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CROSS-REFERENCE TO RELATED APPLICATIONS 
   This is a continuation application of U.S. patent application Ser. No. 10/209,339 filed Jul. 31, 2002 now U.S. Pat. No. 6,904,970 and entitled “Cementing Manifold Assembly”, which claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application Ser. No. 60/310,293 filed Aug. 3, 2001 and entitled “Cementing Manifold”, both hereby incorporated herein by reference for all purposes. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not Applicable. 
   REFERENCE TO A MICROFICHE APPENDIX 
   Not applicable. 
   FIELD OF THE INVENTION 
   The present invention relates generally to apparatus and methods for cementing downhole tubulars into a well bore, and more particularly, the present invention relates to a cementing manifold assembly and method of use. 
   BACKGROUND 
   A well-known method of drilling hydrocarbon wells involves disposing a drill bit at the end of a drill string and rotating the drill string from the surface utilizing either a top drive unit or a rotary table set in the drilling rig floor. As drilling continues, progressively smaller diameter tubulars comprising casing and/or liner strings may be installed end-to-end to line the borehole wall. Thus, as the well is drilled deeper, each string is run through and secured to the lower end of the previous string to line the borehole wall. Then the string is cemented into place by flowing cement down the flowbore of the string and up the annulus formed by the string and the borehole wall. 
   To conduct the cementing operation, typically a cementing manifold is disposed between the top drive unit or rotary table and the drill string. Thus, due to its position in the drilling assembly, the cementing manifold must suspend the weight of the drill pipe, contain pressure, transmit torque, and allow unimpeded rotation of the drill string. When utilizing a top drive unit, a separate inlet is preferably provided to connect the cement lines to the cementing manifold. This allows cement to be discharged through the cementing manifold into the drill string without flowing through the top drive unit. 
   In operation, the cementing manifold allows fluids, such as drilling mud or cement, to flow therethrough while simultaneously enclosing and protecting from flow, a series of darts and/or spheres that are released on demand and in sequence to perform various operations downhole. Thus, as fluid flows through the cementing manifold, the darts and/or spheres are isolated from the fluid flow until they are ready for release. 
   Cementing manifolds are available in a variety of configurations, with the most common configuration comprising a single sphere/single dart manifold. The sphere is dropped at a predetermined time during drilling to form a temporary seal or closure of the flowbore of the drill string, for example, or to actuate a downhole tool, such as a liner hanger, in advance of the cementing operation, as for example. Once the cement has been pumped downhole, the dart is dropped to perform another operation, such as wiping cement from the inner wall of a string of downhole tubular members. 
   Another common cementing manifold comprises a single sphere/double dart configuration. The sphere may be released to actuate a downhole tool, for example, followed by the first dart being launched immediately ahead of the cement, and the second dart being launched immediately following the cement. Thus, the dual darts surround the cement and prevent it from mixing with drilling fluid as the cement is pumped downhole through the drill string. Each dart typically also performs another operation upon reaching the bottom of the drill string, such as latching into a larger dart to wipe cement from the string of downhole tubular members. 
   Many conventional cementing manifolds include external bypass lines such as the manifolds disclosed in U.S. Pat. No. 5,236,035 to Brisco et al. and U.S. Pat. No. 4,854,383 to Arnold et al., both hereby incorporated herein by reference for all purposes. In more detail, Arnold et al. discloses a conventional external bypass cementing manifold for a single dart or double dart configuration. The single dart manifold comprises a tubular enclosure with a longitudinal passageway into which a dart is loaded. The dart holding/dropping mechanism is a ball valve connected via threads to the bottom of the tubular enclosure. An external bypass line with a bypass valve is connected via welds or threads to the tubular enclosure around the dart. For the double dart configuration, an identical arrangement of tubular enclosure, ball valve, and external bypass line with bypass valve is connected below the first tubular enclosure. Each of the darts in the dual dart configuration is separately releasable. 
   When the dart is in the hold position, the ball valve remains closed to prevent flow through the tubular enclosure, and flow is routed around the dart through the bypass line by opening the bypass valve. To release the dart, the bypass valve is closed, and the ball valve is opened to allow flow through the tubular enclosure, thereby causing the dart to drop into the well string. 
