Abstract:
A downhole valve for use in a well has a closure device that is biased closed to seal against flow through the central bore of the valve. A plurality of pistons are each coupled to a respective hydraulic control line into the valve. Each piston is adapted to reside in an actuated position, supporting the closure open, when at least a specified hydraulic pressure is supplied through its control line, and to reside in an unactuated position, not supporting the closure open, when at least the specified hydraulic pressure is not present in its control line. The valve has a chamber containing a hydraulic fluid hydraulically coupling the pistons to support any piston not receiving the specified hydraulic pressure in an unactuated position. In certain aspects, when the piston is in the unactuated position, it seals against communication of fluid with its respective control line using a static-type seal.

Description:
BACKGROUND 
       [0001]    In many instances, a well includes a subsurface safety valve for controlling fluid flow, such as closing the well. The valve is typically designed to failsafe to automatically shut the well in if the hydraulic control line to the valve loses the hydraulic pressure. However, such failsafe mechanism may not distinguish a purposeful shut-in operation from a leak incident that causes a pressure drop in the control line. 
       SUMMARY 
       [0002]    The current disclosure relates to surface controlled valves for controlling fluid flow in a well, including those configured as subsurface control valves. In a general aspect, an independent dual actuated subsurface safety valve is controlled by two independent control systems that can independently actuate the valve. In the event of one of the control systems failing, the other control system can be utilized to maintain full functional control of the valve. 
         [0003]    One aspect encompasses a subsurface safety valve for use in a subterranean well. The valve includes a tubular body defining a flow bore therethrough. A closure is in the tubular body and is changeable between sealing against flow through the flow bore and allowing flow through the flow bore. The valve has a first piston with a first control line inlet arranged to receive a first control pressure from a first control line. The first piston is movable from a first unactuated position to a first actuated position in response to the first control pressure. The valve has a second piston with a second control line inlet arranged to receive a second control pressure from a second control line. The second piston is moveable from a second unactuated position to a second actuated position in response to the second control pressure. The first and second pistons are coupled to the closure to change the closure between sealing against flow through the flow bore and allowing flow through the flow bore when the first and second pistons are respectively moved to the first and second actuated positions. The first and second pistons are hydraulically coupled to one another to support the first piston in an unactuated state when the second control pressure applied to the second piston is greater than an actuation pressure and the first control pressure applied to the first piston is less than the actuation pressure. In certain aspects, the first and second pistons are hydraulically coupled to one another to support the second piston in an unactuated state when the first control pressure applied to the first piston is greater than the actuation pressure and the second control pressure applied to the second piston is less than the actuation pressure. In certain aspects, the first piston has a static-type seal that seals against flow into the first control line when the first piston is in the first unactuated position and the first piston comprises a static-type seal that seals against flow into the first control line when the first piston is in the first actuated position. 
         [0004]    One aspect encompasses a method of operating a downhole valve. In the method actuation pressure from a first control line is received at a first piston and actuation pressure from a second control line is received at a second piston. In response to the actuation pressure, a flow bore closure of the valve is actuated open using the first and second pistons. A reduced pressure, below the actuation pressure, is received from the second control line at the second piston. In response to the reduced pressure, the second piston is supported with a hydraulic pressure created by the first piston. In certain aspects, the second piston is supported to engage a static-type seal against passage of fluid with the second control line. In certain aspects, in response to the actuation pressure, the first piston is moved to engage a static-type seal against passage of fluid with the first control line. 
         [0005]    One aspect encompasses a downhole valve for use in a well. The valve has a closure device in a central bore of the valve. The closure device is biased closed to seal against flow through the central bore. A plurality of pistons are each coupled to a respective hydraulic control line into the valve. Each piston is adapted to reside in an actuated position, supporting the closure open, when at least a specified hydraulic pressure is supplied through its control line, and to reside in an unactuated position, not supporting the closure open, when at least the specified hydraulic pressure is not present in its control line. The valve has a chamber containing a hydraulic fluid hydraulically coupling the pistons to support any piston not receiving at least the specified hydraulic pressure in an unactuated position. In certain aspects, when the piston is in the unactuated position, it seals against communication of fluid with its respective control line using a static-type seal. 
