Abstract:
Apparatus and method for isolating an annular bore below a tubing hanger body disposed in a wellhead assembly. The hanger body is characterized by at least two annulus flowby passages that are capable of fluid communication with areas outside the hanger body through the central bore of the hanger body. An integral valve piston is disposed within the central bore of the hanger body, and is operable to axially displace within the central bore between a closed and opened position. The displacement of the piston is caused by the alternating introduction of pressurizing fluid into separate actuation chambers. In the closed position, the piston is positioned so that at least one of the flowby passages is sealingly engaged in a manner that prevents fluid flow through the hanger body from outside sources such as an annulus bore located below the hanger body.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority to PCT/US2008/061525 filed Apr. 25, 2008, which is hereby incorporated herein by reference in its entirety for all purposes, and claims the benefit of priority to U.S. Provisional Application No. 60/915,178 filed on May 1, 2007. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       BACKGROUND 
       [0003]    1. Field of the Invention 
         [0004]    The invention relates generally to an annulus shutoff valve. More particularly, the present invention relates to an annulus shutoff valve that seals one or more annulus flowby ports. Still more particularly, the present invention relates to a subsea tubing hanger with an integral annulus shutoff valve, which seals one or more annulus flowby ports in the tubing hanger, and is employed to safely close off or isolate an annular space below the tubing hanger. 
         [0005]    2. Background of the Invention 
         [0006]    In general, valves are devices used to regulate the flow of fluids (e.g., gases, liquids, slurries, etc.) through a passage by opening, closing, or partially obstructing the passage. Valves are used in hundreds of industrial, military, commercial, and even residential applications. Depending on the application, the failure of a valve may potentially result in undesirable consequences such as damage to the system of which the valve is a part, an inability to regulate fluid flow through the valve, and significant repair and downtime expenses. 
         [0007]    A common type of valve is a piston-cylinder valve having a piston slidingly disposed within the inner cavity or central bore of a cylinder. Often, an annular seal, which may be seated in a groove around the piston, is provided between the piston and the inside surface of the cylinder wall. By sealingly engaging the inside surface of the cylinder wall, the annular seal prevents the flow of fluids across the piston between the piston and cylinder wall. In addition, an inlet port and an axially spaced apart outlet port are typically provided through the cylinder wall. When the piston is positioned between the inlet port and the outlet port, the piston prevents fluid communication between the inlet port and the outlet port, thereby placing the valve in a “closed” position (e.g., the piston and annular seals block the flow of fluid from the inlet port, through the cylinder cavity, to the outlet port). However, if the piston is not axially positioned between the inlet port and the outlet port, fluid communication between the inlet port and the outlet port is permitted, thereby placing the valve in the “opened” position (e.g., the piston does not block the flow of fluid from the inlet port, through the cylinder cavity, to the outlet port). 
         [0008]    In many conventional piston-cylinder valves, a hydraulic or pneumatic actuator is employed to control the movement of the piston within the cylinder, thereby controlling the status of the valve as “opened” or “closed.” As the piston is moved between the inlet port and outlet port, or outside the inlet port and outlet port, the piston, and any annular seals around the piston, may cross over one or both ports. 
         [0009]    Piston-cylinder valves may be utilized to divert fluid flow in oil or gas production dual-bore wellhead assemblies. For instance, in a subsea oil or gas production well, the wellhead assembly typically includes a tubing hanger having a vertical production bore and at least one vertical annulus bore that is in fluid communication with a tubing annulus and is located below the tubing hanger and between the production tubing and the production casing. In dual-bore wellhead assemblies, two plugs may be required to separately shut off or regulate the flow of fluids into or from the annulus bore, and to shut off the production bore in order to abandon the well or to remove a blow-out preventer (BOP) stack. The running of two separate plugs results in additional equipment and operating costs and time, and decreases the operational flexibility of wellhead assemblies characterized by this dual-bore configuration. 
         [0010]    A piston-cylinder valve with typical annular seals that is known in the art may be used to control access to the vertical annulus bore, or tubing annulus, and to regulate the flow of fluids into or from the annulus through the tubing hanger. Valves of this type are typically not integral to the downhole equipment they are intended to serve, and as a result are not easily installed, accessed, removed or replaced. As such, the installation or removal of these non-integral valves may require the removal of conflicting upstream wellhead equipment, thereby disturbing wellhead operations. Further, the failure of the annular seal in the piston-cylinder valve may require a shutdown of the producing well, and necessitate accessing the piston-cylinder valve by removing upstream equipment such as a Christmas tree or a BOP, pulling the valve from the tubing hanger, and repairing or replacing the valve. A leaking valve or inoperable valve resulting from damage may require replacement or repairs, potentially resulting in significant maintenance costs and downtime. In addition to significant well downtime, such a procedure may result in increased associated cost, such as well as repair and maintenance expenses to access, pull, and repair or replace the failing valve. 
