Patent Publication Number: US-2017370186-A1

Title: Differential fill valve assembly for cased hole

Description:
BACKGROUND 
     In the oil and gas industry, wellbores are drilled into the Earth&#39;s surface in order to access underground reservoirs for the extraction of hydrocarbons. Once a wellbore is drilled, it is often lined with casing or a string of casing sections or lengths, and the casing is then secured into place using cement. In one cementing technique, a cement composition is pumped through the interior of the casing and allowed to flow back toward the surface via the annulus defined between the wellbore wall and the casing. The cement composition within the annulus is then allowed to cure, forming a hardened mass in the annulus. In another cementing technique, commonly referred to as reverse-circulation cementing, the cement composition is pumped through the annulus to the bottom of the wellbore and then back toward the surface via the interior of the casing. Once the cement composition cures within the annulus to form a hardened mass, the casing serves to stabilize the walls of the surrounding subterranean formation to prevent any potential caving into the wellbore. The casing also isolates the various surrounding subterranean formations by preventing the flow or cross-flow of formation fluids via the annulus. The casing further provides a surface to secure pressure control equipment and downhole production equipment, such as a drilling blowout preventer (BOP) or a production packer. 
     When casing is being run into a wellbore, particularly where deep wells are involved, it is desirable to “float” the casing down to its intended location within the wellbore fluid to relieve some of the strain from the derrick, prior to the time the casing is cemented in the well. It is also desirable to have the casing fill automatically at a predetermined rate to save rig time. 
     Float valves are one-way valves (i.e., check valves) that can be installed at or near the interior bottom end of a casing string. Once operational, float valves permit fluid (such as mud or cement) to flow down through the inside of the casing while preventing fluids from flowing in the reverse direction back up the inside of the casing. By doing so, float valves prevent cement that is pumped down through the casing, into the shoe track, and up into the annular space from flowing back up through the valves once the cement is in place, an occurrence known as “reverse flow” or “u-tubing.” U-tube pressure is created by the differential hydrostatic pressure between the fluid column inside the casing and the fluid column in the annulus, in cases where the cement density is close to drilling mud density, the u-tube pressure may be very small—too small to induce backflow or to be detected at the rig. 
     Float shoes and float collars have been developed, which permit automatic filling of the casing and incorporate a backpressure valve to prevent cement back flow into the casing after the cementing operation. Certain backpressure valves also permit the option of terminating the filling of the casing at any point in time. During the insertion of casing into the wellbore, a traditional auto-fill, flapper-type float valve is held open by a pin set across a sleeve in the valve assembly bore. As the casing enters the wellbore, the preset spring tension of the flapper valve spring allows controlled filling of the casing to a predetermined differential pressure between the casing interior and the wellbore annulus. Fluid may be circulated through the casing at any time due to the presence of the circulating flapper valve. When it is desired to actuate the backpressure valve to prevent further filling of the casing as it is being run in, or after circulation has been established prior to initiating of the cementing operation for the casing, a weighted tripping ball is dropped, or carried in with the float valve, which breaks the pin holding the sleeve and thereby freeing the flapper valve to close. After cementing has been completed, the released flapper valve prevents cement flow back into the casing from the wellbore annulus. Due to the close operating pressures of the float valve, premature release of the flapper valve can occur. Additionally, the same operating conditions can cause the flapper valve to not release entirely. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following figures are included to illustrate certain aspects of the present disclosure, and should not be viewed as exclusive embodiments. The subject matter disclosed is capable of considerable modifications, alterations, combinations, and equivalents in form and function, without departing from the scope of this disclosure. 
         FIG. 1A  illustrates a cross-sectional side view of a wellbore system that may employ one or more principles of the present disclosure. 
         FIG. 1B  illustrates a cross-sectional side view of a differential fill valve assembly of the present disclosure, employed in a casing float collar. 
         FIGS. 2A-2C  illustrate a cross-sectional side view of an exemplary differential fill valve assembly, in an unactuated state ( FIG. 2A ), an actuated state ( FIG. 2B ), and a reopened state ( FIG. 2C ). 