   Conventional cementing manifolds often include other external connections, such as the side-mounted sphere dropping mechanisms disclosed in Arnold et al. and U.S. Pat. No. 5,950,724 to Giebeler, hereby incorporated herein by reference for all purposes. In more detail, Arnold et al. discloses a ball dropping mechanism comprising a housing that mounts to the side of the lowermost tubular enclosure. The housing includes a bore in fluid communication with the longitudinal passageway through the tubular enclosure. In the hold position, a ball is positioned on a seat within the housing bore. To drop the ball, a screw shaft pushes the ball through the housing bore and into the longitudinal passageway, thereby dropping the ball down into the well string. 
   A number of disadvantages are associated with cementing manifolds having external connections, such as external bypass lines and side-mounted sphere dropping mechanisms. In particular, several large penetrations are required in the main body of the manifold (i.e. the tubular enclosures) for making the external connections. These penetrations create high stress concentration areas and hydraulically loaded areas that reduce the overall pressure-containing capacity of the cementing manifold. The manifold must also be capable of withstanding fatigue caused by changes in operating conditions, and stress concentration areas minimize the fatigue life of a cementing manifold. Further, the ball drop mechanism and external bypass connections protrude a considerable distance from the main body of the manifold, making these components more prone to damage during well operations. In addition, the external components connect via threads or welds to the main body of the manifold, thereby presenting a safety concern. In particular, the threads could back out or the welds could fail, which would expose rig personnel to high pressure, high velocity fluids. Thus, it would be advantageous to provide a cementing manifold with internal bypass capability and with few external connections to the main body of the manifold. It would also be advantageous to eliminate threaded or welded connections to the main body of the manifold. 
   Some cementing manifolds have internal bypass capability, such as the TDH Top Drive Cementing Head offered by Weatherford/Nodeco. The TDH Head is purpose-built for use with a top-drive system and available in configurations to accommodate either a single ball/single dart, or single ball/dual darts. In both configurations, the TDH Head comprises a main body having a main bore and a parallel side bore, with both bores being machined integral to the main body. The darts are loaded into the main bore, and a dart releaser valve is provided below each dart to maintain it in the hold position. The dart releaser valves are side-mounted externally and extend through the main body. A port in the dart releaser valve provides fluid communication between the main bore and the side bore. The ball drop mechanism is externally side-mounted through one wall of the main body below the lowermost dart and extends into the main bore. The ball is retained by a collet, and to drop the ball, a screw shaft pushes the ball out into the main bore. 
   When circulating prior to cementing, the darts are maintained in the main bore with the dart releaser valves closed. Fluid flows through the side bore and into the main bore below the lowermost dart via the fluid communication port in the dart releaser valve. To release a dart, the dart releaser valve is turned 90 degrees, thereby closing the side bore and opening the main bore through the dart releaser valve. Flow enters the main bore behind the dart, causing it to drop downhole. 
   Although the TDH Top Drive Cementing Head eliminates external bypass lines, it includes large penetrations in the main body for the dart releaser valves and ball drop device. These external components are also welded or threaded to the main body and protrude a significant distance. Thus, many of the concerns associated with external bypass manifolds have not been eliminated. Further, the parallel flow bores restrict the flow capacity of the TDH unit, which could present erosion problems, and also make it more difficult to remove leftover cement that could clog the bores. Thus, it would be advantageous to provide a cementing manifold with internal bypass capability that does not restrict the flow capacity of the manifold. 
   The Model LC-2 Plug Dropping Head offered by Baker Oil Tools, a Baker Hughes Company, is an internal bypass cementing manifold for dropping either a dart or a sphere. The LC-2 comprises a mandrel with a releasable dart/sphere holding sleeve disposed therein, the sleeve being held in place by a rotatable lock pin. The sleeve includes ports that allow fluid bypass into an annular area while the sleeve is in the upper locked position. A pivoting stop extends across the bore of the mandrel below the sleeve to maintain the dart/sphere in the hold position. 
   To drop the dart or sphere, the lock pin is turned 180 degrees to the drop position, which releases the sleeve. The sleeve moves downwardly in response to gravity and fluid flow until it reaches a stop shoulder. The downward movement of the sleeve releases the pivoting stop and restricts flow through the ports leading to the annular bypass area. Thus, the pivoting stop rotates out of the path of the dart or sphere, and all fluid is directed longitudinally through the main bore of the sleeve behind the dart or sphere, causing it to drop down into the drill string. 
   Although the Model LC-2 Plug Dropping Head eliminates external bypass lines and other external components, the releasable sleeve presents disadvantages. Namely, if the sleeve gets hung up in the mandrel, flow will bypass the dart or sphere, thereby preventing its release. Further, because the lock pin provides only limited engagement with the sleeve, improper assembly or maintenance of the lock pin and sleeve connection could cause the sleeve to release prematurely. Thus, it would be advantageous to provide a cementing manifold with internal bypass capability that does not rely on a releasable sleeve as the dropping mechanism. 
