Abstract:
A curved flapper and seat is disclosed for use in a subsurface safety valve. The flapper is biased to a normally closed position to prevent fluid flow through the wellbore. The curved flapper has a sealing surface for engaging a corresponding sealing surface on a seat when the flapper is in its closed position. The sealing surface of the flapper is configured to contact the sealing surface of the seat along a sinusoidal sealing line, or seam, such that the reactive force from the seat is normal to the sinusoidal seating line. In one aspect, the sealing surface of the flapper has a convex spherical configuration relative to the seat. The sealing surface of the seat, in turn, has a concave conical shape relative to the flapper. When well conditions dictate, a resilient soft seat may optionally be used, and is disposed on the seat proximate the sinusoidal seating line.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention is related generally to safety valves. More particularly, this invention pertains to subsurface safety valves which employ a curved flapper for controlling fluid flow through a production tubing string.  
           [0003]    Surface controlled, subsurface safety valves (SCSSVs) are commonly used to shut in oil and gas wells. Such SCSSVs are typically fitted into production tubing in a hydrocarbon producing well, and operate to block the flow of formation fluid upwardly through the production tubing should a failure or hazardous condition occur at the well surface.  
           [0004]    SCSSVs are typically configured as rigidly connected to the production tubing (tubing retrievable), or may be installed and retrieved by wireline, without disturbing the production tubing (wireline retrievable). During normal production, the subsurface safety valve is maintained in an open position by the application of hydraulic fluid pressure transmitted to an actuating mechanism. The hydraulic pressure is commonly supplied to the SCSSV through a control line which resides within the annulus between the production tubing and a well casing. The SCSSV provides automatic shutoff of production flow in response to one or more well safety conditions that can be sensed and/or indicated at the surface. Examples of such conditions include a fire on the platform, a high/low flow line pressure condition, a high/low flow line temperature condition, and operator override. These and other conditions produce a loss of hydraulic pressure in the control line, thereby causing the flapper to close so as to block the flow of production fluids up the tubing.  
           [0005]    2. Description of the Related Art  
           [0006]    Most surface controlled subsurface safety valves are “normally closed” valves. This means that the valves utilize a flapper type closure mechanism which is biased in its closed position. In many commercially available valve systems, the bias is overcome by longitudinal movement of a hydraulic actuator. In some cases the actuator of the SCSSV comprises a concentric annular piston Most commonly, the actuator comprises a small diameter rod piston located in a housing wall of the SCSSV.  
           [0007]    During well production, the flapper is maintained in the open position by a flow tube connected downhole to the actuator. From a reservoir, a pump at the surface delivers regulated hydraulic fluid under pressure to the actuator through a control conduit, or control line. Hydraulic fluid is pumped into a variable volume pressure chamber (or cylinder) and acts against a seal area on the piston. The piston, in turn, acts against the flow tube to selectively open the flapper member in the valve. Any loss of hydraulic pressure in the control line causes the piston and actuated flow tube to retract, which causes the SCSSV to return to its normally closed position by a return means. The return means serves as the biasing member, and typically defines a powerful spring and/or gas charge. The flapper is then rotated about a hinge pin to the valve closed position by the return means, i.e., a torsion spring, and in response to upwardly flowing formation fluid.  
           [0008]    In some wells, high fluid flow rates of as much as 250 million cubic feet or more per day of gas may be produced through the SCSSV. In high flow rate wells, it is well known that curved or arcuate flappers may be used to provide a larger inside diameter, or bore, in the SCSSV as compared to a flat flapper. Examples of such SCSSVs are described in U.S. Pat. Nos. 2,162,578; 4,531,587; 4,854,387; 4,926,945; 5,125,437; and 5,323,859. Curved flapper arrangements enable a larger production tubing inner diameter and, thus, allow for a greater rate of hydrocarbon production through the valve area.  