         [0006]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0007]      FIG. 1  is a schematic illustration of a well having an example independent dual actuated surface controlled subsurface safety valve. 
           [0008]      FIG. 2  is a half, side cross sectional view of an example independent dual actuated subsurface safety valve in an unactuated position. 
           [0009]      FIGS. 3A and 3B  are half, side cross sectional views of the subsurface safety valve of  FIG. 2  in each of the actuated positions. 
           [0010]      FIG. 3C  is a side view of the subsurface safety valve of  FIG. 2  in an actuated position with its spring housing omitted to show features of the valve. 
           [0011]      FIG. 4A  is a detail half, side cross sectional view of the subsurface safety valve of  FIG. 2  in the unactuated position showing details of actuator piston sealing assemblies. 
           [0012]      FIG. 4B  is a detail half, side cross sectional view of the subsurface safety valve of  FIG. 2  showing details of an actuator piston assembly. 
           [0013]      FIG. 4C  is a detail half, side cross sectional view of the subsurface safety valve of  FIG. 2  in the actuated position showing details of the intermix piston assemblies. 
           [0014]      FIG. 5  is a detail half, side cross sectional view of the subsurface safety valve of  FIG. 2  in the unactuated position showing details of intermix annular chamber. 
           [0015]      FIG. 6  is a detail half, side cross sectional view of the subsurface safety valve of  FIG. 2  in the unactuated position showing details of flapper assembly. 
           [0016]      FIG. 7  is a half, side cross sectional view another example independent dual actuated subsurface safety valve in an unactuated position. 
           [0017]      FIG. 8  is a detail half, side cross sectional view of the subsurface safety valve of  FIG. 7  showing details of a pressure balance annular chamber. 
       
    
    
       [0018]    Like reference symbols in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0019]      FIG. 1  is a schematic illustration of a well  100  having an independent dual actuated surface controlled subsurface safety valve  118 . The well  100  has a tubing string  110 , such as a production and/or injection string, that passes fluids between a subterranean zone of interest and the surface. The well  100  may be cased with a casing  112 , and together with the tubing string  110 , form an annulus  114  therebetween. A seal, such as a packer  116 , may be used to seal off the annulus  114  at a subsurface location above the subterranean zone. 
         [0020]    The subsurface safety valve  118  is coupled with control line  120  and control line  121  respectively via control line interface  164  and control line interface  165 . The control lines  120  and  121  may be hydraulic tubing and pass hydraulic fluids respectively from control systems  122  and  123  on the surface. 
         [0021]    As illustrated in  FIG. 1 , the control systems  122  and  123  are two independently powered, and separately located hydraulic control systems that may apply hydraulic pressure to the subsurface safety valve  118  independently. However, in some embodiments, the control systems  122  and  123  may be two different outputs from a common hydraulic control system that achieves similar function as two independent and separated hydraulic power sources. The control systems  122  and  123  supply two separate control pressures to the valve  118 . 
         [0022]    The valve  118  is configured with two separate, independently controlled and redundant actuation systems. In certain instances, the actuation systems can have the same operating characteristics and/or can be of the same physical configuration (although, in some instances, mirror images of one another). One actuation system is coupled to communicate with the control system  122  via control line  120 , and the other actuation system is coupled to communicate with the control system  123  via control line  121 . The valve  118  is biased to default to a closed position, sealing against flow therethrough and through the tubing string  110 . In other words, the valve  118  is configured to fail safe to closed. However, while receiving control pressure from both of the control systems  122  and  123 , the two actuation systems maintain the valve  118  open, allowing passage of flow therethrough and through the tubing string  110 . If the valve loses control pressure from both of the control systems  122  and  123 , for example if the control system  122  and  123  are changed to cease providing pressure or if the control lines  120  and  121  are leaking or ruptured, the actuation systems no longer maintain the valve  118  open and it defaults closed. Notably, if the valve loses control pressure from one but not the other of the control system  122  and  123 , the actuation system receiving pressure retains full function and will maintain the valve  118  open. The valve  118  is configured to positively seal off the hydraulic passages coupled to the leaking or ruptured control line to prevent leakage of hydraulic fluid and/or other fluids from inside the valve  118  into the annulus  114 . 
