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You are an expert at summarizing long articles. Proceed to summarize the following text: 
FIELD OF THE INVENTION 
       [0001]    This technique relates in general to gate valves, and in particular to a gate valve having shearing surfaces independent and apart from sealing surfaces to maintain post-shear seal integrity. 
       BACKGROUND OF THE INVENTION 
       [0002]    A production bore intervention valve may be used on a subsea oil and gas work over riser system. The work over riser system provides safe access to the production bore during well intervention activities. The intervention valve provides a means to control well bore fluids and also to allow the platform to disconnect from the well during an emergency situation. Closure of the valve may be required when downhole tooling such as coiled tubing or wireline are still running through the valve such that the valve must be capable of severing these tools to allow the valve to close, while maintaining its function as a fluid barrier. 
         [0003]    The action of shearing any downhole tooling requires significant loads and very high localized stresses on the gates and seats involved. This stressing can lead to damage or general degradation of the surface finish on any of the sealing faces of the gates and seats adjacent to the edges where this shearing occurs. The sheared downhole tooling, particularly the plug of tooling that becomes trapped in the through hole of the gate, can also cause surface damage as its cut faces are dragged across the sealing faces of the seats as the valve travels to the fully closed position. This surface damage to the sealing faces of the gate and the seat can cause sealing problems when the valve is closed, leading to leakage. 
         [0004]    A need exists for a technique that eliminates or reduces damage and general degradation of the surface finish of any sealing faces of the gates and to the seats adjacent to the edges where shearing occurs. The following technique may solve one or more of these problems. 
       SUMMARY OF THE INVENTION 
       [0005]    In an embodiment of the present technique, a gate valve has a valve body with a flow passage and a central chamber therein. A gate has a hole therein, and is linearly movable within the chamber between a closed position blocking flow through the flow passage and an open position in which the hole registers with the flow passage. First and second valve seats are mounted in the chamber on opposite sides of the gate at the intersection of the flow passage with the central chamber. The valve seats have sealing surfaces that engage the gate when the gate is in the closed position. A shear element is connected to and carried by at least one of the valve seats. The shear element is capable of movement relative to the at least one of the valve seats in response to movement of the gate from the open position to the closed position. 
         [0006]    In an embodiment of the present technique, a gate valve has a valve body with a flow passage and a central chamber therein. A gate has a hole therein, and is linearly movable within the chamber between a closed position blocking flow through the flow passage and an open position in which the hole registers with the flow passage. First and second valve seats are mounted in the chamber on opposite sides of the gate at intersections of the flow passage with the central chamber. The valve seats have sealing surfaces that engage the gate when the gate is in the closed position. A shear element is a partial segment of a circle connected to and carried by each of the valve seats. The shear elements protrude beyond the sealing surfaces of the valve seats in an extended position when the gate is in the closed position. The shear elements are capable of axial movement relative to an axis of the hole in gate to a retracted position in response to movement of the gate from the open position to the closed position. 
         [0007]    In an embodiment of a method of the present technique, a gate is mounted within a valve body having valve seats, a central chamber, and a flow passage extending therefrom. The gate has a hole therein, and a shear element is mounted to at least one of the valve seats with the shear element in an extended position. The gate is moved from an open position toward a closed position, which causes the shear element to move to a retracted position. The gate is engaged with the valve seats, thereby sealing the flow passage. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    So that the manner in which the features and benefits of the technique, as well as others which will become apparent, may be understood in more detail, a more particular description of the technique briefly summarized above may be had by reference to the embodiments thereof which are illustrated in the appended drawings, which form a part of this specification. It is also to be noted, however, that the drawings illustrate only various embodiments of the technique and are therefore not to be considered limiting of the technique&#39;s scope as it may include other effective embodiments as well. 
           [0009]      FIG. 1  is a sectional view illustrating a gate valve that has features constructed in accordance with this technique in an open position. 
           [0010]      FIG. 2  is an enlarged view of the gate and seat portion of  FIG. 1 , showing the gate in an open position. 
           [0011]      FIG. 3  is a sectional view illustrating a gate valve that has features constructed in accordance with this technique in a closed position. 
           [0012]      FIG. 4  is an enlarged view of the gate and seat portion of  FIG. 3 , showing the gate in a closed position. 