           [0009]    In either flat or curved flappers, as the tubular piston and operator tube retract, the flapper closure passes across the lower end of the operator tube and throttles the flow as it rotates toward the closed or “seated” position. At high flow rates, a high differential pressure may be developed across the flapper that may cause distortion and warping of the flapper as it rubs against the operator tube. Also, a flapper seat may be damaged if it is slammed open against the valve housing or slammed shut against the valve seat in response to the high-pressure differentials and production flow regimes. Damage to the flapper seat or leakage around the flapper may also occur if the flapper is closed on any debris in the well, such as sand or other aggregate that may be produced with the hydrocarbons.  
           [0010]    In prior art SCSSVs, the flapper is seated in a variety of configurations. The flapper may be seated against an annular sealing face, either in metal-to-metal contact, or metal against an annular resilient seal.  
           [0011]    In U.S. Pat. No. 3,955,623 discloses a flapper having a flat, annular sealing face. The flapper is engagable against a flat, annular valve seat ring, with sealing engagement being enhanced by an elastomeric seal ring that is mounted on the valve seat.  
           [0012]    U.S. Pat. No. 4,457,376, the valve seat includes a downwardly facing, conical segment having a sloping sealing surface. The flapper closure member has a complimentary, sloping annular sealing surface that is adapted for surface-to-surface engagement against the conical valve seat surface.  
           [0013]    U.S. Pat. No. 5,125,457, (expired) also presents a curved flapper. The flapper has a sealing surface with a convex spherical radius which seats in a matching concave housing. It also has a concave spherical portion constructed of an elastomeric material. The spherical radius flapper/seat has an alternate embodiment shown in U.S. Pat. No. 5,323,859. This patent teaches metal-to-metal sealing surfaces with no resilient seal.  
           [0014]    In U.S. Pat. Nos. 5,682,921, and 5,918,858 a flat sealing surface is provided on both the flapper and the seat, fashioned in a sinusoidal undulating shape and having a combination metal and resilient seal.  
           [0015]    In all these arrangements, the flapper rotates about a hinge assembly that comprises a hinge pin and a torsion spring. It will be appreciated by those of ordinary skill in the art, that structural distortion of the flapper, or damage to the hinge assembly which supports the flapper for rotational movement into engagement with the valve seat, can cause misalignment of the respective sealing surfaces, thereby producing a leakage path around the flapper.  
           [0016]    Misalignment of the flapper relative to the valve seat may also be caused by the deposition of sand particles or other debris on the valve seat and/or sealing surfaces. Sand may be produced in both gas and oil wells, under low flow rate conditions as well as high flow rate conditions. It is particularly difficult to obtain positive sealing engagement of either flat or curved flappers and valve seats in low-pressure, sandy environments.  
           [0017]    The integrity of the sealing engagement between the flapper and valve seat may be compromised under low flow rate conditions, while the same safety valve may provide positive closure and sealing engagement under high flow rate, high differential pressure conditions In this respect, slight misalignment may be overcome by high-pressure impact and engagement of the flapper against the valve seat. However, the same misalignment may produce a leakage path under low differential pressure conditions. Such misalignment will prevent correct seating and sealing of the flapper. The result is that a large amount of formation fluid may escape through the damaged valve, wasting valuable hydrocarbon resources, causing environmental pollution, and creating potentially hazardous conditions for well operations personnel. 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.  
           [0018]    The following U.S. Pat. Nos. pertain to SCSSVs having flapper closure mechanisms and are hereby incorporated by reference: 3,788,595; 3,865,141; 3,955,623; 4,077,473; 4,160,484; 4,161,960; 4,287,954; 4,376,464; 4,449,587; 4,457,376; 4,531,587; 4,583,596; 4,605,070; 4,674,575; 4,854,387; 4,890,674; 4,926,945; 4,983,803; 4,986,358; 5,125,457; 5,137,090; 5,263,847; 5,323,859; 5,423,383; 5,285,851; 5,918,858; 5,682,921.  
