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1. FIELD OF THE INVENTION 
     This invention relates in general to production of oil and gas wells, and in particular to a wellhead assembly having a shuttle valve moveable in one direction by pressurized fluid. 
     2. DESCRIPTION OF RELATED ART 
     Wellheads used in the production of hydrocarbons extracted from subterranean formations typically comprise a wellhead assembly attached at the upper end of a wellbore formed into a hydrocarbon producing formation. Wellhead assemblies usually provide support hangers for suspending production tubing and casing into the wellbore. The casing lines the wellbore, thereby isolating the wellbore from the surrounding formation. The tubing typically lies concentric within the casing and provides a conduit therein for producing the hydrocarbons entrained within the formation. 
     Wellhead assemblies also typically include a wellhead housing adjacent where the casing and tubing enter the wellbore, and a production tree atop the wellhead housing. The production tree is commonly used to control and distribute the fluids produced from the wellbore and selectively provide fluid communication or access to the tubing, casing, and/or annuluses between the tubing and casing. Valves assemblies are typically provided within wellhead production trees for controlling fluid flow across a wellhead, such as production flow from the borehole or circulating fluid flow in and out of a wellhead. 
     In a type of wellhead system, a concentric tubing hanger lands in the wellhead housing thereby extending tubing into the wellbore and forming an annulus between the tubing and casing that lines the wellbore. A tubing annulus valve is generally located in the tubing hanger since a plug cannot be temporarily installed and retrieved from the tubing annulus passage with this type of tree. 
     A prior art example of a wellhead assembly  10  is shown in a side partial sectional view in  FIG. 1 . The wellhead assembly  10  has an annular wellhead housing  12  on its outer periphery and a tubing hanger  14  concentrically landed within the wellhead housing  12 . Tubing  16  depends downward from the tubing hanger  14  to within casing  18 . The casing  18  is suspended from a casing hanger  20  landed within the wellhead housing  12  below the tubing hanger  14 . A tubing annulus  22  is formed in the annular space between the tubing  16  and tubing hanger  14  and casing  18  and casing hanger  20 . A seal in the annulus  22  prevents pressure communication between wellbore pressure and ambient. 
     From time to time access to the annulus  22  and beneath the seal may be required for various downhole operations. Accordingly, shuttle valves  24  are included for fluid communication through the wellhead assembly  10  and into the annulus  22 . In the prior art example of  FIG. 1 , the shuttle valves  24  are shown substantially parallel with the tubing  16  and disposed in cavities formed within the tubing hanger  14 . Typically, the shuttle valves  24  are hydraulically actuated and having a piston therein that selectively moves between an open and closed position. An example of an opening line  26  is shown delivering hydraulic fluid to a side of the piston to move the shuttle valve into an open position that communicates pressure in the annulus  22  through the tubing hanger  14 , where it can then be routed to outside of the wellhead housing  10 . Pressure communication to outside of the wellhead housing  10  is illustrated by a flow passage  30  formed through the wellhead housing and into connection with the shuttle valve  24 . Thus when the shuttle valve is an open position a flow path is formed through the wellhead housing within the passage  30 , through the shuttle valve  24  and into the annulus  22 . Also shown is a closing line  28  that delivers hydraulic fluid for closing the shuttle valve  24  to block pressure communication through the tubing hanger  14 . The opening closing lines  26 ,  28  each occupy space within the wellhead housing  10 . 
     SUMMARY OF THE INVENTION 
     Disclosed herein is an example of a wellhead assembly, that in an example embodiment includes a tubing hanger in a wellhead member and tubing attached to the tubing hanger that inserts into a wellbore. An annulus encircles the tubing. Also included is a shuttle valve assembly in the tubing hanger; where the shuttle valve includes a sleeve, a valve close port, a valve open port, and a piston selectively slidable within the sleeve to a closed position with an amount of pressure applied to the valve close port and the piston selectively slidable to an open position with an amount of pressure applied to the valve open port. In the example embodiment, a flow path extends through the tubing hanger, into the tubing annulus, and through the shuttle valve assembly when the piston is in an open position. An accumulator is included in this example embodiment that is in the annulus and in fluid communication with the valve close port. In an alternative embodiment, the wellhead member can be a wellhead housing or a production tree. The accumulator can be pressurized to a pressure above an ambient pressure. A spring may be optionally included that is biased to urge the shuttle valve into the closed position. In an alternative embodiment, the piston moves into the open position from pressure communication with a passage in the wellhead member and moves into the closed position from pressure communication with the accumulator. In an alternative embodiment, when the valve is moved into the valve open position, pressure in the accumulator maintains a force on the piston to urge the piston toward the closed position. In an alternative embodiment, the shuttle valve assembly is a first shuttle valve assembly and wherein the flow path is a first flow path; in this alternative embodiment a second shuttle valve assembly is included substantially similar to the first shuttle valve assembly. 