         FIGS. 3A-3D  illustrate a cross-sectional side view of an exemplary differential fill valve assembly, in an unactuated state ( FIG. 3A ), partially actuated states ( FIG. 3B-C ), and a fully actuated state ( FIG. 3D ). 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure is related to downhole tools and, more particularly, to the operation of downhole tools during wellbore cementing operations. 
     Traditional fill equipment typically utilizes a match-drilled hole that is pinned with a small diameter brass pin. The pin can be peened and ground flush with the ID of the activation sleeve. These production steps introduce opportunities for errors during assembly, which could produce operational issues. The match-drilled hole and pinning adds considerable time and cost to the assembly of the tool. Moreover, the brass pin may cause premature shifting of the sleeve, or may disable the sleeve from shifting entirely. When a ball lands on the lip of the sleeve, the pin is sheared and the sleeve moves downward. Later, the ball extrudes through the lip. Often, the flow rate of fluid moving past the sleeve does not generate sufficient force to move the sleeve, even when unpinned. 
     The exemplary differential fill valve assemblies disclosed herein provide a mechanism for positive retention of a backpressure valve in an open mode during run-in of the casing, a mechanism for activating a valve during operation, and a mechanism to maintain the valve in an actuated state during operation. 
     The differential fill valve assembly of the present disclosure includes a backpressure flapper valve disposed within a substantially tubular upper housing, and a lower housing containing a slidably disposed activating sleeve therein above a double flapper valve having a fillup flapper valve mounted on a larger, circulating flapper valve, which is attached to the lower housing. 
     As casing is run into the wellbore, the valve assembly of the subject technology is located in a float collar or float shoe, or both, in the casing. The activating sleeve holds the backpressure flapper in an open mode, and is itself maintained in position through use of locking rings. When desired, the backpressure valve can be activated by dropping a weighted tripping ball, which will contact a seat in the bore of the activating sleeve, causing a pressure buildup above the ball, which will drive the activating sleeve downwardly. As the activating sleeve moves downward, the backpressure valve is released. An additional lock ring maintains the activating sleeve in its lower position after the tripping ball is extruded through the tool. 
     The exemplary valve assemblies of the present disclosure allow the activating sleeve to be held in place prior to entry of the weighted tripping ball. The activating sleeve can be held in place without the use of shear pins or other mechanisms that require greater to shear a pin before moving the activating sleeve and releasing the backpressure valve. Mechanisms disclosed herein provide stable securement of the activating sleeve as well as predetermined activation requirements for activating the sleeve and releasing the backpressure valve. Operational consistency is enhanced by maintaining a high retaining force during circulation and requiring only a low pressure to shift the sleeve once the ball seats on the lip. 
     Referring to  FIG. 1A , illustrated is a cross-sectional side view of a wellbore system  100  that may employ one or more of the principles of the present disclosure. More particularly,  FIG. 1A  depicts a wellbore  102  that has been drilled into the Earth&#39;s surface  104  and a surface casing  106  secured within the wellbore  102  and extending from the surface  104 . A wellhead installation  108  is depicted as being arranged at the surface  104  and a casing string  110  is suspended within the wellbore  102  from the wellhead installation  108 . A casing shoe  112  may be attached at the bottom-most portion of the casing string  110 , and an annulus  114  is defined between the wellbore  102  and the casing string  110 . 
     As used herein, the term “casing string.” as in the casing string  110 , may refer to a tubular casing length extending through a wellbore that may include a plurality of tubular casing lengths coupled (e.g., threaded) together to form a continuous tubular conduit of a desired length. It will be appreciated, however, that the casing string  110  may equally refer to a single tubular length or structure, without departing from the scope of the disclosure. 
