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BACKGROUND OF THE INVENTION 
     Subsurface safety valves SSVs are safety devices mounted deep within wells to control flow to the surface. They generally have many components in common. The valve member is generally a flapper, which rotates 90° and is held open by a flow tube shiftable downwardly therethrough to cause the 90° rotation. This direction of movement (opening) is away from a closure or seat. A control system is generally employed to urge the flow tube in the opening direction involving hydraulic pressure from the surface connected to the SSV below via a hydraulic control line. In general, applied pressure opens the valve, while removal of applied pressure from the surface allows a power spring acting on the flow tube to move the flow tube in a direction opposite the opening direction and thereby out of the path of the flapper. This allows the flapper to pivot 90° to a closed position. 
     Various types of control systems have been employed for SSVs in order to address various different issues or interests of an operator. To reduce the size of the closure spring acting on the flow tube, reservoirs pressurized with a gas have been used to counteract the hydrostatic pressure from the column of hydraulic fluid in the control line that runs from the surface down to the SSV. Since the pressurized gas resists the hydrostatic force and offsets it, closure of the SSV is accomplished with a fairly small spring when the actuating piston, acting on the flow tube, is placed in hydraulic pressure balance, thus allowing the small closure spring to shift the flow tube and allow the flapper of the SSV to close. 
     Such systems include pressurized reservoirs having a gas on one side and hydraulic fluid (liquid) acting on the opposite side of an actuating piston. In order to make such systems work, numerous seals are used. Control systems have also been developed that serve to allow normal opening and closing of the SSV while, at the same time, restricting the valve to fail in a predesignated safe position in the event of an occurrence of any of a number of different possible conditions or events relating to component failures in the control system. U.S. Pat. No. 6,109,351 (hereinafter “&#39;351” and which is incorporated herein by reference in its entirety), for example, describes such a control system. 
     With the large number of seals in such a system and the requirement that many of the seals must maintain a seal while statically engaging with a piston and slidably engaging with a cylinder bore; seals are a major source of such component failure. Though the failsafe control system prevents undesirable uphole flow when a seal failure does occur it remains a costly undertaking to withdraw the SSV from downhole to repair and/or replace the defective seal or seals and run the SSV downhole again. As such, the art will welcome seals that exhibit improved durability and reliability. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Disclosed herein is a biased actuator. The actuator includes, a reservoir, at least one piston in operable communication with the reservoir, at least one metal seal disposed about the at least one piston and in substantial sealing communication therewith, the at least one metal seal further being in substantial sealing communication with the reservoir and a biasing system in operable communication with both the reservoir and the at least one piston. 
     Further disclosed herein is a control arrangement for a downhole valve. The control arrangement includes, at least one valve actuating piston having at least one metal seal sealingly engaging a housing in which the at least one valve actuating piston is movable, the at least one valve actuating piston having an opening force and a closing force, the opening force is connectable to a selectively controllable pressure source, a primary biasing arrangement acting on the closing force and a secondary biasing arrangement in selective operable communication with the closing force. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike: 
         FIG. 1  depicts a subsurface safety valve disclosed herein; 
         FIG. 2  depicts a portion of the subsurface safety valve of  FIG. 1  at a higher magnification; 
         FIG. 3  depicts an actuator used in the subsurface safety valve of  FIG. 1 ; 
         FIG. 4  depicts a portion of the actuator of  FIG. 3  shown at a higher magnification; 
         FIG. 5  depicts an even higher magnification of a portion of the actuator of  FIG. 3 ; 
         FIG. 6  depicts a portion of a control arrangement used in the subsurface safety valve of  FIG. 1 ; 
         FIG. 7  depicts a portion of the control arrangement of  FIG. 6  shown at a higher magnification; and 
         FIG. 8  depicts a metallic seal disclosed herein in assembly. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A detailed description of an embodiment of the disclosed apparatus and method are presented herein by way of exemplification and not limitation with reference to the Figures. 
