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BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the invention generally relate to methods and apparatus for use in oil and gas wellbores. More particularly, the invention relates to an isolation valve with debris control and flow tube protection. 
     2. Description of the Related Art 
     An isolation valve is located as part of the casing string and operated through a control line. The isolation valve is configured to temporarily isolate a formation pressure below the isolation valve such that a tool string may be quickly and safely tripped into a portion of the wellbore above the isolation valve that is temporarily relieved to atmospheric pressure. Thus, the isolation valve allows the tool string to be tripped into and out of the wellbore at a faster rate than snubbing in the tool string under pressure. Since the pressure above the isolation valve is relieved, the tool string can trip into the wellbore without wellbore pressure acting to push the tool string out. 
     The isolation valve is movable between an open position and a closed position by selectively actuating a flapper valve of the isolation valve. The flapper valve is actuated by the movement of a flow tube in the isolation valve. In the closed position, the flapper valve obstructs a bore through the isolation valve, and in the open position, the flapper valve resides in a flapper valve cavity. Prior designs for the isolation valve can suffer from various disadvantages. One disadvantage of prior designs is that debris and mud may enter the flapper valve cavity during operation of the isolation valve. The debris and mud may inhibit the function of the flapper valve and thereby affect the opening and/or closing of the isolation valve. Another disadvantage of prior designs is that an end of the flow tube oftentimes becomes damaged while stripping or tripping the drill string through the isolation valve. The damaged flow tube may subsequently cause damage to the flapper valve as the flow tube moves through the isolation valve. Therefore, there exists a need for an improved isolation valve assembly and associated methods. 
     SUMMARY OF THE INVENTION 
     The present invention generally relates to an isolation valve with debris control. In one aspect, an isolation valve for use as part of a casing string is provided. The isolation valve includes a housing having a bore and a valve cavity. The isolation valve further includes a valve member movable between a first position in which the valve member obstructs the bore of the housing and a second position in which the valve member is disposed in the valve cavity. Further, the isolation valve includes a flow tube configured to allow movement of the valve member between the first and second positions. Additionally, the isolation valve includes an engagement assembly adapted to engage the flow tube to substantially prevent debris from entering the valve cavity when the valve member is in the second position. 
     In another aspect, a method of operating an isolation valve in a wellbore is provided. The method includes the step of placing the isolation valve in the wellbore. The isolation valve includes a housing, a valve member, a flow tube, a piston and an engagement assembly. The method further includes the step of moving the valve member into a bore of the housing to obstruct a flow path through the isolation valve. The method also includes the step of moving the flow tube into interference with the valve member to open the flow path through the isolation valve. Additionally, the method includes the step of moving the flow tube into engagement with the engagement assembly to protect the valve member from debris. 
     In yet a further aspect, an isolation valve is provided. The isolation valve includes a housing having a bore. The isolation valve further includes a flapper pivotally movable between a closed position in which the bore is blocked and an opened position in which the bore is open to fluid flow. The isolation valve also includes a movable flow tube for shifting the flapper between the opened position and the closed position. Additionally, the isolation valve includes an engagement assembly adapted to engage the flow tube when the flapper is in the opened position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of 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. 
         FIG. 1  is a cross-section view of an isolation valve in an open position, according to one embodiment of the invention. 
         FIGS. 1A and 1B  are enlarged views of the isolation valve illustrated in  FIG. 1 . 
         FIG. 2  is a cross-section view of the isolation valve in a closed position. 
         FIGS. 2A and 2B  are enlarged views of the isolation valve illustrated in  FIG. 2 . 
         FIG. 3  is a cross-section view of the isolation valve in a locked position. 
         FIGS. 3A and 3B  are enlarged views of the isolation valve illustrated in  FIG. 3 . 
         FIG. 4  is a cross-section view of an isolation valve in an open position, according to one embodiment of the invention. 
         FIGS. 4A and 4B  are enlarged views of the isolation valve illustrated in  FIG. 4 . 
         FIG. 5  is a cross-section view of the isolation valve in a closed position. 
         FIGS. 5A and 5B  are enlarged views of the isolation valve illustrated in  FIG. 5 . 