         [0023]      FIG. 2  is a half, side cross sectional view of an example independent dual actuated subsurface safety valve  200  in an unactuated position. The subsurface safety valve  200  may be used as the subsurface safety valve  118  in  FIG. 1 . In this embodiment, the subsurface safety valve  200  includes a housing, actuation components, a flow tube  250  and a flapper  234 . The actuation components operate the flow tube  250 , which in an actuated position, pushes open the flapper  234  for flow to pass through, and in an unactuated position, remains inside the housing with the flapper  234  shut to seal against flow through the flow tube  250 . 
         [0024]    The housing includes a piston housing  210 , a spring housing  220 , an intermix piston mandrel  240 , and a flapper seat assembly which includes an inner flapper seat  230  and an outer flapper carrier  232 . The spring housing  220  connects to the piston housing  210  at the upper end and the flapper seat assembly at the lower end to form one integral assembly. The intermix piston mandrel  240  is installed inside the spring housing  220 . 
         [0025]    The piston housing  210  encloses actuator piston assemblies which react to control pressure to actuate the flow tube  250  from the unactuated to the actuated position to open the flapper  234 . The piston housing  210  includes two control passages for connection to two control lines: control passage  212  and control passage  214 . The control passages  212  and  214 , respectively extended from two control line inlets  211  and  213 , are connected to the two symmetrical and separate actuation systems which receive actuation hydraulic pressure from these two control passages  212  and  214 . As is discussed in more detail below, the fluid from the control passage  212  and  214  is not comingled within the valve. Further, the symmetrical actuation systems can be configured to have similar, or identical, operational characteristics in actuating the valve. The control passages  212  and  214  receive pressure from control lines to the surface (e.g., control lines  120 ,  121 ). 
         [0026]    An actuator piston rod  216  of one actuation system is coupled with the control passage  212  on its upper end and coupled with a split actuator sleeve  222  of the same actuation system on its lower end. Symmetrically, an actuator piston rod  218  of a second actuation system is coupled with the control passage  214  on its upper end and coupled with a split actuator sleeve  224  of the second actuation system on its lower end. The split actuator sleeves  222  and  224  engage the flow tube  250  (via a spring  450 , discussed below) at the actuation flange  252  of the flow tube  250  and simultaneously couple with an intermix piston rod  226  of the first actuation system (via a sleeve  482 , discussed below) and an intermix piston rod  228  of the second actuation system (via a sleeve  484 , discussed below) respectively. The intermix piston rod  226  and the intermix piston rod  228  are hydraulically interactive via an intermix annular chamber  245  created between the intermix piston mandrel  240  and the spring housing  220 . 
         [0027]    At an unactuated position as shown in  FIG. 2 , the flow tube  250  is refracted within the spring housing  220  by a spring force exerted by a power spring  225 , which pushes the flow tube  250  at the actuation flange  252  and is installed at a spring stop and bearing base  221  affixed to the spring housing  220 . The undeformed length of the power spring  225  is longer than the allowable length after assembly, therefore a pre-stressed compressive force of the power spring  225  continuously pushes the flow tube  250  upwards. The compressive force is greater than the sum of resultant forces of the gravitational force of the flow tube  250  and friction forces, at full extension in the assembly. 
         [0028]    The power spring  225 , shown here as a metallic coil spring, may be any elastic object capable of storing mechanical energy when longitudinally deformed. The force the power spring  225  exerts may be proportional to its change in length: the spring constant of the power spring  225  is then the change in the force it exerts divided by the change in deflection. 