     An alternative example embodiment of a wellhead assembly is described herein, in the alternative example embodiment the wellhead assembly includes a wellhead member with a tubing hanger set therein and tubing hanging from the tubing hanger into a wellbore. The tubing is surrounded by a tubing annulus. In the alternative example embodiment, a shuttle valve is included in the tubing hanger; the shuttle valve is made up of an opening port in fluid communication with pressurized fluid for directing pressure within a portion of the shuttle valve for selectively positioning the shuttle valve into an open position, a flow path extending through the tubing hanger, into the tubing annulus, and through the shuttle valve when the shuttle valve is in the open position, and an accumulator in the tubing annulus in fluid communication with a closing port on the shuttle valve for directing pressure within another portion of the shuttle valve for selectively positioning the shuttle valve into a closed position. In an example embodiment, the shuttle valve has a sleeve and a piston selectively slidable within the sleeve to close the shuttle valve with an amount of pressure applied to the closing port and the piston selectively slidable to open the shuttle valve with an amount of pressure applied to the shuttle valve opening port. In an example embodiment, when the shuttle valve is moved into the shuttle valve open position, pressure in the accumulator maintains a force on the piston to urge the piston in a direction to close the shuttle valve. In an example embodiment, when the valve is moved into the valve open position, pressure in the accumulator maintains a force on the piston to urge the piston toward the closed position. In an example embodiment, the accumulator is suspended in an annulus between a portion of a tubing hanger and a wellhead housing. In an example embodiment, the shuttle valve is actuated into the open position by pressurized fluid in a single line passing through the wellhead housing and actuated into the closed position by pressure in the accumulator. In an example embodiment, another substantially similar shuttle valve is included. In an example embodiment, the accumulator is in fluid communication with a closing port on the second shuttle valve for directing pressure within another portion of the second shuttle valve for selectively positioning the second shuttle valve into a closed position. Optionally, more than one accumulator can be included. 
     Also disclosed herein is a method of controlling flow through a wellhead assembly. In an example embodiment the method includes providing a shuttle valve in a bore of a tubing hanger. The shuttle valve can be opened by applying pressure, such as through a fluid. This allows flow into a flow passage that extends within an annulus between the tubing hanger and a wellhead housing, through the open shuttle valve and outside of the wellhead assembly. Opening the valve also in turn pressurizes a closing side of the shuttle valve. The pressure in the closing side is stored for later use to close the shuttle valve. In an alternative embodiment, storing pressure can involve directing pressure from the closing side of the shuttle valve to an accumulator. In an alternative embodiment, the accumulator is a closed system disposed in the annulus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side partial sectional view of a prior art wellhead assembly. 
         FIG. 2  is a side sectional view of an example embodiment of a shuttle valve in an open position and in accordance with the present disclosure. 
         FIG. 3  is a side sectional view of the valve assembly of  FIG. 2  in a closed position. 
         FIG. 4  is a side sectional view of the valve assembly of  FIG. 2  in a partially closed position. 
         FIG. 5  is a side partial sectional view of a wellhead assembly in accordance with the present disclosure. 
         FIG. 6  is an axial sectional view of an example embodiment of the wellhead assembly of  FIG. 5 . 
         FIG. 7  is a side partial sectional view of an example embodiment of the wellhead assembly of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The apparatus and method of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments are shown. This subject of the present disclosure may, however, be embodied in many different forms and should not be construed as limited to the illustrated embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. For the convenience in referring to the accompanying figures, directional terms are used for reference and illustration only. For example, the directional terms such as “upper”, “lower”, “above”, “below”, and the like are being used to illustrate a relational location. 