         FIG. 6  is a cross-section view of the isolation valve in a locked position. 
         FIGS. 6A and 6B  are enlarged views of the isolation valve illustrated in  FIG. 6 . 
         FIGS. 7A-7C  illustrate a hinge arrangement for a flapper valve. 
         FIG. 8  is a cross-section view of an engagement assembly. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention generally relate to an isolation valve with flow tube protection. The isolation valve may be a downhole deployment valve or a formation deployment valve. To better understand the aspects of the present invention and the methods of use thereof, reference is hereafter made to the accompanying drawings. 
       FIG. 1  shows a cross-section view of an isolation valve  100  in an open position to thereby enable tools such as a drill string to pass through a longitudinal central bore  110  of the isolation valve  100 . The isolation valve  100  includes an outer housing  115  with a flow tube  120  disposed within the housing  115 . The flow tube  120  represents an exemplary mechanism for moving a flapper  105  to open and close the isolation valve  100 , although other types of actuators may be used in some embodiments. In one embodiment, the flapper  105  may be biased in toward the closed position and may reside in a flapper cavity  165  when in the open position. The flow tube  120  may move within the housing  115  based on control signals received to selectively displace the flapper  105  between the open position and the closed position. The flow tube  120  moves across an interface between the flapper  105  and a seat  130  to engage and urge the flapper  105  to the open position or disengage and allow the flapper to return to the closed position. As will be described herein, the flow tube  120  covers the flapper  105  when the isolation valve  100  is in the open position to at least inhibit debris and drilling fluid from collecting around the flapper  105  and the flapper cavity  165 . Build-up of solids between a backside surface of the flapper  105  and the housing  115  can impede the flapper  105  from moving to the closed position after withdrawing the flow tube  120  out of interference with the flapper  105 . 
     The isolation valve  100  includes control line connections  125  at an end of the housing  115  that are in communication with control lines (not shown). The control lines provide fluid via fluid channels  235 ,  240  to first and second piston chambers  135 ,  140  that are defined between the housing  115  and the flow tube  120 . A piston  175  spans an annular area between the housing  115  and the flow tube  120  to define and isolate the first and second chambers  135 ,  140  from one another. The piston  175  is movable to change the relative sizes of the chambers  135 ,  140 . 
     As shown in  FIG. 1A , the piston  175  is attached to the flow tube  120  via a releasable member  150 , such as a shear pin. Fluid pressure can be introduced into the second piston chamber  140  through the channel  240  to act on the piston  175 . The fluid pressure moves the piston  175  and the attached flow tube  120  in a first direction to open the isolation valve  100 . In this respect, the flow tube  120  contacts the flapper  105  and urges the flapper  105  toward the flapper cavity  165 . To return to the closed position, fluid pressure is introduced in the first piston chamber  135  via the channel  235  to act on the piston  175 . The fluid pressure moves the piston  175  and the attached flow tube  120  in a second opposite direction to slide the flow tube  120  out of interference with the flapper  105 . The isolation valve  100  may be movable between the open position and the closed position multiple times by introducing fluid pressure in the respective piston chamber  135 ,  140 . As also shown in  FIG. 1A , a biasing member  160  is disposed between the flow tube  120  and the piston  175 . The biasing member  160  is configured to allow the flow tube  120  to move relative to the piston  175  by compressing the biasing member  160 . The biasing member  160  may be an elastomer, a spring or any other type of biasing member known in the art. As also shown in  FIG. 1A , a lock member  205  (such as a lock ring) is disposed within the housing  115 . The lock member  205  is compressed and held in place by a shear ring  210 . As will be discussed herein, the lock member  205  is configured to interact with a groove  215  in the flow tube  120  when the isolation valve  100  is moved to a locked position ( FIG. 3 ). 
