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TECHNICAL FIELD OF THE INVENTION 
     The present invention relates in general to the field controlling fluid flow in a subterranean well and, more particularly, to a flow control device having redundant sealing capability for regulating fluid flow through a tubing string disposed within a well. 
     BACKGROUND OF THE INVENTION 
     With limiting the scope of the present invention, its background will be described with reference to a regulating fluid flow into a wellbore having one or more subsea completions as an example. 
     In subsea completions, a flow control apparatus, known as a choke, is typically installed in the production tubing string to throttle fluid flow entering the tubing string. The choke is particularly useful where multiple zones are produced into the tubing string and it is desired to regulate the rate of fluid flow from each zone. Additionally, regulatory authorities may require that rates of production from each zone be reported, necessitating the use of the choke or other methods of determining and controlling the rate of production from each zone. Safety concerns may also dictate controlling the rate of production from each zone. 
     Chokes are also useful in subsea well having single zone completions. For example, in a wellbore producing from a single zone, an operator may determine that it is desirable to reduce the flow rate from that zone to limit damage to the reservoir, reduce water coning or enhance ultimate recovery. 
     The typical choke that is placed downhole to limit flow from a certain formation into the tubing string has a fixed orifice which cannot be closed. These conventional chokes require intervention to change the size of the fixed orifice. To compensate for changing well conditions or simply to make adjustments in the flowrate therethrough, these chokes typically require slickline, wireline or other operations, which need a rig for their performance. 
     Attempts have been made to overcome these limitations associated with convention chokes. For example, infinitely variable interval control valves (“IVICV”) have been used. These IVICVs are designed not only for operation in fully closed and fully open configurations, but also, in variable positions that allow for the regulation of fluid flow therethrough. Typically, IVICVs utilize a metal-to-metal sealing surface in the fully closed position to prevent fluid flow therethrough. It has been found, however, that the surfaces of the metal-to-metal seal are susceptible to erosion when the IVICV is in an open position, particularly when the IVICV is throttling flow. Once the sealing surfaces have been eroded, the IVICV is no longer capable of fully preventing fluid flow therethrough even when it is in the fully closed position. 
     Therefore a need has arisen for a flow control apparatus that is rugged, reliable, and capable of withstanding extreme environmental conditions, so that it may be utilized in completions without requiring frequent service, repair or replacement. A need has also arisen for such a flow control apparatus that is capable of accurately regulating fluid flow therethrough and that is resistant to erosion, even when it is configured between its fully open and closed positions. Further, a need has arisen for such a flow control apparatus that is capable fully providing a seal even when erosion of the metallic sealing surfaces has occurred. 
     SUMMARY OF THE INVENTION 
     The present invention disclosed herein comprises a flow regulating apparatus that is rugged, reliable and capable of withstanding extreme environmental conditions, so that it may be utilized in completions without requiring frequent service, repair or replacement. The flow regulating apparatus of the present invention is capable of accurately regulating fluid flow therethrough and is resistant to erosion, even when it is configured between its fully open and closed positions. In addition, the flow regulating apparatus of the present invention is capable of fully providing a seal even when erosion of the metallic sealing surfaces has occurred. 
     The flow regulating apparatus of the present invention comprises a generally tubular outer housing having a housing port formed through a sidewall portion thereof. First and second seats are disposed within the housing. The first seat is substantially fixed relative to the housing. The second seat is slidable relative to the housing. In operation, the second seat is movable relative to the first seat such that a sealing surface of the first seat and a sealing surface of the second seat may be sealingly engaged together, forming a metal-to-metal seal, to substantially prevent fluid flow therebetween. In addition, the first and second seats may be separated from one another to permit fluid flow therebetween. 
     The flow regulating apparatus of the present invention also comprises a sleeve that is slidably disposed within the first and second seats. The sleeve has a flow passage extending generally axially therethrough and has a sleeve port formed through a sidewall portion thereof. The sleeve is variably positionable relative to the first seat to regulate fluid flow through the sleeve port. The sleeve has a seal disposed thereon that selectively provide a redundant seal for the flow regulating apparatus of the present invention. Specifically, the seal, which may be an elastomeric seal, may be positioned between the sleeve and the first seat. In this position, the seal provides redundant sealing capability in addition to the metal-to-metal seal between the first and second seats, thereby fully preventing fluid flow between the housing port and the sleeve port of the flow regulating apparatus of the present invention. 