         SUMMARY OF THE INVENTION  
         [0019]    The present invention provides an improved flapper and seat for a surface controlled subsurface safety valve (SCSSV). The SCSSV of the present invention provides a curved flapper having a novel sealing surface for engaging a novel corresponding sealing surface in the seat. The sealing surface of the flapper is configured to contact the sealing surface of the seat along a sinusoidal sealing line, or seam, such that the reactive force from the seat is normal to the sinusoidal seating line. Thus, a more effective seal is achieved when the flapper pivots to its closed position. In operation, the novel SCSSV will safely and effectively shut in a well below the earth&#39;s surface in the event of damage to the wellhead or flow line, or in the event of a malfunction of any surface equipment, with the shut-in being accomplished whether the well is operating in low flow or in high flow conditions.  
           [0020]    The present invention also provides an improved surface-controlled, subsurface flapper safety valve in which the flapper closure mechanism and valve seat are tolerant of irregularities, such as obstructions or surface distortions caused by sand deposits or erosion of their respective sealing surfaces. The present invention also provides an improved flapper mechanism and seat in an SCSSV assembly having, in one embodiment, a flapper having a spherical sealing surface, and a corresponding metallic seat having a conical sealing surface. In one aspect, the sealing surface of the flapper has a convex spherical configuration relative to the seat. The sealing surface of the seat, in turn, has a concave conical shape relative to the flapper. In such an arrangement, the present invention provides an improved valve seat for an SCSSV adapted to provide a positive seal against a curved or arcuate flapper closure mechanism to overcome imperfect alignment or surface finish of its sealing surface relative to the safety valve seat.  
           [0021]    The present invention also provides an improved flapper mechanism and seat in an SCSSV assembly having, in another embodiment, a flapper having a spherical sealing surface, and a corresponding metallic “hard” seat having a conical sealing surface. Disposed concentrically within the hard seat is also a “soft” valve seat made of a yieldable material such as an elastomer (nitrile, neoprene, AFLAS®, KALREZ®), a thermoplastic polymer (TEFLON®, RYTON®, or PEEK®), or a soft metal (lead, copper, zinc and brass). The soft seat defines a concave spherical or conical segment. The surfaces of the hard seat and the soft seat are configured to lie in sealable contact within the spherical radius that defines the sealing surface on the flapper. The surfaces are configured to provide a positive seal along a continuous interface seam between the conical hard seat, the (optional) resilient soft seat and the concave spherical sealing surface of the flapper.  
           [0022]    According to the foregoing alternative arrangement, a convex spherical sealing segment of the flapper is received in nesting engagement against the surface of the soft seat, and against the conical sealing segment of the hard seat. The nesting arrangement allows for some misalignment of the flapper relative to the valve seat without interrupting surface-to-surface engagement therebetween. In this respect, the surface of the soft seat will tolerate a limited amount of angular misalignment of the flapper that might be caused by structural distortion of the closure or deflection of the hinge assembly, enabling a low-pressure seal. Line contact between the convex spherical segment of the flapper and the conical hard seat serves to realign the flapper as pressure increases. The hard seat also supplies sufficient structural rigidity to enable a pressure seal at high pressures. Positive sealing engagement between the flapper and the hard and soft seats is also obtained in sandy environments by engagement of the yieldable seat which conforms about surface irregularities which may be caused by surface deposits or surface erosion caused by the production of sandy fines.  
           [0023]    It will be appreciated by one of ordinary skill in the art, that the foregoing net result of this interaction, is a flapper and seat system that performs in a sandy environment throughout any pressure range required in a hydrocarbon producing well for both tubing retrievable and wireline retrievable SCSSVs, and for both hydraulic or electrically actuated embodiments thereof.  