         [0029]    The flapper assembly at the end of the housing includes the flapper seat assembly and a flapper  234 . When unactuated, the flapper  234  is forced against the flapper seat assembly by a torsional spring, as well as the pressure of well fluids. The flapper  234  seals to the inner flapper seat  230 . The seal closes the subsurface safety valve  200 . Because the flow tube  250  is biased by the spring in an unactuated state, the valve is biased, fail safe closed. Also, although described in connection with a flapper  234  as the closure mechanism, the valve could alternately be configured with a ball valve type closure mechanism. 
         [0030]      FIGS. 3A and 3B  are half, side cross sectional views of the subsurface safety valve  200  in each of the actuated positions. In  FIG. 3A , the actuator piston rod  216  is actuated, pushing the flow tube  250  down to open the flapper  234  and via a hydraulic mechanism, pushing the actuator piston rod  218  to seal the control line  214 , for example, using a metal-to-metal seal, an elastomer seal, etc. Similarly and symmetrically, in  FIG. 3B , the actuator piston rod  218  is actuated, pushing the flow tube  250  down to open the flapper  234  and via the hydraulic mechanism, pushing the actuator piston rod  216  to seal the control line  212  using, for example, using a metal-to-metal seal, an elastomer seal, etc. 
         [0031]    In  FIGS. 3A and 3B , the piston housing  210  may be connected to a tubing string such as the tubing string  110  in  FIG. 1  by the piston housing inner thread  310 . 
         [0032]    In the implementation in  FIG. 3A , the control line  212  is pressurized to actuate the actuator piston rod  216  to push the flow tube  250  at the actuation flange  252 , compressing the power spring  225  and translating the intermix piston rod  226  downwards, connected via the split actuator sleeve  222 . The downward translation motion of the intermix piston rod  226  forces the intermix piston rod  226  to reach a sealing position at the end of the travel and applies pressure to the hydraulic fluid in the intermix annular chamber  245 , which is formed by the enclosure between the intermix piston mandrel  240  and the spring housing  220 . The hydraulic fluid in correspondence pushes the intermix piston rod  228  upwards, and consequently via the connection through the split actuator sleeve  224 , the actuator piston rod  218  upwards to achieve a seal against the control passage  214 . 
         [0033]    The downward translation motion of the intermix piston rod  226  also pushes the flow tube  250  synchronously downward and opens the flapper  234  to an approximately perpendicular position relative to its closed position. The movement of the flow tube  250  compresses the power spring  225  that is constrained between the actuation flange  252  and the spring stop and bearing base  221 . The compressed power spring  225  stores the elastic potential energy needed to return the flow tube  250  to the unactuated position and allows the flapper  234  to be closed. 
         [0034]    Similarly, in the implementation in  FIG. 3B , the control line  214  is pressurized to actuate the actuator piston rod  218  to push the flow tube  250  at the actuation flange  252 , compressing the power spring  225  and translating the intermix piston rod  228  downwards, connected via the split actuator sleeve  224 . The downward translation motion of the intermix piston rod  228  forces the intermix piston rod  228  to reach a sealing position at the end of the travel and applies pressure to the hydraulic fluid in the intermix annular chamber  245 . The hydraulic fluid in correspondence pushes the intermix piston rod  226  upwards, and consequently via the connection through the split actuator sleeve  222 , pushes the actuator piston rod  216  upwards to achieve a seal against the control passage  212 . 
         [0035]    The downward translation motion of the intermix piston rod  228  also pushes the flow tube  250  synchronously downward and opens the flapper  234  to a perpendicular position relative to its closed position. The movement of the flow tube  250  compresses the power spring  225  that is constrained between the actuation flange  252  and the spring stop and bearing base  221 . The compressed power spring  225  stores the elastic potential energy needed to return the flow tube  250  to the unactuated position and allows the flapper  234  to be closed. 