   In addition to the disadvantages described above, conventional cementing manifolds are typically unitized and purpose-built such that they are not reconfigurable. For example, they can not be converted from a single dart manifold to a double dart manifold and vice versa as the job requires. Further, after the manifold has been used for one job, new darts and/or spheres can not be loaded at the rig site due to the high torques required to disconnect the components to allow reloading. Thus, traditional cementing manifolds must be redressed and reloaded in the shop after each use. In addition, some designs do not enable release of the darts or spheres while pumping fluid downhole due to fluid loads on the release mechanisms. Therefore, known cementing manifolds present various additional operating and maintenance disadvantages. 
   The present invention overcomes the deficiencies of the prior art. 
   SUMMARY 
   The present invention relates to apparatus for cementing a string of tubulars in a borehole, the apparatus comprising an enclosure having a bore therethrough, an axially fixed sphere canister having a sphere aperture therethrough, a sphere valve member having a valve body disposed internally of said bore, and a sphere disposed in said sphere aperture, wherein said sphere valve member has a hold position closing said sphere aperture and a drop position opening said sphere aperture to release said sphere. 
   In another embodiment, an apparatus for cementing a string of tubulars in a borehole comprises an upper member, a first launching unit including a first dart canister and a first dart valve member disposed within a first modular member, a second launching unit including a second dart canister and a second dart valve member disposed with a second modular member, and a third launching unit including a sphere canister and a sphere valve member disposed within a lower member, wherein at least one of said canisters is axially fixed, and wherein at least one of said dart valve members comprises a valve body disposed internally of a bore within at least one of said modular members. 
   Other aspects and advantages of the invention will be apparent from the following description and the appended claims. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more detailed description of the present invention, reference will now be made to the accompanying drawings, wherein: 
       FIG. 1  schematically depicts an exemplary drilling system in which the various embodiments of the present invention may be utilized; 
       FIG. 2  is a cross-sectional side view of a preferred embodiment of a single dart/single sphere cementing manifold of the present invention, with both valves in the closed position; 
       FIG. 3  is a cross-sectional side view of a preferred embodiment of a double dart/single sphere cementing manifold of the present invention, with all valves in the closed position; 
       FIG. 4  is a cross-sectional side view of a preferred embodiment of a single large sphere cementing manifold of the present invention, with the valve in the closed position; 
       FIG. 5  is a cross-sectional bottom view through Section B—B of  FIG. 2 , with 
       FIG. 5A  being an enlargement of a detail of  FIG. 5 ; 
       FIG. 6  is an enlarged view of a valve of the cementing manifold of  FIG. 2 ; 
       FIG. 7  is a cross-sectional top view of the valve of  FIG. 6 , taken along Section A—A; 
       FIG. 8  is an end view of a valve stem of  FIG. 6 ; 
       FIG. 9  is a cross-sectional side view of the single dart/single sphere cementing manifold of  FIG. 2  after the sphere has been dropped, with the first valve closed and the second valve open; 
       FIG. 10  is a cross-sectional side view of the single dart/single sphere cementing manifold of  FIG. 2  after both the sphere and the dart have been dropped, with both valves open; and 
       FIG. 11  is a side view, partially in cross-section, of a preferred embodiment of a cementing swivel of the present invention. 
   

   DETAILED DESCRIPTION 
   Preferred embodiments of the invention are shown in the above-identified Figures and described in detail below. In describing the preferred embodiments, like or identical reference numerals are used to identify common or similar elements. 
     FIG. 1  schematically depicts an exemplary drilling system in which the present invention may be utilized. However, one of ordinary skill in the art will understand that the preferred embodiments are not limited to use with a particular type of drilling system. The drilling rig  100  includes a derrick  102  with a rig floor  104  at its lower end having an opening  106  through which drill string  108  extends downwardly into a well bore  110 . The drill string  108  is driven rotatably by a top drive drilling unit  120  that is suspended from the derrick  102  by a traveling block  122 . The traveling block  122  is supported and moveable upwardly and downwardly by a cabling  124  connected at its upper end to a crown block  126  and actuated by conventional powered draw works  128 . Connected below the top drive unit  120  is a kelly valve  130 , a pup joint  132 , a cementing swivel  900 , and a cementing manifold, such as the single dart/single sphere cementing manifold  200  of the present invention. A flag sub  150 , which provides a visual indication when a dart or sphere passes therethrough, is connected below the cementing manifold  200  and above the drill string  108 . A drilling fluid line  134  routes drilling fluid to the top drive unit  120 , and a cement line  136  routes cement through a valve  138  to the swivel  900 . 