     The sleeve may be slidable repositioned relative to the first and second seats to remove the redundant sealing capability. In this position, the seal is disposed between the sleeve and the second seat. As such, the seal no longer prevents fluid flow between the housing port and the sleeve port leaving only the metal-to-metal seal to prevent fluid flow between the housing port and the sleeve port. While the metal-to-metal seal may be sufficient in some application for some period of time, typical metal-to-metal seals are susceptible to leakage, particularly if the sealing surfaces are subject to erosion. The flow regulating apparatus of the present invention, however, overcomes this limitation through the use of the redundant sealing capability provided by the seal when it is disposed between the sleeve and the first seat. 
     In addition, the seal of the flow regulating apparatus of the present invention is not subject to the hostile environment typically encountered in conventional choke applications. Specifically, the seal is not subject to abrasive wear or erosion either when providing or not providing the redundant seal. More specifically, the sealing surfaces of the first and second seats remain engaged during redundant sealing operations and during movement of the sleeve relative to the second seat which moves the seal out of redundant sealing operations. As such, the seal is never required to seal against high velocity fluid flow and suffer the associated degradation. 
     To regulate the fluid flow through the flow regulating apparatus of the present invention, the sleeve and the second seat are slidably repositionable relative to the first seat. As the sleeve continues to move in the direction that removed the redundant seal, the sleeve and the second seat begin to move together to disengage the seal between the sealing surfaces of the first and second seats. As the sleeve continues movement in this direction, the sleeve port becomes aligned with the end of the first seat such that fluid flow through the sleeve port may occur. The volume of fluid flow may now be infinitely regulated by adjusting the amount of obstruction provided by the first seat relative to the sleeve port. Continued movement of the sleeve in the original direction eventually allows unregulated fluid flow through the sleeve port when the flow regulating apparatus of the present invention is in its fully open position. 
     Reversing the direction of movement of the sleeve may return the flow regulating apparatus of the present invention to its fully closed position. This is achieved by first increasing the level of obstruction of the first seat relative to the sleeve port until it is fully obstructed and bringing the sealing surfaces of the first and second seats into sealing engagement with one another. Once substantially all of the fluid flow is restricted by the sealing engagement of the first and second seats, the sleeve continues its travel in this direction relative to both the first and second seats such that the seal may slide across the sealing engagement of the sealing surfaces of the first and second seats to fully seal the flow regulating apparatus of the present invention. 
     In one embodiment of the present invention, the movement of the sleeve relative to the first and second seats and the movement of the sleeve and second seat relative to the first seat may be achieved using mechanical means such as via wireline or slickline. In another embodiment of the present invention, the movement of the sleeve relative to the first and second seats and the movement of the sleeve and second seat relative to the first seat may be achieved using hydraulic fluid pressure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures in which corresponding numerals in the different figures refer to corresponding parts and in which: 
     FIG. 1 is a schematic illustration of an offshore oil or gas production platform operating a pair of flow control devices of the present invention; 
     FIGS. 2A-2B are quarter sectional views of seccessive axial sections of a flow control device of the present invention depicted in its fully closed and fully sealed position; 
     FIGS. 3A-3B are quarter sectional views of seccessive axial sections of a flow control device of the present invention depicted in its fully closed and unsealed position; 
     FIGS. 4A-4B are quarter sectional views of seccessive axial sections of a flow control device of the present invention depicted in its partially opened position; 
     FIGS. 5A-5B are quarter sectional views of seccessive axial sections of a flow control device of the present invention depicted in its fully opened position; 
     FIGS. 6A-6B are quarter sectional views of seccessive axial sections of a flow control device of the present invention depicted in its fully closed and fully sealed position; 
     FIGS. 7A-7B are quarter sectional views of seccessive axial sections of a flow control device of the present invention depicted in its fully closed and unsealed position; 
     FIGS. 8A-8B are quarter sectional views of seccessive axial sections of a flow control device of the present invention depicted in its partially opened position; and 
     FIGS. 9A-9B are quarter sectional views of seccessive axial sections of a flow control device of the present invention depicted in its fully opened position. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention and do not define the scope of the invention. 