         [0036]      FIG. 3C  is a side view of the subsurface safety valve  200  in an actuated position with its spring housing  220  omitted to show features of the valve.  FIG. 3C  is showing the view from the back side of  FIG. 3A . For example, the housing is integrated by a threaded connection  348  at the piston housing and flapper seat. The split actuator sleeves  222  and  224  conform to the size and shape of the spring housing  220 , as a half cylindrical shape with a thickness giving enough strength for their function.  FIG. 3C  also shows the actuator piston rod  216  at an actuated position, pushing the split actuator sleeve  222  downwards and the split actuator sleeve  224  upwards. 
         [0037]    In  FIG. 3C , the flow tube  250  is translated downwards to open the flapper  234 . The control line  212  is pressurized to actuate the actuator piston rod  216  to push the flow tube  250  at the actuation flange  252 , compressing the power spring  225  and translating the intermix piston rod  226  downwards, connected via the split actuator sleeve  222 . The downward translation motion of the intermix piston rod  226  forces the intermix piston rod  226  to reach a sealing position at the end of the travel and compresses the hydraulic fluid in the intermix annular chamber  245 , which is formed by the enclosure between the intermix piston mandrel  240  and the spring housing  220 . The hydraulic fluid in correspondence pushes the intermix piston rod  228  upwards, and consequently the actuator piston rod  218  upwards to achieve a seal against the control passage  214 . 
         [0038]    The inner flapper seat thread  350  locates at the upper end of the inner flapper seat  230  and couples with the spring housing  220  to seal the well fluid against the flow tube  250  and the intermix annular chamber  245 . It is also shown in  FIG. 3C  that the inner flapper seat  230  is coupled with the outer flapper carrier  232  via a flapper seat joint  346 . 
         [0039]      FIG. 4A  is a detail half, side cross sectional view of the subsurface safety valve  200  in the unactuated position showing details of actuator piston sealing assemblies  422  and  424 . The subsurface safety valve  200  is sealed against well fluid with sealing forces provided by the power spring  225  pushing against the actuation flange  252  of the flow tube  250 . The actuation flange  252  presses against a spring  450  which is coupled with both of the split actuator sleeves  222  and  224 . The split actuator sleeves  222  and  224  are respectively coupled with the actuator piston rod  216  and the actuator piston rod  218 . The actuator piston rods  216  and  218  are respectively connected with piston assemblies  422  and  424  that include seals (e.g., using a metal-to-metal seal, an elastomer seal, etc.) for sealing against control passages  212  and  214  respectively. 
         [0040]    The spring  450 , shown here as a plurality of Belleville washer springs, may be any elastic object capable of storing mechanical energy when longitudinally deformed. The force the spring  450  exerts may be proportional to its change in length. The spring constant of the spring  450  may be the change in the force it exerts divided by the change in deflection. 
         [0041]    With the piston assemblies  422  and  424  in unactuated positions, the power spring  225  transmits compression forces through the flow tube  250 , the spring  450 , the split actuator sleeves  222  and  224 , and the actuator piston rods  216  and  218 , to the piston assemblies  422  and  424  to respectively seal against the upper sealing seats  413  and  415  of the control passages  212  and  214 . As will become apparent from the discussion below, the forces from the power spring  225  are further supplemented by forces from pressure in the intermix chamber  245  acting on intermix piston rods  228 ,  226 , as well as pressure from fluid in the central bore of the tubing string acting on the actuator piston rods  216  and  218 . As a result, the control passages  212  and  214  are sealed against passage of fluid into their respective control lines. In the embodiment depicted in  FIG. 4A , the piston assemblies  422  and  424  employ metal-to-metal static-type seals: using metal components for direct contact with the upper sealing seats  413  and  415 , which are made of metal. Metal-to-metal seals rely on two metal surfaces being brought together under pressure so that any gap remaining between the two surfaces becomes so small that there is no substantial leakage. Such metal-to-metal seal may endure high temperature and high pressure environments and achieve greater reliability than polymer seals. Although discussed here as a metal-to-metal seal, other types of seals could be used. 