     The control system disclosed in &#39;351 has two pistons and two gas charged reservoirs or chambers. One of the pistons is an actuating piston and the other is a balancing piston. Both pistons may be made of metal. The actuating piston moves a flow tube in a downhole direction in response to a pressure increase supplied from surface via a control line. The flow tube is moved in an uphole direction in response to urging from a power spring when the pressure in the control line is reduced below a predetermined value. The other piston is a pressure-balancing piston that isolates a primary gas charged pressure from the control line when all seals are properly sealing. The pressure-balancing piston allows the pressure of the primary gas charge to bleed to the control line and thereby equalize with the control line pressure in response to leakage of any of a plurality of control system seals. Each of the pistons in the control system has at least one seal that sealably engages with the piston and slidably sealably engages with cylinders in which the pistons are axially moveable. Disclosed herein is an exemplary embodiment of a subsurface safety valve with metallic seals employing the control system of &#39;351. 
     Referring to  FIGS. 1 and 2  an embodiment of the subsurface safety valve  10  is illustrated. The safety valve  10  among other things includes a control arrangement  14 , a flow tube  18 , a flapper  22  and a power spring  26 . The control arrangement  14  includes pistons, seals and gas charged reservoirs or chambers that are too small to be seen in  FIGS. 1 and 2  and will be described with reference to  FIGS. 2 through 8  below. In  FIGS. 1 and 2  the safety valve  10  is shown in an open position thereby allowing fluid to flow through the safety valve  10  in either an uphole or a downhole direction. The flow tube  18  is repositionable between an uphole and a downhole position (shown in downhole position). When the flow tube  18  is in the downhole position the flow tube  18  locks the flapper  22  between the flow tube  18  and a housing  30  in an orientation substantially parallel to an axis of the flow tube  18 , thereby holding the safety valve  10  open. The power spring  26  is positioned between the housing  30  and a shoulder  34  of the flow tube  18  such that it presents a biasing force to urge the flow tube  18  in the uphole direction toward the closed valve position. When the safety valve  10  is in the closed position (not shown), the flapper  22  is pivoted 90° such that the flapper  22  is substantially perpendicular to the axis of the flow tube  18  and seals against a seat  38 . An optional biasing member (not shown), such as a torsion spring, for example, can be mounted about a hinge of the flapper  22  to urge the flapper  22  toward the closed position. With the valve  10  in the closed position fluid is prevented from flowing through the safety valve  10  in either the uphole or downhole direction. It should be noted however that in instances when there is a greater pressure on an uphole side of the flapper  22  than a downhole side of the flapper  22  the pressure differential may be able to urge the flapper  22  off the seat  38  allowing flow in a downhole direction even while the flow tube  18  is in the uphole position. In contrast when pressure on the uphole side of the flapper  22  is less than on a downhole side of the flapper  22  the difference in pressures urges the flapper  22  against the seat  38  thereby increasing the sealing engagement force of the flapper  22  against the seat  38 . 
     Through the foregoing structure the movement of the flow tube  18  between the uphole position and the downhole position facilitates the operation of the safety valve  10  between an open and closed position. As such, by controlling the position of the flow tube  18  the opening and closing of the safety valve  10  can be controlled. 
     Referring to  FIGS. 2-5  one embodiment of an actuator  40  of the Control arrangement  14  including a pair of actuating pistons  42  is illustrated. Although two actuating pistons  42  are disclosed alternate embodiments could have more than two actuating pistons  42  or a single actuating piston  42 . Each of the actuating pistons  42  is functionally engaged with the flow tube  18 . In this embodiment the functional engagement includes a shoulder  50  on each actuating piston  42  that is contactable with a shoulder  46  on the flow tube  18  such that movement of the actuating pistons  42  in a downhole direction causes a corresponding movement of the flow tube  18  in a downhole direction. The biasing force of the power spring  26  on the flow tube  18 , in an uphole direction, assures that the opposing shoulders  46  and  50  remain in continuous contact. The actuating pistons  42 , in this embodiment, are each housed within a longitudinal cylinder  54  formed in a housing  58 , which may be made of metal, and are sealably engaged with the cylinder  54  by a plurality of seals  60 ,  62 , and  64 . Each of the seals  60 ,  62 ,  64  sealably engages with the piston  42  and the cylinder  54  to which they are engaged. The engagement of the seals  60 ,  62 ,  64  with the cylinders  54  is also slidable such that the piston  42  and the seals  60 ,  62 ,  64  can move axially within the cylinder  54  while maintaining a seal with the cylinder  54 . While each of the sealing locations of this embodiment incorporates a single metal seal  60 ,  62 ,  64  it should be understood that alternate embodiments could use multiple metal seals  60 ,  62  and  64  at each sealing location. 
