Abstract:
Valve assemblies and methods of designing valve assemblies having a valve ball, an upper seat, and a lower seat. When fluid pressure is applied to the valve ball, the valve ball will float in the axial direction between an upper position, a neutral position, and a lower position in response to fluid pressure. The valve assembly may be made suitable for internal blowout preventers.

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The invention relates generally to blowout preventers used in the oil and gas industry. Specifically, the invention relates to a ball valve blowout preventer with two biasing seats, allowing the valve ball to float. 
     2. Background Art 
     Well control is an important aspect of oil and gas exploration. When drilling a well, for example, in oil and gas exploration applications, safety devices must be put in place to prevent injury to personnel and damage to equipment resulting from unexpected events associated with the drilling activities. 
     Drilling wells in oil and gas exploration involves penetrating a variety of subsurface geologic structures, or “layers.” Occasionally, a wellbore will penetrate a layer having a formation pressure substantially higher than the pressure maintained in the wellbore. When this occurs, the well is said to have “taken a kick.” The pressure increase associated with the kick is generally produced by an influx of formation fluids (which may be a liquid, a gas, or a combination thereof) into the wellbore. The relatively high pressure kick tends to propagate from a point of entry in the wellbore uphole (from a high pressure region to a low pressure region). If the kick is allowed to reach the surface, drilling fluid, well tools, and other drilling structures may be blown out of the wellbore. These “blowouts” often result in catastrophic destruction of the drilling equipment (including, for example, the drilling rig) and substantial injury or death of rig personnel. 
     Because of the risk of blowouts, blowout preventers (“BOP”) are typically installed at the surface or on the sea floor in deep water drilling arrangements to effectively seal a wellbore until active measures can be taken to control the kick. Blowout preventers may be activated so that kicks may be adequately controlled and circulated out of the system. 
     Just as a kick will propagate up the well, it may also enter the drill string and propagate through the inside of the drill string. To control a kick inside the drill string, a drill string internal blowout preventer (“IBOP”), sometimes called a “kelly valve” or a “kelly cock,” is used to seal off the drill string until measures can be taken to control the kick. An example of a Kelly cock is disclosed in U.S. Pat. No. 4,480,813, and incorporated herein by reference in its entirety. (An IBOP is sometimes called a “kelly valve” because, on older-style rigs, the IBOP was typically located near the “kelly,” which is a non-circular part of the drill string that is used to impart rotary motion to the drill string.) 
     An IBOP can be formed from of a variety of different types of valves, but a ball valve configuration is the most standard type. Ball valve type IBOPs typically include a valve ball that is located between two seats in the middle of a passage. The valve ball has a through hole, and can be rotated between two positions. U.S. Pat. No. 5,246,203 issued to McKnight et al. (“McKnight”), incorporated by reference in its entirety, discloses an oilfield valve that incorporates a ball valve mechanism. The mechanics of a basic ball valve mechanism are demonstrated in the McKnight patent. In a first position, as demonstrated in  FIG. 1A , the through hole of the valve ball will align with the passage of the pipe or drill string. This position will allow complete, undisrupted fluid flow through the passage. The valve ball can then be rotated from this position into a second position, as demonstrated in  FIG. 1B , to be misaligned with the passage of the pipe, disrupting fluid flow. Each of the seats surrounding the valve ball, one upper seat and one lower seat, seal against the valve ball, not allowing flow between the valve ball and the seat. Thus, the valve ball, coupled with the two seats sealing against the valve ball, can completely stop flow through the pipe passage when the valve ball is positioned to misalign with the through hole passage by having the seats seal up against the valve ball. In  FIG. 1B , a seal is made between the seats,  101  and  102 , and the ball valve,  105 , to completely prohibit flow through the passage. The valve ball has the ability to seal against the seats to be effective against even the highest of pressures, allowing the arrangement to be used as an IBOP. 
     One issue with this type of ball valve arrangement is that when the valve ball is in the second position, blocking flow through the passage, as seen in  FIG. 1B , a force is being transmitted to the valve ball across the diameter of the valve ball, placing the valve ball under a compressive force. The compressive force is translated through the ball to act at highest stress on points  110  and  111 , where the points  110 ,  111  are relatively thin metal sections. The high stress may lead to deformation, crushing the ball. When the valve ball begins to deform, the generally round flow through passage of the ball warps into a narrower passage, representing that of an ellipse. After being deformed, when the valve ball is in the “closed” second position, the seal between the valve ball and the seats is undermined because of uneven contact between the seats and the valve ball. As well, in the “open” first position, the deformed valve ball will disrupt fluid flow through the passage because the elliptical through hole of the valve ball will not completely align with the round passage. 