     Referring to FIG. 1, a pair of flow regulating devices in use with an offshore oil and gas production platform is schematically illustrated and generally designated  10 . Semi-submersible platform  12  is centered over submerged oil and gas formations  14 ,  16  located below sea floor  18 . A well  20  extends from platform  12  through the sea and penetrates the various earth strata including formations  14 ,  16  forming, wellbore  22 . A casing string  24  extends from wellhead  26  into wellbore  22  and is cemented in place. Also extending from wellhead  26  is a tubing string  28 . Well  20  is completed in formations  14 ,  16  such that hydrocarbon fluids may be produced into tubing sting  28 . Incorporated into tubing string  28  is a pair of flow regulating devices  30 . One flow regulating device  30  is associated with the completion of formation  16  including seal assemblies  32 ,  34  while the other flow regulating device  30  is associated with the completion of formation  14  including seal assemblies  36 ,  38 . As will be discussed in more detail below, flow regulating devices  30  allow the operator of platform  12  to regulate the fluid flow from formation  14 ,  16 . For example, by operating flow regulating devices  30 , the operator may produce from formation  14  while shutting in production from formation  16 . When it is desirable to stop producing from formation  14 , the operator may shut in production from formation  14  and produce from formation  16 . Alternatively, both flow regulating devices  30  may be fully or partially opened such that fluids may be produced from both formations  14 ,  16 . In fact, using flow regulating devices  30 , the operator may determine the individual production rates from formations  14 ,  16 . 
     Representatively illustrated in FIGS. 2A-2B is a flow regulating device  30  which embodies principles of the present invention. In the following description of flow regulating device  30  it should be noted that directional terms, such as “above,” “below,” “upper,” “lower,” etc., are used for convenience in referring to the accompanying drawings as it is to be understood that the various embodiments of the present invention described herein may be utilized in various orientations, such as inclined, inverted, horizontal, vertical, etc., without departing from the principles of the present invention. Additionally, although flow regulating device  30 , shown in the accompanying drawings, is depicted in successive axial sections, it is to be understood that the sections form a continuous assembly. 
     Flow regulating device  30  has an axially extending generally tubular outer housing  32 . Housing  32  is threadedly and sealingly attached to the lower end of an upper connector  34  that coupled to a section of tubing string  28  on the upper end thereof. Housing  32  is threadedly and sealingly attached to the upper end of a lower connector  36  that coupled to a section of tubing string  28  on the lower end thereof. 
     Housing  32  includes a series of spaced apart openings  38 , only one of which is shown, that are circumferentially distributed about housing  32 . Openings  38  are formed through housing  32  and thereby provide fluid communication between the area external to flow control device  30  and the interior of housing  32 . Housing  32  also includes a series of spaced apart slots  40 , only one of which is shown, that are circumferentially distributed about housing  32 . Housing  32  has a radially reduced interior region  42 , thereby forming upper and lower internal shoulders above and below the region  42 . Housing  32  also has a lower shoulder  43 . 
     Flow regulating device  30  has an axially extending generally tubular inner sleeve  44 . Sleeve  44  is substantially disposed within housing  32  and axially moveable relative to housing  32 . Sleeve  44  includes a series of axially spaced apart openings  46 ,  48 ,  50 , that are circumferentially distributed about sleeve  44 . Openings  46 ,  48 ,  50  are formed through sleeve  44  and thereby provide fluid communication between the area internal to flow control device  30  and the exterior of sleeve  44 . Sleeve  44  has a radially extended exterior region  52 , thereby forming upper and lower external shoulders  54 ,  56 , respectively, above and below region  52 . A series of radially outwardly extending lugs  58  extends from region  52  and travel within slots  40  of housing  32  to define the limits of relative axial movement between sleeve  44  and housing  32  as will be described in greater detail below. Sleeve  44  has a radially extended exterior region  60 , thereby forming upper and lower external shoulders above and below region  60 . In addition, sleeve  44  has a radially extended exterior region  62 , thereby forming upper and lower external shoulders above and below region  62 . 
     Sleeve  44  carries an upper seal element  64  and a lower seal element  66 . In the illustrated embodiment, seal elements  64 ,  66  include an elastomeric ring that is held in place with a metal sleeve. It should be understood, however, by those skilled in the art that seal elements  64 ,  66  may be selected from a variety of sealing members that are common in the art. For example, seal elements  64 ,  66  may be spring loaded lip seals or simple O-ring seals. Likewise, seal elements  64 ,  66  may be elastomeric seals or may alternatively be non-elastomeric seals. 
