Abstract:
A tubing hanger having two actuators  34, 70  disposed at opposite ends of a valve gate  28,  to provide pressure balancing. Each actuator has a piston  36  acted on by a hydraulic fluid supplied to sealed spaces  54, 60  through ports  62, 66, 76, 74.  Ports  66, 74  may be linked to prevent the two pistons from being forced apart by line pressure leaking from the passage  12  into cavity  72.  A single actuator may be used for low pressure applications. A bias spring  68  provides fail safe closure of the valve gate  28.

Description:
FIELD OF THE INVENTION 
     This invention concerns the control of annulus fluid flows in subsea oil or gas wells, and gate valves for this and other uses. 
     BACKGROUND OF THE INVENTION 
     Tubing hangers for use with conventional (i.e. non-horizontal) subsea Christmas trees are known which have an annulus passage for providing fluid communication between the tubing annulus and the tree. Current methods of controlling annulus flow in this passage include: 
     (a) setting or removing wireline plugs in the passage; 
     (b) use of an annular sliding sleeve that is arranged to block or allow fluid access to the passage; 
     (c) a wireline actuated shuttle valve provided in the passage; 
     (d) an hydraulically actuated shuttle valve provided in the passage; 
     (e) hydraulically or electrically operated ball valves provided in the passage. 
     Methods (a) and (c) require wireline access to the passage. Due to reliability problems with the remote actuators concerned, it has been common to additionally provide secondary wireline actuation capability in at least method (d). Wireline valve actuation or plug is itself time consuming and unreliable, especially at increased water depths. Moreover, providing the required wireline access means that the tubing hanger annulus passage and the corresponding annulus conduit in the tree cannot be unduly convoluted or offset from the wellhead center line. These design constraints will generally enlarge the tubing hanger and tree needed for a given production bore diameter. As maximum tree weight is limited by the supply vessel crane capacity, providing tubing hanger annulus passage wireline accessibility will reduce the maximum possible production bore diameter. 
     An alternative approach to the control of tubing annulus fluid flows is to provide a flow loop bypassing the tubing hanger and containing suitable flow control valves. This method is used in horizontal trees as well as some conventional completion designs, for example some tubing head completions. 
     British Patent Reference GB 2287263 (FMC) concerns a tubing hanger having an annulus bore closeable by a rotatable disc disposed horizontally so as to intersect with the production bore. 
     European Patent Reference EP 0624711 (Cooper Cameron) discloses a tubing hanger having a central bore opened and closed by a gate valve. Opposed valve actuators are located on the outside of a wellhead into which the tubing hanger is run. Actuating stems extending from the actuators push the valve gate between the open and closed positions. 
     There is thus a need for a remotely operable, high integrity tubing annulus isolation seal system, capable of selectively sealing or providing fluid communication with the tubing annulus, preferably without the need for wirelines. It would be of further advantage if this isolation seal system could be applied to either conventional or horizontal trees, providing greater standardization design between tree types. 
     SUMMARY OF THE INVENTION 
     Accordingly the invention provides a tubing hanger having a flow passage for well fluids; a cavity contained within the tubing hanger and intersecting with the flow passage; a valve gate linearly movably received in the cavity and containing a through bore, and an actuator substantially wholly contained in the cavity arranged to move the valve gate between a position in which the through bore is aligned with the flow passage to permit fluid flow through the flow passage and through bore, and a position in which the through bore and flow passage are out of alignment so that the valve gate seals the flow passage. 
     Thus, the flow passage (preferably a tubing annulus passage) in the tubing hanger is effectively furnished with a gate valve, which is the preferred oil industry flow control valve, having a proven record of high sealing integrity and long term reliability. The valve gate also provides shearing capability for wirelines, coiled tubing or other objects that may be lowered through the flow passage. The tubing hanger may be for use in a conventional wellhead in conjunction with a conventional tree, or for use in a horizontal tree. 
     Preferably the actuator has a piston received within the cavity so as to define, in conjunction with the cavity, an enclosed space to which hydraulic fluid may be supplied to move the piston. Apart from any necessary hydraulic service line penetrations, the tubing hanger and its integral gate valve and valve actuator will thus be entirely independent of the wellhead housing; readily installable in or retrievable from the wellhead housing as a self-contained unit. 