           [0024]    As has been described in detail above, the present invention has been contemplated to overcome the deficiencies of the prior equalizing safety valves specifically by disclosing significant improvements to the flapper closure mechanism and the corresponding seat. The novel features of the invention are set forth with particularity in Detailed Description of Preferred Embodiments and The Claims. The invention will best be understood from the following description when read in conjunction with the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    So that the manner in which the above recited features of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
         [0026]    [0026]FIG. 1 is a semi-diagrammatic schematic, in cross section, of a typical production well having a surface controlled, tubing retrievable subsurface safety valve installed according to the present invention;  
         [0027]    [0027]FIG. 2 is an isometric view, in partial section, of a tubing retrievable subsurface safety valve of the present invention shown in the open position;  
         [0028]    [0028]FIG. 3 is an isometric view, in partial section, of a tubing retrievable subsurface safety valve of the present invention shown in the closed position;  
         [0029]    [0029]FIG. 4 is a close-up detailed isometric view, in partial section, of a flapper and seat in the all-metal configuration (without a soft seat) in a subsurface safety valve of the present invention, shown in the closed position;  
         [0030]    [0030]FIG. 5 is an exploded isometric view of a flapper/seat subassembly of the present invention, shown in the closed position and without a soft seat;  
         [0031]    [0031]FIG. 6 illustrates a sphere and cone sealing method and seal interface line in accordance with prior art.  
         [0032]    [0032]FIG. 7 is an exploded isometric view of a flapper/seat subassembly of the present invention, shown in the closed position and with a combination soft/hard seat;  
         [0033]    [0033]FIG. 8 is a cross-sectional view of a flapper/seat subassembly of the present invention, shown in the closed position and with soft seat/hard seat configuration;  
         [0034]    [0034]FIG. 9 is a cross-sectional view of a flapper/seat subassembly of the present invention, shown in the open position and with the soft seat/hard seat configuration;  
         [0035]    [0035]FIG. 10 is an isometric view of a flapper and seat in the soft seat/hard seat configuration of the present invention shown in the open position, incorporated into a substrate safety valve;  
         [0036]    [0036]FIG. 11 is a close-up detailed isometric view, in partial section, of a flapper and seat in the soft seat/hard seat configuration of the present invention shown in the closed position, incorporated into a subsurface safety valve;  
         [0037]    [0037]FIG. 12 is an isometric view of a flapper and seat in the soft resilient seat/hard seat configuration in a subsurface safety valve of the present invention shown in the closed position with a flapper closing means;  
         [0038]    [0038]FIG. 13 is an exploded isometric view of a metal-to-metal flapper and seat in a subsurface safety valve of the present invention shown in the open position with a flapper closing means and an equalizing means;  
         [0039]    [0039]FIG. 14 is an exploded isometric view of a metal-to-metal flapper and seat in a subsurface safety valve of the present invention shown in the closed position with a flapper closing means and an equalizing means; and  
         [0040]    [0040]FIG. 15 is an enlarged isometric view of a closed flapper/seat subassembly in partial section, which illustrates details of the all-metal flapper and seat of the present invention.  
         [0041]    [0041]FIGS. 16, 17,  18  and  19  are rotated isometric views of the flapper closure mechanism.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0042]    In the description that follows, like parts are marked throughout the specification and drawings with the same reference numerals, respectively. The drawings may be but are not necessarily to scale and the proportions of certain parts have been exaggerated to better illustrate details and features of the invention. One of normal skill in the art of subsurface safety valves will appreciate that the present invention can and may be used in all types of subsurface safety valves, including but not limited to tubing retrievable, wireline retrievable, injection valves, subsurface controlled valves (such as storm chokes), or any type of flapper safety valve that benefits from a larger flow area by the employment of a curved or arcuate flapper closure mechanism.  
         [0043]    Referring now to FIG. 1, a subsurface safety valve  10  is shown in place in a typical well completion schematic  12 . A land well is shown for the purpose of illustration; however, it is understood that a subsurface safety valve  10  of the present invention may be commonly used in offshore wells. Visible in the well  12  of FIG. 1 are a wellhead  20 , a master valve  22 , a flow line  24 , a casing string  26 , production tubing  28 , and a packer  30 . In operation, opening the master valve  22  allows pressurized hydrocarbons residing in the producing formation  32  to flow through a set of perforations  34  and into the well  12 . The packer  30  seals an annulus  35  between the casing  26  and the production tubing  28  in order to direct the flow of hydrocarbons. Hydrocarbons (illustrated by arrows) flow into the production tubing  28 , through the subsurface safety valve  10 , through the wellhead  20 , and out into the flow line  24 .  
