Abstract:
A flapper valve assembly ( 120 ) for controlling fluid flow therethrough is disclosed. The flapper valve assembly ( 120 ) comprises a tubular valve housing having a valve chamber. A valve seat ( 124 ) is mounted within the housing. The valve seat ( 124 ) has a valve seat sealing surface ( 126 ). The valve seat ( 124 ) also has an internal load bearing shoulder ( 134 ). A flapper closure plate ( 122 ) is rotatably disposed within the valve chamber. The flapper closure plate ( 122 ) is rotatable between a valve open position in which the flapper closure plate ( 122 ) is removed from the valve seat ( 124 ) and a valve closed position in which the sealing surface ( 128 ) of the flapper closure plate ( 122 ) sealingly engages the valve seat sealing surface ( 126 ) for preventing flow through the flapper valve assembly ( 120 ). The maximum travel of the flapper closure plate ( 122 ) in the closed position is defined by the internal load bearing shoulder ( 134 ) of the valve seat ( 124 ).

Description:
This application is a division of application Ser. No. 09/309,716 filed on May 11, 1999 now U.S. Pat. No. 6,196,261. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     This invention relates in general to subsurface safety valves and, in particular, to a subsurface safety valve that includes a flapper closure plate for controlling fluid flow therethrough having a maximum travel position defined by a load bearing shoulder of the flapper seat. 
     BACKGROUND OF THE INVENTION 
     Without limiting the scope of the invention, the background will describe surface controlled, subsurface safety valves, as an example. 
     Surface controlled, subsurface safety valves are commonly used to shut in oil and gas wells in the event of a failure or hazardous condition at the well surface. Such safety valves are typically fitted into the production tubing and operate to block the flow of formation fluid upwardly therethrough. The subsurface safety valve provides automatic shutoff of production flow in response to a variety of out of range safety conditions that can be sensed or indicated at the surface. For example, the safety conditions include a fire on the platform, a high or low flow line temperature or pressure condition or operator override. 
     During production, the subsurface safety valve is typically held open by the application of hydraulic fluid pressure conducted to the subsurface safety valve through an auxiliary control conduit which extends along the tubing string within the annulus between the tubing and the well casing. Flapper type subsurface safety valves utilize a closure plate which is actuated by longitudinal movement of a hydraulically actuated, tubular piston. The flapper valve closure plate is maintained in the valve open position by an operator tube which is extended by the application of hydraulic pressure onto the piston. A pump at the surface pressurizes a reservoir which delivers regulated hydraulic control pressure through the control conduit. Hydraulic fluid is pumped into a variable volume pressure chamber and acts against the crown of the piston. When, for example, the production fluid pressure rises above or falls below a preset level, the control pressure is relieved such that the piston and operator tube are retracted to the valve closed position by a return spring. The flapper plate is then rotated to the valve closed position by a torsion spring or tension member, 
     In conventional subsurface safety valves of the type utilizing an upwardly closing flapper plate, the flapper plate is seated against an annular sealing face, either in metal-to-metal contact or metal against an annular elastomeric seal. In one design, the flapper closure plate has a flat, annular sealing face which is engagable against a flat, annular valve seat ring, with sealing engagement being enhanced by an elastomeric seal ring which is mounted on the valve seat. In another design, the valve seat includes a downwardly facing, conical segment having a sloping sealing surface and the flapper closure plate has a complementary, sloping annular sealing surface which is adapted for surface-to-surface engagement against the conical valve seat surface. 
     Typically, the flapper closure plate is supported for rotational movement by a hinge assembly which includes a hinge pin and a torsion spring or tension member. It will be appreciated that structural distortion of the flapper valve closure plate, or damage to the hinge assembly which supports the flapper closure plate, can cause misalignment of the respective sealing surfaces, thereby producing a leakage path through the safety valve. 
     Such misalignment will prevent correct seating and sealing of the flapper closure plate, and a large amount of formation fluid may escape through the damaged valve, causing waste and pollution. During situations involving damage to the wellhead, the well flow must be shut off completely before repairs can be made and production resumed. Even a small leak through the flapper safety valve in a gas well can cause catastrophic damage. 
     Attempts have been made to overcome this misalignment problem. For example, one design involves the use of a valve seat and an upwardly closing flapper plate each having a sealing surface with a matched spherical radius of curvature. That is, the valve seat is a concave spherical segment and the sealing surface of the flapper plate is a convex spherical segment. In this arrangement, the spherical radius of curvature of the concave valve seat spherical segment is matched with the spherical radius of curvature of the convex spherical segment which defines the sealing surface on the flapper plate. The matching spherical surfaces are lapped together to provide a metal-to-metal seal along the interface between the nested convex and concave sealing surfaces. 
