  to drive the flow tube  100  up so that the flapper  114  closes. Since there is a balance of hydrostatic forces on piston  102  the spring  114  does not have to be sized to oppose any hydrostatic force acting on piston  102  since there is no such force acting on it in this embodiment. 
   If seal  104  breaks then the flapper  112  will close under the force of spring  114 . Failure of seal  122  will allow tubing pressure from passage  110  into chamber  108  forcing the flow tube  100  up and the flapper  112  will close. Failure of seal  120  will send tubing pressure from passage  110  to chamber  106  and will likely overpower spring  114  to hold the flapper  112  open unless pressure is applied to the control line  118 . 
   Those skilled in the art will appreciate that a variety of control systems are disclosed that use a single control line and a pressure balanced piston with respect to the annulus. The designs that fail safe closed are also pressure balanced to tubing pressure as well. Pressure balance to the annulus can occur at opposed ends with bore through the piston or with separate exposure of opposed ends of the piston to annulus pressure. In the preferred embodiment the piston can be one or more rod pistons but other piston shapes are contemplated. Pressurized chambers or offsets for control line hydrostatic pressure are not needed. The annulus pressure is used to at least in part offset the control line hydrostatic pressure and the closure spring  18  is sized to overcome net force on the piston from the net difference in pressure acting on it from the control line trying to push it down and the annulus pressure trying to push it back up. 
   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.

Technology Classification (CPC): 4