   Any cementing swivel may be utilized, but preferably the cementing swivel  900  is configured as shown in  FIG. 11 . Referring now to  FIGS. 1 and 11 , the swivel  900  includes a mandrel  910 , a housing  920 , and a cap  930 , with upper and lower seal assemblies  950  disposed above and below a cement port  960  and between the mandrel  910  and the housing  920 . The swivel  900  preferably provides cement line connections  940  and tie-off connections  942 ,  944  (shown in  FIG. 1 ) that are integral to the housing  920 , thereby avoiding the disadvantages of conventional swivel connections that are typically threaded, welded, or bolted on. The threaded and bolted connections can come loose over time, and the welded connections are subject to damage or failure due to corrosion at the weldment. Conventional swivel connections are further subject to fatigue caused by the weight of the overhanging cement line  136  and cement valve  138  that connect thereto. Mandrel  910  includes upper and lower threaded connections for connecting the upper end of mandrel  910  to top drive unit  120  and the lower end to the cementing manifold  200  connected to the upper end of drill string  108 . 
   The housing  920  includes one or more radially projecting integral conduits  924  with a cement port  926  extending through conduit  924  and the wall  928  of housing  920 . Housing  920  and conduits  924  are preferably made from a common tubular member such that conduits  924  are integral with housing  920  and do not require any type of fastener including welding. Conduit  924  provides a connection means, such as threads  932 , for connecting cement line  136  to swivel  900 . 
   The preferred swivel  900  also includes two swivel connections  940  for redundancy in case one connection  940  becomes damaged. The cement ports  960  within the mandrel  910  are preferably angled so that as cement flows through the connection  940 , it enters the throughbore  905  of the mandrel  910  generally in the downwardly direction. This allows the cement to impinge on the wall of throughbore  905  at an angle and minimizes erosion of the ports  960  and mandrel  910 . 
   An additional feature of the preferred swivel  900  is that the mandrel  910  includes a common cylindrical outer surface  912  in the areas of the bearings  951  and seal assemblies  950 , which are disposed in recessed areas in the housing  920 . Conventional mandrels included a step shoulder on the mandrel for the seals, requiring individual seal placement. The common cylindrical outer surface  912  of the mandrel  910  allows for the bearings  951  and seal assemblies  950  to be positioned within the housing  920  as one unit, such that the mandrel  910  can then slide through the bore  922  of the housing  920  and assembled cap  930 . A groove  911  is provided at each end of the mandrel  910 , and an externally threaded, split cylindrical ring  914  is positioned within the grooves  911 . An internally threaded ring  913  is screwed onto the split ring  914 , and these rings  913 ,  914  hold the assembled housing  920  and cap  930  in place on the mandrel  910 . 
   Referring again to  FIG. 1 , in operation, drilling fluid flows through line  134  down into the drill string  108  while the top drive unit  120  rotates the drill string  108 . The housing  920  of cementing swivel  900  is tied-off to the derrick  102  via lines or bars  140 ,  142  such that the swivel housing  920  cannot rotate and remains stationary while the mandrel  910  of the swivel  900  rotates within housing  920  to enable the top drive unit  120  to rotate the drill string  108 . 
   To perform an operation such as, for example, actuating a downhole tool to suspend a tubular  144 , such as a casing string or liner, from existing and previously cemented casing  146 , a sphere may be dropped from the cementing manifold  200 . Then, once the tubular  144  is suspended from the casing  146  via a rotatable liner hanger  151 , cement will be pumped down through the drill string  108  and through the tubular  144  to fill the annular area  148  in the uncased well bore  110  around the tubular  144 . To start the cementing operation, the kelly valve  130  is closed, and the valve  138  to the cement line  136  is opened, thereby allowing cement to flow through the swivel  900  and down into the drill string  108 . Thus, the swivel  900  enables cement flow to the drill string  108  while bypassing the top drive unit  120 . 