     At the surface  104 , a feed line  116  may be operably and fluidly coupled to the wellhead installation  108  and in fluid communication with an interior  118  of the casing string  110 . The feed line  116  may have a feed valve  120  configured to regulate the flow of cement  122  into the interior  118  of the casing string  110 , and the feed line  116  may be fluidly coupled to a source  124  of cement  122 . In the depicted embodiment, the source  124  of the cement  122  is a cement truck, but could equally be a cement head, a standalone pump, or any other pumping mechanism known to persons skilled in the art and capable of introducing the cement  122  into the interior  118  of the casing string  110 . A return line  126  may also be connected to the wellhead installation  108  and in fluid communication with the annulus  114 . In some cases, as illustrated, the return line  126  may include a return valve  128  configured to regulate the flow of fluids returning to the surface  104  via, the annulus  114 . 
     In order to secure the casing string  110  within the wellbore  102 , cement  122  may be pumped from the source  124  and into the interior  118  of the casing string  110  via the feed line  116 . The cement  122  flows to the bottom of the casing string  110  and is diverted at the casing shoe  112  back toward the surface  104  within the annulus  114 . 
     Referring to  FIG. 1B , with continued reference to  FIG. 1A , a differential fill valve assembly  200  may be provided within a float collar  136  of a casing string  110 . The float collar  136  can be suspended in a wellbore from upper casing  132 , having a bore  142 . Float collar  136  can include a generally cylindrical tubing section, which can interface with the upper casing  132  by a mating interface (e.g., threads, etc.). A collar  136  can be attached at its lower end to lower casing  134 , having a bore  144 , by another mating interface (e.g., threads, etc.). The float collar  136  has a substantially uniform inner diameter at an inner surface thereof to hold cement casting  140  in place. The differential fill valve assembly  200  may be securely maintained in place, relative to the float collar  136 , by the cement casting  140 . 
     Referring to  FIGS. 2A-2C , the valve assembly  200  can include a substantially tubular upper housing  210  defining an axial entry bore  212 . Below entry bore  212 , a frustoconical bore wall  216  can extend radially outward to a larger diameter in the downward direction. The interior of the lower housing  296  also forms a frustoconical surface  236  that tapers from an upper, larger diameter bore wall to a lower, smaller diameter bore wall. 
     A backpressure flapper  220  may be provided on one side of the valve assembly  200 . The flapper  220  is pivotable on a pin  222 , and is biased toward a closed position by a torsion spring, or other biasing mechanism, acting thereupon. In some embodiments, one surface of the flapper  220  can include a slight annular undercut surface  228  at its periphery to engage an outer wall  270  of the sleeve  250 . An outwardly flaring frustoconical surface  224  extends from the surface  228  to an elastomeric seal  226 . The elastomeric seal  226  can extend annularly and provide a flexible lip at an outer extent thereof. 
     An activating sleeve  250  is slidably contained within the lower housing  296 , and can include an annular lip  256  extending from an inner wall thereof. The annular lip  256  can have an inner cross-sectional dimension (e.g., a diameter) that is smaller than an outer cross-sectional dimension (e.g., a diameter) of a weighted tripping ball  299  ( FIG. 2B ), as described further herein. The annular lip  256  may be flexible and otherwise configured to bend, expand, or bow radially outwardly upon application of a force corresponding to a predetermined threshold, as described further herein. The exterior of the activating sleeve  250  provides an annular shoulder  260  having a radially flat upper face and a frustoconical lower face. One or more ports  280  extend through the wall of activating sleeve  250  from a radially outer wall  270  of the activating sleeve  250  to a radially inner surface of the activating sleeve  250 . 
     According to some embodiments, the activating sleeve  250  can be initially secured to lower housing  296  by one or more shear fasteners  292  that extend into corresponding apertures defined in the annular shoulder  260 . The shear fastener  292  can extend from a first radial side of the annular shoulder  260  through the lower housing  296  and the shoulder  260 . The shear fastener  292  can be peened and ground flush with the inner diameter of the activation sleeve  250 . 
     According to some embodiments, a split lock ring  240  surrounds an exterior surface of the activating sleeve  250 , and is contained within an annular recess  234 . An upper inner frustoconical surface of the lock ring  240  is configured to flare upwardly and radially outwardly. A lower surface can extend in a radial plane. 
     With continued reference to  FIGS. 1A-2C , exemplary operation of the valve assembly  200  will now be provided, according to one or more embodiments. 