     As such, the convex spherical sealing segment of the flapper plate is received in nesting engagement within the concave spherical segment surface of the valve seat, which allows some angular displacement of the flapper plate relative to the valve seat without interrupting surface-to-surface engagement therebetween. Thus, the concave spherical seating surface of the safety valve seat will tolerate a limited amount of misalignment of the flapper plate which might be caused by structural distortion of the closure plate or warping of the hinge assembly. 
     It has been found, however, the even when using spherical sealing surfaces leakage may occur. Specifically, applications using large diameter tubing and having a low ratio between the outer diameter and the inner diameter of the sealing surfaces, distortion of the flapper closure plate caused by increased loads on the flapper closure plate may result in a loss of the seal. These increased loads are developed as a consequence of using larger safety valves having larger flapper closure plates in larger tubing. 
     Therefore, a need has arisen for a flapper valve that maintains a seal in a well requiring a large diameter flapper valve having a low ratio between the outer diameter and the inner diameter of the sealing surfaces. A need has also arisen for such a flapper valve that does not experience a loss of the seal in response to distortion of the flapper closure plate caused by the increased loads associated with such designs. 
     SUMMARY OF THE INVENTION 
     The present invention disclosed herein comprise a flapper valve that maintains a seal in a well requiring a large diameter flapper valve having a low ratio between the outer diameter and the inner diameter of the sealing surfaces. The flapper valve of the present invention does not experience a loss of the seal in response to distortion of the flapper closure plate caused by the increased loads associated with such applications. 
     The flapper valve assembly of the present invention comprises a tubular valve housing having a valve chamber. A valve seat is mounted within the housing. The valve seat has an internal load bearing shoulder or surface. A flapper closure plate is disposed within the valve chamber and rotates between a valve open position, in which the flapper closure plate is removed from the valve seat, and a valve closed position, in which the sealing surface of the flapper closure plate sealingly engages the valve seat sealing surface for preventing flow therethrough. When the flapper closure plate is in the valve closed position, the internal load bearing shoulder of the valve seat defines the maximum travel of the flapper closure plate. 
     The internal load bearing shoulder may be machined as an integral part of the valve seat. Alternatively, the valve seat of the flapper valve assembly of the present invention may include a seal ring insert. The seal ring insert may comprise a material having a hardness greater than that of the valve seat. The seal ring insert may be a solid ring. Alternatively, the seal ring may be a machined weld bead. In either case, the seal ring insert forms a portion of the valve seat sealing surface and the internal load bearing shoulder. 
     The flapper valve assembly of the present invention may, in addition to having the internal load bearing shoulder or as an alternative to having the internal load bearing shoulder, utilize a valve seat having an external load bearing surface. In this embodiment, the flapper closure plate includes a ballast member extending from the end of the flapper closure plate opposite the pivot pin, such that the external load bearing surface of the valve seat and the ballast member of the flapper closure plate defines the maximum travel of the flapper closure plate. 
     The flapper valve assembly of the present invention may, for example, be incorporated into a subsurface safety valve that is adapted to be placed in a well tubing string to control flow therethrough. In this case, the flapper valve assembly is disposed within a housing. An operator tube is movable within the bore of the housing for controlling movement of the flapper closure plate. A tubular piston is movably mounted within the housing that is designed for longitudinal extension and retraction. The piston is coupled to the operator tube for extending the operator tube relative to the flapper closure plate so that the flapper closure plate may be operated between the valve open and valve closed positions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, including its features and advantages, reference is now made to the detailed description of the invention, taken in conjunction with the accompanying drawings of which: 
     FIG. 1 is a schematic illustration of an offshore oil or gas production platform operating a subsurface safety valve of the present invention; 
     FIGS. 2A-2B are half sectional views of a subsurface safety valve of the present invention in the valve open position; 
     FIGS. 3A-3B are half sectional views of a subsurface safety valve of the present invention in the valve closed position; 
     FIG. 4 is a cross sectional view of a flapper valve assembly of the present invention in the valve closed position; 
     FIG. 5 is a perspective view of a flapper closure plate of a flapper valve assembly of the present invention; 
     FIG. 6 is a cross sectional view of a flapper valve assembly of the present invention in the valve closed position under typical load conditions; 
     FIG. 7 is a cross sectional view of a flapper valve assembly of the present invention in the valve closed position under high load conditions; 
     FIG. 8 is a cross sectional view of a flapper valve assembly of the present invention in the valve closed position under typical load conditions; 
     FIG. 9 is a cross sectional view of a flapper valve assembly of the present invention in the valve closed position; and 
     FIG. 10 is a perspective view of a flapper closure plate of a flapper valve assembly of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the making and using of various embodiments of the present invention is 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 delimit the scope of the invention. 