   It is preferable to rotate the drill string  108  during cementing to ensure that cement is distributed evenly around the tubular  144  downhole. More specifically, because the cement is a thick slurry, it tends to follow the path of least resistance. Therefore, if the tubular  144  is not centered in the well bore  110 , the annular area  148  will not be symmetrical, and cement may not completely surround the tubular  144 . Thus, it is preferable for the top drive unit  120  to continue rotating the drill string  108  through the swivel  900  while cement is introduced from the cement line  136 . When the appropriate volume of cement has been pumped into the drill string  108 , a dart is typically dropped from the cementing manifold  200  to latch into a larger dart  152  to wipe cement from the tubular  144  and land in the landing collar  153  adjacent the bottom end of the tubular  144 . 
   Although  FIG. 1  depicts one example drilling environment in which the preferred embodiments of the present invention may be utilized, one of ordinary skill in the art will readily appreciate that the preferred embodiments of the present invention may be utilized in other drilling environments such as, for example, to cement casing into an offshore wellbore. 
   Referring now to  FIG. 2–4 , the preferred embodiments of the cementing manifold of the present invention may be provided in a variety of different configurations including a single dart/single sphere manifold  200  as shown in  FIG. 2 , a double dart/single sphere manifold  300  as shown in  FIG. 3 , or a single large sphere manifold  400  as shown in  FIG. 4 . 
   Referring now to  FIG. 2 , the single dart/single sphere manifold  200  comprises an upper cap  210 , a housing  220 , and a lower cap  230 . The upper cap  210  comprises a body  212  having a longitudinal throughbore  214 , a box connection end  216  for attachment to another tool, such as the swivel  900  shown in  FIG. 11 , and a lower threaded box end  218  which is castellated forming preferably six circumferentially disposed slots  219  for aligning with the upper end of housing  220 . The housing  220  comprises a body  222  having a longitudinal throughbore  224 , an upper threaded pin end  226  which is also castellated forming preferably six circumferentially disposed slots  227  for aligning with the lower castellated end of upper cap  210 , and a lower threaded box end  228  which is castellated having preferably six circumferentially disposed slots  229  for aligning with the upper castellated end of lower cap  230 . The lower cap  230  comprises a body  232  having a longitudinal throughbore  234 , an upper threaded pin end  236  which is castellated having preferably six circumferentially disposed slots  237  for aligning with the lower castellated end of housing  220 , and a lower pin connection end  238  for attachment to another tool, such as a flag sub  150 , or directly to the drill string  108 . 
   The upper cap  210 , housing  220 , and lower cap  230  form an enclosure that is load bearing and pressure containing. The box end of upper cap  210  connects to the pin end of housing  220  preferably via threads  215 , and high pressure seals  211  are provided therebetween. The high pressure seals  211  are provided for pressure and fluid containment. The respective slots  219 ,  227  in the upper cap  210  and housing  220  are also aligned, then dogs  280  are installed in every other set of aligned slots  219 ,  227 , and a cap screw  282  fixes each dog  280  into place. A circumferential ring  284  maintains all dogs  280  in place circumferentially. 
   Similarly, the box end of housing  220  and the pin end of lower cap  230  connect via threads at  225  with high pressure seals  221  provided therebetween, and dogs  280  are preferably positioned in every other set of aligned slots  229 ,  237  of the housing  220  and lower cap  230 , respectively. Each dog  280  is held in place via a cap screw  282 , and a circumferential ring  284  maintains all dogs  280  in position. 
   Disposed within the throughbores  214 ,  224  of the upper cap  210  and housing  220  is a dart canister  240  having a cylindrical body  242  with a throughbore  244  into which a dart  290  is loaded. The cylindrical body  242  includes flow slots  246  circumferentially disposed around the upper end, an equalizing port  247  adjacent the lower end, and a seal  248  at the lowermost end. The flow slots  246  provide a fluid path from the throughbore  214  of the upper cap  210  to the annular area  249  in the housing throughbore  224  around the dart canister  240 . The equalizing port  247  enables pressure equalization when the fins  292  of the dart  290  form a seal with canister  240  that traps pressure in the canister  240 . 
   At the upper end of the dart canister  240 , a retention mechanism  500  prevents the dart  290  from floating upwardly out of the upper end of canister  240 .  FIG. 5  depicts a cross-sectional bottom view of the retention mechanism  500  taken at Section B–B of  FIG. 2 , and  FIG. 5A  depicts an enlarged view of the connection details. The retention mechanism  500  comprises two fingers  510 , each finger  510  extending approximately halfway across the diameter of the throughbore  244  of the dart canister  240 . The fingers  510  are connected such that they are only capable of a hinged movement downwardly into the canister  240 , and the fingers  510  are biased to the position shown in  FIG. 2  and  FIG. 5  by a torsional spring  520 . The fingers  510  connect to the dart canister  240  by a clevis pin  530  that extends through the body  242  of the dart canister  240 , through the end of the finger  510 , and through the torsional spring  520 . A cotter pin  540  is provided at the end of the clevis pin  530  to prevent pin  530  from backing out. 