         [0042]    In actuated positions, the piston assemblies  422  and  424  seal against lower sealing seats  432  and  434  that are installed into the lower end of the piston housing  210 , as illustrated in  FIG. 3A  where the piston assembly  422  is sealing against the lower sealing seat  432 . The arrangement of the components is such that once the actuator piston rods  216  and  218  have compressed the power spring  225  and moved the flow tube  250  to open the flapper  234 , the actuator piston rods  216  and  218  can be moved further to compress the spring  450  and accomplish the seal of the piston assemblies  422  and  424  with their respective sealing seats  432  and  434 . In the embodiment depicted in  FIG. 4A , the piston assemblies  422  may employ a metal-to-metal static seal that uses metal components for direct contact with metal components of the lower sealing seat  432 . Although discussed here as metal-to-metal seals, other types of seals could be used. 
         [0043]      FIG. 4B  is a detail half, side cross sectional view of the subsurface safety valve  200  showing details of an actuator piston assembly  422 . The actuator piston assembly  422  is symmetrically identical to the actuator piston assembly  424 ; however, in some implementations, minor modifications may be made. For example, if the two separate control paths are assigned a primary and a secondary role, the actuator piston assembly  422  may be different from the actuator piston assembly  424  in dimensions, materials, etc. In the current embodiment, the actuator piston assembly  422  is symmetrically identical to the actuator piston assembly  424 . 
         [0044]    The upper metal seal  425  forms a static metal-to-metal seal with the upper sealing seat  413  at the lower end of the control passage  212 , and connects to the middle connector  431  by screw thread  445  received in a female thread  441 . Similarly, the lower portion of the piston assembly  422  is the actuator piston rod  216 . The upper end of the actuator piston rod  216  is a flange structure that forms a static metal-to-metal seal with the lower sealing seat  432  ( FIG. 4A ), and has an inner screw thread  443  to receive the middle connecter outer thread  447  and assemble with the middle connecter  431 . 
         [0045]    An upper hydraulic seal support  427  and a lower hydraulic seal support  435  are provided to support and locate an upper dynamic-type seal assembly  429  and a lower hydraulic dynamic-type seal assembly  433 , respectively to seal with the sidewall of the piston cylinder. The dynamic-type seal assemblies  429 ,  433  are “dynamic” in that they are configured to seal against the wall of the piston cylinder while the piston is traversing the cylinder. 
         [0046]      FIG. 4C  is a detail half, side cross sectional view of the subsurface safety valve  200  in the actuated position showing details of the intermix piston assemblies. The intermix piston assembly of one control path may include the split actuator sleeve  222 , the intermix piston rod sleeve  482 , the intermix piston rod  226 , and the inter mix piston  488 . The split actuator sleeve  222  can translate up and down in the longitudinal direction of the flow tube  250 , constrained by the inner wall of the spring housing  220  and the outer wall of the spring stop and bearing base  221 . The spring stop and bearing base  221  is coupled with the intermix piston mandrel  240  using bearing base thread  480 . The intermix piston mandrel  240  is further coupled with the spring housing  220  at the lower end using screw thread shown in  FIG. 6 . Therefore the spring stop and bearing base  221  is affixed to the spring housing  220  at a relatively permanent position. 
         [0047]      FIG. 4C  shows the control passage  214  pressurizing and actuating the actuator piston rod  218  to push down the flow tube  250  and the split actuator sleeve  224 , the same as that of  FIG. 3B . The split actuator sleeve  224  is coupled with the intermix piston rod sleeve  484  at a step created by diameter difference of the sleeve  484 . The connection between the split actuator sleeve  224  and the intermix piston rod sleeve  484  may be keyed to prevent relative rotation. The intermix piston rod sleeve  484  is affixed to the intermix piston rod  228 , translating downwards with the intermix piston  490  (shown in  FIG. 5 ) and displacing hydraulic fluids to actuate the intermix piston  488  upwards. 
         [0048]    The intermix pistons  488  and  490  may include a number of hydraulic seals inside the intermix cylinders  491  and  493  respectively. Each the intermix cylinders  491  and  493  may be connected with the intermix annular chamber  245  through a tubular passage with an opening diameter smaller than that of the intermix cylinders. The tubular passages serve as a travel stop for the intermix pistons  488  and  490 . 