     McKnight discloses an oilfield valve that includes a ball valve mechanism that seeks to reduce any valve ball deformation. An embodiment disclosed by McKnight is shown in  FIG. 2 , which shows an oilfield ball valve  200  that includes a valve ball  205 , and an upper and a lower seat  210 ,  220  sealed against the valve ball  205 . The lower seat  220  is supported by a spring  221  underneath, while the upper seat  210  is firmly positioned in the passage. When a fluid force is applied from above, the valve ball  205  translates the fluid force to the lower seat  220 , and the lower seat  220  translates the fluid force to the spring  221  supporting it. If the fluid force is large enough, the spring  221  will compress. During compression, the lower seat  220  moves with the spring  221 , and the valve ball  205  “floats” down to stay in sealing contact with the lower seat  220 . Because the upper seat  210  is firmly positioned in the passage, the upper seat  210  does not move with the valve ball  205 . The space between the upper seat  210  and the valve ball  205  provides an opening for the fluid to travel through. The fluid then fills up the valve ball  205 , which translates the fluid force through the lower end of the valve ball  205  into the lower seat  220 . 
     The translation of the fluid force is shown in  FIG. 1B . As the fluid enters the valve ball  105 , the fluid force moves from position  151 , on the outer diameter on top of the valve ball  105 , to position  152 , on the inner diameter inside of the valve ball  105 . In this fashion, the valve ball  105  will no longer experience a crushing load. Accordingly, the stress on the valve ball  105  is moved to section  115 , one arcuate side of the valve ball. As a result, the fluid force is more directly translated to the lower seat  101 . 
     An issue with having a ball valve arrangement with a spring loaded lower seat, as described above, is that the valve ball can only float in one direction. When fluid force comes from above the passage, translating down on the valve ball, the valve ball is able to float against the lower seat if necessary to prevent the valve ball from deforming. However, such an assembly does not allow the ball to float in the upward direction. Thus, when the valve ball is in its closed position and the fluid force coming from below the valve ball is too great, the valve ball will still deform from stress. The valve ball deformation is because the ball valve arrangement does not provide an opening for the upward flowing fluid, fluid flowing opposite to the direction the valve ball is allowed to float in, to travel into the valve ball and redistribute the stresses on the valve ball. 
     U.S. Pat. No. 6,662,886 issued to Russell (“Russell”) discloses another type of ball valve assembly and is incorporated by reference in its entirety. Russell discloses a ball valve that comes in a preassembled valve cartridge for use as a mudsaver valve. The cartridge ball valve assembly includes a sealing spring loaded lower seat, similar to that of the McKnight patent. A difference is that the ball valve disclosed by Russell does not have an upper seat that seals against fluid pressure exerted from below. Russell discloses that allowing pressure to be transmitted past the upper seat is advantageous because it allows for the pressure to be monitored by a pressure sensor upstream from the ball valve. For an IBOP, allowing pressure to transmit across the closed valve ball in either direction is undesirable. 
     For an IBOP, crushing amounts of fluid pressure may come from above and below. Accordingly, relief of the fluid pressure while preventing communication of the fluid pressure may be desirable in IBOP applications. What is still needed, therefore, are improved ball valve assemblies, particularly for use in IBOPs. 
     SUMMARY OF INVENTION 
     In one aspect, the present invention relates to a valve assembly. The valve assembly includes a housing having a passage therethrough and a valve ball having a through hole. The valve ball is inside the housing and rotatable between an open and a closed position with the open position aligning with the passage and the closed position misaligning with the passage. The valve assembly further includes an upper seat inside the housing located above the valve ball and biased towards the valve ball, and a lower seat inside the housing located below the valve ball and biased towards the valve ball. The valve ball is moveable in an axial direction between a lower position, a neutral position, and an upper position. 