           [0013]      FIG. 5  is a perspective view of the valve seat and shear element of the gate valve of  FIG. 1 . 
           [0014]      FIG. 6  is a perspective view of the gate of the gate valve of  FIG. 1 . 
           [0015]      FIG. 7  is an enlarged view of the gate valve of  FIG. 1 , showing the gate in an open position and one embodiment of how the shear elements are retained. 
           [0016]      FIG. 8  is an enlarged view of the gate valve as shown in  FIG. 7 , showing the gate in a closed position. 
           [0017]      FIG. 9  is an enlarged view of the gate valve of  FIG. 1 , showing the gate in an open position and a second embodiment of how the shear elements are retained. 
           [0018]      FIG. 10  is an enlarged view of the gate valve as shown in  FIG. 9 , showing the gate in a closed position. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    The present technique now will be described more fully hereinafter with reference to the accompanying drawings in which a preferred embodiment of the technique is shown. This technique may, however, be embodied in many different forms and should not be construed as limited to the embodiment set forth herein; rather, this embodiment is provided so that this disclosure will be thorough and complete, and will fully convey the scope of the technique to those skilled in the art. Like numbers refer to like elements throughout. 
         [0020]    Referring to  FIG. 1 , a valve  11  is a standard gate valve except for features in accordance with this invention. The valve  11  has a body  13 , a base  15 , a bonnet  17 , and a flow passage  19  that extends transversely through the body  13 . As illustrated, coiled tubing  20 , or in other embodiments, wire line, may extend through the flow passage  19 . The valve  11  has a gate  21  with a hole  23  therethrough. The gate  21  is located in a sealed chamber  25  in the body  13  and is shown in the open position in  FIG. 1 . The gate  21  is connected to an actuation device  27  by a stem  29  that extends between the gate  21  and the actuation device  27 . In this embodiment, the actuation device  27  is an actuator piston  31  that connects to the stem  29  to stroke the gate  21  between its open and closed positions. Also shown in  FIG. 1  are ring-shaped valve seats  33  mounted in the body  13 , which have holes  34  that register with the flow passage  19  of the valve  11 . Each valve seat  33  is fitted with a shear element  35 . In an embodiment, each shear element  35  may be either a complete circumferential ring or a partial section thereof. In other embodiments, the shear element  35  may have other shapes, for example, the shear element  35  may be rectangular or may have a variable cross section. In the embodiments illustrated, shear element  35  is a partial section of a ring. The shear elements  35  may be either releasably or movably connected to the valve seats  33 . 
         [0021]    When the gate  21  is in the open position ( FIGS. 1 and 2 ), the hole  23  of the gate  21  registers with the flow passage  19  of the valve  11 , thereby allowing flow through the valve  11 . When the gate  21  is closed ( FIGS. 3 and 4 ), the hole  23  no longer registers with the flow passage  19 . Instead, the solid portion  36  of the gate  21  registers with the flow passage  19  and comes into contact with the seats  33 . 
         [0022]    Referring to  FIGS. 2 and 4 , each seat  33  rests in a recess  37  formed in the valve body  13 . Each seat  33  has a flat sealing face  39  that interfaces with the gate  21  when the gate  21  is in the closed position to create a seal. The sealing face  39  is located outboard of the shear elements  35 . Each shear element  35  rests in a profile  40  on each valve seat  33  and projects beyond the face  39  of each seat  33  when in an extended position. The profile  40  appears on only one side of the hole  34 , as illustrated in  FIG. 5 . The upper and lower sides of the gate  21  are profiled with arcuate recesses  41  such that when the gate  21  is in the open position, the gate  21  does not contact the shear elements  35 . The upper and lower sides of the gate  21  have acutely angled surfaces  43  on their closing sides that are adapted to contact the shear elements  35  and force them apart axially relative to the axis of the valve seat hole  34  when the gate  21  is moved from an open position as illustrated in  FIGS. 1 and 2  to a closed position as illustrated in  FIGS. 3 and 4 .  FIG. 5  further illustrates the valve seat  33  having a hole  34  extending therethrough and the shear element  35  resting in the profile  40  and projecting beyond the sealing face  39  as it is shown in the extended position. The outer edges of the valve seat  33 , outboard of the sealing face  39 , are tapered.  FIG. 6  further illustrates the gate  21  with the hole  23  extending therethrough and the recess  41  and acutely angled surface  43  opposite one another with the solid or main sealing portion  36  being adjacent to the hole  23 . The recesses  41  are not annular and in the illustrated embodiment, extend only about forty-five (45) degrees. 