         [0044]    Referring now to FIG. 2, a subsurface safety valve  10  of the present invention is shown in the open position. An upper nipple  36  and a lower sub  38  serve to sealingly connect the safety valve  10  to the production tubing  28 . The safety valve  10  is maintained in the open position by hydraulic pressure. Hydraulic pressure is supplied by a pump (not shown) in a control panel  14  through a control line  16  to the safety valve  10 . The hydraulic pressure holds a flapper closure mechanism  18  within the safety valve  10  in the open position. Because the safety valve  10  is a “fail closed” device, loss of hydraulic pressure in the control line  16  will cause the flapper closure mechanism  18  to actuate, thereby blocking the upward flow of hydrocarbons to the surface.  
         [0045]    As noted, the safety valve  10  shown in FIGS. 1 and 2 is hydraulically actuated. In this respect, the safety valve  10  includes a hydraulic chamber housing  40  and a piston  42  therein. The piston  42  is typically a small diameter piston which moves within a bore of the housing  40  in response to hydraulic pressure from the surface. Alternatively, the piston may be a large concentric piston which is pressure actuated. It is within the scope of the present invention, however, to employ other less common actuators such as electric solenoid actuators, motorized gear drives and gas charged valves (not shown). Any of these known or contemplated means of actuating the subsurface safety valve  10  of the present invention may be used.  
         [0046]    Energizing the actuating means  42  serves to open the subsurface safety valve  10 . In the arrangement of the safety valve  10  shown in FIG. 2, the application of hydraulic pressure through the control line  16  serves to force the piston  42  within the chamber housing  40  downward. The piston  42 , in turns, acts upon a flow tube  44 , translating the flow tube  44  longitudinally. In FIG. 2, the flow tube  44  is shown shifted fully downward due to the energy from the actuating means  42 . In this position, the flow tube maintains the flapper closure mechanism  18  (obscured by flow tube  44  in this figure) in an open position.  
         [0047]    [0047]FIG. 3 presents the safety valve of the present invention in its closed position. In this position, the flapper  18  is blocking the wellbore. A power spring  46  is shown in its fully compressed position acting on a connecting means  48 , allowing the power spring  46  to bias the flow tube to an upward position.  
         [0048]    When pressure (or energy) is released from the piston  42  as shown in FIG. 3, the power spring  46  moves the flow tube  44  longitudinally upward, allowing the flapper closure mechanism  18  to close, and thereby preventing flow from the well.  
         [0049]    [0049]FIG. 4 depicts, in quarter section, a close up view of a portion of the closed subsurface safety valve  10  of FIG. 3. Features illustrated are the flow tube  44 , a lower end of the power spring  46 , and the flapper closure mechanism  18 , all arranged inside the lower sub  38 .  
         [0050]    Referring now to FIG. 5, FIG. 5 presents an exploded isometric view of a flapper/seat subassembly of the present invention. The flapper  18  is shown in the closed position with a metal-to-metal seal. A hard seat  50  adapted for use in a safety valve  10  has a concave conical sealing surface  58  formed therearound. A flapper mount  60  is affixed to the hard seat  50  by a plurality of attachment screws  62  threaded into a plurality of threaded holes  63 . Close tolerance alignment pins  64  assure a precision alignment between a centerline of the flapper mount  60  and the hard seat  50 . A clevis pair  66  is fashioned into the flapper mount  60  wherein a mounting hole  68  is drilled through for receiving at least one flapper pin  70 . The curved flapper  18  is rotatably mounted on the at least one flapper pin  70  by a hinge  72 , having pin hole  74  drilled therethrough. Thus, the flapper  18  pivots between its open and closed positions about the flapper pin  70 .  
