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TECHNICAL FIELD OF THE INVENTION 
     This invention relates in general to flapper type valves and, in particular, to a flapper type valve that includes a flapper closure plate for controlling fluid flow therethrough that is biased into sealing engagement with the flapper seat by a flapper closure assembly that pushes on the back face of the flapper closure plate. 
     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. 
     It has been found, however, that in tight safety valve applications having a large inner diameter and a small outer diameter, typical torsion spring or tension member designs provide insufficient bias or closure force to lift the flapper plate to the closed position. In the case of a torsion spring, the size and therefore the closure force of the torsion spring are limited by the space available to house the torsional spring. In the case of a tension member, closure force is limited by length of the lever arm between the hinge pin of the flapper plate and the location where in the tension member is attached to the linkage that extends from the hinge in the direction opposite of the flapper plate. 
     Therefore, a need has arisen for a flapper valve that has sufficient bias or closure force to lift the flapper plate into the closed position in tight applications. A need has also arisen for such a flapper valve the can produce the required closure force without increasing the space available for a spring within the flapper valve. A need has further arisen for such a flapper valve that can take advantage of a longer lever arm to exert a greater closure force to the flapper plate. 
     SUMMARY OF THE INVENTION 
     The present invention disclosed herein comprises a flapper valve assembly that has sufficient closure force to move the flapper closure plate to the closed position in tight applications. The flapper valve assembly disclosed herein produces the required closure force without the need for increasing the size of the spring within the flapper valve. The flapper valve assembly of the present invention utilizes a longer lever arm to exert an increased closure force to the flapper plate. 
     The flapper valve assembly of the present invention comprises a tubular valve housing having a hinge and a longitudinal hole. A valve seat is mounted within the housing. The valve seat has a valve seat sealing surface and defines a flow passage therethrough. A flapper closure plate is rotatable about the hinge between a valve open position and a valve closed position. In the valve open position, the flapper closure plate is removed from the valve seat. In the valve closed position the sealing surface of the flapper closure plate sealingly engages the valve seat sealing surface for preventing flow through the flow passage. 
     The flapper closure plate has a notch on the side opposite the sealing surface of the flapper closure plate that is spaced a distance from the pivot point of the hinge. A flapper closure assembly biases the flapper closure plate toward the closed position. The flapper closure assembly includes a spring that is mounted within the longitudinal hole and a connection member that is operably coupled to the spring and engages the notch such that when the flapper closure plate is moved toward the open position, the spring is compressed causing the connection member to urge the flapper closure plate toward the closed position. The distance between the notch and the hinge provides a moment arm sufficient for the flapper closure assembly to bias the flapper closure plate from the open position to the closed position which may be about 90 degrees. 
     The flapper closure assembly may include a piston member that is at least partially disposed within the hole. The piston member couples the spring to the connection member. The connection member may be rotatably coupled to the piston member and may be rotatably received within the notch of the flapper closure plate. The connection member may have an arcuate shape such that the connection member exerts a moment on the flapper closure plate when the flapper closure plate is in the open position. 
     In one embodiment of the present invention, the flapper valve assembly may be incorporated into a subsurface safety valve that is adapted to be placed in a well tubing string to control flow therethrough. The subsurface safety valve comprises a valve housing having a bore therethrough, a hinge and a longitudinal hole. A flapper closure plate is mounted within the bore and is rotatable about the hinge. The flapper closure plate is movable between an open position and a closed position. The flapper closure plate has a sealing surface on one side and a notch on the opposite side. An operator is movably disposed within the bore for controlling movement of the flapper closure plate from the closed position to the open position. A flapper closure assembly biases the flapper closure plate toward the closed position. The flapper closure assembly includes a spring mounted within the longitudinal hole and a connection member that is operably coupled to the spring and that engages the notch such that when the flapper closure plate is moved toward the open position, the spring is compressed causing the connection member to urge the flapper closure plate toward the closed position. A valve seat is disposed within the valve housing. The valve seat has a sealing surface such that when the subsurface safety valve is in the closed position, the sealing surface of the flapper closure plate sealingly engaging the sealing surface of the valve seat. 
    
    
     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 half sectional view of a flapper valve assembly of the present invention in the valve open position; 
     FIG. 5 is a half sectional view of a flapper valve assembly of the present invention in the valve closed position; 
     FIG. 6 is a top view of a flapper closure plate of a flapper valve assembly of the present invention; 
     FIG. 7 is a perspective view of a flapper closure plate of a flapper valve assembly of the present invention; 
     FIG. 8 is a plan view of a flapper closure plate of a flapper valve assembly of the present invention; 
     FIG. 9 is a perspective view of a connection member of a flapper valve assembly of the present invention; 
     FIG. 10 is a half sectional view of a flapper valve assembly of the present invention in the valve open position; and 
     FIG. 11 is a half sectional view of a flapper valve assembly of the present invention in the valve closed position. 
    