     The seals  60 ,  62 ,  64  divide the cylinder  54  into four cavities  70 ,  72 ,  74 , and  76 . The seal  60  isolates the cavity  70  from the cavity  72 , the seal  62  isolates the cavity  72  from the cavity  74 , and the seal  64  isolates the cavity  74  from the cavity  76 . The cavity  74  is fluidically connected, via a port not shown, to a downhole environment within which the safety valve  10  is located. Similarly, the cavity  70  is fluidically connected to a control line through a port not shown that ports control pressure from the surface to the safety valve  10 . A longitudinal port  80  within each of the actuating pistons  42  fluidically connects the cavities  72  and  76  such that the cavities  72  and  76  are maintained at equal pressures at all times. Additionally, the cavity  76  is ported to a primary charge pressure via a portion of the control arrangement  14  that will be described with reference to  FIGS. 6 and 7 . 
     Referring to  FIGS. 6 and 7  a portion of the control arrangement  14  is illustrated in cross section at two different levels of magnification. The control arrangement  14  includes, a pressure-equalizing piston  84  with two seals  90 ,  92  thereon, a fill block  96  and a housing  100 . The fill block  96  and the housing  100 , which may both be made of metal, have cylindrical ports  104  and  108 , respectively, formed therein that are receptive of the pressure-balancing piston  84 . The housing  100  is sealably connected to the fill block  96  with the cylindrical ports  104 ,  108  in axial alignment with one another. The seals  90 ,  92  are in sealing engagement with the piston  84  and are in slidable sealing engagement with the cylindrical ports  104 ,  108  such that the piston  84  and seals  90 ,  92  can move axially within the cylindrical ports  104 ,  108  while maintaining sealing engagement with both the piston  84  and the ports  104 ,  108 . The cylindrical port  108 , however, has two portions  112 ,  114 , the first portion  112  is dimensionally smaller than the second portion  114 , and as such the seal  92  is sealably engagable with the first portion  112  while not being sealably engagable with the second portion  114 . The second portion  114  is displaced axially from the first portion  112  such that axial movement of the piston  84  such that the seal  92  moves from the first portion  112  to the second portion  114  will result in a loss of seal between the seal  92  and the cylindrical port  108 . 
     The seals  90 ,  92  divide the cylindrical ports  104 ,  108  into three cavities  120 ,  122 , and  124 . The seal  90  isolates the cavity  120  from the cavity  122 , and the seal  92  isolates the cavity  122  from the cavity  124  when the seal  92  is in sealing engagement with the first portion  112 . The cavity  122  is fluidically connected through porting not shown to the control line, which is also fluidically connected to cavity  70  of  FIGS. 3-5 . Similarly, cavity  124  is fluidically connected to the cavity  76  of  FIGS. 3-5  through porting not shown. Cavities  124  and  76  are further fluidically connected to a pressurized gas charged primary reservoir or chamber not shown. Similarly, the cavity  120  is fluidically connected to a pressurized gas charged secondary reservoir or chamber, depicted herein as the cavity  120  in the fill block  96 . 
     The foregoing structure is in accord with the failsafe control system of &#39;351. As such failure of any of the seals  60 ,  62 ,  64 ,  90 , and  92  will result in a pressure differential across the pressure-equalizing piston  84  so that it moves the seal  92  from sealing engagement with the first portion  112  to non-sealing engagement with the second portion  114 . At this point the higher pressure of the pressurized gas charged primary reservoir (the cavities  124  and  76 ) is able to bleed to the control line (the cavities  122  and  70 ) thereby equalizing pressure between the pressurized gas charged primary reservoir and the control line. After which, the cavities  124 ,  76 ,  122  and  70  all have the same pressure therein. Once the pressure in cavities  124 ,  76 ,  122  and  70  is equalized the urging force of the power spring  26 , which is set high enough to overcome the frictional and gravitational forces acting against it, is able to move the flow tube  18  to its uphole, or failsafe position, allowing the flapper  22  to close thereby preventing undesirable uphole fluid flow. Additionally, with the pressure-equalizing piston  84  no longer isolating the (control line) cavity  122 ,  70  from the cavity  124 ,  76 , any increase in pressure in the control line will equalize about the actuating piston  42  preventing subsequent actuations of the safety valve  10  with increases in pressure in the control line. It should be that while the embodiment disclosed herein uses gas charged chambers to create biasing forces on the pistons  42 ,  84 , alternate embodiments could create biasing forces using other sources of stored energy than chambers filled with a gas charge. 