     Differential fill float collar  136 , as previously noted, is run into the open wellbore as suspended from the casing  132 . The wellbore is generally filled with fluid such as drilling mud, and the casing is thereby “floated” into the wellbore. The casing bore  142  above the differential fill float collar  136  is filled with wellbore fluid at a gradual rate, so that the casing  132  above float collar  136  is only partially filled and “floated” into the hole, and thereby lessening strain on the derrick that introduces the casing  132  downhole. The fluid level above float collar  136  will thus be below that outside the casing. More particularly, the difference in fluid level is a function of the weight of the drilling fluid and the fillup spring size and the fillup spring may be selected to provide a desired fill rate. 
     While the casing is being run, the top end of activating sleeve  250  maintains backpressure flapper  220  in an open position. Consequently, circulation can be established at any time during the running of the casing without releasing activating sleeve  250 . 
     As shown in  FIG. 2B , a weighted tripping ball  299  may be dropped down the casing bore  142  until locating and landing on the annular lip  256  defined within the activating sleeve  250 . The pressure above the ball  299  will build until shear pin  292  shears (if installed), and activating sleeve  250  will travel downward releasing backpressure flapper  220 . Activating sleeve  250  can be prevented from rotating by the shear fastener  292 . 
     As shown in  FIG. 2C , after the activating sleeve  250  reaches the full extent of its travel, ball  299  can extended past the annular lip  256  and be pumped out of the float collar  136  to the bottom of the wellbore. Ports  280  in the wall of activating sleeve  250  permit any fluid trapped near the annular shoulder  260  of the activating sleeve  250  to escape when the activating sleeve  250  moves down. The activating sleeve  250  is prevented from moving back to its original position by the lock ring  240 . More specifically, as the shoulder  260  on activating sleeve  250  contacts the frustoconical upper face on the lock ring  240 , the lock ring  240  is forced apart and over the shoulder  260  so that when differential pressure is released (as when ball  299  leaves the float collar  136 ), the radially flat lower face of the lock ring  240  will engage the shoulder  260  of the activating sleeve  250 . 
     As the cementing operation is performed, the released backpressure flapper  220  is able to control any back flow of cement up into casing bore  142 , as the elastomeric seal  226  seats on the annular surface  216  of the upper housing  210  as the hydrostatic pressure in the casing bore  144  and the force of the spring  222  urges the backpressure flapper  220  into a closed position. At the resumption of cement pumping, pump pressure in the casing bore  142  overcomes the spring force and hydrostatic pressure below the float collar  136 , and the backpressure flapper  220  reopens. 
     After the cementing operation is completed, the interior components of the float collar  136  can be drilled out by means known in the art to provide an open casing bore to the bottom of the casing. 
     Referring now to  FIGS. 3A-D , with continued reference to  FIGS. 1A-1B , an exemplary valve assembly  300  can include substantially tubular upper housing  310  defining an axial entry bore  312 . Below entry bore  312 , a frustoconical bore wall  316  can extend radially outward to a larger diameter in a downward direction. The interior of the lower housing  396  also forms a frustoconical surface  336  that tapers from an upper, larger diameter bore wall to a lower, smaller diameter bore wall. 
     A backpressure flapper  320  is provided on one side of the valve assembly  300 . The flapper  320  is pivoted on pin  322 , and is biased toward a closed position by a torsion spring, or other biasing mechanism, acting thereupon. One surface of the flapper  320  can include a slight annular undercut surface  328  at its periphery to engage an outer wall  370  of the sleeve  350 . An outwardly flaring frustoconical surface  324  extends from the surface  328  to an elastomeric seal  326 . The elastomeric seal  326  can extend annularly and provide a flexible lip at an outer extent thereof. 