         [0049]      FIG. 5  is a detail half, side cross sectional view of the subsurface safety valve  200  in the unactuated position showing details of intermix annular chamber  245 . The intermix annular chamber  245  is formed from the enclosure of the outer surface of the intermix piston mandrel  240 , the inner surface of the spring housing  220 , the thread seal  510  connecting the intermix piston mandrel  240  and the spring housing  220 , and the intermix piston housing seal  520 . The intermix annular chamber  245  connects the intermix cylinder  491  with the intermix cylinder  493 . 
         [0050]    In an unactuated position as shown in  FIG. 5 , the intermix cylinders  491  and  493  and the intermix annular chamber  245  contain an incompressible or compressible hydraulic fluid (e.g., liquid). In certain instances, the hydraulic fluid is silicon oil. The chamber  245  can further contain a compressible fluid pressurized to a specified pressure that is above the expected downhole pressures to ensure any leakage will be directed from the chamber  245  outward and will avoid potential pollution of the hydraulic fluid from the well fluids and to cause the hydraulic fluid to operate as a liquid spring. The hydraulic fluid enables the intermix piston rods  226  and  228  to be responsively coupled with each other: when one is fully displaced to an actuated position, the other actuates the actuator piston rod  216  or  218  up to form a metal-to-metal seal with the control passages  212  or  214 , respectively. 
         [0051]    When the control passage  212  pressurizes and actuates the actuator piston rod  216  to push down the flow tube  250 , the split actuator sleeve  222  is forced downwards as depicted in  FIG. 3A . The split actuator sleeve  222  is coupled with the intermix piston rod sleeve  482  (shown in  FIG. 4C ) at a step created by diameter difference of the sleeve  482 . The connection between the split actuator sleeve  222  and the intermix piston rod sleeve  482  may be keyed to prevent relative rotation. The intermix piston rod sleeve  482  is affixed to the intermix piston rod  226 , translating downwards with the intermix piston  488  and displacing hydraulic fluids to actuate the intermix piston  490  upwards. 
         [0052]      FIG. 6  is a detail half, side cross sectional view of the subsurface safety valve  200  in the unactuated position showing details of flapper assembly. The flapper assembly includes the flapper  234 , the flapper pin  610 , the outer flapper carrier  232 , the flapper seat joint, the inner flapper seat  230 , and the flapper seal  630 . The flapper  234  is biased closed with a spring carried about the flapper pin  610 . The flapper pin  610  is assembled and affixed to the outer flapper carrier  232 . The outer flapper carrier  232  maybe connected to the inner flapper seat  230  by screw thread and sealed with the flapper seat joint  620 . The inner flapper seat  230  is assembled to the spring housing  220  by screw thread at the lower end of the spring housing  220 . The upper circumferential end of the inner flapper seat  230  engages the guide  640  of the spring housing  220  and forms a seal. The flow tube  250  may apply downwards forces on the flapper  234  and cause the flapper  234  to rotate and open. 
         [0053]      FIG. 7  is a half, side cross sectional view another example independent dual actuated subsurface safety valve  700  in an unactuated position. The subsurface safety valve  700  may be used as the subsurface safety valve  118  in  FIG. 1 . In this embodiment, the subsurface safety valve  700  includes a housing, actuation components, a float balancing piston  780  and chamber  775 , a flow tube  750  and a flapper  744  as in the configuration of  FIG. 2  above. Further elements that are similar to elements of subsurface safety valve  200  are similarly numbered with a 7XX prefix. However, the embodiment additionally includes a pressure balance chamber  775 . 
         [0054]      FIG. 8  is a detail half, side cross sectional view of the subsurface safety valve  700  showing details of the pressure balance annular chamber  775 . Unlike in the first embodiment where the subsurface safety valve  200  having the intermix annular chamber  245 , the subsurface safety valve  700  includes the pressure balance annular chamber  775  filled with hydraulic fluid for the float balancing piston  780  that can respond to changes in pressure within the bore of the tubing string. The pressure in the pressure balance annular chamber  775  is balanced to this bore pressure, therefore compensating for variations of well and tubing string pressure and/or temperature fluctuations on the function of the subsurface safety valve  700 . 