     Flow regulating device  30  has an axially extending generally tubular upper seat  68 . Seat  68  is disposed between sleeve  44  and housing  32 . Seat  68  is axially moveable relative to sleeve  44 . Likewise, seat  68  is axially moveable relative to housing  32 . Seat  68  includes an upper shoulder  70 . Seat  68  also includes a collet section having a plurality of circumferentially distributed collet fingers representatively illustrated by collet finger  72 . Collet finger  72  has an upper end that terminates as collet finger head  74 . Seat  68  includes a lower sealing surface  76 . Seat  68  also includes a series of slots circumferentially distributed around seat  68  and representatively illustrated by slot  77 . Each slot  77  receives one of the lugs  58  radially outwardly extending from region  52  of sleeve  44  to define the limits of relative axial movement between sleeve  44  and seat  68  as will be described in greater detail below. 
     Flow regulating device  30  has an axially extending generally tubular lower seat  78 . Seat  78  is disposed between sleeve  44  and housing  32 . Seat  78  is substantially fixably positioned relative to housing  32  due to the interaction between upper shoulder  80  of seat  78  and lower shoulder  43  of housing  32 . Seat  78  is axially moveable relative to sleeve  44 . Seat  78  includes an upper sealing surface  82  and an axially extending and internally inclined lip  84 . 
     Lip  84  acts to prevent, or at least greatly reduce, erosion of sealing surface  82  as well as erosion of sleeve  44 . Sealing surface  82  of seat  78  is cooperatively shaped to sealingly engage sealing surface  76  of seat  68 , and, in the configuration of flow regulating device  30  shown in FIGS. 2A-2B, sealing surface  82  is contacting and sealingly engaging sealing surface  76 . Preferably, sealing surfaces  76 ,  82  are formed of hardened metal or carbide for erosion resistance, although other materials, such as elastomers, resilient materials, etc., may be utilized without departing from the principles of the present invention. 
     As such, seat  68  and seat  78  form a trim set. As used herein, the term “trim set” describes an element or combination of elements which perform a function of regulating fluid flow. For example, a Master Flo Flow Trim manufactured by, and available from, Master Flo of Ontario, Canada may be used to perform this function, although other trim sets may be utilized without departing from the principles of the present invention. 
     Flow regulating device  30  has a packing stack  86  that is disposed between sleeve  44  and housing  32 . Flow regulating device  30  also has packing stack  88  that is disposed between seat  78  and housing  32 . Supporting packing stack  86  within flow regulating device  30  is a bearing ring  90 . An axially extending generally tubular collet extends downwardly from bearing ring  90 . The collet has a plurality of circumferentially distributed collet fingers representatively illustrated by collet finger  92 . Each collet finger has a radially extended interior region that is representatively illustrated as region  94  on collet finger  92 . Flow regulating device  30  has an axially extending generally tubular biasing device  96  that is axially disposed between lower connector  36  and seat  78  to bias seat  78  toward seat  68 . 
     In operation, flow regulating device  30  has four principal operating configurations. Specifically, flow regulating device  30  has a fully closed, fully sealed configuration, a fully closed, unsealed configuration, a partially opened configuration and a fully open configuration. Referring to FIGS. 2A-2B, flow regulating device  30  is representatively illustrated in a fully closed, fully sealed configuration in which sealing surface  76  of seat  68  is in sealing engagement with sealing surface  82  of seat  80  and seal elements  64 ,  66  straddle the location of the sealing engagement. Fluid is, thus, prevented from flowing through openings  46 ,  48 ,  50 . 
     Referring next to FIGS. 3A-3B, flow regulating device  30  is representatively illustrated in a fully closed, unsealed configuration in which sealing surface  76  of seat  68  is in sealing engagement with sealing surface  82  of seat  80  but seal elements  64 ,  66  no longer straddle the location of the sealing engagement. In the illustrated embodiment, a shifting tool engages shifting profile  98  of sleeve  44  to upwardly shift sleeve  44  relative to seat  68 , seat  78  and housing  32 . In this configuration of flow control device  30 , the sealing engagement of sealing surface  76  of seat  68  with sealing surface  82  of seat  80  substantially prevents fluid flow through openings  46 ,  48 ,  50 . It is to be understood, however, that it is not necessary and somewhat unlikely that the sealing engagement of sealing surface  76  of seat  68  with sealing surface  82  of seat  80  completely prevents fluid communication between the area external to flow regulating device  30  and the flow passageway within flow regulating device  30  through openings  46 ,  48 ,  50 . 