         [0011]    Thus, there remains a need to develop methods and apparatus for more flexible wellhead assembly with integral annulus valve configurations, as well as more easily accessible piston-cylinder valve assemblies that overcome some of the foregoing difficulties while providing more advantageous overall results. 
       BRIEF SUMMARY OF SOME OF THE PREFERRED EMBODIMENTS 
       [0012]    These and other needs in the art are addressed in one embodiment by an integral annulus shutoff valve. In an embodiment, the integral annulus shutoff valve comprises a sliding piston disposed within a tubing hanger body. In addition, the tubing hanger body comprises a plurality of flowby conduits allowing for fluid flow through the tubing hanger body. The sliding piston is operable to prevent fluid flow between at least two spaced apart flowby conduits, where one of the flowby conduits is capable of being fluidly connected with an annulus bore located below the tubing hanger body. Further, the tubing hanger body may comprise a plurality of pressurizing conduits capable of delivering pressurizing fluid to respective actuation chambers such that the sliding piston may be shifted from an opened to closed position and vice versa. 
         [0013]    These and other needs in the art are addressed in another embodiment wherein the integral annulus shutoff valve is disposed as part of a wellhead assembly. In one embodiment, the wellhead assembly comprises a tubing hanger assembly and a Christmas tree. In addition, the tubing hanger assembly comprises a wireline plug disposed in a central production bore. The tubing hanger assembly comprises the sliding piston embodiment described above, which is operable to selectively control fluid flow through the tubing hanger body and into the annulus bore. Further, the tubing hanger assembly and Christmas tree are separately retrievable from the wellhead assembly as a result of the positioning of the wireline plug and integral annulus shutoff valve. 
         [0014]    Thus, embodiments described herein comprise a combination of features and advantages intended to address various shortcomings associated with certain prior devices. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments, and by referring to the accompanying drawings. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the embodiments described herein. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    For a detailed description of the preferred embodiments of the invention, reference will now be made to the accompanying drawings in which: 
           [0016]      FIG. 1  is a cross-sectional view of an embodiment of a tubing hanger assembly with integral annulus shut off valve; 
           [0017]      FIG. 2A  is an enlarged cross-sectional view of the annulus shut off valve piston of  FIG. 1  in the “opened” position; 
           [0018]      FIG. 2B  is an enlarged cross-sectional view of the annulus shut off valve piston of  FIG. 1  in the “closed” position; 
           [0019]      FIG. 2C  is a cross-sectional view of an annulus valve piston and valve sleeve combination in the “closed” position; 
           [0020]      FIG. 2D  is a cross-sectional view of an annulus valve piston and valve sleeve combination in the “opened” position; 
           [0021]      FIG. 3  is a cross-sectional view of a wellhead assembly; 
           [0022]      FIG. 4A  is a cross-sectional view of the wellhead assembly of  FIG. 3  with the tubing hanger assembly of  FIG. 1  employed therein and the annulus shut off valve in the “closed” position; 
           [0023]      FIG. 4B  is a cross-sectional view of the wellhead assembly of  FIG. 3  with the tubing hanger assembly of  FIG. 1  employed therein and the annulus shut off valve in the “opened” position; and 
           [0024]      FIG. 5  is an embodiment of a tubing hanger assembly in a wellhead assembly including a wireline plug. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]    The following discussion is directed to various embodiments of the invention. Although one or more of these embodiments may be preferred, the embodiments disclosed should not be interpreted, or otherwise used, as limiting the scope of the disclosure, including the claims. In addition, one skilled in the art will understand that the following description has broad application, and the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to intimate that the scope of the disclosure, including the claims, is limited to that embodiment. 
         [0026]    Certain terms are used throughout the following description and claims to refer to particular features or components. As one skilled in the art will appreciate, different persons may refer to the same feature or component by different names. This document does not intend to distinguish between components or features that differ in name but not function. The drawing figures are not necessarily to scale. Certain features and components herein may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in interest of clarity and conciseness. 
         [0027]    In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ” Also, the term “couple” or “couples” is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection, or through an indirect connection via other devices and connections. Further, as used herein, the phrase “fluidly connected” or “in fluid communication” means that the components are interconnected in a manner that permits fluid flow therebetween. 