     It is to be understood that the subject of the present disclosure is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. In the drawings and specification, there have been disclosed illustrative embodiments of the subject disclosure and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation. Accordingly, the subject disclosure is therefore to be limited only by the scope of the appended claims. 
     An example of a shuttle valve assembly  40  in accordance with the present disclosure is shown in a side sectional view in  FIG. 2 . The shuttle valve assembly  40  of  FIG. 2  is set in a bore  42  formed axially through a tubing hanger  44 . An annular sleeve  46  is retained within a portion of the bore  42  by an annular locknut  48  shown abutting an end of the sleeve  46 . Gallery ports  50  project radially through the sleeve  46  into fluid communication with an annular groove  52  formed in the outer wall of the bore  42 . An elongate piston  54  is shown coaxial within the bore  42 ; an end of the piston  54  inserts within the sleeve  46 . A shoulder  55  is provided along a portion of the outer surface of the piston  54  and disposed a distance from the sleeve  46 . The shoulder  55  is defined along a region where the piston  54  extends radially outward to form a forward and a rearward face  56 ,  57 ; where the forward face  56  is on the end of the shoulder  55  proximate the sleeve  46 , and the rearward face  57  is on the end of the shoulder distal the sleeve  46 . 
     Still referring to  FIG. 2 , an opening port  58  projects radially into the housing  42  and into fluid communication with the bore  42 . The opening port  58  intersects the bore  42  adjacent the side of the sleeve  46  that faces the shoulder  55 . A closing port  60  projects radially inward from an outer surface of the body  42  and into fluid communication with the bore  42 . The closing port  60  intersects the bore  42  adjacent the forward face  56  of the shoulder  55 . A flow line  61  is shown that couples on the closing port  60  for providing fluid communication between the port  60  and a source of pressurized fluid (not shown). A spring  62  is shown having an end inserted into an axial bore  63  formed in an end of the piston  54  distal from the sleeve  46 . An opposite end of the spring  62  abuts a stem  64  secured within the bore  42  to compress the spring  62  between the piston  54  and stem  64 . A bolt head  66  on the outer end of the stem  64  threadingly secures the stem  64  into the body  42  so that the compressed spring  62  imparts a biasing force to further insert the piston  54  into the sleeve  46 . The shuttle valve  40  of  FIG. 2  is in an open position thereby providing fluid communication from within the bore  42 , through the gallery ports  50  and into the annular groove  52 . The annular groove  52  couples with a flow path (not shown) in fluid communication with or ambient to a wellhead assembly (not shown). 
       FIG. 3  illustrates in a side sectional view the shuttle valve  40  of  FIG. 2  in a closed position. In this example, the piston  54  is shown further inserted within the sleeve  46  and blocking fluid communication from within the bore  42  and through the gallery ports  50 . In an example of use, the piston  54  is urged within the bore  42  by controlling pressure in the opening and/or closing ports  58 ,  60  to create a pressure differential across the shoulder  56 . When pressure at the closing port  60  exceeds pressure at the opening port  58  sufficiently to overcome sliding friction, the piston  54  moves from the position of  FIG. 2  to the position of  FIG. 3  to close the shuttle valve  40 . Optionally, such as in a case when pressures are substantially equal at the opening/closing portions  58 ,  60 , the force in the compressed spring  62  may urge the piston  54  into a closing position. Moving the piston  54  away from the stem  64  allows the spring  62  to elongate from the compressed configuration of  FIG. 2 . When the piston  54  is in the closed position, the shoulder  55  is in contact with an end of the sleeve  46  opposite the locknut  48 . 
     Shown in a side sectional view in  FIG. 4  is an example of the shuttle valve  40  in a partially closed position. In this example, the piston  54  is urged from the open position of  FIG. 2  toward the closed position of  FIG. 3 . However, due to friction between the piston  54  and bore  42 , the front face  56  of the shoulder  55  is spaced apart from the sleeve  46  thereby leaving a gap  67  between the front face  56  and sleeve  46 . A partially closed configuration may occur when an ability to apply pressure at the open and/or closing ports  58 ,  60  is lost and the closing function is performed solely by expansion of the spring  62 . 