     As can be seen in comparing the configuration of flow regulating device  30  in FIGS. 2A-2B with the configuration of flow regulating device  30  in FIGS. 3A-3B, as sleeve  44  upwardly shifts relative to seat  68 , seat  78  and housing  32 , lug  58  travels upwardly within slot  40  of housing  32 . In addition, lug  58  travels upwardly within slot  77  of seat  68 . Also, as best seen in FIGS. 3A-3B, when lug  58  reaches its point of maximum upward travel within slot  77 , thus defining the maximum upward travel of sleeve  44  relative to seat  68 , the lower shoulder of radially extended exterior region  60  is nearing the top of collet finger head  74 . Once radially extended exterior region  60  clears the top of collet finger head  74 , collet finger head  74  may radially inwardly shift out of radially reduced interior region  42  of housing  32 . Further upward travel of sleeve  44  will thereafter carry seat  68  upwardly due to the contact between lug  58  and the top of slot  77 . 
     Referring additionally now to FIGS. 4A-4B, flow regulating device  30  is representatively illustrated in a partially open configuration in which the upper openings  46  are partially exposed to direct fluid flow between the exterior of flow regulating device  30  and the fluid passage within flow regulating device  30  through openings  38  of housing  32 . In this configuration, the shifting tool has shifted sleeve  44  and seat  68  relative to seat  78  and housing  32 , thus permitting fluid flow through the exposed portion of openings  46  of sleeve  44 . It should be noted that, as representatively illustrated in the accompanying drawings, openings  46  of sleeve  44  are small compared to openings  38  of housing  32 , in order to provide an initial relatively highly restricted fluid flow therethrough when seat  68  is displaced axially away from seat  78 . As such, the flowrate of fluid through flow regulating device  30  may be precisely controlled by increasing or decreasing the separation between seat  68  and seat  78 . 
     It should also be noted that while openings  46  are shown identically dimensioned and positioned axially spaced apart, it is to be understood that opening  46  may be otherwise dimensioned, otherwise positioned, otherwise dimensioned with respect to each other, and otherwise positioned with respect to each other, without departing from the principles of the present invention. Additionally, while openings  46  are shown identically dimensioned as openings  48 ,  50 , openings  46  may alternatively have larger or smaller ports, or may have a different orientation with respect to openings  48 ,  50 . Thus, openings  46 ,  48 ,  50  shown in the accompanying drawings are merely illustrative and additions, modifications, deletions, substitutions, etc., may be made thereto without departing from the principles of the present invention. 
     Referring now to FIGS. 5A-5B, flow regulating device  30  is representatively illustrated in a fully open configuration in which seat  78  has completely uncovered openings  46 ,  48 ,  50 . Fluid is, thus, permitted to flow unobstructed through openings  46 ,  48 ,  50  and into the fluid passage of flow regulating device  30 . In this configuration, radially extended exterior region  62  of sleeve  44  has moved across radially extended interior region  94  of collet finger  92  which, along with fiction, helps to prevent downward movement of sleeve  44  relative to housing  32  until such downward movement is desired. 
     To return flow regulating device  30  to one of the prior configurations, the shifting tool engages shifting profile  98  of sleeve  44  to downwardly shift sleeve  44  and seat  68  relative to seat  78  and housing  32 . Initially, sleeve  44  and seat  68  move together as the lower shoulder of radially extended exterior region  60  of sleeve  44  acts on the upper surface of collet finger head  74 . Seat  68  travels downwardly until sealing surface  76  contacts sealing surface  82  and collet finger head  74  reaches radially reduced interior region  42 . Further downward shifting of sleeve  44  results in relative movement between sleeve  44  and seat  68  as lugs  58  slide within slots  77 . Such downward relative movement may continue until shoulder  56  contacts shoulder  70 . Thereafter, further downward movement of sleeve  44  downwardly urges seat  78  which is upwardly biased by biasing device  96  to increase the force between sealing surfaces  76 ,  82  to thereby improve their sealing capability. 
     It should be noted that a particular benefit of this embodiment of the present invention is that portions thereof may erode during normal use, without affecting the ability of flow regulating device  30  to be fully closed and fully sealed to fluid flow therethrough. For example, openings  38 , lip  84 , opening  46 ,  48 ,  50 , etc., may erode without damaging sealing surfaces  76 ,  82  and without damaging seal elements  64 ,  66 . In fact, even if sealing surfaces  76 ,  82  were eroded, seal elements  64 ,  66  are never required to seal against high velocity fluid flow and suffer the associated degradation. Thus, where it is important for safety purposes to ensure the fluid tight sealing integrity of the wellbore, flow regulating device  30  preserves its ability to shut off fluid flow therethrough even after its fluid choking elements and its sealing surfaces  76 ,  82  have been degraded as seal elements  64 ,  66  are isolated from the flow paths. 