     An activating sleeve  350  is slidably contained within a lower housing  396 , and can include an annular lip  356  extending from an inner wall thereof. The annular lip  356  can have an inner cross-sectional dimension (e.g., a diameter) that is smaller than an outer cross-sectional dimension (e.g., a diameter) of a weighted tripping ball  399 , as described further herein. The annular lip  356  may be flexible and otherwise configured to bend, expand, or bow radially outwardly upon application of a force corresponding to a programmed threshold. One or more ports  380  extend through the wall of activating sleeve  350  from a radially outer surface of the activating sleeve  350  to a radially inner surface of the activating sleeve  350 . The activating sleeve  350  can be formed from one or more of a variety of materials, including brass, aluminum, composite materials, elastomers, and thermoplastic or thermoset polymers. Material selection for the activating sleeve  350  can provide predetermined retention of the ball  399  up to selected force limits, beyond which the annular lip  356  can be elastically or plastically deformed to allow passage of the ball  399 . Material selection for the activating sleeve  350  can facilitate drilling through the valve assembly  300  at the completion of an operation. 
     The exterior of the activating sleeve  350  provides an annular shoulder  360  with an upper face  364  in a radial plane. For example, the upper face  364  can face axially toward the axial entry bore  312  at any point thereon. Alternatively, the upper face  364  can be frustoconical by flaring upwardly and radially outwardly. Other shapes of the upper face  364  are contemplated, such as concave and/or convex contoured surfaces. The upper face  364  can be configured to securely engage opposing and/or complementary surfaces on an upper side of the shoulder  360 . 
     The annular shoulder  360  can further have a frustoconical lower face  362 . For example, the lower face  362  can face radially outwardly and downwardly (i.e., toward the axial exit bore  3141 ) at any point thereon. By further example, the lower face  362  can form an oblique angle relative to a longitudinal axis of the valve assembly  300 . Such an angle can be selected to determine, at least in part, the force required to shift the activating sleeve  350  past a lower lock ring  340 . For example, the angle can be between 10° and 80°. An exemplary lower face  362  can form an angle of about 27°. As will be appreciated, greater angles can result in a greater force being required to expand the lower lock ring  340 , and smaller angles can result in a smaller force being required. The required force can be significant enough to avoid premature movement of the activating sleeve  350 , yet still be less than a force required to both shear a pin and move an activating sleeve. Other shapes of the lower face  362  are contemplated, such as concave and/or convex contoured surfaces. The lower face  362  can be configured to engage and/or separate structures providing opposing and/or complementary surfaces on a lower side of the shoulder  360 . 
     According to some embodiments, an upper split lock ring  382  surrounds an exterior surface of the activating sleeve  350 , and is contained within an annular recess  332 . The upper split lock ring  382  can be formed as a circumferentially discontinuous ring that can expand to increase an opening there through. Other radial locking mechanisms can be used to controllably retain the activating sleeve  350 . For example, one or more retractable protrusions, biased radially inwardly, can individually engage the shoulder  360 . By further example, a radial locking mechanism can be provided to receive the activating sleeve  350  from the entry bore  312  when a force by the activating sleeve  350  causes elastic or plastic deformation of such a radial locking mechanism. Other locking methods could include collet mechanisms, j-slots, snap-fit, interference fit, or friction alone. The upper split lock ring  382  can be formed from one or more of a variety of materials, including brass, aluminum, steel, composite materials, elastomers, and thermoplastic or thermoset polymers. Material selection for the upper split lock ring  382  can provide predetermined retention of the shoulder  360  of the activating sleeve  350  up to selected force limits, beyond which the upper split lock ring  382  can be elastically or plastically deformed to allow passage of the shoulder  360 . Material selection for the upper split lock ring  382  can facilitate drilling of the components at the completion of an operation. 
     An upper inner frustoconical surface  384  of the upper lock ring  340  flares radially upwardly and outwardly. For example, the upper surface  384  can face radially inwardly and upwardly (i.e., toward the axial entry bore  312 ) at any point thereon. A lower surface  386  can extend in a radial plane. For example, the lower surface  386  can face axially toward the axial exit bore  314  at any point thereon. The upper split lock ring  382  can have an inner cross-sectional dimension (e.g., a diameter) that is smaller than an outer cross-sectional dimension (e.g., a diameter) of the shoulder  360  of the activating sleeve  350 . 