   Referring again to  FIG. 2 , a first valve  250  is positioned within the housing  220  and below the dart canister  240  to act as a dart holding/dropping mechanism. The first valve  250  comprises a body  252 , a rotatable plug  254 , and an actuating stem  256  to enable manual or remote actuation of the plug  254  within the body  252  of valve  250 . Retainer rings  251 ,  253  are disposed in shoulders of the housing  220  above and below the body  252  to properly position the valve  250  in the housing  220 . 
   Below the first valve  250 , and disposed within the housing  220  and the lower cap  230  is a sphere canister  260 , which has a cylindrical body  262  with a throughbore  264 . A sphere  295  fits within the throughbore  264 , and the cylindrical body  262  includes an equalizing port  266  adjacent the lower end, and a seal  268  at the lowermost end. The equalizing port  266  enables pressure equalization should the sphere  295  form a seal with canister  260  that traps pressure in the canister  260 . A second valve  270  is positioned within the lower cap  230  and below the sphere canister  260  to act as a sphere holding/dropping mechanism. The second valve  270  is preferably identical to the first valve  250  so as to be interchangeable and comprises a body  272 , a rotatable plug  274 , and an actuating stem  276  for manual or remote actuation of plug  274  within body  272  of the valve  270 . A retainer ring  271  is disposed in a shoulder of the lower cap  230  above the valve body  272  to properly position the second valve  270  in the lower cap  230 . A sleeve  297  is provided as a spacer to fit between the counterbore in the body  272  of the valve  270  and the lower cap  230 , which enables adjustable spacing and interchangeable parts. 
     FIGS. 6–8  depict enlarged views of the components of the first valve  250  in more detail. Preferably the second valve  270  is identical to the first valve  250  in construction and operation so that the valves  250 ,  270  are interchangeable. Thus, only the first valve  250  is described in detail.  FIG. 6  provides an enlarged view of the first valve  250  within the manifold of  FIG. 2 ,  FIG. 7  provides a cross-sectional top view of the same valve  250  taken along Section A—A of  FIG. 6 , and  FIG. 8  provides an end view of the valve stem  256 . Valve  250  includes an upper milled slot  610  along the length of the body  252  to enable installation of the valve  250  into the housing  220 . Slots  612 ,  614  are also milled into the lower portion of the body  252  to accept a plug retainer plate  620 , which is a split plate disposed above and below the plug  254  to position the plug  254  with respect to the body  252 . The retainer plate  620  is designed to encircle a boss  630  on one side of the plug  254  that enables rotation between the valve body  252  and valve plug  254 . O-rings  712 ,  714  are provided between the valve body  252  and plug  254  primarily to protect the valve  250  from contamination caused by debris rather than to provide pressure containment. 
   The plug  254  includes a throughbore  750  with a first end  752  and a second end  754 , a transverse bore  660  having an open port  652  with a fouling bar  665  disposed across the diameter of the open port  652 , and a closed side  650  opposite transverse bore  660 . The transverse bore  660  extends perpendicularly to the throughbore  750  and communicates therewith. The fouling bar  665  is provided to prevent the sphere  295  from floating into the valve  750  and interfering with its operation. Although the plug  254  is depicted as being cylindrical in shape, one of ordinary skill in the art will appreciate that the plug  254  may be provided in a variety of shapes such as, for example, a spherical shape. 
   A pin  625  is provided between the valve body  252  and the valve plug  254 . The pin  625  enables proper alignment of the valve plug  254  within the body  252  so that the valve  250  is installed in the closed or hold position as shown in  FIG. 2  and in  FIG. 7 . The pin  625  is shown in top view in  FIG. 8  disposed in a travel slot  810  that only allows a 90° rotation of the valve  250  from the closed, dart holding position to the open, dart dropping position. Thus, the pin  625  aligns the valve  250  properly to be installed in the closed position and also allows the valve  250  to travel only 90° between the hold and the drop positions. 