     It should also be noted that fluids entering flow regulating device  30  from below lower connector  63  may be commingled with fluids entering flow regulating device  30  through openings  46 ,  48 ,  50 , and the rate of flow of each may be accurately regulated utilizing one or more of the flow regulating devices  30  of the present invention. For example, as seen in FIG. 1, multiple the flow regulating devices  30 , may be installed within wellbore  22  to regulate the rate of flow of the fluids therein. Alternatively, a flow regulating device  30  may be used in an injection operation to regulate the rate of fluid flow outward through opening  46 ,  48 ,  50  and, alone or in combination with additional flow regulating devices  30 , may be utilized to accurately regulate fluid flow rates into multiple zones of well  20 . Of course, flow regulating devices  30  may be useful in single zone completions to regulate fluid flow into or out of the zone. 
     Even though FIGS. 2A-2B,  3 A- 3 B,  4 A- 4 B and  5 A- 5 B, have depicted flow regulating device  30  as being mechanically actuated, it should be understood by those skilled in the art that flow regulating device  30  of the present invention may be otherwise actuated without departing from the principles of the present invention. For example, flow regulating device  30  may be actuated electrically, magnetically, hydraulically or the like. 
     In fact, a hydraulically operated flow regulating device  130  is representatively illustrated in FIGS. 6A-6B. Flow regulating device  130  has an axially extending generally tubular outer housing  132 . Housing  132  is threadedly and sealingly attached to the lower end of an upper connector  134  that coupled to a section of tubing string  28  on the upper end thereof. Housing  132  is threadedly and sealingly attached to the upper end of a lower connector  136  that coupled to a section of tubing string  28  on the lower end thereof. 
     An axially extending hydraulic communication port  110  extends through upper connector  134 . Hydraulic communication port  110  is attached to a hydraulic line (not pictured) that may extend to platform  12 . Hydraulic communication port  110  is in fluid communication with the upper end of a hydraulic chamber  112 . Disposed within hydraulic chamber  112  is a packing stack  114  having rings  116 ,  118  located on either side thereof. Also disposed within hydraulic chamber  112  is a magnetic position sensor  120  that allows for precise locating of the position of sleeve  144  relative to housing  132 . As best seen in FIG. 7A, a second axially extending hydraulic communication port  124  extends through upper connector  134  and is attached to a second hydraulic line (not pictured). Hydraulic communication port  124  is in fluid communication with the lower end of hydraulic chamber  112 . 
     Housing  132  includes a series of spaced apart openings  138 , only one of which is shown, that are circumferentially distributed about housing  132 . Openings  138  are formed through housing  132  and thereby provide fluid communication between the area external to flow control device  130  and the interior of housing  132 . Housing  132  also includes a series of spaced apart slots  140 , only one of which is shown, that are circumferentially distributed about housing  132 . Housing  132  has a radially reduced interior region  142 , thereby forming upper and lower internal shoulders above and below the region  142 . Housing  132  also has a lower shoulder  143 . 
     Flow regulating device  130  has an axially extending generally tubular inner sleeve  144 . Sleeve  144  is substantially disposed within housing  132  and axially moveable relative to housing  132 . Sleeve  144  includes a series of axially spaced apart openings  146 ,  148 ,  150 , that are circumferentially distributed about sleeve  144 . Openings  146 ,  148 ,  150  are formed through sleeve  144  and thereby provide fluid communication between the area internal to flow control device  130  and the exterior of sleeve  144 . Sleeve  144  has a radially extended exterior region  152 , thereby forming upper and lower external shoulders  154 ,  156 , respectively, above and below region  152 . A series of radially outwardly extending lugs  158 , only one of which is shown, extends from region  152  and travel within slots  140  of housing  144  to define the limits of relative axial movement between sleeve  144  and housing  132  as will be described in greater detail below. Sleeve  144  has a radially extended exterior region  160 , thereby forming upper and lower external shoulders above and below region  160 . In addition, sleeve  144  has a radially extended exterior region  162 , thereby forming upper and lower external shoulders above and below region  162 . Sleeve  144  includes a pair of radially reduces exterior regions  167 ,  169  that respective receive portions of ring  116  and ring  118 , thereby coupling ring  116  and ring  118  to sleeve  144 . 
     Sleeve  144  carries an upper seal element  164  and a lower seal element  166 . In the illustrated embodiment, seal elements  164 ,  166  include an elastomeric ring that is held in place with a metal sleeve. It should be understood, however, by those skilled in the art that seal elements  164 ,  166  may be selected from a variety of sealing members that are common in the art. For example, seal elements  164 ,  166  may be spring loaded lip seals or simple O-ring seals. Likewise, seal elements  164 ,  166  may be elastomeric seals or may alternatively be non-elastomeric seals. 