         [0028]      FIG. 1  illustrates an embodiment of a tubing hanger assembly  100 . Tubing hanger assembly  100  comprises a generally tubular hanger body  110 , inner sleeve  120 , outer sleeve  130 , load ring  140 , wedge ring  150 , annulus valve piston  160 , and locking ring  180 . An integral pup for attaching to production tubing pup  170  is disposed at one end of tubing hanger assembly  100 . Hanger body  110  is characterized by longitudinal central production bore  190  that is coaxial with production bore  174  of production tubing pup  170 , with both production bore  190  and production bore  174  collectively referred to as a production bore. An inner bore  111  is also formed in hanger body  110 . A first flow conduit  112   a  and a second flow conduit  112   b  extend longitudinally through hanger body  110 , and are fluidly connected on one end with inner bore  111  via first annulus port  113   a  and second annulus port  113   b,  respectively. First flow conduit  112   a  is further fluidly connected with inner sleeve bowl  122 , and second flow conduit  112   b  is further fluidly connected with lower annulus bore  172  surrounding production tubing pup  170 . First pressurizing conduit  114   a  and second pressurizing conduit  114   b  are provided through hanger body  110 . 
         [0029]    Although a variety of locking means are contemplated for use with the present device to secure hanger body  110  of tubing hanger assembly  100  within a wellbore tubular member,  FIG. 1  illustrates a single representative embodiment of locking means. A split “C” locking ring  180  extends around hanger body  110  and rests on locking shoulder  116  of hanger body  110 . As an alternative to the split “C” ring, locking ring  180  may be in the form of a segmented ring. The upper neck portion  117  of hanger body  110  is surrounded by generally tubular inner sleeve  120 . Inner sleeve  120  is attached to, coaxial and aligned with hanger body  110 , and rests on an upper shoulder  118  of hanger body  110 . A generally tubular outer sleeve  130  surrounds and is positioned concentric to inner sleeve  120 , and is longitudinally slidable with respect to hanger body  110  and inner sleeve  120 . A load ring  140  is disposed between outer sleeve  130  and inner sleeve  120 , and is threadingly connected to a lower inner surface of outer sleeve  130 . An upper end of load ring  140  initially engages a lower shoulder  124  of inner sleeve  120 . A wedge ring  150  is connected to outer sleeve  130  and is located below load ring  140 , such that a lower end of load ring  140  engages an upper end of wedge ring  150 . The lower outer portion of wedge ring  150  is formed with a tapered surface  152 . The lower inner surface of wedge ring  150  contacts an intermediate neck  119  of hanger body  110  directly above locking shoulder  116 . 
         [0030]    A generally tubular annulus valve piston  160  is disposed concentrically within central production bore  190  of hanger body  110 . A vertical bore  161  extends through the annulus valve piston  160  and opens at a first end into the central production bore  190  of hanger body  110 . The second end of vertical bore  161  is aligned with and opens to bore  174  of production tubing pup  170 . Annulus valve piston  160  may be used to control fluid flow through hanger body  110 , and specifically to control fluid flow through first flow conduit  112   a  and second flow conduit  112   b . To that end, annulus valve piston  160  may be characterized by at least a first radial protrusion extending from the outer surface  162   a,  and a second radial protrusion extending from outer surface  162   b.  In certain embodiments, an upper shoulder  163  may serve as the first radial protrusion, and a lower shoulder  164  may serve as the second radial protrusion. Outer surfaces  162   a  and  162   b  of annulus valve piston  160  have a first diameter, and upper shoulder  163  and lower shoulder  164  are at a second, larger diameter. The second diameter value of upper shoulder  163  and lower shoulder  164  is such that the outer surfaces of upper shoulder  163  and lower shoulder  164  engage the inner surface  111   a  of inner bore  111 . 
         [0031]    Upper and lower annular seals  167   a  and  167   b  may be disposed in upper and lower circumferential grooves  168   a  and  168   b  located respectively on the outer surfaces of upper shoulder  163  and lower shoulder  164 . Upper and lower annular seals  167   a  and  167   b  sealingly engage the inner surface  111   a  of inner bore  111 , thereby preventing the flow of fluid axially across annulus valve piston  160  and between annulus valve piston  160  and hanger body  110 . In certain embodiments, upper and lower annular seals  167   a  and  167   b  may comprise O-ring type seals. Upper and lower annular seals  167   a  and  167   b  may alternatively be disposed in grooves in inner surface  111   a  of inner bore  111  such that seals  167   a  and  167   b  sealingly engage the outer surfaces of upper shoulder  163  and lower shoulder  164 , respectively. 