   Referring to  FIG. 7 , the stem  256  is installed in an aperture in the wall of housing  220  and includes a high-pressure seal  716  engaging housing  220  for pressure and fluid containment, and a flange  720  that prevents the stem  256  from being forced out of the aperture of housing  220  via fluid pressure. Thrust bearings  725  between the flange  720  and housing  220  offset the frictional load exerted on the interior face  727  of the flange caused by fluid pressure inside of the valve  250 . Thus, the bearings  725  eliminate the pressure-induced frictional load, thereby allowing the stem  256  to rotate. 
   Referring to  FIG. 6 , any voids in the cementing manifold  200 , such as the void  640  below the retainer plate  620  in the body  252  of the valve  250  and the gap  645  between the plug  254  and the milled slot  610  in the valve body  252  can potentially become filled with cement or other debris. If the cement hardens in such voids and gaps, then the manifold  200  will require excessive torque to actuate and will not otherwise operate properly. Thus, in the preferred embodiments of the present invention, all voids, such as void  640 , and all gaps, such as gap  645 , would be filled with a solid metal part or a flexible filler material, such as urethane, or a silicone or a rubber boot so that cement and other debris can not enter the area and harden. 
   Referring to  FIG. 6  and  FIG. 7 , to assemble the valve  250  into the housing  220 , the retainer ring  251  is installed. Then the stem  256 , with the high pressure seal  716  and thrust bearings  725 , is installed from inside the housing  220 , thereby ensuring that the stem  256  can never be removed or loosened inadvertently. Due to the milled slot  610  along the length of the valve  250 , the valve body  252  and plug  254  can be assembled into the housing  220  as shown in  FIG. 7 , oriented such that the protruding key  730  of the stem  256  fits into the protruding slot portion  710  of the plug  254 , which ensures that the valve  250  is installed in the closed position. 
   Referring now to  FIG. 2 , the single dart/single sphere cementing manifold  200  is depicted in the holding position before the sphere  295  or the dart  290  are dropped, with both the first valve  250  and the second valve  270  in the closed position. To load the dart  290  and sphere  295  into the cementing manifold  200  as shown in  FIG. 2 , the first valve  250  is opened and the second valve  270  is closed. The sphere  295  is rolled into the manifold  200  through the upper cap  210 , through the dart canister  240 , through the first valve  250 , and into the sphere canister  260  until the sphere  295  engages the closed second valve  270 . Then the first valve  250  is closed, and a dart  290  is installed into the throughbore  214  of the upper cap  210 . The fins  292  of the dart  290  engage the body  242  and collapse within the dart canister  240  such that the dart  290  must be pushed down into the throughbore  244  of the dart canister  240  until the bottom of the dart  290  engages the closed side  650  of first valve  290 . 
   Preferably, once the sphere  295  and dart  290  have been dropped from the manifold  200 , the manifold  200  can then be reloaded in the field. However, in larger sizes, the dart  290  may be too large to be forced into the througbore  244  of the dart canister  240  without mechanical assistance. Therefore, in an alternative embodiment, the dart canister  240  is provided as a two-piece component having upper and lower portions such that the upper portion of the dart canister  240  is removable to enable loading of larger-sized darts  290 . Thus, the cementing manifold  200  is preferably designed to allow for reloading in the field so that the manifold  200  may be moved from rig to rig and only returned to the shop when necessary for redressing and workover rather than after each job for reloading. 
   As previously described, the upper cap  210  is threadingly connected at  215  to the housing  220 , and the housing  220  is threadingly connected at  225  to the lower cap  230 . During operation, the top drive unit  120  exerts high torque on the cementing manifold  200 , which tends to tighten up the threaded connections  215 ,  225 . Then, to reload the cementing manifold  200  after the sphere  295  and dart  290  have been dropped, the upper cap  210 , the housing  220 , and the lower cap  230  must be broken out from one another at the threads  215 ,  225 , which would typically require high torques, such as those exerted by the top drive unit  120 . 
   To enable isolation of the threaded connections  215 ,  225  without fully preloading the connections  215 ,  225  with make-up torque, the slots  219  of the castellated box end  218  of upper cap  210  are matched up with the slots  227  of the castellated pin end  226  of the housing  220 . Similarly, the slots  219  of the castellated box end  228  of housing  220  are matched up with the slots  237  of castellated pin end  236  in the lower cap  230 . For purposes of preventing tightening at the threads  215 ,  225 , only three sets of mating slots disposed 120 degrees apart is preferred, but three additional sets of mating slots are preferably provided circumferentially on each of the upper cap  210 , housing  220  and lower cap  230  to enable alignment of the valve stems  256 ,  276  that extend through the housing  220  and lower cap  230 , respectively, to within 30 degrees. It is preferred, but not required, that the valve stems  256 ,  276  extend from the same side of the manifold  200  for ease of manual actuation. 