         [0023]    As previously mentioned, the shear elements  35  may be either rigidly or flexibly connected to the valve seats  33 . In an embodiment illustrated in  FIGS. 7 and 8 , the shear elements  35  are flexibly connected to the valve seats  33 . While the upper and lower shear elements  35  operate in the same manner, only the operation of the upper shear element  35  will be discussed for illustration purposes. The shear element  35  has a window  53  and an elongated aperture  55  extending through an inner surface  57 . A retaining screw  59  passes through the window  53  and the aperture  55  in the shear element  35  and into the valve seat  33 , thereby connecting the shear element  35  to the valve seat  33 . The window  53  and the aperture  55  allow the shear element  35  to move relative to the retaining screw  59  and the valve seat  33 . A resilient member  61  extends between the shear element  35  and the valve seat  33  and acts to bias the shear element  35  in an extended position relative to the valve seat  33 . In an embodiment, the resilient member  61  is a spring that is positioned in a recessed pocket  63  of the valve seat  33 . In other embodiments, the resilient member  61  may be an insert made from an elastic material. An upward facing shoulder  62  is formed in the valve seat  33 . A stop tab  64  extends radially outward from the shear element  35  and is adapted to engage the upward facing shoulder  62  to limit the travel of the shear element  35  relative to the axis of the valve seat hole  34 . As illustrated, the shear element  35  moves axially relative to the axis of the valve seat hole  34  beyond the sealing face  39  of the valve seat  33  as the spring  61  biases the shear element  35  to an extended position ( FIG. 7 ). The profiles  41  in the gate  21  are such that the shear elements  35  do not contact the gate  21  in their extended position. In the extended position, the shear elements  35  are closer to each other than in the retracted position. However, the shear elements  35  do not contact the sealing surfaces of the gate  21  while in the extended position and the gate  21  is in the open position. While the gate  21  is being closed and moving from the open position to the closed position, the angled surfaces  43  push the shear elements  35  to the retracted position. 
         [0024]    In an embodiment illustrated in  FIGS. 9 and 10 , shear elements  65  are rigidly, but releasably connected to the valve seat  33 . While the upper and lower shear elements  65  operate in the same manner, only the operation of the upper shear element  65  will be discussed for illustration purposes. The shear element  65  has an aperture  67  extending through an inner surface  69 . A shear pin  71  passes through the aperture  67  in the shear element  65  and into the valve seat  33 , thereby rigidly connecting the shear element  65  to the valve seat  33 . A void  73  exists between the shear element  65  and the valve seat  33  when the gate  21  is in the open position ( FIG. 9 ), and allows for the shear element  65  to move axially relative to the axis of the valve seat hole  34  when the gate  21  is moved to a closed position ( FIG. 10 ), thereby shearing the shear pin  71 . As illustrated, the shear element  65  projects beyond the flat sealing face  39  of the valve seat  33  when the gate  21  is in an open position. The profile  41  in the gate  21  is such that the shear element  65  does not contact the gate  21  when the gate  21  is in an open position. 
         [0025]    In operation, while in the open position shown in  FIG. 1 , fluid flows unrestricted through the flow passage  19 . As further illustrated, in an embodiment, tubing  20  extends through the flow passage  19  unrestricted. In the closed position shown in  FIG. 4 , the downstream side of the gate  21  will seal against the sealing face  39  of the downstream valve seat  33 . 
         [0026]    In the event of an emergency situation, closure of the valve  11  may be required when downhole tooling such as coiled tubing or wireline are still running through the valve  11 . When the valve  11  is required to prevent the flow of fluid, the actuation device  27  strokes the stem  29 , which in turn strokes the gate  21  in a direction transverse to the flow passage  19 , from an open position to a closed position. The side of the gate passage  23 , opposite the shear elements  35 , pushes the tubing  20  laterally into contact with the extended shear elements  35 . 