     As shown in  FIG. 1B , the isolation valve  100  further includes an engagement assembly  170 . The engagement assembly  170  is configured to interact with the flow tube  120  when the isolation valve  100  is in the open position in order to protect (and/or seal) the flapper cavity  165  from debris. In this respect, the engagement assembly  170  may be placed below the flapper cavity  165 . In one embodiment, the engagement assembly  170  includes a guide member  180  and a sleeve member  195  that are interconnected via a shearable member  190 . The guide member  180  may include a tapered inner surface that is configured to centralize a drill string and/or other tools passing through the isolation valve  100 . In addition, the engagement assembly  170  is configured to protect the end of the flow tube  120  from damage due to the movement of the drill string and other tools through the isolation valve  100 . In this embodiment, the engagement assembly  170  may absorb impact from the drill string because it is the first (or lowest) component in the isolation valve  100 , which is in contact with the drill string as the drill string moves upward through the bore  110  of the isolation valve  100 , the engagement assembly  170  substantially shields the flow tube  120  from any damage that may occur. In addition, the engagement assembly may direct the drill string (using the tapered surface) into the inner diameter of the flow tube  120 , thereby protecting the end profile of the flow tube  120 . The engagement assembly  170  may also direct debris into the inner diameter of the flow tube  120  to prevent packing off of the flapper cavity  165 . In this manner, the engagement assembly  170  may shield the flow tube  120  from damage that may occur as the drill string or fluid moves upward through the bore  110  of the isolation valve  100 . 
     During opening of the isolation valve  100 , the piston  175  moves the flow tube  120  in a direction toward the engagement assembly  170  when fluid pressure is introduced into the second piston chamber  140  via the channel  240 . The flow tube  120  continues in the direction until a lower portion  155  of the flow tube  120  contacts an upper portion  185  of the guide member  180 . In one embodiment, the lower portion  155  of the flow tube  120  includes a shaped surface, such as a bull-nosed shape, which is configured to contact with a surface on the upper portion  185  of the guide member  180 . In one embodiment, the surface on the upper portion  185  is shaped to mate with the shaped surface. The flow tube  120  and the guide member  180  are optionally biased against each other to maintain contact between the flow tube  120  and the guide member  180  after the isolation valve  100  is in the open position. In the embodiment illustrated, the biasing member  160  ( FIG. 1A ) is used to bias the flow tube  120  against the guide member  180  of the engagement assembly  170 . In another embodiment, a biasing member may be placed in between the components of the engagement assembly  170 . In further embodiment, a biasing member may be placed in the engagement assembly  170  and in the flow tube  120 . The biased contact arrangement is optionally used to maintain contact between the flow tube  120  and the guide member  180 , and in this manner the flapper cavity  165  is protected from debris that may restrict the operation of the flapper  105 . 
       FIG. 2  illustrates a cross-section view of the isolation valve  100  in a closed position. As shown, the flapper  105  is obstructing the longitudinal central bore  110  through the isolation valve  100 . To close the isolation valve  100 , fluid pressure is supplied to the first piston chamber  135  (see  FIG. 2A ) via the channel  235 , which moves the flow tube  120  out of interference with the flapper  105 . Because the flapper  105  is biased toward the seat  130 , movement of the flow tube  120  out of interference with the flapper  105  allows the flapper  105  to move toward the seat  130 . The seat  130  is a portion of the isolation valve  100  that engages the flapper valve  105  when the isolation valve  100  is in the closed position. The seat  130  may be part of the housing  115 , or the seat  130  may be a separate component in the isolation valve  100 . Additionally, as the flow tube  120  moves through the housing  115 , the flow tube  120  disengages from the guide member  180  of the engagement assembly  170  (see  FIG. 2B ). 