     Before the activating sleeve  350  moves downwardly, the upper split lock ring  382  can prevent the activating sleeve  350  from moving upwardly by engaging the shoulder  360 . According to some embodiments, no shear fastener is required to prevent the activating sleeve  350  from moving upwardly. For example, as shown in  FIG. 3A , the upper lock ring  382  can be biased to contract radially inwardly such that the lower surface  386  of the upper lock ring  382  can contact and engage the upper face  364  of the shoulder  360 . The surface contours of the lower surface  386  and the upper face  364  can be such that an upward force applied by the upper face  364  to the lower surface  386  does not tend to cause radial expansion of the upper lock ring  382 . 
     According to some embodiments, a lower split lock ring  340  surrounds an exterior surface of the activating sleeve  350 , and is contained within an annular recess  334 . An upper inner frustoconical surface  342  of the lower lock ring  340  flares radially upwardly and outwardly. For example, the upper surface  342  can face radially inwardly and upwardly (i.e., toward the axial entry bore  312 ) at any point thereon. By further example, the upper surface  342  can form an oblique angle relative to a longitudinal axis of the valve assembly  300 . Such an angle can be selected to determine, at least in part, the force required to shift the activating sleeve  350  past the lower lock ring  340 . An angle formed by the upper surface  342  relative to a longitudinal axis can be equal to an angle formed by the lower face  362  relative to the same longitudinal axis. A lower surface  344  can extend in a radial plane. For example, the lower surface  344  can face axially toward the axial exit bore  314  at any point thereon. The lower split lock ring  340  can have an inner cross-sectional dimension (e.g., a diameter) that is smaller than an outer cross-sectional dimension (e.g., a diameter) of the shoulder  360  of the activating sleeve  350 . 
     When the activating sleeve  350  moves downwardly, the lower face  362  is configured to apply a force against the upper surface  342  of the lower split lock ring  340 . The lower split lock ring  340  can be discontinuous or otherwise sufficiently flexible to move radially outwardly into the annular recess  334  and allow passage of the shoulder  360 . The lower split lock ring  340  can be formed as a circumferentially discontinuous ring that can expand to increase an opening there through. Other radial locking mechanisms can be used to controllably retain the activating sleeve  350 . For example, one or more retractable protrusions, biased radially inwardly, can individually engage corresponding portions of the shoulder  360 . By further example, a radial locking mechanism can be provided to retain the activating sleeve  350  until a force by the activating sleeve  350  causes elastic or plastic deformation of such a radial locking mechanism. Other locking methods could include collet mechanisms, j-slots, snap-fit, interference fit, or friction alone. The lower split lock ring  340  can be formed from one or more of a variety of materials, including brass, aluminum, steel, composite materials, and polymers. Material selection for the lower split lock ring  340  can provide predetermined retention of the shoulder  360  of the activating sleeve  350  up to selected three limits, beyond which the lower split lock ring  340  can be elastically or plastically deformed to allow passage of the shoulder  360 . Material selection for the lower split lock ring  340  can facilitate drilling of the components at the completion of an operation. The lower face  362  and the upper surface  342  can provide complementary surface contours to maximize an amount of surface contact between the lower face  362  and the upper surface  342 . 
     After the activating sleeve  350  moves downwardly, the lower split lock ring  340  can prevent the activating sleeve  350  from moving upwardly again by engaging the shoulder  360 . For example, as shown in  FIG. 3D , the lower face  362  of the shoulder  360  can settle upon the frustoconical surface  336  of the lower housing  396 . The lower face  362  and the frustoconical surface  336  can provide complementary surface contours to maximize an amount of surface contact between the lower face  362  and the frustoconical surface  336 . After the activating sleeve  350  complete such downward travel, the lower lock ring  340  can contract radially inwardly such that the lower surface  344  of the lower lock ring  340  can contact and engage the upper face  364  of the shoulder  360 . The surface contours of the lower surface  344  and the upper face  364  can be such that an upward force applied by the upper face  364  to the lower surface  344  does not tend to cause radial expansion of the lower lock ring  340 . 
     With reference to  FIGS. 1A-1B and 3A-3D , exemplary operation of the valve assembly  300  is now provided, according to one or more embodiments. 