     Referring to  FIG. 8  an embodiment of the seals  60 ,  62 ,  64 ,  90 , and  92  disclosed herein is illustrated. Each of the seals  60 ,  62 ,  64 ,  90 , and  92  is made of a metal tubular member  128 . The tubular member  128  includes three frustoconical portions  132 ,  134 , and  136 . The first frustoconical portion  132  and the second frustoconical portion  134  increase the radial dimension of the tubular member  128  to a greatest radial dimension portion  140  that has a greater radial dimension than all other portions of the tubular member  128  when in a non-energized position. Similarly, the second frustoconical portion  134  and a third frustoconical portion  136  decrease the radial dimension of the tubular member  128  to a smallest radial dimension portion  144  that has a smaller radial dimension than all other portions of the tubular member  128  when in a non-energized position. The seal  60 ,  62 ,  64 ,  90 , and  92  is in a non-energized position (not shown) when the seal  60 ,  62 ,  64 ,  90 , and  92  is not in an assembly and is therefore not constrained in any way. In the energized position (as shown in  FIG. 8 ), the seal  60 ,  62 ,  64 ,  90 , and  92  is constrained by an inner dimension and an outer dimension by a cavity into which it is assembled. As such, the tubular member  128  has a maximum radial dimension of the portion  140  that is greater when the tubular member  128  is in the non-energized position than the portion  140  has when it is in the energized position. Similarly, the tubular member  128  has a minimum radial dimension of the portion  144  that is smaller when the tubular member  128  is in the non-energized position than the portion  144  has when it is in the energized position. 
     The tubular member  128 , therefore, is in the energized position when the portions  140 ,  144  are constrained within an assembly. In the energized position the portion  140  is sealably engagable with an inside sealing surface  148  of the housing  58 , the fill block  96  or the housing  100 , depending upon which seal  60 ,  62 ,  64 ,  90 , and  92  is being used. A sealing force between the portion  140  and the inside sealing surface  148  is due to the energizing force of the tubular member  128  being in the energized position. This energizing force results from the elasticity of the metal from which the tubular member  128  is fabricated. Similarly, in the energized position the tubular member  128  has the portion  144  sealably engaged with an outside sealing surface  152  of the actuating piston  42  or the pressure-equalizing piston  84 . A sealing force between the portion  144  and the outside sealing surface  152  is due to the energizing force of the tubular member  128  being in the energized position. This energizing force results from the elasticity of the metal from which the tubular member  128  is fabricated. 
     The elasticity of the metal tubular member  128  is such that the seal created between the tubular member  128  and the sealing surface  148 ,  152  is flexible enough to allow for minor movements of the pistons  42 ,  84  relative to the housings  58 ,  96 ,  100  without resulting in leakage therebetween. Additionally, the pistons  42 ,  84  and the tubular members  128  are axially slidably movable within the housings  58 ,  96 ,  100  while maintaining sealing engagement therebetween. The metal of the tubular member  128  can be highly resistant to degradation with long term exposure to high temperatures and high pressures that are commonly found in downhole environments. The metal of the tubular member  128  can also be highly resistant to corrosion and caustic fluids that may be encountered downhole as well. As such the sliding seal created between the seals  60 ,  62 ,  64 ,  90 , and  92  and the housings  58 ,  96 ,  100 , can have a high level of reliability and durability in very challenging applications. 
     While the invention has been described with reference to an exemplary embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the claims.

Summary:
A biased actuator includes, a reservoir, at least one piston in operable communication with the reservoir, at least one metal seal and a biasing system in operable communication with both the reservoir and the at least one piston. The at least one metal seal is disposed about the at least one piston and is sealed to both the piston and the reservoir.