         [0051]    In operation, the curved flapper  18  swings in an arc of substantially 80-90 degrees between its opened and closed positions about the pin  70 . In its open position, the flapper  18  is positioned essentially vertically so as not to obstruct the upward flow of hydrocarbons from the well. In its closed position, the flapper  18  seals essentially horizontally within the well so as to obstruct the upward flow of fluids. The flapper  18  is configured to meet a sealing surface  58  in the seat  50 . In the arrangement shown in FIG. 5, the flapper  18  includes a convex spherical sealing surface which engages a corresponding convex spherical sealing surface in the seat  50 .  
         [0052]    The convex spherical sealing surface  76  formed on the curved flapper  18  results in a slightly elliptical flapper shape. FIGS.  16 - 19  more clearly depict the elliptical shape.  
         [0053]    The geometrical configurations of the sealing surfaces  58 ,  76  in the present invention are complex. Visualization of the complexity of this geometry in a two dimensional environment for most requires illustration of a simpler and well-known sealing device. Reference is thus made to the sealing device often employed in “poppet type” valves. FIG. 6 shows a simplified prior art arrangement of a convex spherical poppet seal  52  and a convex conical seat  54 , the sealing surface of the seat being tangent to the spherical radius of the poppet seal  52 . The interface between the spherical poppet  42  and the convex conical seat  54  forms a flat circular sealing line  56 . Pressure forces acting on the spherical poppet  42  creates very high local stresses along the sealing line  56 , thereby affecting a fluidic seal along the flat circular sealing line  56 . The seating line  56  represents every point on the convex conical seat  54  that is tangent to the surface of the spherical poppet seal  52 . Visualizing this tangency is helpful in understanding the geometry of the present invention. The flapper and seat seal of the present invention is related to the ball and cone poppet seal, but is more complex. The flat circular sealing line  56  of the poppet seal will not transcribe onto the geometry of a curved flapper with a spherical sealing segment. In this respect, the curved flapper is designed to maximize the inside diameter of a SCSSV.  
         [0054]    In recent years, engineers and designers have employed highly advanced computerized software known generically as parametric solid modeling. Parametric solid modeling software is marketed under various brand names including: PRO-ENGINEER™, SOLID WORKS™, and SDRC-IDEAS™. Use of such software allows the designer to create and visualize geometries that are difficult or even impossible to describe in two-dimensional media, including two-dimensional drawings. Manufacturers first realized the difficulty where traditional drawings could not be used to either build or inspect parts. Means were created to translate the computerized electronic geometry directly to machine code. This increases capability, and efficiency and saves time over manufacturing processes that require drawings. It also provides the only means for reliably manufacturing a flapper and seat arrangement of the present invention.  
         [0055]    The present invention, and specifically the interaction of the convex spherical sealing surface  76  and the concave conical sealing surface on the hard seat  50 , can more easily be visualized in the “soft seat” embodiment hereinafter described in FIG. 7.  
         [0056]    In FIG. 7, the hard seat  50  again has a concave conical sealing surface  58 . However, it also has a seat recess  78  for receiving a soft seat  80 . As before, flapper mount  60  is affixed to the hard seat  50  by a plurality of attachment screws  62  threaded into a plurality of threaded holes  63 . Close tolerance alignment pins  64  assure a precision alignment between a centerline of the flapper mount  60  and the hard seat  50 . A clevis pair  66  is fashioned into the flapper mount  60  wherein a mounting hole  68  is drilled through for receiving at least one flapper pin  70 . The curved flapper closure mechanism  18  is rotatably mounted on the at least one flapper pin  70  by a hinge  72 , having pin hole  74  drilled therethrough.  
         [0057]    In operation, the curved flapper closure mechanism  18  pivots in an arc of substantially 80-90 degrees between its opened and closed positions about the pin  70 . The concave conical sealing surface  58  of the seat  50  is adapted to receive the closed flapper closure mechanism  18  of the present invention upon which a convex spherical sealing surface  76  is formed.  