     Differential fill float collar  136 , as previously noted, is run into the open wellbore suspended from casing  132 . The wellbore is generally filled with fluid such as drilling mud, and the casing is “floated” into the wellbore. The casing bore  142  above the differential fill float collar  136  is filled with wellbore fluid at a gradual rate, so that the casing  132  above float collar  136  is only partially filled and “floated” into the hole, lessening strain on the derrick. The fluid level above float collar  136  will thus be below that outside the casing. The difference in fluid level is a function of the weight of the drilling fluid and the fillup spring size; the fillup spring may be selected to provide the desired fill rate. 
     While the casing is being run, the top end of activating sleeve  350  maintains backpressure flapper  320  in an open position. Circulation can be established at any time during the running of the casing without releasing activating sleeve  350 . 
     Referring to  FIGS. 3B-3C , a weighted tripping ball  399  is dropped down the casing bore  142 , where it travels downward until it seats on annular lip  356  in activating sleeve  350 . The pressure above ball  399  will build until the activating sleeve  350  travels downward, releasing backpressure flapper  320 . According to some embodiments, the only force required to allow travel of the activating sleeve  350  is the force required to actuate the lower lock ring  340 . According to some embodiments, the activating sleeve  350  is not secured to the lower housing  396  or any portion of the valve assembly  300 . Rather, the only limits on axial movement of the activating sleeve  350  are imposed by the upper lock ring  382  and a lower lock ring  340 . 
     As shown in  FIG. 3D , after the activating sleeve  350  reaches the full extent of its travel, ball  399  can extended past the annular lip  356  and be pumped out of the float collar  136  to the bottom of the wellbore. Ports  380  in the wall of activating sleeve  350  permit any fluid trapped near the annular shoulder  360  of the activating sleeve  350  to escape when the activating sleeve  350  moves down. The activating sleeve  350  is prevented from moving back to its original position by the lock ring  340 . As the shoulder  360  on activating sleeve  350  contacts the frustoconical upper face  342  on the lock ring  340 , the lock ring  340  is forced apart and over the shoulder  360 . When differential pressure is released (as when ball  399  leaves the float collar  136 ), the lower face  344  of the lock ring  340  will engage corresponding portions of the shoulder  360  of the activating sleeve  350 . 
     As the cementing operation is performed, the released backpressure flapper  320  is able to control any back flow of cement up into casing bore  142 , as the elastomeric seal  326  seats on the annular surface  316  of the upper housing  310  as the hydrostatic pressure in the casing bore  144  and the force of the spring  322  urges the backpressure flapper  320  into a closed position. At the resumption of cement pumping, pump pressure in the casing bore  142  overcomes the spring force and hydrostatic pressure below the float collar  136 , and the backpressure flapper  320  reopens. 
     After the cementing operation is completed, the interior components of the float collar  136  can be drilled out by means known in the art to provide an open casing bore to the bottom of the casing. 
     Embodiments disclosed herein include: 
     A. A valve assembly, including: a flapper valve biased to move from a restrained position to a released position to cover an entry bore; an activating sleeve retaining the flapper valve in the restrained position and having a shoulder; an upper radial lock mechanism configured to prevent movement of the shoulder toward the entry bore and past the upper radial lock mechanism; a lower radial lock mechanism between the shoulder and an exit bore, the lower radial lock mechanism configured to prevent movement of the shoulder toward the exit bore and past the lower radial lock mechanism until a force threshold is exceeded. 
     B. A tool string, including: a casing; a float collar within the casing; a valve assembly within the float collar, the valve assembly including: a flapper valve biased to move from a restrained position to a released position to cover an entry bore; an activating sleeve retaining the flapper valve in the restrained position and having a shoulder; an upper radial lock mechanism configured to prevent movement of the shoulder toward the entry bore and past the upper radial lock mechanism; a lower radial lock mechanism between the shoulder and an exit bore, the lower radial lock mechanism configured to prevent movement of the shoulder toward the exit bore and past the lower radial lock mechanism until a force threshold is exceeded. 