         [0027]    As the gate  21  moves from the open position shown in  FIGS. 1 and 2  to the closed position shown in  FIGS. 3 and 4 , shear elements  35  will shear tubing  20 , and then, as the gate  21  approaches the fully closed position, the acutely angled surfaces  43  on either side of the gate  21  contact the shear elements  35  and force the shear elements  35  apart, along the axis of the flow passage  19 . The sealing faces on the solid portion  36  of the gate  21  interface directly with the sealing faces  39  on the either valve seat  33 . As the hole  23  in the gate  21  no longer aligns with the holes  34  in the valve seats  33 , the flow of fluid is restricted and a pressure differential across the gate  21  is created, which in turn creates a sealing contact stress between the sealing face  36  on the gate  21  and the sealing face  39  on the valve seat  33 . 
         [0028]    In further explanation and with respect to the embodiment illustrated in  FIGS. 7 and 8 , as the gate  21  approaches the fully closed position after shearing tubing  20 , the acutely angled surfaces  43  on either side of the gate  21  contact the shear elements  35 . While the upper and lower shear elements  35  operate in the same manner, only the operation of the upper shear element  35  will be discussed for illustration purposes. The angled surface  43  contacts the shear element  35  and causes the shear element  35  to move upward relative to the valve seat  33 . The force the gate  21  applies to the shear element  35  causes the resilient member  61  to compress and for the shear element  35  to move upward to such point that the retaining screw  59  is positioned in the bottom of the window  53  and elongated aperture  35 . The ability of the shear element  35  to move axially upward relative to the valve seat  33  allows the gate  21  to shear the tubing  20  on the thinner profiled section on the hole  23 , but to seal on the main solid section  36  of the gate  21 . As a result, the shear element  35  shears, but is not involved in the sealing. This technique also produces a plug of tubing  75  ( FIG. 4 ) that is shorter than the gap between the opposing sealing faces  39  on the two valve seats  33  such that there is no interference between the tubing plug  75  and the valve seats  33  that may cause further/additional surface damage to the valve seats  33 . 
         [0029]    With respect to the embodiment illustrated in  FIGS. 9 and 10 , as the gate  21  approaches the fully closed position after shearing tubing  20 , the acutely angled surfaces  43  on either side of the gate  21  contact the shear elements  65 . While the upper and lower shear elements  65  operate in the same manner, only the operation of the upper shear element  65  will be discussed for illustration purposes. The angled surface  43  contacts the shear element  65  and causes the upper shear element  65  to move axially upward relative to the valve seat  33 . The force the gate  21  applies to the shear element  65  causes the shear pin  71  to shear, which allows the shear element  65  to move upward to fill the void  73  between the shear element  65  and the valve seat  33 . The ability of the shear element  65  to move axially upward relative to the valve seat  33  allows the gate  21  to shear the tubing  20  on the thinner profiled section around the hole  23 , but to seal on the main solid section  36  of the gate  21 . As a result, the shear element  65  shears, but is not involved in the sealing. This technique also produces a plug of tubing  75  ( FIG. 4 ) that is shorter than the gap between the opposing sealing faces  39  on the two valve seats  33  such that there is no interference between the tubing plug  75  and the valve seats  33  that may cause further/additional surface damage to the valve seats  33 . 
         [0030]    In the drawings and specification, there have been disclosed a typical preferred embodiment of the technique, and although specific terms are employed, the terms are used in a descriptive sense only and not for purposes of limitation. The technique has been described in considerable detail with specific reference to these illustrated embodiments. It will be apparent, however, that various modifications and changes can be made within the spirit and scope of the technique as described in the foregoing specification and as set forth in the following claims.

Summary:
A gate valve capable of shearing through any downhole tooling and maintaining its seal capability after the shearing operation is complete. The gate valve has a body with a flow passage therein and a gate being linearly movable between a closed position blocking flow through the flow passage and an open position. First and second valve seats are mounted on opposite sides of the gate and have sealing surfaces that engage the gate when the gate is in the closed position. Shear elements are connected to and carried by each of the valve seats and are capable of axial movement relative to the valve seats in response to movement of the gate from the open position to the closed position.