     Flow regulating device  130  has an axially extending generally tubular upper seat  168 . Seat  168  is disposed between sleeve  144  and housing  132 . Seat  168  is axially moveable relative to sleeve  144 . Likewise, seat  168  is axially moveable relative to housing  132 . Seat  168  includes an upper shoulder  170 . Seat  168  also includes a collet section having a plurality of circumferentially distributed collet fingers representatively illustrated by collet finger  172 . Collet finger  172  has an upper end that terminates as collet finger head  174 . Seat  168  includes a lower sealing surface  176 . Seat  168  also includes a series of slots circumferentially distributed around seat  168  and representatively illustrated by slot  177 . Each slot  177  receives one of the lugs  158  radially outwardly extending from region  152  of sleeve  144  to define the limits of relative axial movement between sleeve  144  and seat  168  as will be described in greater detail below. 
     Flow regulating device  130  has an axially extending generally tubular lower seat  178 . Seat  178  is disposed between sleeve  144  and housing  132 . Seat  178  is substantially fixably positioned relative to housing  132  due to the interaction between upper shoulder  180  of seat  178  and lower shoulder  143  of housing  132 . Seat  178  is axially moveable relative to sleeve  144 . Seat  178  includes an upper sealing surface  182  and an axially extending and internally inclined lip  184 . 
     Lip  184  acts to prevent, or at least greatly reduce, erosion of sealing surface  182  as well as erosion of sleeve  144 . Sealing surface  182  of seat  178  is cooperatively shaped to sealingly engage sealing surface  176  of seat  168 , and, in the configuration of flow regulating device  130  shown in FIGS. 6A-6B, sealing surface  182  is contacting and sealingly engaging sealing surface  176 . Together, seat  168  and seat  178  form a trim set such as that described above. 
     Flow regulating device  130  has a packing stack  186  that is disposed between sleeve  144  and upper connector  134 . Flow regulating device  130  also has packing stack  188  that is disposed between seat  178  and housing  132 . Supporting packing stack  186  within flow regulating device  130  is a bearing ring  190 . An axially extending generally tubular collet extends downwardly from bearing ring  190 . The collet has a plurality of circumferentially distributed collet fingers representatively illustrated by collet finger  192 . Each collet finger has a radially extended interior region that is representatively illustrated as region  194  on collet finger  192 . Flow regulating device  130  has an axially extending generally tubular biasing device  196  that is axially disposed between lower connector  136  and seat  178  to bias seat  178  toward seat  168 . 
     In operation, flow regulating device  130  has four principal operating configurations. Specifically, flow regulating device  130  has a fully closed, fully sealed configuration, a fully closed, unsealed configuration, a partially opened configuration and a fully open configuration. Referring to FIGS. 6A-6B, flow regulating device  130  is representatively illustrated in a fully closed, fully sealed configuration in which sealing surface  176  of seat  168  is in sealing engagement with sealing surface  182  of seat  180  and seal elements  164 ,  166  straddle the location of the sealing engagement. Fluid is, thus, prevented from flowing through openings  146 ,  148 ,  150 . 
     Referring next to FIGS. 7A-7B, flow regulating device  130  is representatively illustrated in a fully closed, unsealed configuration in which sealing surface  176  of seat  168  is in sealing engagement with sealing surface  182  of seat  180  but seal elements  164 ,  166  no longer straddle the location of the sealing engagement. In the illustrated embodiment, hydraulic fluid enters the bottom of hydraulic chamber  112  from hydraulic communication port  124  to upwardly shift sleeve  144  relative to seat  168 , seat  178  and housing  132 . In this configuration of flow control device  130 , the sealing engagement of sealing surface  176  of seat  168  with sealing surface  182  of seat  180  substantially prevents fluid flow through openings  146 ,  148 ,  150 . It is to be understood, however, that it is not necessary and somewhat unlikely that the sealing engagement of sealing surface  176  of seat  168  with sealing surface  182  of seat  180  completely prevents fluid communication between the area external to flow regulating device  130  and the flow passageway within flow regulating device  130  through openings  146 ,  148 ,  150 . 