     In another aspect, the present invention relates to an internal blowout preventer. The internal blowout preventer includes a housing having a passage therethrough with the housing configured to be connected to a drill string and a valve ball having a through hole. The valve ball is inside the housing and is rotatable between an open and closed position with the open position aligning with the passage and the closed position misaligning with the passage. The valve ball is moveable in an axial direction between a lower position, a neutral position, and an upper position. The internal blowout preventer further includes a rotation means for rotating the valve ball between the open position and the closed position, an upper seat inside of the housing located above the valve ball, and a lower seat inside the housing located below the valve ball. Also, the internal blowout preventer includes a biasing means for biasing the upper seat into sealing contact with the valve ball when the valve ball is in the upper position and the neutral position and a biasing means for biasing the lower seat into sealing contact with the valve ball when the valve ball is the neutral position and the lower position. A spacer inside the housing between the upper seat and the lower seat and at least partially surrounding the valve ball is also included in the internal blowout preventer. 
     In another aspect, the present invention relates to a valve assembly. The valve assembly includes a housing having a passage therethrough and a valve ball having a through hole. The valve ball is inside the housing and is rotatable between an open and closed position with the open position aligning with the passage and the closed position misaligning with the passage. The valve ball is moveable in an axial direction between a lower position, a neutral position, and an upper position. The valve assembly further includes a rotation means for rotating the valve ball between the open position and the closed position, an upper seat inside of the housing located above the valve ball, and a lower seat inside the housing located below the valve ball. Also, the valve assembly includes a biasing means for biasing the upper seat into sealing contact with the valve ball when the valve ball is in the upper position and the neutral position and a biasing means for biasing the lower seat into sealing contact with the valve ball when the valve ball is the neutral position and the lower position. 
     In another aspect, the present invention relates to a method of designing a valve assembly with a valve ball, an upper seat, and a lower seat. The method includes determining an upper position pressure at which the valve ball changes from the neutral position to a lower position and determining a lower position pressure at which the valve ball changes from the neutral position to a lower position. The method further includes selecting a distance the valve ball may move in an axial direction between the lower position and the upper position. Then, at least one of the group consisting of an upper spring coefficient, a lower seat travel distance, and an inner diameter of the lower seat is selected based on the upper position pressure. At least one of the group consisting of a lower spring coefficient, an upper seat travel distance, and an inner diameter of the upper seat is selected based on the lower position pressure. 
     Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A and 1B  are cross-sectional views of a prior ball valve assembly. 
         FIG. 2  is an exploded view of a prior art internal blowout preventer. 
         FIG. 3  is a cross-sectional view of a ball valve mechanism in accordance with an embodiment of the present invention. 
         FIG. 4  is an isometric view of a ball valve mechanism in accordance with an embodiment of the present invention. 
         FIG. 5  is a cross-sectional view of a prior art ball valve actuation device. 
         FIG. 6  is a cross-sectional view of a ball valve mechanism in accordance with an embodiment of the present invention. 
         FIGS. 7A ,  7 B, and  7 C are cross-sectional views of a ball valve mechanism in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention relates to ball valve assemblies. More specifically, embodiments of the present invention provide high and low pressure sealing ability on both sides of the ball valve in a manner suitable for use in an IBOP. 
       FIG. 3  is a cross-sectional view of a ball valve assembly  301 , in accordance with one embodiment of the invention. The major components of the ball valve assembly  301  are a housing  350 , an upper seat assembly  310 , a lower seat assembly  320 , a valve ball  305 , and a spacer  332 . In this embodiment, the housing  350  includes an upper passage section  351  and a lower passage section  352 . The upper passage section  351  may be larger than the lower passage section  352 . 
     In this embodiment, the lower seat assembly  320  may have an upper end  321  and a lower end  322 . As shown, the lower end  322  is received in the lower passage section  352  by the housing  350 . In this embodiment, the lower seat  320  sits on a biasing mechanism, in which the biasing mechanism is a wave spring  323 . The wave spring  323  is in a partially-compressed state between a housing shoulder  356  and the lower seat  320 . The wave spring  323 , when partially-compressed, provides a spring force that translates up through the lower seat  320 . The upper end  321  of the lower seat assembly  320  generally has a rounded arcuate surface  324 . The rounded arcuate surface  324  of the lower seat  320  is in contact with the valve ball  305 , and is able to seal against pressure in either direction. The force that the wave spring  323  applies to the lower seat  320  helps keep the valve ball  305  in sealing contact with the lower seat  320 . Those having ordinary skill in the art will appreciate that other biasing mechanisms, such as Bellville washers, helical springs, and elastomeric material, may be used instead of a wave spring without departing from the scope of the present invention. 