     Referring now to  FIG. 5 , an example of a wellhead assembly  68  is illustrated in accordance with present disclosure. The wellhead assembly  68  is shown having an annular wellhead housing  70  on its outer periphery and a tubing hanger  44  concentric within the wellhead housing  70 . Tubing  74  depends downward into a well bore (not shown) from the tubing hanger  44 . Casing  76  circumscribes the tubing  74  and a portion of the tubing hanger  44  thereby defining an annulus  80  between the tubing  74  and tubing hanger  44 , and casing  76 . A casing hanger  78  is illustrated for supporting the casing  76  within the wellhead assembly  68 . Cavities are formed in the tubing hanger  44  and configured to receive shuttle valves  40  therein. An opening line  79  is shown formed axially from an upper end of the tubing hanger  44  downward adjacent to the shuttled valve  40 , and into fluid communication with the shuttle valve  40 . Supplying pressurized fluid into the opening line  79  can actuate the shuttle valve  40  into the open position. In the open position, a flow passage  86  is put into fluid communication with the annulus  80  through the shuttle valve  40 , thereby providing access to the annulus  80  from ambient to the wellhead assembly  68 . The flow passage  86  is formed through the tubing hanger  44  and has an end facing the annular groove  52 . Coursing upward through the tubing hanger  44 , and then radially outward, the flow passage  86  intersects an annular groove  87  in the wellhead assembly shown circumscribing an upper portion of the tubing hanger  44 . The groove  87  is in fluid communication with an annulus  88  in the wellhead assembly and above the groove  87 . 
     In the example of  FIG. 5  an accumulator  82  is illustrated in fluid communication with the flow line  61  coupled to the closing port  60  of the shuttle valve  40  ( FIG. 2 ). In an example, the accumulator  82  is a closed system, such as a vessel or piping circuit, that may be pressurized for storing a potential closing force for closing the shuttle valve  40 . In an example embodiment, the accumulator  82  is pressurized prior to being installed. When the shuttle valve  40  is moved to the open position, the piston  54  urges fluid through the flow line  61  towards the accumulator  82  thereby increasing pressure in the accumulator  82 . Thus, energy for closing the shuttle valve  40  can be stored each time the shuttle valve  40  is opened and also made available apart from spring actuation. Optionally, tubing (not shown) could connect to the accumulator  82  for pressurizing the accumulator  82  after installation. By providing sufficient pressure to the accumulator  82 , the shuttle valve  40  can be moved into the closed position when pressure is removed from the opening line  79 ; where removing pressure from the opening line  79  can be intended or from a loss of pressure supply. An additional advantage of the accumulator  82  is that a closing function to the shuttle valve  40  is provided without consuming space in the wellhead assembly  68 . Accordingly, other control lines may be included within the space no longer occupied by the flow line for closing the shuttle valve  40 . 
     Shown in  FIG. 6 , is an axial partial sectional view of the wellhead assembly  68  looking upward from below the shuttle valves  40  and accumulator  82 . In this example, multiple shuttle valves  40  are shown disposed within the annulus  80  at various angular positions. Also, more than one accumulator  82  is illustrated provided within the annulus  80 . In the example embodiment of  FIG. 6 , the accumulators  82  may be offset from a shuttle valve  40 , or may be directly below a shuttle valve  40 . Shown in a side partial sectional view in  FIG. 7  is an example embodiment of the wellhead assembly  68  wherein more than one shuttle valve  40  is in fluid communication with an accumulator  82 . In this example an accumulator circuit  83  stems between the shuttle valves  40  and into pressure communication with an accumulator  82 . 
     While the invention has been shown or described in only some of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention.

Summary:
A wellhead assembly made up of a wellhead housing, a production tree mounted on the wellhead housing, tubing suspended into a wellbore from within the wellhead housing, and an annulus between the tubing and the wellhead housing. A shuttle valve is provided within a tubing hanger that supports the tubing from within the wellhead housing. An accumulator is disposed in the annulus that is in fluid communication with a closed position port on the shuttle valve. Pressure is maintained in the accumulator for closing the shuttle valve when a force for opening the valve is removed.