     Referring to FIG. 1, a subsurface safety valve in use with an offshore oil and gas production platform is schematically illustrated and generally designated  10 . A semi-submersible platform  12  is centered over a submerged oil and gas formation  14  located below sea floor  16 . Wellhead  18  is located on deck  20  of platform  12 . Well  22  extends through the sea  24  and penetrates the various earth strata including formation  14  to form wellbore  26 . Disposed within wellbore  26  is casing  28 . Disposed within casing  28  and extending from wellhead  18  is production tubing  30 . A pair of seal assemblies  32 ,  34  provide a seal between tubing  30  and casing  28  to prevent the flow of production fluids therebetween. During production, formation fluids enter wellbore  26  through perforations  36  of casing  28  and travel into tubing  30  through sand control device  38  to wellhead  18 . Subsurface safety valve  40  is located within the production tubing  30  and seals the wellhead  18  from the well formation  14  in the event of abnormal conditions. Subsurface safety valve  40  includes a flapper valve closure plate that, during production from formation  14 , is maintained in the valve open position by hydraulic control pressure received from a surface control system  42  through a control conduit  44 . 
     Referring now to FIGS. 2A,  2 B,  3 A and  3 B, a subsurface safety valve  50  is illustrated. Safety valve  50  has a relatively larger production bore and is therefore, intended for use in high flow rate wells. Safety valve  50  is connected directly in series with production tubing  30 . Hydraulic control pressure is conducted in communication with a longitudinal bore  52  formed in the sidewall of the top connector sub  54 . Pressurized hydraulic fluid is delivered through the longitudinal bore  52  into an annular chamber  56  defined by a counterbore  58  which is in communication with an annular undercut  60  formed in the sidewall of the top connector sub  54 . An inner housing mandrel  62  is slidably coupled and sealed to the top connector sub  54  by a slip union  64  and seal  66 , with the undercut  60  defining an annulus between inner mandrel  62  and the sidewall of top connector sub  54 . 
     A piston  68  is received in slidable, sealed engagement against the internal bore of inner mandrel  62 . The undercut annulus  60  opens into a piston chamber  70  in the annulus between the internal bore of a connector sub  72  and the external surface of piston  68 . The external radius of an upper sidewall piston section  74  is machined and reduced to define a radial clearance between piston  68  and connector sub  72 . An annular sloping surface  76  of piston  68  is acted against by the pressurized hydraulic fluid delivered through control conduit  44 . In FIGS. 2A-2B, piston  68  is fully extended with the piston shoulder  78  engaging the top annular face  80  of an operator tube  82 . In this valve open position, a return spring  84  is fully compressed. 
     A flapper plate  86  is pivotally mounted onto a hinge sub  88  which is threadably connected to the lower end of spring housing  90 . A valve seat  92  is confined within a counterbore formed on hinge sub  88 . The lower end of safety valve  50  is connected to production tubing  30  by a bottom sub connector  94 . The bottom sub connector  94  has a counterbore  96  which defines a flapper valve chamber  98 . Thus, the bottom sub connector  94  forms a part of the flapper valve housing enclosure. Flapper plate  86  pivots about pivot pin  100  and is biased to the valve closed position as shown in FIGS. 3A-3B by coil spring  102 . In the valve open position as shown in FIGS. 2A-2B, the spring bias force is overcome and flapper plate  86  is retained in the valve open position by operator tube  82  to permit formation fluid flow up through tubing  30 . 
     When an out of range condition occurs and subsurface safety valve  50  must be operated from the valve open position to the valve closed position, hydraulic pressure is released from conduit  44  such that return spring  84  acts on the lower end of piston  68  which retracts operator tube  82  longitudinally through flapper valve chamber  98 . Flapper closure plate  86  will then rotate through chamber  98 . As flapper closure plate  86  nears the valve closed position within flapper valve chamber  98  where significant throttling of fluid flow occurs, the high magnitude reaction forces may distort the operator tube  82 , flapper closure plate  86  or pivot pin  100 . Moreover, the alignment of flapper plate  86  relative to valve seat  92  may be disturbed in response to slamming impact of flapper closure plate  86  against valve seat  92 . 