         [0032]    Referring now to  FIGS. 2A and 2B , upper hanger seal  169   a  is disposed below inner shoulder  111   b  of hanger body  110 , and is located between inner surface  111   a  of hanger body  110  and outer surface  162   a  of annulus valve piston  160 , and engages an upper portion of outer surface  162   a  of annulus valve piston  160 . Similarly, lower hanger seal  169   b  is disposed above upper end  176  of production tubing pup  170 , and between an inner upper surface  178  of production tubing pup  170  and outer surface  162   b  of annulus valve piston  160 , and engages a lower portion of outer surface  162   b  of annulus valve piston  160 . Upper hanger seal  169   a  and lower hanger seal  169   b  provide sealing engagement at the respective contact points between annulus valve piston  160  and hanger body  110 , as well as annulus valve piston  160  and production tubing pup  170 , so as to prevent fluid flow across annulus valve piston  160  and out of production bore  190 . 
         [0033]    Upper and lower annular seals  167   a  and  167   b  may comprise any suitable material, including without limitation non-metals (e.g., polymer, elastomer, ceramic, etc.), composites, or combinations thereof. In certain embodiments, upper and lower annular seals  167   a  and  167   b  preferably comprise an elastomer such as nitrile rubber, or a polymer, or PEEK®. In some embodiments in which upper and lower annular seals  167   a  and  167   b  comprises PEEK®, a resilient member (not shown) may be included in upper and lower circumferential grooves  168   a  and  168   b  between seals  167   a  and  167   b  and annulus valve piston  160  to exert forces on upper and lower annular seals  167   a  and  167   b,  thereby tending to maintain upper and lower annular seals  167   a  and  167   b  in sealing engagement with inner surface  111   a  of hanger body  110 . Upper and lower hanger seals  169   a  and  169   b  may be comprised of pack-offs of conventional design, and may comprise any suitable material, including without limitation metals (e.g., tin, copper, etc.), composites, or combinations thereof. 
         [0034]    Referring still to  FIGS. 2A and 2B , hanger body  110  is shown with annulus valve piston  160  disposed within central production bore  190  of hanger body  110 . When annulus valve piston  160  is disposed within hanger body  110 , an inner flowby chamber  111   c  is created between upper shoulder  163  and lower shoulder  164 . Additionally, first actuation chamber  165  is created in the annular space above upper shoulder  163  and below inner shoulder  111   b  of hanger body  110 . The first actuation chamber  165  is further defined by the annular space between outer surface  162   a  of annulus valve piston  160  and the inner surface  111   a  of inner bore  111 . A second actuation chamber  166  is created in the annular space between lower shoulder  164  and an upper end of production tubing pup  170 . The second actuation chamber  166  is further defined by a second, downhole annular space between outer surface  162   b  of annulus valve piston  160  and the inner surface  111   a  of inner bore  111 . First actuation port  115   a  is located to correspond with the position of first actuation chamber  165 , such that first actuation port  115   a  is in fluid communication with first actuation chamber  165 . Second actuation port  115   b  is located to correspond with the position of second actuation chamber  166 , such that second actuation port  115   b  is in fluid communication with second actuation chamber  166 . First pressurizing conduit  114   a  is in fluid communication with first actuation chamber  165  via first actuation port  115   a,  and second pressurizing conduit  114   b  is in fluid communication with second actuation chamber  166  via second actuation port  115   b.    
         [0035]    Annulus valve piston  160  is slidable longitudinally with respect to hanger body  110  between a first, opened position (as shown in  FIG. 2A ), and a second, closed position (as shown in  FIG. 2B ). Specifically, the outer surfaces of upper shoulder  163  and lower shoulder  164  of annular valve piston  160  slidingly engage inner surface  111   a  of inner bore  111 . Thus, annular valve piston  160  is permitted to move axially within production bore  190  of hanger body  110 . Referring to  FIG. 2A , the engagement between lower shoulder  164  and upper end  176  of production tubing pup  170  prevents additional downward movement of annulus valve piston  160  beyond the opened position. Similarly, referring to  FIG. 2B , the engagement between upper shoulder  163  and inner shoulder  111   b  prevents additional upward movement of annulus valve piston  160  beyond the closed position. As will be described in more detail below, pressurized fluid from an outside source (not shown) is transferred via upper pressurizing conduit  114   a  and lower pressurizing conduit  114   b  through upper actuation port  115   a  and lower actuation port  115   b  to first actuation chamber  165  and second actuation chamber  166  in order to actuate annulus valve piston  160  from the “opened” position illustrated in  FIG. 2A  to the “closed” position illustrated in  FIG. 2B , and vice versa. 