     In this embodiment, the upper seat assembly  310  is designed to have sealing contact with the valve ball  305 . The upper seat  310  generally has a rounded arcuate surface  312 , which corresponds to the surface of the valve ball  305  to provide better sealing contact between the valve ball  305  and the upper seat  310 . The upper seat  310  has a biasing mechanism located above it, in which the biasing mechanism is a wave spring  314 . The wave spring  314  is partially-compressed between the upper seat  310  and a securing mechanism  340 . Similar to the lower wave spring  323 , the upper wave spring  314  translates a spring force through the upper seat  310  and assists in the sealing contact between the valve ball  305  and the upper seat  310 . Those having ordinary skill in the art will appreciate that other biasing mechanisms, such as Bellville washers, helical springs, and elastomeric material, may be used instead of a wave spring without departing from the scope of the present invention. 
     The valve ball  305  has a through hole  306 . In the pictured embodiment, the through hole  306  forms a generally cylindrical passage. The valve ball  305  rotates between an “open” position, in which the through hole  306  of the valve ball  305  is aligned with the passage  351 , and a “closed” position, in which the through hole  306  of the valve ball  305  is misaligned with the passage  351 . The valve ball may have a smooth exterior as to provide a better surface for sealing against the upper seat  310  and the lower seat  320 . Because the upper seat  310  and the lower seat  320  are biased towards the valve ball  305 , spring loaded in this embodiment, the valve ball  305  has the ability to move up and down in the housing  310  and keep sealing contact with each of the seats  310 ,  320  at the sealing surfaces  312 ,  324 . The position in which the valve ball  305  is in sealing contact with the upper valve seat  310  and the lower valve seat  320  is referred to as the “neutral position” of the valve ball  305 . 
     The particular embodiment shown in  FIG. 3  further includes an upper seat stop  331  below the upper seat  310  that fits into a groove  355  of the housing  350 . The upper seat stop  331  limits the movement of the upper seat  310 . Adjacent to a surface of the upper seat stop  331  is a spacer  332 . The spacer  332  is positioned between the upper seat stop  331  and the lower seat  320 . The spacer  332  partially encompasses the valve ball  305 , keeping the valve ball  305  from moving in the radial direction.  FIG. 4  is an isometric view of a ball valve assembly  401 , in accordance with one embodiment of the invention. The ball valve assembly  401  includes a rotation device  610  for rotating the ball valve  305 . The rotation device  610  is discussed more fully below. When the spacer  332  is positioned adjacent to the upper seat stop  331 , the spacer  332  limits the movement of the lower seat  320 . With the upper seat stop  331  and the spacer  332  in place, the upper seat  310  and lower seat  320  are limited in axial movement up and down the passage  351 . 
     When the valve ball  305  is in the closed position and blocking flow through the passage  351  from above, the fluid applies pressure against the valve ball  305 . The pressure against the valve ball  305  translates as a downward force to the lower wave spring  323 . If the pressure is great enough, the lower wave spring  323  compresses and the lower seat  320  moves downward in the axial direction. As the lower seat  320  moves downward, the valve ball  305  moves downward with the lower seat  320 , staying in sealing contact with the lower seat  320 . As the valve ball  305  moves downward, the upper seat  310  moves downward with the valve ball  305 . As the upper seat  310  moves downward, the upper wave spring  312  extends from the original partially-compressed state. As the upper wave spring  312  extends, the wave spring  312  continues to bias the upper seat  310  towards to the valve ball  305 , keeping the upper seat  310  and the valve ball  305  in sealing contact. As the upper seat  310  moves downward, the upper seat stop  331  allows the upper seat  310  to move only a selected distance before contact is made between the upper seat  310  and the upper seat stop  331 , preventing any further movement of the upper seat  310 . If the fluid applies any additional pressure to the valve ball  305 , when the upper seat  310  is in contact with the upper seat stop  331 , the valve ball  305  and the lower seat  320  continue to move down the passage  351  and the upper seat  310  stays in position against the upper seat stop  331 . The sealing contact between the valve ball  305  and the upper seat  310  is lost and fluid enters into the valve ball  305  and the spacer  332 . As the fluid enters the valve ball  305  through the opening between the valve ball  305  and the upper seat  310 , the sealing contact between the lower seat  320  and the valve ball  305  is maintained, so little or no fluid escapes the ball valve assembly  301 . The position in which sealing contact is lost between the upper seat  310  and the valve ball  305 , and sealing contact is maintained between the lower seat  320  and the valve ball  305  is defined as the “lower position” of the valve ball. 