         [0055]    The pressure balance annular chamber  775  functions similar to the intermix annular chamber  245  in that the intermix piston rod  726  is hydraulically linked with the intermix piston rod  728 . The two intermix piston rods  726  and  728  are housed in the balanced intermix piston cylinders  762  and  764  respectively, each having a piston at the lower end for hydraulic sealing. The balanced intermix piston cylinders  762  and  764  are connected to the pressure balance annular chamber  775  via the balanced intermix flow passage  766  and  768 , the intermix mandrel inner chamber  754 , the pressure relief valve  772  and the check valve  774 . 
         [0056]    The intermix mandrel inner chamber  754  is enclosed by the outer surface of the balanced intermix piston housing  760 , the inner surface of the spring housing  720 , the intermix mandrel seal  767  and the inner balanced line housing  770 . The clearance between the spring housing  220  and the inner balanced line housing  770  allows hydraulic fluids to flow between the intermix mandrel inner chamber  754  and the fluid ports in the inner balanced line housing  770 . The inner balanced line housing  770  includes an overflow relief valve port  771  and a check valve port  773 . 
         [0057]    A check valve  774  and a pressure relief valve  772  are installed in the inner balanced line housing  770 , respectively connected with the check valve port  773  and the relief valve port  771 . The check valve  774  allows hydraulic fluid to pass in only one direction—from the pressure balance annular chamber  775  toward the intermix piston cylinders  762  and  764 . The pressure relief valve  772  allows hydraulic fluid to pass from the intermix cylinders  762  and  764  to the pressure balance annular chamber  775  if the fluid pressure reaches and/or exceeds a specified pressure value. This enables the pressure in the pressure balance annular chamber  775  to fluctuate: if the pressure is lower than the pressure in the central bore of the tubing string, the check valve  774  allows the float balancing piston  780  to charge the pressure balance annular chamber  775 ; if the pressure exceeds the pressure in the central bore of the tubing string by a specified value, the pressure relief valve  772  allows hydraulic fluid to vent. 
         [0058]    The float balancing piston  780  separates the pressure balance annular chamber  775  from the balanced flow chamber  783  keeping the hydraulic fluid separate from well fluids in the central bore of the tubing string. The pressure balance annular chamber  775  is connected to the check valve  774  and the pressure relief valve  772  as described above. The balanced flow chamber  783  is connected to the fluid in the central bore of the tubing string via a plurality of balanced flow pressure ports  782 . The float balancing piston  780  has inner and outer float balancing piston seals  781 . The float balancing piston  780  therefore can transmit pressure from either one of the chambers until a balanced position (i.e., static pressure balance between the fluids in the pressure balance annular chamber  775  and the fluid in the bore of the tubing string) is reached. That pressure is then communicated (as limited by the check valve  774  and pressure relief valve  772 ) to the intermix cylinders  762  and  764 . 
         [0059]    The specified value of the pressure relief valve  772  is selected to ensure that when one of the intermix piston rods  726  and  728  is compressed, sufficient pressure can be retained in the intermix cylinders  762  and  764  to overcome friction of the system and drive the other unactuated piston upwards without venting to the reservoir. 
         [0060]    In addition, the pressure relief valve  772  ensures that the intermix cylinders  762  and  764  maintain a higher pressure than the pressure balance annular chamber  775 . A higher pressure in the intermix cylinders  762  and  764  ensures long term slow leeching effects will not allow leakage of well fluids into the hydraulic fluid. The use of a pressure balance chamber  775  and the pressure relief valve  772  can result in a very low pressure differential at different sealing locations, further reducing long term leeching effect due to seal by-pass. 
         [0061]    Notably, although the configurations described above are described with only two actuation systems, three or more actuation systems could be provided. 
         [0062]    A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made. Accordingly, other embodiments are within the scope of the following claims.