    
     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 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  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  such that a return spring  84  is fully compressed and valve  50  is in the valve open position. 
     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 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 on hinge  100  and is biased about 90 degrees to the valve closed position as shown in FIGS. 3A-3B by flapper valve closure assembly  102  housed partially within hole  104  in tubing  30 . In the valve open position as shown in FIGS. 2A-2B, the bias force of flapper valve closure assembly  102  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 about 90 degrees through chamber  98  in response to the bias force of flapper valve closure assembly  102 . 
     Referring now to FIGS. 4 and 5, a flapper valve assembly is depicted and generally designated  110 . Flapper valve assembly  110  includes a flapper closure plate  86  that is bias to the closed position by flapper valve closure assembly  102 . In the illustrated embodiment, the sealing surfaces of flapper closure plate  86  and valve seat  92  have mating spherical segments which are matched in curvature to provide a metal-to-metal seal. It has been found that the use of metal-to-metal contact between nesting spherical segments provides a continuous, positive seal that is maintained completely around the spherical segment interface. It should be understood by those skilled in the art, however, that other flapper closure plate and valve seat arrangements having other sealing surface configurations that are known in the art are equally well-suited for use in the present invention, these arrangements and configurations being contemplated and falling within the scope of the present invention. 
     Flapper valve closure assembly  102  includes one or more compression springs  116 , only one of which is shown, that are disposed within longitudinal hole  104 . Flapper valve closure assembly  102  also includes a piston  118  that has shoulder  120  which contacts compression spring  116  within longitudinal hole  104 . A connection member  122  is coupled to piston  118  on the end opposite spring  116  with coupling  124 . In the illustrated embodiment, connection member  122  is rotatably coupled to piston  118 . On the end opposite coupling  124 , connection member  122  engages flapper closure plate  86  as will be more fully discussed below. 
     When flapper valve assembly  110  is operated from the close position as depicted in FIG. 5 to the open position as depicted in FIG. 4, operator tube  82  is extended downwardly through valve chamber  98  rotating flapper closure plate  86  about 90 degrees. As flapper closure plate  86  rotates, compression spring  116  is compressed due to the downward movement of connection member  122  and piston  118  thereby biasing flapper closure plate  86  toward the closed position. As flapper closure plate  86  nears its fully open position, connection member  122  contacts counterbore  96  of flapper valve chamber  98 . As such, connection member  122  acts like a leaf spring and further biases flapper closure plate  86  toward the closed position. This result is achieved due to the arcuate shape of connection member  122  which assures that a moment will be applied about hinge  100  even when flapper closure plate  86  is in the fully opened position. 
     Referring next to FIGS. 6-8, therein is depicted various views of flapper closure plate  86 . Flapper closure plate  86  has a arcuate shaped face  126  which receives operator tube  82  when flapper closure plate  86  is operated from the closed position to the open position. Flapper closure plate  86  also includes a pair of hinge members  128  through which a pin is inserted along axis  130  to couple hinge members  128  of flapper closure plate  86  with hinge  100 . Flapper closure plate  86  includes a notch  132 . Notch  132  has an axis of rotation  134  located therein. The distance  136  between axis  130  and axis  134  defines the length of the moment arm used to bias flapper closure plate  86  from the open position to the closed position. Distance  136  may be adjusted as necessary depending upon the size and weight of flapper closure plate  86 . 
     Referring now to FIGS. 7 and 9, connection member  122  is inserted into notch  132  of flapper closure plate  86 . Connection member  122  includes a pair of pins  138 ,  140  that are received within notch  132  and assure that connection member  122  does not become separated from flapper closure plate  86  during operation. As explained above, connection member  132  has an arcuate shape which allows connection member  122  to act as a leaf spring and bias flapper closure plate  86  toward the closed position when flapper closure plate  86  is in its fully open position. 
     Referring now to FIGS. 10 and 11, a flapper valve assembly is depicted and generally designated  150 . Flapper valve assembly  150  includes a flapper closure plate  86  that is bias to the closed position by flapper valve closure assembly  102 . Flapper valve closure assembly  102  includes a compression spring  154  that is disposed within annular hole  152 . Flapper valve closure assembly  102  also includes a piston  156  that has sleeve  158  which contacts compression spring  154  within annular hole  152 . A connection member  122  is coupled to piston  156  on the end opposite spring  154  with coupling  124 . In the illustrated embodiment, connection member  122  is rotatably coupled to piston  156 . On the end opposite coupling  124 , connection member  122  engages flapper closure plate  86  as described above. 
     When flapper valve assembly  150  is operated from the close position as depicted in FIG. 11 to the open position as depicted in FIG. 10, operator tube  82  is extended downwardly through valve chamber  98  rotating flapper closure plate  86  about 90 degrees. As flapper closure plate  86  rotates, compression spring  154  is compressed due to the downward movement of connection member  122  and piston  156  thereby biasing flapper closure plate  86  toward the closed position. As flapper closure plate  86  nears its fully open position, connection member  122  contacts counterbore  96  of flapper valve chamber  98 . As such, connection member  122  acts like a leaf spring and further biases flapper closure plate  86  toward the closed position. This result is achieved due to the arcuate shape of connection member  122  which assures that a moment will be applied about hinge  100  even when flapper closure plate  86  is in the fully opened position. 
     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.

Summary:
A flapper valve assembly ( 110 ) having a biasing flapper closure assembly ( 102 ) is disclosed. The flapper valve assembly ( 110 ) comprises tubular valve housing ( 94 ) having a hinge ( 100 ) and a longitudinal hole ( 104 ). A valve seat ( 92 ) is mounted within the housing ( 94 ) having a flow passage therethrough. A flapper closure plate ( 86 ) is rotatably disposed within the housing ( 94 ) about the hinge ( 100 ). The flapper closure plate ( 86 ) is rotatable between a valve open position and a valve closed position. The flapper closure plate ( 86 ) has a notch ( 132 ) on the side opposite its sealing surface ( 114 ) that is spaced a distance ( 136 ) from the hinge ( 100 ). The flapper closure assembly ( 102 ) includes a spring ( 116 ) mounted within the longitudinal hole ( 104 ) and a connection member ( 122 ) operably coupled to the spring ( 116 ) and engaging the notch ( 132 ) such that when the flapper closure plate ( 86 ) is moved toward the open position, the spring ( 116 ) is compressed causing the connection member ( 122 ) to urge the flapper closure plate ( 86 ) toward the closed position.