         [0036]    In general, when annulus valve piston  160  is in the “closed” position illustrated in  FIG. 2B , first flow conduit  112   a  and second flow conduit  112   b  are not in fluid communication. Moreover, second flow conduit  112   b  is not in fluid communication with inner flowby chamber  111  c of inner bore  111 . Specifically, lower annulus port  113   b  is blocked by the combination of lower shoulder  164  and lower annular seal  167   b,  thereby preventing fluid flow from or into lower annulus port  113   b.  Thus, as used herein, the term “closed” and “closed position” refer to configurations of annulus valve piston  160 , lower shoulder  164 , and lower annular seal  167   b,  in which fluid communication between spaced apart annulus flowby ports (e.g., upper annulus port  113   a  and lower annulus port  113   b ) and annular spaces outside of hanger body  110 , such as lower annulus bore  172  below hanger body  110 , and production bore  190  in hanger body  110 , is prevented. Annulus valve piston  160  is maintained in its “closed” position by fluid pressure introduced into second actuation chamber  166  via second actuation port  115   b . Pressurized fluid from an outside source acts on lower shoulder  164 , thereby constraining annulus valve piston  160  in the closed position with upper shoulder  163  engaging inner shoulder  111   b  of hanger body  110 . 
         [0037]    Still referring to  FIGS. 2A and 2B , by controlling the location of annulus valve piston  160  within production bore  190  of hanger body  110  with the balancing of fluid pressures in first actuation chamber  165  and second actuation chamber  166 , annulus valve piston  160  may be configured between “opened” and “closed” positions. Prior to operation, annulus valve piston  160  may be pressure balanced with respect to production bore  190  and lower annulus bore  172 . In operation, annulus valve piston  160  may be moved downward from the closed position to the opened position by increasing the amount of pressurized fluid in first actuation chamber  165  via first actuation port  115   a  and first pressurizing conduit  114   a,  and the reducing the amount of pressurized fluid in second actuation chamber  166  via second actuation port  115   b  and second pressurizing conduit  114   b.  In certain embodiments, hydraulic fluid may be used as the pressurizing fluid. The flow of hydraulic fluid may be controlled by any suitable means, and without limitation may be manually controlled, electronically controlled, computer controlled, remotely controlled, or combinations thereof. 
         [0038]    Annulus valve piston  160  may be moved from the “closed” position to the “opened” position. When piston  160  is in the closed position, the hydraulic pressure in second actuation chamber  166  is greater than that in first actuation chamber  165 . In order to move piston  160  to the opened position, the fluid pressure in second actuation chamber  166  is reduced at the same time the fluid pressure in first actuation chamber  165  is increased via first pressurizing conduit  114   a  and first actuation port  115   a.  As a result of the change in relative fluid pressure in first actuation chamber  165  and second actuation chamber  166 , piston  160  moves downward axially due to the fluid pressure gradient acting on upper shoulder  163 . 
         [0039]    As piston  160  moves axially within production bore  190  due to the fluid pressure gradient across first actuation chamber  165  and second actuation chamber  166 , lower shoulder  164  and lower annular seal  167   b  begin to also move downward and begin to clear and unseal lower annulus port  113   b.  The fluid pressure in first actuation chamber  165  is increased until piston  160  attains the opened position, such that lower shoulder  164  and lower annular seal  167   b  of piston  160  have moved sufficiently to permit fluid communication through lower annulus port  113   b  between second flow conduit  112   b,  first flow conduit  112   a  via inner chamber  111   c  of inner bore  111 . In the “opened” configuration, the flow of fluids is thereby permitted from first flow conduit  112   a  to second flow conduit  112   b,  or vice versa depending on the relative pressures between first flow conduit  112   a  and second flow conduit  112   b.    
         [0040]    Once lower shoulder  164  of piston  160  engages upper end  176  of production tubing pup  170 , annulus valve piston  160  is prevented from further axial movement relative to hanger body  110  and is constrained in the opened position. At this point, the fluid pressure present in first actuation chamber  165  is stabilized, thereby holding piston  160  in the opened position due to the force of the fluid pressure on upper shoulder  163 . As shown in  FIG. 2A , piston  160 , lower shoulder  164 , and lower annular seal  167   b  have achieved the full “opened” position in which there are no obstructions between lower annulus port  113   b,  upper annulus port  113   b,  and inner bore  111  (e.g., lower annulus port  113   b  is fully opened). 