     C. A method, including: providing a valve assembly with (i) an activating sleeve retaining a flapper valve in a restrained position and (ii) an upper radial lock mechanism preventing movement of a shoulder of the activating sleeve toward an entry bore of the valve assembly and past the upper radial lock mechanism; advancing the activating sleeve toward an exit bore by delivering a tripping ball to the activating sleeve; and releasing the flapper valve to move from a restrained position to a released position to cover the entry bore. 
     Each of embodiments A, B, and C may have one or more of the following additional elements in any combination: 
     Element 1: when the shoulder is between the lower radial lock mechanism and the exit bore, the lower radial lock mechanism can be configured to prevent movement of the shoulder toward the entry bore and past the lower radial lock mechanism. Element 2: the upper radial lock mechanism can be between the shoulder and the entry bore. Element 3: the upper radial lock mechanism can have an inner cross-sectional dimension that is smaller than an outer cross-sectional dimension of the shoulder. Element 4: the lower radial lock mechanism can have an inner cross-sectional dimension that is smaller than an outer cross-sectional dimension of the shoulder. Element 5: the flapper valve, in the released position, can allow fluid flow from the entry bore to the exit bore and prevents fluid flow from the exit bore to the entry bore. Element 6: the activating sleeve can provide an annular lip having an inner cross-sectional dimension that is smaller than an outer cross-sectional dimension of a tripping ball size to travel through the entry bore to the activating sleeve. Element 7: a lower radial lock mechanism between the shoulder and the exit bore can be configured to prevent the advancing until a force threshold is exceeded. Element 8: the advancing can include moving the shoulder toward the exit bore and past a lower radial lock mechanism. Element 9: fluid flow can be allowed from the entry bore to the exit bore through the flapper valve, in the released position, and preventing fluid flow from the exit bore to the entry bore. Element 10: advancing the activating sleeve can include seating a tripping ball on an annular lip of the activating sleeve and creating a pressure differential across the activating sleeve. Element 11: the tripping ball can be advanced through the annular lip. 
     Therefore, the disclosed systems and methods are well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. The particular embodiments disclosed above are illustrative only, as the teachings of the present disclosure may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular illustrative embodiments disclosed above may be altered, combined, or modified and all such variations are considered within the scope of the present disclosure. The systems and methods illustratively disclosed herein may suitably be practiced in the absence of any element that is not specifically disclosed herein and/or any optional element disclosed herein. While compositions and methods are described in terms of “comprising,” “containing,” or “including” various components or steps, the compositions and methods can also “consist essentially of” or “consist of” the various components and steps. All numbers and ranges disclosed above may vary by some amount. Whenever a numerical range with a lower limit and an upper limit is disclosed, any number and any included range falling within the range is specifically disclosed. In particular, every range of values (of the form, “from about a to about b,” or, equivalently, “from approximately a to b,” or, equivalently, “from approximately a-b”) disclosed herein is to be understood to set forth every number and range encompassed within the broader range of values. Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and clearly defined by the patentee. Moreover, the indefinite articles “a” or “an,” as used in the claims, are defined herein to mean one or more than one of the element that it introduces. If there is any conflict in the usages of a word or term in this specification and one or more patent or other documents that may be incorporated herein by reference, the definitions that are consistent with this specification should be adopted. 
     As used herein, the phrase “at least one of” preceding a series of items, with the terms “and” or “or” to separate any of the items, modifies the list as a whole, rather than each member of the list (i.e., each item). The phrase “at least one of” allows a meaning that includes at least one of any one of the items, and/or at least one of any combination of the items, and/or at least one of each of the items. By way of example, the phrases “at least one of A, B, and C” or “at least one of A, B, or C” each refer to only A, only B, or only C; any combination of A, B, and C; and/or at least one of each of A, B, and C. 
     The use of directional terms such as above, below, upper, lower, upward, downward, left, right, uphole, downhole and the like are used in relation to the illustrative embodiments as they are depicted in the figures, the upward direction being toward the top of the corresponding figure and the downward direction being toward the bottom of the corresponding figure, the uphole direction being toward the surface of the well and the downhole direction being toward the toe of the well.