       FIG. 3  illustrates a cross-section view of the isolation valve  100  in a locked position. As set forth herein, the isolation valve  100  is movable between the open position and the closed position multiple times by introducing fluid pressure in the respective piston chamber  135 ,  140 . The isolation valve  100  may be locked in the open position by manipulating the location of the flow tube  120 . The flow tube  120  includes inner mating profiles  145  that enable engagement of the flow tube  120  with a corresponding profile tool (not shown) for manipulating the location of the flow tube  120 . To permit free movement of the flow tube  120  relative to the piston  175 , a predetermined force is required to break the releasable member  150  between the flow tube  120  and the piston  175 . Upon application of the predetermined force using the profile tool, the releasable member may break into a first portion  150 A and a second portion  150 B (see  FIG. 3A ). Thereafter, the flow tube  120  is allowed to move through the housing  115  a distance that is greater than a distance traveled when the isolation valve  100  is moved to the open position. As the flow tube  120  moves through the housing  115 , the groove  215  moves to a location adjacent the lock member  205  to allow the lock member  205  to engage the groove  215 . Upon engagement of the lock member  205  in the groove  215 , the flow tube  120  is locked in the open position, and the flow tube  120  will no longer be able to move to close the isolation valve  100 . In addition, as the flow tube  120  moves through the housing  115 , the sleeve contacts and acts on the guide member  180 , which causes the shearable member  190  to shear. Thereafter, the guide member  180  moves relative to the sleeve member  195  until the guide member  180  contacts a shoulder  220 , as shown in  FIG. 3B , to accommodate the extra travel required for the flow tube  120  during the locking operation. 
       FIG. 4  shows a cross-section view of another embodiment of an isolation valve  300 . Similar to the isolation valve  100 , the isolation valve  300  includes an engagement assembly  370  that is configured to interact with a flow tube  320  disposed within a housing  315 . The engagement assembly  370  and the flow tube  320  interact when the isolation valve  300  is in the open position in order to protect (and/or seal) a flapper cavity  365  from debris that may restrict the operation of a flapper valve  305 . The flow tube  320  is also used to allow a flapper valve  305  to open and close the isolation valve  300 . 
     The isolation valve  300  includes control line connections  325  that are in communication with control lines (not shown). The control lines provide fluid to first and second piston chambers  335 ,  340  via fluid channels  470 ,  475 . The first piston chamber  335  (see  FIG. 5 ) and the second piston chamber  340  (see  FIG. 4 ) are defined between the housing  315  and a piston sleeve  375 . A piston sleeve  375  is movable in response to the introduction of fluid into the piston chambers  335 ,  340 . The piston sleeve  375  includes a first piston surface  435  and a second piston surface  440 . 
     As shown in  FIG. 4A , the piston sleeve  375  is connected to the flow tube  320  via a releasable member  350 . As will be described herein, the releasable member  350  will release the connection between the piston sleeve  375  and the flow tube  320  when the isolation valve  300  is moved to the locked position. 
     Referring back to  FIG. 4 , the isolation valve  300  is in the open position, which allows drill string and/or other tools to pass through a longitudinal central bore  310  of the isolation valve  300 . To move the isolation valve  300  to the open position, fluid pressure is introduced into the second piston chamber  340  via the fluid channel  470 . The fluid pressure in the second piston chamber  340  acts on the second piston surface  440  of piston sleeve  375 , which moves the flow tube  320  in a first direction to open the isolation valve  300 . To return to the closed position, fluid pressure is introduced in the first piston chamber  335  via the fluid channel  475 , and the fluid pressure acts on the first piston surface  435  of the piston sleeve  375  which moves the flow tube  320  in a second opposite direction to slide the flow tube  320  out of interference with the flapper valve  305 . In this manner, the isolation valve  300  is movable between the open position and the closed position multiple times by introducing fluid pressure in the respective piston chamber  335 ,  340 . 
     As shown in  FIG. 4A , a biasing member  360  is disposed between the flow tube  320  and the piston sleeve  375 . The biasing member  360  is configured to allow the flow tube  320  to move relative to the piston sleeve  375  by compressing the biasing member  360 . In one embodiment, the biasing member  360  is a wave spring. In other embodiments, the biasing member  360  may be an elastomer, a helical spring or any other type of biasing member known in the art. As also shown in  FIG. 4A , a lock member  405 , such as a lock ring, is disposed within the flow tube  320 . The lock member  405  is compressed and held in place by a shear ring  410  disposed around an outer surface of the flow tube  320 . As will be discussed herein, the lock member  405  is configured to interact with a groove  415  in the housing  315  when the isolation valve  300  is moved to a locked position. 