         [0041]    From the fully “opened” position illustrated in  FIG. 2A , annulus valve piston  160  may be repositioned to the “closed” position shown in  FIG. 2B  by adjusting the relative fluid pressure gradient in first actuation chamber  165  and second actuation chamber  166 . In operation, the fluid pressure in first actuator chamber  165  is reduced as the pressure in second actuation chamber  166  is increased. In response to the change in fluid pressure gradient in first actuation chamber  165  relative to second actuation chamber  166 , piston  160  begins to move axially relative to hanger body  110  in an upward direction. 
         [0042]    Referring now to  FIG. 2C , in alternative embodiments an annulus valve piston  260  in combination with a valve sleeve  264  may be used to control fluid flow through hanger body  110 . Annulus valve piston  260  may be characterized by an outer surface  262  and a circumferential shoulder  263 , which functions as the first radial protrusion from outer surface  262  of annulus valve piston  260 . Valve sleeve  264  is secured at one end to outer surface  262  and disposed below circumferential shoulder  263  on annulus valve piston  260 , and may function as the second radial protrusion from outer surface  262 . Valve sleeve  264  is further characterized by circumferential seals  265   a  and  265   b,  which are disposed between outer surface  262  and sleeve  264 , and sleeve  264  and production tubing pup  170 , respectively. An inner passage  266  is disposed through sleeve  264 , and is defined on opposed ends by a longitudinal opening  267  and a radial opening  268 . Inner passage  266  may be described as being “L-shaped” in that the central axes defining longitudinal opening  267  and radial opening  268  are oriented perpendicular to each other. 
         [0043]    Referring still to  FIG. 2C , hanger body  110  is shown with the annulus valve piston  260  and valve sleeve  264  combination disposed within central production bore  190  of hanger body  110 . A first actuation chamber  165  is created in the annular space above circumferential shoulder  263  and below inner shoulder  111   b  of hanger body  110 . A second actuation chamber  166  is created in the annular space between valve sleeve  264  and an upper end of production tubing pup  170 . First actuation port  115   a  is located to correspond with the position of first actuation chamber  165 , such that first actuation port  115   a  is in fluid communication with first actuation chamber  165 . Second actuation port  115   b  is located to correspond with the position of second actuation chamber  166 , such that second actuation port  115   b  is in fluid communication with second actuation chamber  166 . 
         [0044]    When the annulus valve piston  260  and valve sleeve  264  combination is in the “closed” position illustrated in  FIG. 2C , first flow conduit  112   a  and second flow conduit  112   b  are not in fluid communication. Specifically, lower annulus port  113   b  is blocked by valve sleeve  264 , thereby preventing fluid flow from or into second flow conduit  112   b  via lower annulus port  113   b.  Further, in the “closed” configuration shown, radial opening  268  is aligned with and in fluid communication with upper annulus port  113   a.  As a result, any fluid flow through first flow conduit  112   a  travels through upper annulus port  113   a  and into inner passage  266  via radial opening  268 . Because longitudinal opening  267  is adjacent to circumferential shoulder  263  in this configuration, the fluid flow is blocked by valve sleeve  264  and constrained between inner passage  266  and circumferential shoulder  263 . 
         [0045]    Similar to the manner of operation described above, the embodiment comprising the annulus valve piston  260  and valve sleeve  264  combination may be alternated between “opened” and “closed” positions by controlling the fluid pressure gradient across first actuation chamber  165  and second actuation chamber  166 . Referring now to  FIG. 2D , the annulus valve piston  260  and valve sleeve  264  combination is shown in the “opened” position. In the “opened” position, fluid flow is permitted between first flow conduit  112   a  and second flow conduit  112   b  through an inner flowby chamber  111   c  created adjacent to upper annulus port  113   a  and between circumferential shoulder  263  and valve sleeve  264 . Inner flowby chamber  111   c  is in fluid communication on one end with upper annulus port  113   a,  and on an opposite end with longitudinal opening  267 . Further, radial opening  268  is aligned with and in fluid communication with lower annulus port  113   b.  As a result, fluid flow from first flow conduit  112   a  to second flow conduit  112   b  is allowed through inner passage  266 . When the annulus valve piston  260  and valve sleeve  264  combination are oriented in the “opened” configuration, fluid from first flow conduit  112   a  may enter inner flowby chamber  111   c,  and pass through inner passage  266  into second flow conduit  112   b.    