         [0058]    The interaction between the concave conical sealing surface  58  of the seat  50  and the convex spherical sealing surface  76  of the flapper  18  is along a pair of sinusoidal sealing lines. First, a hard sinusoidal sealing line  82  is formed in the hard seat  50 ; second, a soft sinusoidal sealing line  84  is formed on the soft seat  80 . Not obvious in this figure is the “angle” of the concave conical sealing surface. A single conical angle is represented by line  86 . In order to provide the desired seal with the flapper  18 , this conical angle  86  must be substantially tangent to a flapper sealing line  88  on the convex spherical sealing surface of the flapper  18 . It must also be substantially tangent to a sinusoidal sealing line  82  formed in the hard seat  50  and the soft sinusoidal sealing line  84  formed on the soft seat  80 . (The flapper sealing line  88  is illustrated in FIGS.  16 - 19 .) This means that the conical angle  86  depicted must be variable circumferentially around a cross-sectional perimeter of the hard seat  50 .  
         [0059]    As earlier discussed, the variable conical angle  86  cannot be accurately depicted in this 2-D format. Computer software was used to generate the required solid model geometry to depict the part, as well as the machining code necessary to manufacture the part. A Coordinate Measuring Machine or CMM may be used to inspect manufactured parts for accuracy. For purposes of this disclosure, it must be understood that the angle of intersection between the sealing surfaces  58 ,  76  varies along the perimeter of the flapper  18 .  
         [0060]    When it becomes necessary to close, the flapper  18  rotates about the pin  70  until it begins to nest in the hard seat. The flapper sealing line  88  on the convex spherical sealing surface  76  first contacts the sinusoidal sealing line  84  formed on the soft seat  80 . This interaction allows for an effective seal at low pressures. The soft seal  80  is fabricated from a resilient material. Preferably, the resilient seat is constructed of an elastomeric material having a durometer hardness in the range of 60 to 99. Other materials, however, are satisfactory for the soft seat  80 . Acceptable examples include a thermoplastic polymeric material, e.g., tetrafluoroethylene (TFE) fluorocarbon polymer or polyetheretherkeytone (PEEK), a reinforced thermoplastic containing carbon or glass, or a soft metallic material, e.g., lead, copper, zinc, gold or brass.  
         [0061]    At higher pressures, the resilient nature of the soft seat material deforms. The flapper sealing line  88  on the flapper seating surface  76  engages the sinusoidal sealing line  82  formed in the hard seat  50 . This interaction allows for a high-pressure seal. Forces along the sinusoidal sealing line due to pressure are resolved very efficiently in the present invention. The reactive force from the hard seat normal to the sinusoidal sealing line inhibits and virtually eliminates the metaphorically descriptive “Taco Effect”, or tendency of prior art curved flappers to bend like the familiar food item when subjected to high pressure. Any such bending in a flapper can cause undesirable leakage and possible failure. The present invention also resolves stresses in the flapper and seat in a very efficient manner.  
         [0062]    Reference is now made to FIGS. 8 and 9. FIGS. 8 and 9 present cross-sectional views of a flapper  18  of the present invention, along with a resilient soft seat  80 , the hard seat  50 , the flapper mount  60 , and the hinge  72 . In FIG. 8, the flapper  18  is in its closed position. In FIG. 9, the flapper  18  is shown in the open position. FIG. 9 also clearly shows an interface between the hard sinusoidal seating line  82  and the soft sinusoidal seating line  84 .  
         [0063]    [0063]FIG. 10 provides an assembled isometric view of a flapper closure mechanism  18 , a hard seat  50 , and a soft seat  80  for use in a subsurface safety valve  10  of the present invention, shown in the open position. Also visible in this view is an interface between the hard sinusoidal seating line  82  and the soft sinusoidal seating line  84 , as well as the convex spherical sealing surface  76  on the flapper  18 .  