     Allowing the fluid to enter the valve ball  305  causes the fluid pressure to no longer be applied to the top of the valve ball  305  from the outside, but to be applied to the bottom of the valve ball  305  from the inside. By applying pressure to the inner surface of the valve ball  305 , the fluid force is more directly translated to the lower seat  320  and the lower wave spring  323 , which allows crushing forces across the diameter of the valve ball  305  to be avoided. Relocating the fluid force more directly to the lower seat  320  helps to avoid deforming of the valve ball  305  when the fluid pressure becomes too great. 
     The ball valve assembly  301  goes through an analogous reverse movement when the fluid pressure is coming up from below the valve ball  305  through the lower passage  352 . The fluid pressure pushes the valve ball  305  and the upper seat  310  upward in the axial direction, further compressing the upper wave spring  314  from the original partially-compressed state of the wave spring  314 . As the valve ball  305  and the upper seat  310  move upward, the lower seat  320  contacts the spacer  332 . The spacer  332  prevents any further upward movement of the lower seat  310 . If the valve ball  305  and the upper seat  310  continue to move upward from additional fluid pressure, the lower seat  320  will stay in contact with the spacer  332  and not move upward with the valve ball  305  and the upper seat  310 . The sealing contact between the valve ball  305  and the lower seat  320  is lost, and fluid enters into the valve ball  305  and the spacer  332 . As the fluid is entering the valve ball  305  through the opening between the valve ball  305  and the lower seat  320 , the sealing contact between the lower seat  320  and the valve ball  305  is maintained, so little or no fluid escapes the ball valve assembly  301 . The position in which sealing contact is lost between the lower seat  320  and the valve ball  305 , and sealing contact is maintained between the upper seat  310  and the valve ball  305  is defined as the “upper position” of the valve ball  305 . In the upper position of the valve ball  305 , when the fluid enters the valve ball  305  in the opening between the valve ball  305  and the lower seat  320 , the fluid force is applied to the inner surface of the valve ball  305 . 
     The ball valve assembly  301  may be retained into the housing  350  by a securing mechanism  340 . In one embodiment, the securing mechanism  340  may comprise a split ring  341 , a support ring  342 , and a snap ring  343 . The split ring  341 , which may include a plurality of ring segments, is received into groove  353  of the housing  350 . The split ring  341  segments may be placed down into the passage  351  one at a time, and then moved radially outward into the groove  353 . After the segments of the split ring  341  are in the groove  353 , the support ring  342  is placed into the passage  351  to be radially inside of the split ring  341 . The support ring  342  helps to prevent the segments of the split ring  341  from moving radially inward. A snap ring  343  may be used to fix the support ring  342  in place. The snap ring  343  is radially compressed, placed in alignment with the groove  344  in the split ring  341 , and released to radially expand into groove  344 . The snap ring  343  sits above the support ring  342  and keeps the securing mechanism  340  in a fixed position. Those having ordinary skill in the art will appreciate that other securing mechanisms, such as set screw, may be used without departing from the scope of the present invention. 
     Shown in this embodiment, a spacer  360  is disposed between the securing mechanism  340  and the upper wave spring  314 . The spacer  360  disposed between the securing mechanism  340  and the upper wave spring  314  is not necessary to the design, but it may be present to properly fit the wave springs  314 ,  323  into the ball valve assembly  301 . If the spacer  360  is not present, then the upper wave spring  314  may be positioned between the upper seat  310  and the securing mechanism  340 . If the spacer  360  is present, as shown in  FIG. 3 , the upper wave spring  314  may be positioned between the upper seat  310  and the spacer  360 . 