         [0046]    Referring now to  FIG. 3 , one embodiment of wellhead assembly  200  is shown. Wellhead assembly  200  comprises a high-pressure housing  210 , wherein high-pressure housing  210  is installed on housing  205 . High-pressure housing  210  is generally tubular and comprises a longitudinal central bore  212 . Generally tubular casing hangers  208  and  220  are landed within bore  212  of high-pressure housing  210 . A first annular pack-off  230  of conventional design seals the casing hanger  220  to the inner wall  214  of the high-pressure housing  210 . The first annular pack-off  230  comprises a first locking ring  232 , engaged with an upper circumferential groove  215  in the inner wall  214  of the high-pressure housing  210 . Second annular pack-off  240  locks down casing hanger  208 , and comprises a second locking ring  242 , engaged with a lower circumferential groove  216  in the inner wall  214  of the high-pressure housing  210 . The first and second locking rings  232  and  242  serve to retain the casing hangers  220  and  208  in position within the high-pressure housing  210 , and permit casing hangers  220  and  208  only limited movement longitudinally within the central bore  212  of high-pressure housing  210 . 
         [0047]    The upper portion of casing hanger  220  is formed with a seating surface  222  that allows an annular ring  250  to seat on casing hanger  220  within the central bore  212  of high-pressure housing  210 . Annular ring  250  has an inner tapered surface  252 , forming a seat on which may be landed annular snap collar  260 . Annular snap collar  260  is generally tubular, and is characterized by a central bore therethrough and a circumferential lip  262  at a first end. The first end of annular snap collar  260  is further comprised of a series of equally-spaced spring elements  264 , wherein circumferential lip  262  is present on each spring element  264 . The annular snap collar  260  is slidable longitudinally with respect to annular ring  250  between a first, upper position, and a second, lower position. 
         [0048]    A running tool (not shown) is used to engage an internal groove  266  on annular snap collar  260 , and to apply a downward force to insert annular snap collar  260  into annular ring  250 , such that spring elements  264  deflect inward, and then spring back to their original configuration as an outer tapered surface  268  of annular snap collar  260  engages complimentary inner tapered surface  252  of annular ring  250 . Simultaneous with annular snap collar  260  coming to rest on inner tapered surface  252 , the deflection of spring elements  264  cause circumferential lip  262  to engage a lower circumferential groove  254  located within the central bore of annular ring  250 , such that the engagement of circumferential lip  262  with inner circumferential groove  254  locks annular snap collar  260  into position. 
         [0049]    A split “C” locking ring  270  extends around an outer surface of annular snap collar  260  and rests on an upper shoulder  256  of annular ring  250 . The locking ring  270  is formed to be engageable with a pair of locking grooves  218  formed in the inner wall  214  of high-pressure housing  210 . As an alternative to the split “C” ring, the locking ring  270  may be in the form of a segmented ring. As annular snap collar  260  is forced downward into position such that it is seated with respect to annular ring  250 , the outer surface of annular snap collar  260  engages with locking ring  270  and urges it radially outward into engagement with the locking grooves  218 . As described above, the downward movement of annular snap collar  260  is limited by outer tapered surface  268  contacting the corresponding inner tapered surface  256  on annular ring  250 . Annular ring  250 , annular snap collar  260 , and locking ring  270  are provided in order to accommodate pressure uploads tending to push tubing hanger assembly  100  upward when positioned in its “as-installed” position. 
         [0050]    Referring now to  FIG. 4A , in one embodiment tubing hanger assembly  100  is shown landed within a wellbore tubular member such as casing hanger  220 , retained by an internal form  226  within the central bore  224  of casing head  220 . Tubing hanger running tool  500  (partially shown) is inserted into outer sleeve  130  and applies a downward force to outer sleeve  130 . The downward force applied to outer sleeve  130  causes the downward longitudinal movement of outer sleeve  130 , which is translated to load ring  140  and wedge ring  150 . As a result, load ring  140  and wedge ring  150  are forced downward with respect to hanger body  110 . Outer sleeve  130 , load ring  140 , and wedge ring  150  are slidable longitudinally with respect to hanger body  110  between a first, upper position, and a second, lower position. In the first position, the tapered surface  152  of wedge ring  150  contacts the upper edge of locking ring  180 . Upon downward movement of outer sleeve  130 , load ring  140 , and wedge ring  150  from the first position to the second position, the tapered surface  152  of the wedge ring  150  engages with the locking ring  180  and urges it radially outward into engagement with the locking grooves  228  in the inner surface  227  of casing hanger  220 . As a result, hanger body  110  is constrained in place due to the engagement between the lower edge of locking ring  180  with upper shoulder  118  of hanger body  110 , and the engagement of locking ring  180  within the locking grooves  228  of casing hanger  220 . 