       FIG. 4B  is an enlarged view of the engagement assembly  370 . The engagement assembly  370  is configured to interact with the flow tube  320  when the isolation valve  300  is in the open position to substantially protect a flapper cavity  365  from debris that may restrict the operation of the flapper valve  305 . As shown, the engagement assembly  370  includes a guide member  380  and a sleeve member  395  that are interconnected via a shearable member  390 . In one embodiment, the guide member  380  includes a tapered surface that is configured to centralize a drill string and/or other tools passing through the longitudinal central bore  310  of the isolation valve  300 . In addition, the engagement assembly  370  is configured to protect the end of the flow tube  320  from damage due to the movement of the drill string and other tools upward through the isolation valve  300 . Since the engagement assembly  370  is the first (or lowest) component in the isolation valve  300 , which is in contact with the drill string as the drill string moves upward through the bore  310  of the isolation valve  300 , the engagement assembly  370  substantially shields the flow tube  320  from any damage that may occur. 
     As set forth herein, the piston sleeve  375  moves the flow tube  320  in a direction toward the engagement assembly  370  when fluid pressure is introduced into the second piston chamber  340  from the fluid channel  470 . The flow tube  320  moves within the housing  315  until a lower portion  355  of the flow tube  320  is in contact with an upper portion  385  of the guide member  380 . In one embodiment, the lower portion  355  of the flow tube  320  includes a shaped surface, such as a bull-nosed shape, which is configured to mate with a corresponding shaped surface on the upper portion  385  of the guide member  380 . The flow tube  320  and the guide member  380  are optionally biased against each other to maintain contact between the flow tube  320  and the guide member  380  while the isolation valve  300  is in the open position. In the embodiment illustrated, the biasing member  360  ( FIG. 4A ) is used to bias the flow tube  320  against the guide member  380  of the engagement assembly  370 . In other embodiments, the biasing member  360  may be placed at other locations in the isolation valve  300 , such as between the components of the engagement assembly  370 . In another embodiment, there may be more than one biasing member at various locations in the isolation valve  300 . In this manner, the biased contact arrangement is used to maintain contact between the flow tube  320  and the guide member  380  to protect the flapper cavity  365  from debris that may restrict the operation of the flapper valve  305 . 
       FIG. 5  illustrates a cross-section view of the isolation valve  300  in a closed position. As shown, the flapper valve  305  is obstructing the longitudinal central bore  310  through the isolation valve  300 . To move the isolation valve  300  to the closed position, fluid pressure is supplied to the first piston chamber  335  through the fluid channel  475 , which acts on the first piston surface  435  of the piston sleeve  375  to move the flow tube  320  out of interference with the flapper valve  305 . The flapper valve  305  is biased toward the seat  330 . Therefore, the movement of the flow tube  320  out of interference with the flapper valve  305  allows the flapper valve  305  to move toward the seat  330 . In addition, the movement of the flow tube  320  through the housing  315  causes the flow tube  320  to disengage from the guide member  380  of the engagement assembly  370  (see  FIG. 5B ). 
       FIG. 6  illustrates a cross-section view of the isolation valve  300  in a locked position. The isolation valve  300  is movable between the open position and the closed position multiple times by introducing fluid pressure in the respective piston chamber  335 ,  340 . The isolation valve  300  may also be locked in the open position by manipulating the location of the flow tube  320  by mechanical force. The flow tube  320  includes inner mating profiles  345  that enable engagement of the flow tube  320  with a corresponding profile tool (not shown) for manipulating the location of the flow tube  320 . To permit free movement of the flow tube  320  relative to the piston sleeve  375 , a predetermined force is required to break the releasable member  350  between the flow tube  320  and the piston sleeve  375 . Upon application of the predetermined force, the releasable member breaks  350  into a first portion  350 A and a second portion  350 B (see  FIG. 6A ), which allows the flow tube  320  to move relative to the piston sleeve  375 . In addition, the application of the predetermined force shears the ring  410 . The movement of the flow tube  320  through the housing  315  also moves the lock member  405  to a location adjacent the groove  415  in the housing, and thereafter the lock member  405  engages the groove  415 . The flow tube  320  is locked in the open position upon engagement of the lock member  405  in the groove  415 . At this point, the flow tube  320  will no longer be able to move through the housing  315  to close the isolation valve  300 . As shown in  FIG. 6B , the movement of the flow tube  320  through the housing causes the flow tube  320  to contact and act on the guide member  380 , which causes the member  390  to shear. Thereafter, the guide member  380  moves relative to the sleeve member  395  to accommodate the extra travel required for the flow tube  320  during the locking operation. 