     A groove  326  may be formed in the lower seat  320 . The purpose of the groove  326  is to fit a lower seal  327  around the lower seat  320 . The lower seal  327 , which may be an O-ring, prevents fluid pressure from bypassing the ball valve assembly  301  entirely. The housing  350  may also have a groove  354  for an upper seal  315 , which may be an O-ring. The upper seal  315  seals between the upper seat  310  and the housing  350 . The seals  327 ,  315  help to prevent fluid from flowing around the ball valve assembly  301 . Fluid flow around the ball valve assembly  301  can erode or corrode the components, which could degrade the performance of the ball valve. 
     Shown in  FIG. 6  is another embodiment of the present invention. Similar to the embodiment of  FIG. 3 , a ball valve assembly  601  includes an upper seat  310  and a lower seat  320 , biased towards a valve ball  305 . In this embodiment, the seats  310 ,  320  are biased with an upper wave spring  314  and a lower wave spring  323 . The embodiment has a lower seat stop  621  which limits the axial movement of the lower seat  320 , instead of having a spacer  332  extending down from the upper seat stop  331  as shown in  FIG. 3 . The valve ball  305  is rotated between the open and closed positions of the valve ball  305  by a rotation device  610  fitting into a recess  605  on the valve ball  305 . The rotation device includes a tongue shaped rib  611 , a rotating disc  612 , and an actuator  613 . In the recess  605  fits the tongue shaped rib  611 , where the tongue shaped rib is attached to the rotating disc  612 . The rotating disc  612  is interlocking with the actuator  613 , where the means of interlocking between the rotating disc  612  and the actuator  613  is obvious to those of ordinary skill in the art.  FIG. 2  shows a method used in the prior art, which may be used for this invention. The actuator  613  has a stem  614 , in which the stem  614  has a wrench fitting  615  to receive an Allen wrench or other interlocking shape. The stem  614  may have ball bearings  616  located around the stem  614  to reduce friction in the rotation of the rotation device  610 .  FIG. 4  shows how the rotation device  610  may fit into the embodiment of the ball valve assembly  401  when a spacer  332  is present. Those having ordinary skill in the art will appreciate that other rotation means may be designed to rotate the valve ball  305  without departing from the scope of the present invention. 
     The rotation device  610  can be rotated manually with a wrench, or by a machine drive. For the use in an IBOP, it is preferable for the ball valve assembly to be rotated by a machine drive for reliability and speed. An example of a prior art machine driven actuation device for the ball valve assembly  301  is shown in  FIG. 5 . An example of a machine actuation device is disclosed in U.S. Pat. No. 5,507,467, issued to Mott and incorporated herein by reference in its entirety. In this configuration, the ball valve assembly  301  can be remotely operated in the event the valve ball  305  needs to be rotated quickly and correctly. 
     In one or more embodiments, the ball valve assembly may be designed for sealing contact between the seats and the valve ball may be designed to “break”, or lose sealing contact, at a selected pressure. When the selected pressure is reached, the valve ball will move from the neutral position into the upper position or lower position to prevent too high of pressure from being applied across the valve ball. Those having ordinary skill in the art will appreciate that the pressure limits (i.e. pressure that is “too high”) may vary depending on the strength of the valve ball, in addition to other factors. 
     The neutral position of the valve ball assembly  301  is shown in  FIG. 7B . The valve ball  305  is in sealing contact with the upper seat  310  and the lower seat  320 , in which the valve ball  305  “floats” between the two seats. The ball valve assembly  301  seals between the valve ball  305  and the upper seat  310  and the lower seat  320  in both axial directions of flow, in which the flow pressure is of a value less than a selected pressure. When the fluid pressure reaches a selected pressure, the ball valve assembly will move into either a lower position or an upper position, depending on the direction of the fluid pressure. In the lower position, shown in  FIG. 7A , the valve ball  305  is pushed against the lower seat  320  from the fluid pressure from above the ball valve assembly  301 . In the upper position, shown in  FIG. 7C , the valve ball  305  is pushed against the upper seat  310  from the fluid pressure from below the valve ball assembly  301 . To be most effective, the valve ball  305  should have an axial travel greater than axial travel of the upper seat  310  and the lower seat  320 . 