         [0051]    The embodiment of tubing hanger assembly shown in  FIGS. 4A-4B  is further comprised by an annulus valve piston  160  that is substantially the same, and operates in substantially the same manner, as annulus valve piston  160  described above with reference to  FIGS. 2A-2B . In certain embodiments, annulus valve piston  160  is “opened” and “closed” as described above in reference to  FIGS. 2A-2B , respectively. As employed in certain embodiments of tubing hanger assembly  100 , annulus valve piston  160  permits selective and controlled access to lower annulus bore  172  defined by production tubing pup  170  and casing hanger  220  located within a wellbore. Specifically, annulus valve piston  160  is used to control the flow of fluids to and from lower annulus bore  172  when tubing hanger assembly  100  is disposed within wellhead assembly  200 . 
         [0052]    As described above, hanger body  110  of tubing hanger assembly  100  comprises a first flow conduit  112   a  capable of providing fluid communication between inner chamber  111   c  of inner bore  111  and a region above hanger body  110  (e.g., inner sleeve bowl  122 ) through first annulus port  113   a.  Hanger body  110  also comprises a second flow conduit  112   b  capable of providing fluid communication between inner chamber  111   c  of inner bore  111  and a region below hanger body  110  (e.g., lower annulus bore  172 ) through second annulus port  113   b.  In the manners previously described, by controlling the position of annulus valve piston  160  within production bore  190 , tubing hanger assembly  100  may be placed in an “opened” or “closed” position as desired. When annulus valve piston  160  is in the “closed” position (e.g.,  FIGS. 2A and 2B ), second annulus port  113   b  and second flow conduit  112   b  are not in fluid communication with inner chamber  111   c  of inner bore  111 , and therefore fluid flow is prevented to or from lower annulus bore  172  via second flow conduit  112   b.  However, when annulus valve piston  160  is in the “opened” position (e.g.,  FIGS. 2A and 4B ), second flow conduit  112   b  is fluidly connected with inner chamber  111   c  of inner bore  111  via second annulus port  113   b , thereby placing lower annular bore  172  into fluid communication with inner chamber  111   c.  As a result, when annulus valve piston  160  is in the “opened” position, fluid may flow through hanger body  110  into lower annular bore  172 . Thus, by selectively actuating annular valve piston  160  (i.e., controlling the position of annular valve piston  160  with respect to second annulus port  113   b ), tubing hanger assembly  100  may be used to control flow fluids to or from lower annulus bore  172 . 
         [0053]    In the manner described, embodiments of the tubing hanger assembly  100  described herein (e.g., those including integral annulus valve piston  160 ) permit the selective control of fluid flow between two physically separated environments (e.g., inner sleeve bowl  122  and lower annulus bore  172 ). The integral configuration of annulus valve piston  160  within tubing hanger assembly  100  results in a flexible apparatus that can be used to overcome several of the shortcomings in previous dual-bore tubing hanger designs. In the current embodiments, tubing hanger assembly  100  may be removed from casing hanger  220  without disturbing a separately retrievable Christmas tree that may be mounted above. Moreover, the current embodiments allow for the removal of the Christmas tree without disturbing tubing hanger assembly  100 . 
         [0054]    Referring now to  FIG. 5 , tubing hanger assembly  100  is shown disposed within casing hanger  220 . Annulus valve piston  160  is shown in the “closed” position, such that fluid flow through hanger body  110  to lower annulus bore  172  through first flow conduit  112   a  and second flow conduit  112   b  is prevented. A wireline plug  300  of conventional design is shown in  FIG. 5  installed in the central production bore  190  of hanger body  110 . With annulus valve sleeve piston  160  in the “closed” position and plug  300  positioned to shut off any fluid flow from production tubing pup  170 , the wellbore may be completed sealed. As such, any equipment located above tubing hanger assembly  100 , such as a Christmas tree or blow-out preventer (BOP) may be removed without disturbing tubing hanger assembly  100 . 
         [0055]    Additionally, it should be noted that the integral configuration of annulus valve sleeve  160  within tubing hanger assembly  100  provides the benefit of allowing a relative reduction in the diameter of hanger body  110 . The relatively small diameter of hanger body  110  with respect to a Christmas tree or BOP located on the wellhead makes it possible to fit tubing hanger assembly  100  through either a Christmas tree or BOP. As a result, the need for a separate trip to plug a wellbore, or the need for an additional piece of large, expensive equipment, to completely seal fluid flow in the previous dual-bore wellhead configurations is eliminated. 
         [0056]    While preferred embodiments have been shown and described, modifications thereof can be made by one skilled in the art without departing from the scope or teachings herein. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the system and apparatus are possible and are within the scope of the invention. For example, the relative dimensions of various parts, the materials from which the various parts are made, and other parameters can be varied. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims.