Abstract:
A completion system for a subsea well includes a tree having a generally cylindrical wall forming an internal bore therethrough and a production port extending laterally through the wall in communication with the internal bore. The internal wall has a landing arranged to support a tubing hanger having seals for sealing the production port between the tubing hanger and the internal wall, the production port being arranged to communicate with a lateral production fluid outlet port in the tubing hanger. A workover port extends laterally from an opening in the internal wall below the production port and the production port seals and a tubing annulus seal sealing the workover port from the tubing annulus. A tubing annulus port extends from an opening in the tree below the tubing annulus seal and the tubing annulus port and workover port being arranged to be in fluid communication externally of the internal bore.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a flow completion apparatus for producing oil or gas from a subsea well. More particularly, the invention relates to a flow completion apparatus which comprises a tubing hanger having an annulus bore which is adapted to communicate with a choke and kill line of a blowout preventer which is installed over the tubing hanger during installation and workover of the flow completion apparatus.  
           [0002]    Flow completion assemblies for producing oil or gas from subsea wells may generally be categorized as either conventional or horizontal. A typical horizontal flow completion assembly is disclosed in U.S. Pat. No. 6,039,119, hereby incorporated herein by reference.  
           [0003]    International Publication No. WO 01/73259 of International Application No. PCT/US01/09607 filed Mar. 22, 2001 and published Oct. 4, 2001, shows a tubing hanger with an annulus bore. International Publication No. WO 01/73259 is hereby incorporated herein by reference.  
         SUMMARY OF THE INVENTION  
         [0004]    The flow completion apparatus comprises a wellhead housing which is installed at the upper end of the wellbore; a tubing spool which is connected over the wellhead housing and which includes a central bore which extends axially therethrough, a production outlet which communicates with the central bore, and an annulus passageway which communicates with the tubing annulus; a tubing hanger which is supported in the central bore and is connected to an upper end of the tubing string, the tubing hanger including a production bore which extends axially therethrough and a production passageway which communicates between the production bore and the production outlet; a first closure member which is positioned in the production bore above the production passageway; production seals positioned between the tubing hanger and central bore above and below the production passageway; and a tubing annulus seal which is positioned between the tubing hanger and the central bore below the production passageway and production seals. Furthermore, the tubing spool also comprises a workover passageway which extends between the annulus passageway and a portion of the central bore that is located between the production seals and the tubing annulus seal, and the tubing hanger also comprises an annulus bore which extends between the workover passageway and the upper end of the tubing hanger. In this manner, fluid communication between the tubing annulus and the upper end of the tubing hanger may be established through the annulus passageway, the workover passageway, and the annulus bore.  
           [0005]    The flow completion apparatus further comprises a blowout preventer which is removably connectable to the top of the tubing spool and which includes a BOP bore, at least one set of BOP rams, and at least one choke and kill line that communicates with a portion of the BOP bore which is located below the BOP rams; and a tubing hanger running tool which is removably connectable to the top of the tubing hanger and which includes a generally cylindrical outer diameter surface and a production port that communicates with the production bore in the tubing hanger. An annulus passageway extends between the annulus bore in the tubing hanger and the BOP choke and kill line. This passageway may either be the annular area around the tubing hanger running tool or may include an annulus port through the running tool that communicates between the annulus bore and an opening which is formed in the outer diameter surface of the tubing hanger running tool to communicate with the BOP choke and kill line. In this manner, fluid communication between the tubing annulus and the BOP choke and kill line may be established through the annulus passageway, the workover passageway, the annulus bore, either the annular area around the tubing hanger running tool or an annulus port in the tubing hanger running tool, and the portion of the BOP bore which is located below the closed BOP ram.  
           [0006]    The annulus bore in the tubing hanger provides a convenient means for connecting the tubing annulus with the BOP choke and kill line. An annulus port in the tubing hanger running tool provides a closed path between the annulus bore in the tubing hanger and the BOP choke and kill line.  
           [0007]    A first barrier between the wellbore and the environment is provided by both the first closure member in the production bore and the tubing annulus seal between the tubing hanger and the tubing spool. In addition, a second barrier between the wellbore and the environment is provided by both a second closure member that is positioned in the production bore above the first closure member and the production seals that are positioned between the tubing hanger and the tubing spool above the tubing annulus seal. In this manner, both the first and second barriers between the wellbore and the environment are mounted in or on the tubing hanger.  
           [0008]    These and other objects and advantages of the present invention will be made apparent from the following detailed description, with reference to the accompanying drawings. In the drawings, the same reference numerals are used to denote similar components in the various embodiments. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is a representation of one embodiment of the flow completion apparatus shown in the production mode of operation with the tubing hanger annulus bore extending between a first and second closure member;  
         [0010]    [0010]FIG. 2 is a representation of the flow completion apparatus of FIG. 1 shown in the installation and workover mode of operation with an annulus passageway extending from the tubing hanger annulus bore to the choke and kill line;  
         [0011]    [0011]FIG. 3 is a representation of another embodiment of the flow completion apparatus shown in the production mode of operation with an annulus bore extending from the workover passageway to that portion of the internal bore of the spool tree above the first and second closure members;  
         [0012]    [0012]FIG. 4 is a representation of the flow completion apparatus of FIG. 3 shown in the installation workover mode of operation with an annulus passageway extending from the tubing hanger annulus bore to the choke and kill line;  
         [0013]    [0013]FIG. 5 is a representation of a still another embodiment of the flow completion apparatus shown in the production mode of operation with an annulus bore extending to the top of the tubing hanger and sealed with a seal stab;  
         [0014]    [0014]FIG. 6 is a representation of the flow completion apparatus of FIG. 5 shown in the installation and workover mode of operation with a passageway extending through the running tool between the tubing hanger annulus bore and the choke and kill line;  
         [0015]    [0015]FIG. 7 is a representation of the flow completion apparatus of FIG. 5 shown in the installation and workover mode of operation with a passageway extending around the running tool between the tubing hanger annulus bore and the choke and kill line;  
         [0016]    [0016]FIG. 8 is a representation of a further embodiment of the flow completion apparatus shown in the production mode of operation with an annulus bypass bore extending between the annulus bore and the production bore;  
         [0017]    [0017]FIG. 9 is a cross section at plane  9 - 9  in FIG. 8; and  
         [0018]    [0018]FIG. 10 is a still further embodiment of the flow completion apparatus shown in the production mode of operation with a tubing suspension conduit below the tubing hanger.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]    The present invention relates to methods and apparatus for flow completion and particularly for circulation in the borehole of a well during installation and workover. The present invention is susceptible to embodiments of different forms. There are shown in the drawings, and herein will be described in detail, specific embodiments of the present invention with the understanding that the present disclosure is to be considered an exemplification of the principles of the invention, and is not intended to limit the invention to that illustrated and described herein.  
         [0020]    In particular, various embodiments of the present invention provide a number of different constructions and methods of operation of the completion system. The embodiments of the present invention also provide a plurality of methods for circulation in the borehole of a well. It is to be fully recognized that the different teachings of the embodiments discussed below may be employed separately or in any suitable combination to produce desired results. Reference to up or down will be made for purposes of description with up meaning away from the bottom of the well and down meaning toward the bottom of the well.  
         [0021]    In the description which follows, the use of the same reference numerals throughout the specification and drawings indicates like parts. The drawing figures are not necessarily to scale. Certain features of the invention may be shown in exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in the interest of clarity and conciseness.  
         [0022]    Referring initially to FIG. 1, one embodiment of a flow completion apparatus according to the present invention, is generally indicated by reference numeral  10 . The flow completion apparatus  10  comprises a wellhead  12 , a tubing spool  14  which is connected and sealed to the wellhead and which includes a central bore  16  extending axially therethrough, a generally annular tubing hanger  18  which is supported on a shoulder (not shown) located in the central bore, and a tree cap  20  which is installed in the central bore above the tubing hanger. The tubing hanger  18  is secured to the tubing spool  14  by a lockdown mechanism (not shown) and suspends a tubing string  22  that extends into the well bore and defines a tubing annulus  24  surrounding the tubing string. Tubing hanger  18  also includes a production bore  26  which communicates with the flowbore of the tubing string  22  and a lateral production passageway  28  which extends between the production bore  26  and the outer diameter of the tubing hanger. The tubing spool  14  includes a production outlet  30  which communicates with the production passageway  28 , an annulus passageway  32  which communicates with the tubing annulus  24 , and an annulus outlet  34  which is connected to the annulus passageway  32  and a workover passageway  36  which extends between the annulus passageway  32  and an area  86  of the central bore  16  above the tubing hanger  18 . In addition, the tubing hanger  18  is sealed to the tubing spool  14  by upper and lower, preferably metal, production seal rings  40 ,  38 , each of which engages a corresponding annular sealing surface formed on the wall forming central bore  16 . The communication between the workover passageway  36  and the tubing annulus  24  is sealed by a tubing annulus seal ring  57 . Furthermore, the production bore  26  is sealed above the production passageway  28  by a suitable closure member  42 , such as a plug, which directs the flow of oil or gas from the tubing string  22  into the production passageway  30 .  
         [0023]    The tubing hanger  18  also includes an annulus bore  80  which extends between the upper end and lower end of the tubing hanger  18 . In this manner, communication between the tubing annulus  24  and area  86  above the the upper end of tubing hanger  18  is provided by the annulus passageway  32 , the workover passageway  36 , and the annulus bore  80 . This arrangement permits communication between the tubing annulus  24  and area  86  and also a choke and kill line in a BOP with tree cap  20  removed as shown in FIG. 2.  
         [0024]    The flow completion apparatus  10  may also comprise a production master valve  44  and a production wing valve  46  to control flow through the production outlet  30 , and an annulus master valve  48 , an annulus wing valve  50  and a workover valve  52  to control flow through the annulus passageway  32 , the annulus outlet  34  and the workover passageway  36 , respectively. While these valves may be any suitable closure members, they are preferably remotely operated gate valves. Moreover, some or all of the valves may be incorporated into the body of the tubing spool  14 , into separate valve blocks which are bolted onto the tubing spool, or into individual valve assemblies which are connected to their respective outlets or passageways in the tubing spool with separate lengths of conduit. Furthermore, the production outlet  30  and the annulus outlet  34  are preferably connected to respective flow loops which communicate with a surface vessel, either directly or via a manifold, in a manner that is well known in the art.  
         [0025]    In the production mode of operation of the flow completion apparatus  10 , shown in FIG. 1, a first barrier between the well bore and the environment is provided by the closure member  42  production seals  38 ,  40 , and the tubing annulus seal  57 , which together serve to isolate the fluid in the wellbore from the environment above the tubing hanger. The second barrier is provided by the tree cap  20  and by a typically metal seal ring  54  which is disposed between the tree cap  20  and the tubing spool  14  and a wireline plug  56  which is positioned in an axial bore  58  extending through the tree cap. Thus, in the completion assembly  10 , the first barrier is associated with the tubing hanger  18  while the second barrier is associated with the tree cap  20 . Although not shown in FIG. 1, the tree cap  20  also includes a lockdown mechanism to secure the tree cap to the tubing spool  14 .  
         [0026]    Referring now to FIG. 2, the flow completion assembly  10  is shown in the installation or workover mode of operation. In either of these modes of operation, a blowout preventer  60  is connected to the top of the tubing spool  14  and a tubing hanger running tool  62  is attached to the top of the tubing hanger  18 . The BOP includes an internal BOP bore  64 , at least one set of rams  66  which is capable of sealing against the tubing hanger running tool  62 , and at least one choke and kill line  68  for providing communication between the BOP bore below the rams  66  and a surface vessel (not shown). In addition, the tubing hanger running tool  62  comprises an internal bore  70 , or production port, which connects to the production bore  26  via a production stab (not shown). Also, although the BOP rams are described herein as sealing against the tubing hanger running tool, it should be understood that the rams could instead seal against another member, such as an extension member or a work string, which comprises a production port that communicates with the production port of the tubing hanger running tool.  
         [0027]    During both installation and workover of the flow completion assembly  10 , communication between the tubing annulus  24  and the surface vessel may be established through the annulus passageway  32 , the workover passageway  36 , the annulus bore  80 , the central bore  16 , the BOP bore  64 , and the choke and kill line  68 . For example, deep well circulation can be accomplished by pumping fluid down the tubing hanger running tool bore  70 , through the production bore  26 , through the flowbore of tubing string  22 , around or through the lower end of the tubing string  22 , up the tubing annulus  24 , through the annulus passageway  32 , through the workover passageway  36 , through the annulus bore  80 , into the central bore  16  above the tubing hanger  18 , into the BOP bore  64  and through the BOP choke and kill line  68  to the surface.  
         [0028]    Referring now to FIGS. 3 and 4, there is shown an alternative embodiment of the flow completion assembly  10  of FIGS. 1 and 2. Annulus bore  80  may be made in flow communication with a secondary annulus bore  82  extending through tree cap  20  by including an annulus stab  84  that seals into the top of the annulus bore  80  and into the secondary annulus bore  82  in the bottom of tree cap  20 . This provides an annulus bore which extends from workover port  36  to that portion of the internal bore  16  of tubing spool  14  above both the first and second closure members, namely tree cap  20  and wire line plug  42 , respectively. Secondary annulus bore  82  may be closed and sealed by a seal stab (not shown) installed in the upper end of secondary annulus bore  82   
         [0029]    Referring now to FIG. 5, another embodiment of a flow completion apparatus according to the present invention is generally indicated by reference numeral  110 . The flow completion apparatus comprises a wellhead  112 , tubing spool  114  which is mounted on the wellhead which includes a central bore  116  extending axially therethrough, and a generally annular tubing hanger  118  which is supported on a shoulder (not shown) located in the central bore and from which is suspended a tubing string  120  that extends into the well bore and defines a tubing annulus  122  surrounding the tubing string. The tubing hanger  118  is secured to the tubing spool  114  by a lockdown mechanism (not shown) and includes a production bore  124  which communicates with the flowbore of the tubing string  120  and a lateral production passageway  126  which extends between the production bore  124  and the outer diameter of the tubing hanger. Similarly, the tubing spool  114  includes a production outlet  128  which communicates with the production passageway  126 , an annulus passageway  130  which communicates with the tubing annulus  122 , and an annulus outlet  132  which is connected to the annulus passageway. In addition, the tubing hanger  118  is sealed to the tubing spool  114  by an upper and lower, preferably metal, production seal rings  134 ,  136 , each of which engages a corresponding annular sealing surface formed on the wall of central bore  116 . Furthermore, the production bore  124  is sealed above the production passageway  126  by a suitable closure member  138  which directs the flow of oil or gas from the tubing string  120  into the production passageway  126 . Ring seals  156 ,  157 , located above and below production port  128  and production seals  134 ,  136 , sealingly engage a corresponding annular sealing surface formed by the wall of the central bore  116 .  
         [0030]    The tubing hanger  118  also includes an annulus bore  140  which extends between the top and the lower outer diameter of the tubing hanger  118 , and the tubing spool  114  comprises a workover passageway  142  that extends between the annulus passageway  130  and the annulus bore  140 . The communication between the workover passageway  142  and the tubing annulus  122  is sealed by tubing annulus seal ring  157 . In this manner, communication between the tubing annulus  122  and the top of tubing hanger  118  is provided by the annulus passageway  130 , the workover passageway  142 , and the annulus bore  140 . This arrangement permits communication between the tubing annulus  122  and a BOP to be routed through a tubing hanger running tool, shown in FIG. 6, rather than in the area  186  of the central bore  116  above the tubing hanger  118 .  
         [0031]    The flow completion apparatus  110  may also comprises a production master valve  144  and a production wing valve  146  to control flow through the production outlet  128 , and an annulus master valve  148 , an annulus wing valve  150  and a workover valve  152  to control flow through the annulus passageway  130 , the annulus outlet  132  and the workover passageway  142 , respectively. While these valves may be any suitable closure members, they are preferably remotely operated gate valves. Moreover, some or all of the valves may be incorporated into the body of the tubing spool  114 , into separate valve blocks which are bolted onto the tubing spool, or into individual valve assemblies which are connected to their respective outlets or passageways in the tubing spool with separate lengths of conduit. Furthermore, the production outlet  128  and the annulus outlet  132  are preferably connected to respective flow loops which communicate with a surface vessel, either directly or via a manifold, in a manner that is well known in the art.  
         [0032]    In the production mode of operation of the flow completion apparatus  110 , shown in FIG. 5, production seal  134  and tubing annulus seal  157  together function as a double barrier to isolate the fluid in the production passageway  126  from the environment below the tubing hanger  118  and production seal  134  and secondary seal  156  together function as a double barrier to isolate the fluid in the production passageway  126  from the environment above the tubing hanger  118 .  
         [0033]    In accordance with the present invention, a first barrier between the well bore and the environment is provided by the closure member  138  and the production seals  134 ,  136 , which together serve to isolate the fluid in the production bore from the environment above and below the tubing hanger  118 . A second barrier between the well bore and the environment is provided by a suitable second closure member  154 , which is mounted in the production bore  124  above the closure member  138 , and secondary seal  156  and tubing annulus seal  157 , preferably a metal ring seals, which are mounted on the tubing hanger  118  above and below production passageway  126 . Thus, the necessary first and second barriers for isolating the production passageway  126  from the environment are provided by components which are mounted on or in the tubing hanger  118 .  
         [0034]    The present invention also provides for isolating the tubing annulus  122  from the environment above the tubing hanger  118  during the production mode of operation. Provided the annular master valve  148  and the workover valve  152  are closed, the production seals  134 ,  136 , the secondary seal  156 , and the tubing annulus seal  157  will provide the required first and second barriers between the tubing annulus and the environment. However, when pressure in the tubing annulus  122  needs to be bled off through the annulus passageway  130  and the annulus outlet  132 , or when gas is introduced into the tubing annulus through the annulus outlet and the annulus passageway during gas lift applications, the annulus master valve  148  must be opened.  
         [0035]    Therefore, the flow completion apparatus preferably also comprises a tree cap  158  which includes an annulus stab  160  that seals into the top of the annulus bore  140  to provide a second barrier, in conjunction with the workover valve  152 , between the tubing annulus  122  and the environment when the environment master valve  148  is open. While the tree cap  158  may include an annular, preferably non-metallic seal (not shown) to seal against the tubing spool  114  and thereby prevent sea water from entering the central bore  116 , the tree cap is not intended to provide a barrier against well pressure in the production bore. The tree cap  158  is preferably landed on the tubing hanger  118  and locked to the tubing spool  114  with a convention lockdown mechanism  162 . This lockdown mechanism will provide a backup to the lockdown mechanism used to secure the tubing hanger to the running tool. It should be noted that, although the tree cap  158  is depicted as an internal tree cap, it could instead be configured as an external tree cap.  
         [0036]    Referring now to FIG. 6, during installation and workover of the flow completion apparatus  110 , a BOP  164  is lowered on a riser (not shown) and connected and sealed to the top of the tubing spool  114 . The BOP  164  includes an internal BOP bore  166 , at least one choke and kill line  168 , and one or more sets of BOP rams  170 ,  172 . In addition, a tubing hanger running tool  174  is connected to the top of the tubing hanger  118 . The tubing hanger running tool  174  is either connected to the tubing hanger at a surface vessel and used to lower the tubing hanger into the tubing spool during installation of the tubing hanger, or lowered through a riser and the BOP and connected to the tubing hanger in the tubing spool in anticipation of a workover operation. The tubing hanger running tool  174  is shown to comprise a generally cylindrical outer diameter surface, a production port  176  which is connected to a production bore  124  in the tubing hanger  118  by a suitable production seal stab  178 , and an annulus port  180  which extends from a portion of the outer diameter surface of tubing hanger running tool  174  to a suitable annulus seal stab  182  that engages the tubing hanger annulus bore  140 .  
         [0037]    Thus, with the BOP rams  170 ,  172  sealed against the tubing hanger running tool  174 , communication between the tubing annulus  122  and the BOP choke and kill line  168  may be established through the annulus passageway  130 , the workover passageway  142 , the annulus bore  140 , the annulus port  180 , and the portion  167  of the BOP bore  166  which is located between the BOP rams  170 ,  172 . For example, with the annulus wing valve  150  closed, pressure can be transmitted from the surface vessel down the choke and kill line  168 , through the annulus portion  180 , through the tubing hanger annulus bore  140 , through the workover passageway  142 , through the annulus passageway  130 , and into the tubing annulus  122 . The well circulation may be accomplished by closing both the annulus wing valve  150  and the production master valve  144  and pumping fluid down the choke and kill line  168  through the annulus port  180 , through the annulus bore  140 , through the workover passageway  142 , through the annulus passageway  130 , down the tubing annulus  122 , past the downhole packer, up the tubing string  120 , through the production bore  124 , and up the production port  176 . Moreover, since the flow between the tubing hanger annulus bore  140  and the choke and kill line  168  is restricted by the tubing hanger running tool  174 , no possibility exists that the flow will foul the tubing hanger lockdown mechanism or erode the central bore  116 .  
         [0038]    Referring now to FIG. 7, there is shown an alternative embodiment of the flow completion assembly  110  of FIG. 6. Annulus bore  140  communicates with the choke and kill line  168  through an annular passageway  187  between the tubing hanger running tool  174  and the internal bores of the BOP  166  and the tubing spool  114 .  
         [0039]    Referring now to FIG. 8, an alternate embodiment of a flow completion apparatus according to the present invention as described in FIG. 5, is generally indicated by reference numeral  110 . The flow completion apparatus comprises a wellhead  112 , tubing spool  114  which is mounted on the wellhead which includes a central bore  116  extending axially therethrough, and a generally annular tubing hanger  118  which is supported on a shoulder (not shown) located in the central bore and from which is suspended a tubing string  120  that extends into the well bore and defines a tubing annulus  122  surrounding the tubing string. The tubing hanger  118  is secured to the tubing spool  114  by a lockdown mechanism (not shown) and includes a production bore  124  which communicates with the flowbore of the tubing string  120  and a lateral production passageway  126  which extends between the production bore and the outer diameter of the tubing hanger. Similarly, the tubing spool  114  includes a production outlet  128  which communicates with the production passageway  126 , an annulus passageway  130  which communicates with the tubing annulus  122 , and an annulus outlet  132  which is connected to the annulus passageway. In addition, the tubing hanger  118  is sealed to the tubing spool  114  by an upper and lower, preferably metal, production seal rings  134 ,  136 , each of which engages a corresponding annular sealing surface formed by the wall of the central bore  116 . Furthermore, the production bore  124  is sealed above the production passageway  126  by a suitable closure member  138  which directs the flow of oil or gas from the tubing string  120  into the production passageway  126 . Ring seals  156 ,  157 , located above and below production port  128  and production seals  134 ,  136 , sealingly engage a corresponding annular sealing surface formed by the wall of the central bore  116 .  
         [0040]    The tubing hanger  118  also includes an annulus bore  140  which extends between the top and the outer diameter of the tubing hanger  118 , and the tubing spool  114  comprises a workover passageway  142  that extends between the annulus passageway  130  and the annulus bore  140 . The communication between the workover passageway  142  and the tubing annulus  122  is sealed by an annular seal ring  157 . Tubing hanger  118  also includes an annulus bypass bore  141  extending from annulus bore  140  through valve  139  and continuing through bypass bore  143  to production bore  124 . In this manner fluid communication between the tubing annulus  122  and the production bore  124  above closure member  138  is provided.  
         [0041]    Referring now to FIG. 9, a section view of FIG. 8 generally indicated a valve actuation member  147  and valve stem  149  for valve  139 . Valve actuation as indicated here is described in U.S. Pat. No. 5,992,527 which is hereby incorporated herein by reference.  
         [0042]    Similar valve actuation member  151  and valve stem  153  are shown as an alternate for valve closure member  154 . The valve actuation member  151  attached to tubing spool  114  may be used outside or inside the necessary second barriers for isolating the production bore  124  from the environment as described earlier.  
         [0043]    The flow completion apparatus  110  may also comprise a production master valve  144  and a production wing valve  146  to control flow through the production outlet  128 , and an annulus master valve  148 , an annulus wing valve  150  and a workover valve  152  to control flow through the annulus passageway  130 , the annulus outlet  132  and the workover passageway  142 , respectively. While these valves may be any suitable closure members, they are preferably remotely operated gate valves. Moreover, some or all of the valves may be incorporated into the body of the tubing spool  114 , into separate valve blocks which are bolted onto the tubing spool, or into individual valve assemblies which are connected to their respective outlets or passageways in the tubing spool with separate lengths of conduit. Furthermore, the production outlet  128  and the annulus outlet  132  are preferably connected to respective flow loops which communicate with a surface vessel, either directly or via a manifold, in a manner that is well known in the art.  
         [0044]    In the production mode of operation of the flow completion apparatus  110 , shown in FIG. 8, production seal  136  and tubing annulus seal  157  together function as a double barrier to isolate the fluid in the production passageway  126  from the environment below the tubing hanger  118  and production seal  134  and secondary seal  156  together function as a double barrier to isolate the fluid in the production passageway  126  from the environment above the tubing hanger  118 .  
         [0045]    In accordance with the present invention, a first barrier between the well bore and the environment is provided by the closure member  138  and the production seals  134 ,  136 , which together serve to isolate the fluid in the production bore from the environment above and below the tubing hanger  118 . A second barrier between the well bore and the environment is provided by a suitable second closure member  154 , which is mounted in the production bore  124  above the closure member  138 , and secondary seal  156  and tubing annulus seal  157 , preferably a metal ring seals, which are mounted on the tubing hanger  118  above and below production passageway  126 . Thus, the necessary first and second barriers for isolating the production passageway  126  from the environment are provided by components which are mounted on or in the tubing hanger  118 .  
         [0046]    The present invention also provides for isolating the tubing annulus  122  from the environment above the tubing hanger  118  during the production mode of operation. Provided the annular master valve  148  and the workover valve  152  are closed, the production seals  134 ,  136 , the secondary seal  156 , and the tubing annulus seal  157  will provide the required first and second barriers between the tubing annulus and the environment. However, when pressure in the tubing annulus  122  needs to be bled off through the annulus passageway  130  and the annulus outlet  132 , or when gas is introduced into the tubing annulus through the annulus outlet and the annulus passageway during gas lift applications, the annulus master valve  148  must be opened.  
         [0047]    Therefore, the flow completion apparatus preferably also comprises a tree cap  158  which includes an annulus stab  160  that seals into the top of the annulus bore  140  to provide a second barrier, in conjunction with the workover valve  152 , between the tubing annulus  122  and the environment when the environment master valve  148  is open. While the tree cap  158  may include an annular, preferably non-metallic seal (not shown) to seal against the tubing spool  114  and thereby prevent sea water from entering the central bore  116 , the tree cap is not intended to provide a barrier against well pressure in the production bore. The tree cap  158  is preferably landed on the tubing hanger  118  and locked to the tubing spool  114  with a convention lockdown mechanism  162 . This lockdown mechanism will provide a backup to the lockdown mechanism used to secure the tubing hanger to the running tool. It should be noted that, although the tree cap  158  is depicted as an internal tree cap, it could instead be configured as an external tree cap.  
         [0048]    Referring now to FIG. 10, one embodiment of a flow completion apparatus according to the present invention, is generally indicated by reference numeral  210 . The flow completion apparatus  210  comprises a wellhead  212 , a tubing spool  214  which is connected and sealed to the wellhead and which includes a central bore  216  extending axially therethrough, a generally annular tubing hanger  218  which is supported on a shoulder (not shown) located in the central bore, and a tree cap  220  which is installed in the central bore above the tubing hanger. The tubing hanger  218  is secured to the tubing spool  214  by a lockdown mechanism (not shown) and is in communication with a tubing string  222  that extends into the well bore and defines a tubing annulus  224  surrounding the tubing string. Tubing hanger  218  also includes a production bore  226  which communicates with the flowbore of the tubing string  222  and a lateral production passageway  228  which extends between the production bore and the outer diameter of the tubing hanger. The tubing spool  214  includes a production outlet  230  which communicates with the production passageway  228 , an annulus passageway  232  which communicates with the tubing annulus  224 , and an annulus outlet  234  which is connected to the annulus passageway  232  and a workover passageway  236  which extends between the annulus passageway  232  and an area  286  of the central bore  216  above the tubing hanger  218 . In addition, the tubing hanger  218  is sealed to the tubing spool  214  by a lower, preferably metal production seal ring  238  and an upper, preferably metal production seal ring  240 , each of which engages a corresponding annular sealing surface formed by the wall of the central bore  216 . The communication between the workover passageway  236  and the tubing annulus  224  is sealed by tubing annulus seal ring  257  and by seal ring  259  in sealing relationship with tubing suspension conduit  219 . Furthermore, the production bore  226  is sealed above the production passageway  228  by a suitable closure member  242 , such as a plug, which directs the flow of oil or gas from the tubing string  222  into the production passageway  230 .  
         [0049]    In a similar manner as described for FIG. 1, the tubing hanger  218  also includes an annulus bore  280  which extends between the upper end and lower end of the tubing hanger  218 . In this manner, communication between the tubing annulus  224  and the upper end of tubing hanger  218  is provided by the annulus passageway  32 , the workover passageway  36 , and the annulus bore  280 . This arrangement permits communication between tubing annulus  224  and area  286  and also a choke and kill line in a BOP with tree cap  220  removed.  
         [0050]    The flow completion apparatus  210  may also comprise a production master valve  244  and a production wing valve  246  to control flow through the production outlet  230 , and an annulus master valve  248 , an annulus wing valve  250  and a workover valve  252  to control flow through the annulus passageway  232 , the annulus outlet  234  and the workover passageway  236 , respectively. While these valves may be any suitable closure members, they are preferably remotely operated gate valves. Moreover, some or all of the valves may be incorporated into the body of the tubing spool  214 , into separate valve blocks which are bolted onto the tubing spool, or into individual valve assemblies which are connected to their respective outlets or passageways in the tubing spool with separate lengths of conduit. Furthermore, the production outlet  230  and the annulus outlet  234  are preferably connected to respective flow loops which communicate with a surface vessel, either directly or via a manifold, in a manner that is well known in the art. See U.S. Pat. No. 5,372,199, hereby incorporated herein by reference.  
         [0051]    In the production mode of operation of the flow completion apparatus  210 , shown in FIG. 10, a first barrier between the well bore and the environment is provided by the closure member  242  production seals  238 ,  240 , and the tubing annulus seal  257 , which together serve to isolate the fluid in the wellbore from the environment above the tubing hanger. The second barrier is provided by the tree cap  220  and by a typically metal seal ring  254  which is disposed between the tree cap  220  and the tubing spool  214  and a wireline plug  256  which is positioned in an axial bore  258  extending through the tree cap. Thus, in the completion assembly  210 , the first barrier is associated with the tubing hanger  218  while the second barrier is associated with the tree cap  220 . This embodiment allows the removal of tubing hanger  218  while leaving tubing string and tubing suspension conduit  219  in tubing spool  214 .  
         [0052]    The embodiments described above assume a requirement for double barriers. It should be appreciated that one of the barriers may be eliminated should only one barrier be required in a particular jurisdiction.  
         [0053]    It should be recognized that, while the present invention has been described in relation to the preferred embodiments thereof, those skilled in the art may develop a wide variation of structural and operational details without departing from the principals of the invention. For example, the various elements shown in the different embodiments may be combined in a manner not illustrated above. Therefore, the appended claims are to be construed to cover all equivalents falling within the true scope and spirit of the invention.