     Referring now to FIG. 4, a flapper valve assembly is depicted and generally designated  120 . Flapper valve assembly  120  includes a flapper closure plate  122  and a valve seat  124 . In the illustrated embodiment, the sealing surfaces of flapper closure plate  122  and valve seat  124  have mating segments which are matched in curvature to provide a metal-to-metal seal. Sealing surface  126  of valve seat  124  is a concave spherical segment. Sealing surface  128  of flapper closure plate  122  is a convex spherical segment. Convex flapper closure plate sealing surface  128  and concave valve seat sealing surface  126  are both generally a surface of revolution produced by revolving a semi-circular arc having an arc length  130  and radius of curvature  132 . As shown in FIG. 4, the radius of curvature of convex flapper closure plate sealing surface  128  is substantially equal to the radius of curvature of concave valve seat sealing surface  126 . 
     Specifically, the spherical radius of curvature of the concave valve seat sealing surface  126  is matched with the spherical radius of curvature of the convex flapper closure plate sealing surface  128 . As used herein, “matched radius of curvature” means that the radius of curvature of the flapper plate convex sealing surface  128  is substantially the same as, but not greater than, the radius of curvature of the concave valve seat sealing surface  126 . Preferably, the convex and concave surfaces are matched in curvature to provide smooth, non-binding surface engagement of convex flapper closure plate sealing surface  128  against concave valve seat sealing surface  126 . The matching convex and concave spherical surfaces  128 ,  126  are lapped together to permit close nesting engagement of flapper closure plate  122  within valve seat  124 . This arrangement permits smooth angular displacement of flapper closure plate  122  relative to valve seat  124  without interrupting surface-to-surface engagement therebetween. 
     Valve seat  124  includes an internal load bearing shoulder  134  extending generally radially inwardly from concave valve seat sealing surface  126 . As explained in more detail below, internal load bearing shoulder  134  defines the maximum travel of flapper closure plate  122  relative to valve seat  124 . 
     Referring now to FIG. 5, flapper closure plate  122  has a convex spherical sealing surface  128  and a semi-cylindrical channel  136  across the top of flapper closure plate  122  in alignment with its longitudinal axis  138 . The radial projection of flapper closure plate  122  is minimized, so that in the valve open position as shown in FIGS. 2A-2B, operator tube  82  is received within semi-cylindrical channel  136 , with convex spherical sealing surface  128  projecting into the annulus between operator tube  82  and bottom sub connector  130 . 
     Referring now to FIGS. 6 and 7, flapper valve assembly  120  is depicted in a view that is rotated 90 degrees from that in FIG.  4 . Flapper valve assembly  120  includes flapper closure plate  122  and valve seat  124 . As explained above with reference to FIG. 4, sealing surface  126  of valve seat  124  is a concave spherical segment and sealing surface  128  of flapper closure plate  122  is a convex spherical segment. Concave sealing surface  126  of valve seat  124  has a radius of curvature that is substantially equal to that of convex flapper closure plate sealing surface  128 . Valve seat  124  includes an internal load bearing shoulder  134  extending generally radially inwardly from concave valve seat sealing surface  126  which defines the maximum travel of flapper closure plate  122  relative to valve seat  124 . 
     Under typical flow rate regimes, the matching convex and concave spherical surfaces  128 ,  126  are lapped together to permit close nesting engagement of flapper closure plate  122  within valve seat  124  as shown in FIG. 6 wherein a gap  140  exists between the upper surface  142  of flapper closure plate  122  and internal load bearing shoulder  134  of valve seat  124 . In applications where large diameter tubing and large diameter flapper closure plates are necessary and where the ratio of the outer and inner diameters of the sealing surfaces are low, the loads on flapper closure plate  122  tend to deform flapper closure plate  122  about axis  138  which may result in a loss of seal. Specifically, as flapper closure plate  122  deforms about axis  138 , the seal area between flapper closure plate  122  and valve seat  124  could be reduced. As best seen in FIG. 7, internal load bearing shoulder  134  of valve seat  124  defines the maximum travel of flapper closure plate  122  such that any deformation of flapper closure plate  122  about axis  138  that closes gap  140  will not reduce the seal area between flapper closure plate  122  and valve seat  124  and will not interrupt surface-to-surface engagement between the nested spherical segments, but will merely shift the region of overlapping engagement. Consequently, a continuous, positive metal-to-metal seal is maintained completely around the spherical segment interface. 
     Referring next to FIG. 8, therein is depicted another embodiment of a flapper valve assembly of the present invention that is generally designated  150 . Flapper valve assembly  150  has a flapper closure plate  122  and valve seat  152 . As with flapper valve assembly  120  of FIGS. 6 and 7, flapper valve assembly  150  has concave valve seat sealing surface  126  and a convex flapper closure plate sealing surface  128 . Concave sealing surface  126  of valve seat  152  has a radius of curvature that is substantially equal to that of convex flapper closure plate sealing surface  128 . 
     Valve seat  152  includes a seal ring insert  154 . Seal ring insert  154  forms a portion of concave sealing surface  126  and forms internal load bearing shoulder  134  that extends generally radially inwardly from concave valve seat sealing surface  126 . Internal load bearing shoulder  134  defines the maximum travel of flapper closure plate  122  relative to valve seat  152 . Preferably, seal ring insert  154  comprises a material that has a higher hardness than valve seat  152 . As seal ring insert  154  must withstand extreme loads exerted by flapper closure plate  122 , the hardness of seal ring insert  154  is an important feature of the present invention. For example, seal ring insert  154  may be formed by machining out a section of valve seat  152  and laying a weld bead therein. The weld bead is then machined smooth to form a portion of concave sealing surface  126  and internal load bearing shoulder  134 . Alternatively, seal ring insert  154  may be a solid ring that is welded in place within valve seat  152  then machined smooth to form a portion of concave sealing surface  126  and internal load bearing shoulder  134 . 
     Referring now to FIG. 9, a flapper valve assembly is depicted and generally designated  160 . Flapper valve assembly  160  includes a flapper closure plate  162  and a valve seat  164 . In the illustrated embodiment, the sealing surfaces of flapper closure plate  162  and valve seat  164  have mating segments which are matched in curvature to provide a metal-to-metal seal. Sealing surface  166  of valve seat  164  is a concave spherical segment. Sealing surface  168  of flapper closure plate  162  is a convex spherical segment. The radius of curvature  170  of convex flapper closure plate sealing surface  168  is substantially equal to the radius of curvature of concave valve seat sealing surface  166 . 
     Specifically, the radius of curvature of the flapper plate convex sealing surface  168  is substantially the same as, but not greater than, the radius of curvature of the concave valve seat sealing surface  166 . Preferably, the convex and concave surfaces are matched in curvature to provide smooth, non-binding surface engagement of convex flapper closure plate sealing surface  168  against concave valve seat sealing surface  166 . The matching convex and concave spherical surfaces  168 ,  166  are lapped together to permit close nesting engagement of flapper closure plate  162  within valve seat  164 . This arrangement permits smooth angular displacement of flapper closure plate  162  relative to valve seat  164  without interrupting surface-to-surface engagement therebetween. 
     Valve seat  164  includes an internal load bearing shoulder  172  extending generally radially inwardly from concave valve seat sealing surface  166 . Valve seat  164  also includes an external load bearing shoulder  174  extending generally radially outwardly from concave valve seat sealing surface  166 . External load bearing shoulder  174  is axially aligned with ballast member  176  of flapper closure plate  162 . Ballast member  176  is integral with flapper closure plate  162  and is disposed opposite of pivot pin support member  178 . Together, internal load bearing shoulder  172  and external load bearing shoulder  174  defines the maximum travel of flapper closure plate  162  relative to valve seat  124 . It should be noted by those skilled in the art that even though ballast member  176  is depicted as integral with flapper closure plate  162 , a ballast member could be attached to flapper closure plate  162  using a variety of methods including, but not limited to, welding. 
     In application where large diameter tubing and large diameter flapper closure plates are necessary and wherein the ratio between the outer and inner diameters of the sealing surfaces is low, the loads on flapper closure plate  162  tend to deform flapper closure plate  162  about both axis  180  and axis  182 , as best seen in FIG.  10 . As flapper closure plate  162  deforms about axis  180  and gap  184  is closed, internal load bearing shoulder  172  of valve seat  164  defines the maximum travel of flapper closure plate  162 . Likewise, as flapper closure plate  162  deforms about axis  182  and gap  186  is closed, external load bearing shoulder  174  of valve seat  162  defines the maximum travel of ballast member  176  of flapper closure plate  162 . As such, any deformation of flapper closure plate  162  about axis  180  or axis  182  will not reduce the seal area between flapper closure plate  162  and valve seat  164  and will not interrupt surface-to-surface engagement between the nested spherical segments, but will merely shift the region of overlapping engagement. Consequently, a continuous, positive metal-to-metal seal is maintained completely around the spherical segment interface. 
     While this invention has been described with a 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.