         [0064]    [0064]FIG. 11 is a close-up detailed isometric view, in partial section, of a flapper closure mechanism  18 , a hard seat  50 , and a soft seat  80  for use in a subsurface safety valve of the present invention. In this view, the valve  10  is shown in the closed position. The soft seat  80  is configured to protrude above the hard seat  50 . As the flapper  18  closes, the resilient soft seat  50  initially engages the flapper  18  to provide a low-pressure seal. As pressure increases, the flapper closure mechanism  18  moves to contact the hard seat  50 , thereby providing the valve with a high-pressure seal.  
         [0065]    [0065]FIG. 12 is an assembled isometric view of a safety valve of the present invention, shown in the closed position. A flapper spring means  92  for biasing the flapper  18  to the closed position is seen. One of ordinary skill in the art of safety valve design will understand that there are many well-known means to bias a flapper  18  to the closed position. . Use of any type of spring means to close the flapper  18  of the present invention is regarded within the scope and spirit of the present invention.  
         [0066]    [0066]FIG. 13 is an assembled isometric view of the safety valve of FIG. 12, shown in the open position. A flapper spring means  92  for biasing the flapper closure mechanism  18  to the closed position is again shown. Also depicted, is an optional equalizing valve means  94 . In FIG. 13, the pressure equalizing means  94  is a dart.  
         [0067]    The equalizing means  94  shown in FIG. 13 is a well-known device for equalizing differential pressures across the flapper  18  When the flapper  18  is closed, pressure builds up below, and acts on the flapper&#39;s surface area. This pressure force may be as high as 20,000 psig. This amount of force is too great for the flow tube  44  to overcome. Therefore, a means of equalizing pressure is required in order for the flapper  18  to open. When it becomes necessary to open the SCSSV, the flow tube  44  (not shown in this view) translates downward and contacts the dart  94 . Dart  94  includes an opening which permits fluid to bleed through the valve  10 , thereby equalizing pressure above and below the flapper  18 . When pressure substantially equalizes across the flapper  18 , the flow tube  44  translates axially downward and fully opens the SCSSV.  
         [0068]    [0068]FIG. 14 is an exploded isometric view of a safety valve  10  of the present invention, shown in the closed position. The valve  10  also includes a pressure equalizing means  94 . The valve  10  of FIG. 14 utilizes metal-to-metal contact between the flapper  18  and the seat  50 . Visible are the flapper mount  60 , the flapper pin  70 , a leaf spring  96 , an equalizing dart  94 , and at least one dart spring  100 . A hole  102  is machined through the flapper for receiving the dart  98 . The at least one dart spring  100  biases the dart  94  to a closed position.  
         [0069]    [0069]FIG. 15 is an enlarged isometric view of a flapper  18 , a hard seat  50 , and a flapper mount  60 . . This Figure illustrates details of the all-metal flapper and seat engagement of the present invention, in one aspect.  
         [0070]    [0070]FIGS. 16, 17,  18 , and  19  are rotated isometric views of the curved flapper  18  used in a valve  10  of the present invention. These Figures show the substantially elliptical shape of flapper  18 . Also shown in these rotated views are the convex spherical sealing surface  76  of the flapper  18 , and the sinusoidal shape of the flapper sealing line  88 .  
         [0071]    It should be noted that while a tubing retrievable embodiment is shown and discussed herein, the curved flapper and seat of the present invention might also be adapted for use in a wireline retrievable subsurface safety valve. Operation of the tubing retrievable subsurface safety valve  10  is otherwise in accord with the operation of any surface controllable, wireline retrievable safety valves that employ this invention.  
         [0072]    Although the invention has been described in part by making detailed reference to specific embodiments, such detail is intended to be and will be understood to be instructional rather than restrictive. As has been described in detail above, the present invention has been contemplated to overcome the deficiencies of the prior equalizing safety valves specifically by improving the sealing capabilities of curved flapper subsurface safety valves.  
         [0073]    Whereas the present invention has been described in relation to the drawings attached hereto, it should be understood that other and further modifications, apart from those shown or suggested herein, might be made within the scope and spirit of the present invention.