     Embodiments of the present invention may be designed in which the selected pressures to move the valve ball into the upper position and the lower position are different from one another. For example, in an embodiment in which the ball valve assembly is used as an IBOP, higher fluid pressures are expected to enter from below the ball valve assembly, as compared to the fluid pressures entering from above the ball valve assembly. The selected pressure to move the ball valve assembly into the upper position in  FIG. 7C , an “upper position pressure,” could be more than the selected pressure to move the ball valve assembly into the lower position in  FIG. 7A , a “lower position pressure,” to accommodate for the higher pressure entering from the bottom. 
     In the upper position in  FIG. 7C , the upper position pressure for the ball valve assembly  301  is established from a balance of several factors: an upper spring coefficient, a lower seat travel distance, and a lower piston area. The upper spring coefficient, k upper , is the “stiffness” or “spring rate” of the upper biasing mechanism, in which a biasing mechanism is stiffer with a higher coefficient. The lower seat travel distance is the distance the lower seat  320  may travel between the lower seat stop  621  and the lower wave spring. The lower piston area is the area inside the diameter of the seal between the lower seat  320  and the valve ball  305 , which is the effective area upon which the fluid pressure acts. The upper position pressure may be increased by, for example, an increase in the upper spring coefficient, an increase in the lower seat travel distance, a decrease in the lower piston area, or a combination thereof. The upper position pressure may be decreased by an inverse of the previous example, in which the upper spring coefficient may be decreased, the lower seat travel may be decreased, the lower piston area may be increased, or a combination thereof. Those having ordinary skill in the art will appreciate that a spring coefficient may refer to any biasing mechanism, such as Bellville washers, helical springs, or elastomeric material, in which the biasing mechanism may exert a force over a compressed or extended distance. 
     In the lower position in  FIG. 7A , the lower position pressure for the ball valve assembly  301  is established from a balance of several factors: a lower spring coefficient, an upper seat travel distance, and an upper piston area. The lower spring coefficient, k lower , is the “stiffness” or “spring rate” of the lower biasing mechanism. The upper seat travel distance is the distance the upper seat  310  may travel between the upper seat stop  331  and the upper wave spring. The upper piston area is the area inside the diameter of the seal between the upper seat  310  and the valve ball  305 , which is the effective area upon which the fluid pressure acts. The lower position pressure may be increased by, for example, an increase in the lower spring coefficient, an increase in the upper seat travel distance, a decrease in the upper piston area, or a combination thereof. The lower position pressure may be decreased by an inverse of the previous example, in which the lower spring coefficient may be decreased, the upper seat travel may be decreased, the upper piston area may be increased, or a combination thereof. 
     Embodiments of the present invention have one or more of the following advantages. By avoiding crushing loads on the valve ball, more economical valve balls may be used because of the reduced strength requirements. For example, a valve ball may have a thinner wall, thereby requiring less material to manufacture. Further, the valve ball may be made with a material having a relatively lower tensile strength than prior art valve balls. The economic advantage is particularly pronounced in high pressure applications, in which the ball valve assembly must withstand greater than 15,000 psi. 
     As shown above, the ball valve assembly makes sealing contact in both directions of the passage, not allowing fluid flow when the valve ball is in its closed position. Having both the upper and lower seats being biased towards the valve ball can be effective in a wide range fluids with a wide range of pressures. Low pressure fluids will not provide enough fluid force as to move the valve ball from the neutral position, sealing the fluid between the upstream seat and the valve ball. High pressure fluids will provide enough fluid force as to move the valve ball in the direction of the fluid flow, sealing the fluid between the downstream seat and the valve ball. As well, the range of fluids which may be used in the present invention include that of both high and low viscosity. The openings made between the valve ball and the lower and upper seats when the valve ball is in the upper or lower position can be designed to allow high viscosity fluid to be allowed to flow into the valve ball, or designed to allow fluids with particles in them to flow through into the valve ball. Being adaptable to such a range of fluids is ideal for an IBOP because the expected drilling mud that can be passing through the ball valve assembly varies in consistency and may contain sediment or other solids within the drilling mud. 
     Embodiments of the present invention may also be designed to replace existing ball valve mechanisms. For example, a housing having a ball valve mechanism similar to that disclosed by McKnight can have its upper seat removed. An upper seat and biasing mechanism can then be installed with a spacer to restrict movement of the upper seat. Although such an embodiment may require modification to the housing, significant savings can be achieved by reusing components. 
     While this invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.