     As can be seen in comparing the configuration of flow regulating device  130  in FIGS. 6A-6B with the configuration of flow regulating device  130  in FIGS. 7A-7B, as sleeve  144  upwardly shifts relative to seat  168 , seat  178  and housing  132 , lug  158  travels upwardly within slot  140  of housing  132 . In addition, lug  158  travels upwardly within slot  177  of seat  168 . Also, as best seen in FIGS. 7A-7B, when lug  158  reaches its point of maximum upward travel within slot  177 , thus defining the maximum upward travel of sleeve  144  relative to seat  168 , the lower shoulder of radially extended exterior region  160  is nearing the top of collet finger head  174 . Once radially extended exterior region  160  clears the top of collet finger head  174 , collet finger head  174  may radially inwardly shift out of radially reduced interior region  142  of housing  132 . Further upward travel of sleeve  144  will thereafter carry seat  168  upwardly due to the contact between lug  158  and the top of slot  177 . 
     Referring additionally now to FIGS. 8A-8B, flow regulating device  130  is representatively illustrated in a partially open configuration in which openings  146  are partially exposed to direct fluid flow between the exterior of flow regulating device  130  and the fluid passage within flow regulating device  130  through openings  138  of housing  132 . In this configuration, hydraulic fluid has shifted sleeve  144  and seat  168  relative to seat  178  and housing  132 , thus permitting fluid flow through the exposed portion of openings  146  of sleeve  144 . Additional upward or downward movement of sleeve  144  relative to seat  178  using hydraulic fluid pressure within hydraulic chamber  112  allows for precise control of the flowrate of fluid through flow regulating device  130  by increasing or decreasing the separation between seat  168  and seat  178  which is monitored using magnetic positioning sensor  120 . Specifically, hydraulic fluid may enter the top of hydraulic chamber  112  from hydraulic communication port  110 , as best seen in FIG. 6A, to downwardly shift sleeve  144 . 
     Referring now to FIGS. 9A-9B, flow regulating device  130  is representatively illustrated in a fully open configuration in which seat  178  has completely uncovered openings  146 ,  148 ,  150 . Fluid is, thus, permitted to flow unobstructed through openings  146 ,  148 ,  150  and into the fluid passage of flow regulating device  130 . In this configuration, radially extended exterior region  162  of sleeve  144  has moved across radially extended interior region  194  of collet  192  which, along with hydraulic fluid in the lower portion of hydraulic chamber  112 , prevent downward movement of sleeve  144  relative to housing  132  until such downward movement is desired. 
     To return flow regulating device  130  to one of the prior configurations, hydraulic fluid enters the upper section of hydraulic chamber  112  from hydraulic communication port  110  to downwardly shift sleeve  144  and seat  168  relative to seat  178  and housing  132 . Initially, sleeve  144  and seat  168  move together as the lower shoulder of radially extended exterior region  160  of sleeve  144  acts on the upper surface of collet finger head  174 . Seat  168  travels downwardly until sealing surface  176  contacts sealing surface  182  and collet finger head  174  reaches radially reduced interior region  142 . Further downward shifting of sleeve  144  results in relative movement between sleeve  144  and seat  168  as lugs  158  slide within slots  177 . Such downward relative movement may continue until shoulder  156  contacts shoulder  170 . Thereafter, further downward movement of sleeve  144  downwardly urges seat  178  which is upwardly biased by biasing device  196  to increase the force between sealing surfaces  176 ,  182  to thereby improve their sealing capability. 
     While this invention has been described in reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.

Summary:
An apparatus ( 30 ) and method for regulating fluid flow through a downhole tubing string ( 28 ) is disclosed. The apparatus ( 30 ) comprises a generally tubular outer housing ( 32 ) having a housing port ( 38 ) formed through a sidewall portion thereof. First and second seats ( 78, 68 ) are disposed within the housing ( 32 ), each having sealing surface ( 82, 76 ) that are sealingly engageable to one another to substantially prevent fluid flow therebetween. A sleeve ( 44 ) is slidably disposed within the first and second seats ( 68, 78 ). The sleeve ( 44 ) has a flow passage extending generally axially therethrough and is variably positionable relative to the first seat ( 78 ) to regulate fluid flow through a sleeve port ( 46 ). The sleeve ( 44 ) has first and second positions relative to the second seat ( 68 ). In the first position, a seal ( 66 ) is disposed between the sleeve ( 44 ) and the first seat ( 78 ), thereby preventing fluid flow through the sleeve port ( 46 ). In the second position, the seal ( 66 ) is disposed between the sleeve ( 44 ) and the second seat ( 68 ), thereby not preventing fluid flow through the sleeve port ( 46 ).