       FIGS. 7A-7C  illustrate a hinge arrangement  425  for the flapper valve  305 . As shown in  FIG. 7A , the hinge arrangement  425  connects the flapper valve  305  to the housing  315 . During the manufacturing process of the isolation valve  300 , the flapper valve  305  is aligned to allow for proper engagement of the flapper valve  305  and the seat  330 . The seat  330  may be part of the housing  315 , or the seat  330  may a separate component in the isolation valve. The design permits for small alignment movement along a seat/hinge mating surface  430  due to the connection members. Once the hinge arrangement  425  is aligned, the hinge arrangement  425  is fastened to the housing  315  by a plurality of connection members  420 , such as screws. Further, an adjustment locking connection member  445  may be used to fine tune the alignment of the hinge arrangement  425  and/or to prevent axial direction movement along the plane of the seat/hinge mating surface  430 . As shown in  FIG. 7C , the adjustment locking connection member  445  is attached to a portion of the housing  315 , and the adjustment locking connection member  445  is tightened in a direction along the plane of the seat/hinge mating surface  430  and therefore prevents axial direction movement along the plane of the seat/hinge mating surface  430 . Additionally, as shown in  FIG. 7C , a safety connection member  495 , such as a screw, snap ring or pin, is disposed at a location adjacent the adjustment locking connection member  445 . The safety connection member  495  is configured to substantially prevent the adjustment locking connection member  445  from inadvertently falling out during operation of the flapper valve  305 . Although the hinge arrangement  425  was described in relation to the flapper valve  305 , the hinge arrangement  425  applies to other valves such as the flapper  105 . 
       FIG. 8  is a cross-section view of an engagement assembly  450 . The engagement assembly  450  functions in a similar manner as described herein with regards to the engagement assemblies  170 ,  370 . The primary difference is that the engagement assembly  450  is made from a single piece rather than two pieces (e.g., guide member and sleeve member). The engagement assembly  450  is attached to the housing  315  via a connection member  460 , such as a resilient connection member (e.g. o-ring) or a non-resilient connection member (e.g. shear screw). One advantage of the connection member  460  being a resilient connection is that the connection member  460  may be used to bias the engagement assembly  450  in contact with the flow tube  320  while the isolation valve  300  is in the open position. The engagement assembly  450  includes a tapered surface  465 . The engagement assembly  450  is configured to take impact from a drill string and direct the drill string (using the tapered surface  465 ) into the inner diameter of the flow tube  320 , thereby protecting the end profile of the flow tube  320 . The engagement assembly  450  also directs debris into the inner diameter of the flow tube  320  to prevent packing off of the flapper cavity  365 . 
     Similar to as described herein, the engagement assembly  450  is configured to interact with the flow tube  320  when the isolation valve  300  is in the open position in order to protect the flapper cavity  365  from debris that may restrict the operation of a flapper valve  305 . To maintain contact between the flow tube  320  and the engagement assembly  450  while the isolation valve  300  is in the open position, one or both of the flow tube  320  and the engagement assembly  450  are biased toward each other. Additionally, when the isolation valve  300  is moved to the locked position, the flow tube  320  contacts and acts on the engagement assembly  450 , which causes the member  460  to shear to accommodate the extra travel required for the flow tube  320  during the locking operation. 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Summary:
The present invention generally relates to an isolation valve with debris control. In one aspect, an isolation valve for use as part of a casing string is provided. The isolation valve includes a housing having a bore and a valve cavity. The isolation valve further includes a valve member movable between a first position in which the valve member obstructs the bore of the housing and a second position in which the valve member is disposed in the valve cavity. Further, the isolation valve includes a flow tube configured to allow movement of the valve member between the first and second positions. Additionally, the isolation valve includes an engagement assembly adapted to engage the flow tube to substantially prevent debris from entering the valve cavity when the valve member is in the second position. In another aspect, a method of operating an isolation valve in a wellbore is provided.