     The actuator may comprise a plug which closes an outer end of the cavity. The piston may be coupled to the valve gate, for example by a pin and slot connection, allowing limited float in the valve gate in the direction of the tubing annulus passage axis. A pair of such pistons may be provided, opposed and substantially identical, with one at each end of the gate, to define a pressure balanced, double acting actuator system. One or both parts of the flow passage on either side of the valve gate may be provided with a seat pocket containing a floating valve seat for sealing co-operation with the adjacent face of the valve gate. The valve gate is preferably resiliently biased to provide fail safe closure. 
     Further preferred features are described below in connection with illustrative embodiments of the invention shown in the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic part section through a tubing hanger according to the invention, showing a preferred valve gate and actuators; 
     FIG. 2 is a perspective view of the valve gate shown in FIG. 1; 
     FIG. 3 is a perspective view of a piston shown in FIG. 1; 
     FIG. 4 is a sectional view of the seat/gate interface taken on line IV—IV in FIG. 1, but showing the valve gate in the open position; 
     FIG. 5 is a diagrammatic, part sectional plan view of a tubing hanger embodying the invention, for use with a conventional tree; 
     FIG. 6 is a diagrammatic, part sectional side view of the tubing hanger of FIG. 5; 
     FIG. 7 is a diagrammatic, part sectional side view of a tubing hanger embodying the invention, for use with a horizontal tree; and 
     FIG. 8 is a fluid circuit diagram showing external valves for use with the tubing hanger of FIG.  7 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIGS. 1-4, a tubing hanger  10  includes a flow passage  12  (hereinafter “tubing annulus passage”) in communication with a tubing annulus. A cavity  14  of slightly larger diameter than the tubing annulus passage  12  is bored transversely through the tubing hanger  10  so as to intersect the tubing annulus passage  12 . Seat pockets  16 ,  18  are formed in the tubing annulus passage  12  adjacent to the cavity  14  for reception of floating valve seats  20 ,  22 . Wave springs  24 ,  26  at the bottoms of the pockets  16 ,  18  bias the seats into sealing engagement with opposed faces of a valve gate  28  received in the cavity  14 . Sealing rings  30 ,  32  seal the seats  20 ,  22  in the pockets  16 ,  18 . 
     One end of the valve gate  28  is connected to an actuator  34  in the form of a piston  36  received in the cavity  14 . A stub shaft  38  projects from the inner end of the piston  36  and is cross bored for reception of a connecting pin  40 . The projecting end of the stub shaft  38  is loosely received in a socket  42  formed in the end of the gate  28 , with the ends of the connecting pin  40  received in slots  44  cut transversely through the gate  28  so as to intersect the socket  42 . In this way the gate  28  is attached to the piston  36 . The slots  44  are elongated in the direction of the tubing annulus passage axis, to allow limited floating movement of the gate  28  in that direction, as required for proper sealing with the seats  20 ,  22 . 
     The cavity  14  is stepped at  46  to define a relatively small diameter inner portion and a relatively larger diameter outer portion. The piston  36  is similarly stepped at  48  to define a relatively large diameter outer end and a relatively small diameter inner end. The larger end of the piston  36  is received in the larger diameter portion of the cavity  14  and sealed to it by a seal ring  50 . The smaller inner end of the piston  36  is received in the smaller diameter portion of the cavity  14  and sealed to it by a seal ring  52 . A sealingly enclosed annular space  54  is thereby defined between the steps  46  and  48 . 
     The outer end of the cavity  14  behind the piston  36  is sealed by a screw threaded plug  56  and seal ring  58 , so as to define a further enclosed space  60 . Hydraulic fluid may be supplied to the space  54  through a port  62 , so as to move the piston  36  and gate  28  to the left as viewed in FIG.  1 . This brings a through bore  64  in the gate  28  into alignment with the tubing hanger annulus passage  12 , opening it for fluid flow. Hydraulic fluid may also be supplied to the space  60  through a port  66 , moving the piston  36  and gate  28  to the right, into the position shown in FIG. 1, where the tubing annulus passage is closed by a solid portion of the gate  28 . A coil spring  68  is used to provide fail safe closure bias for the gate  28 . 
     The foregoing arrangement is satisfactory for flow control with low tubing annulus pressures. However, higher tubing annulus line pressures supplied through the passage  12  will in the usual way leak past the upstream valve seat to act on the inner end of the piston  36  in the valve cavity  72 , requiring higher hydraulic pressures at the port  66  in order to close the valve gate  28 . Such high line pressures may completely overcome the force of the spring  68 , so that the gate is no longer fail safe closing. To solve these problems, a pressure balanced system is preferably provided. As shown in FIG. 1, this is achieved by attaching a further actuator  70  to the valve gate  28 , on the opposite side of the tubing annulus passage to the actuator  34 . Actuators  34  and  70  and their attachments to the gate  28  are substantially identical, save that actuator  70  does not have a bias spring  68 . In FIG. 1, actuator  34  and plug  56  are shown in section, whereas actuator  70  and plug  57  are shown in elevation. Equal and opposite line pressures act on the respective pistons of the actuators  34  and  70 , thereby cancelling each other out. The valve gate  28  is opened by hydraulic fluid supplied to port  62  of actuator  34  and is closed by hydraulic fluid supplied to a corresponding port  76  of actuator  70 . 
     To relieve tension imposed on the gate-to-piston connections (including the pin  40  and stub shaft  38  and the corresponding components of actuator  70 ) caused by the line pressure, the port  66  of actuator  34  may be directly connected to a corresponding port  74  of the actuator  70 . The closed system thereby created is filled with substantially incompressible hydraulic fluid, preventing the pistons of the respective actuators from being forced apart by the line pressure. The stub shaft  38  and pin  40  of each actuator and the socket  42  and slots  44  in the valve gate  28  may therefore be omitted if desired. Then each piston simply pushes on the gate and is returned to a centralized position by the closed hydraulic circuit comprising the ports  66  and  74 . Alternatively, if such tension relief for the valve gate/actuator connections is not required, the ports  62 ,  66 ,  74 ,  76  may be used in various readily apparent combinations to move the valve gate as desired. The four ports used in this way provide redundancy or backup capability (port  74  backing up port  62  and port  66  backing up port  76 ), further increasing the safety and reliability of the tubing annulus isolation system. 
     Thus the invention may be used to provide a tubing hanger with one or more integral gate valves. FIGS. 5 and 6 show the positioning of such a valve  80  for tubing annulus isolation in a conventional (i.e. non-horizontal) tubing hanger  82  having a concentric production bore  84  and highly radially offset tubing annulus conduit  12 . 
     FIG. 7 is a sectional view corresponding to FIG. 6, but shows a tubing hanger  86  for a horizontal tree, for example a high pressure horizontal tree. It includes a concentric, vertical production bore  88 , closed above a horizontal production wing branch  90  by a plug  92 . Penetrations  94  are provided for downhole service lines. In contrast to usual horizontal tree tubing hanger designs, a tubing annulus passage  96  is provided, extending vertically through the tubing hanger  86 . A pair of gate valves  98 ,  100 , formed substantially as described with reference to FIGS. 1-5, are provided in the passage  96 , on either side of an annulus wing branch  102 . These perform the functions of the annulus master valve and annulus access valve respectively, of known horizontal trees. 
     As shown in FIG. 8, the tubing hanger  86  is housed in a horizontal tree  104 . Annular seals  106 ,  108 ,  110  isolate the production and annulus wing branches  90 ,  102  and connect them to corresponding annulus wing  112  and production wing  114  conduits terminating in the tree  104 . The top of the tree above the tubing hanger  86  is sealed by an internal cap  116  in the usual way. The production wing conduit  114  contains a production master valve  122  and a production wing valve  124 . One end of a crossover conduit  118  containing a crossover valve  120  is joined to the production wing conduit  114 , between the valves  122 ,  124 . The other end of the crossover conduit  118  is joined to the annulus wing conduit  112 , inwardly of an annulus wing valve  126 . 
     Although the production master valve  122  is shown to be internal to the tree block and the remaining valves  120 ,  124 ,  126  external to the tree block, any of these valves may be positioned either internally or externally as desired, as is well known. These valves are preferably remotely operable gate valves of the kind commonly used in subsea completions.