   In more detail, when the housing  220  and the lower cap  230  are threaded together at  225 , for example, the mating slots  229 ,  237  on the housing  220  and the lower cap  230 , respectively, may be mis-aligned. In that circumstance, the threaded connection  225  is backed off enough to align the slots  229 ,  237  so that dogs  280  can be installed in every other set of the slots  229 ,  237 . Although the slots  229 ,  237  may be aligned, however, it is also preferred that the valve stems  256 ,  276  extend from the same side of the cementing manifold  200 . Therefore, the threads  225  may need to be backed off 180° to achieve the preferred position of the two valve stems  256 ,  276 . Positioning the valve stems  256 ,  276  is especially preferred when the valves  250 ,  270  are physically opened and closed by manual operation. Thus, with the valve stems  256 ,  276  on the same side of the manifold  200 , an operator that goes up on a line to open the valves  250 ,  270  in the proper sequence can easily identify which is the second valve  270  and which is the first valve  250 . 
   Once proper alignment has been achieved, dogs  280 , that are capable of withstanding the rated torque of the top-drive unit  120 , are installed into the aligned sets of slots to isolate the threaded connections  215 ,  225 . The dogs  280  are installed and held in place by a circumferential ring  284  that fits over all of the dogs  280 . The ring  284  includes equally spaced apertures (not shown) that equal the number of dogs  280  to be installed, such that the dogs  280  may be installed one at a time. The ring  284  fits over all of the mated slots between two components, such as slots  229 ,  237  between the housing  220  and the lower cap  230 . The apertures through the ring  284  are positioned to allow for a dog  280  to be installed into preferably every other set of slots  229 ,  237 . Then a cap screw  282  is threaded through each dog  280  to hold the dogs  280  in position. Once all the dogs  280  have been installed, the ring  284  is rotated to dispose the apertures over empty sets of slots  229 ,  237 . In this position, the ring  284  will prevent the loaded dogs  280  from backing out, even if the cap screws  282  come loose. The dogs  280  and ring  284  are designed to be flush with the exterior surface of the manifold  200 . An identical procedure is followed to install dogs  280  into mated slots  219 ,  227  between the upper cap  210  and the housing  220  utilizing another circumferential ring  284 . 
   To describe the flow path through the cementing manifold  200 , reference will now be made to  FIG. 2 ,  FIG. 6 , and  FIG. 7 .  FIG. 2  provides a cross-sectional view of the cementing manifold  200  in the holding position, with first and second valves  250 ,  270  closed. Referring to  FIG. 6 , which depicts an enlarged view of the first valve  250  in the position shown in  FIG. 2 , the closed side  650  of the valve plug  254  is positioned against the dart canister  240 , the throughbore  750  is disposed perpendicular to the longitudinal axis  205  of the manifold  200 , and the transverse bore  660  is facing downwardly in fluid communication with the throughbore  264  of the sphere canister  260 . The fouling mechanism  665  is positioned in the transverse bore  660  so as to prevent the sphere  295  from floating upwardly to inhibit the operation of the first valve  250 . The design of the valve plug  254  ensures that no hydraulically induced loads are exerted on the valve body  252  when the valve  250  is in the closed position. 
     FIG. 7  depicts the first valve  650  in cross-section through Section A—A of  FIG. 6 . In this cross-section, the full throughbore  750  and the fowling mechanism  665  of the valve  250  is more clearly depicted. The body  252  of the valve  250  includes a D-shaped cutout section  760  that can not be seen in  FIG. 2 . The D-shaped cutout section  760  enables fluid flow through annular area  249  past the plug  254  of the valve  250  through the valve body  252  when the valve  250  is in the closed position. Although the cutout section  760  is depicted as being D-shaped in  FIG. 7 , one of ordinary skill in the art will readily appreciate that the section  760  could be any other shape that would allow fluid to bypass the plug  254 . 
   With the cementing manifold  200  in the holding position as shown in  FIG. 2 , the fluid flows along the path represented by the flow arrows. Namely, the drilling fluid would first flow into the throughbore  214  of the upper cap  210 , then out through the flow slots  246  in the dart canister  240 , and down through the annular area  249  between the dart canister  240  and housing  220  in the housing throughbore  224 . Because both valves  250 ,  270  are closed, there is no flow path through the plug  254  of the first valve  250 , so the flow will bypass the plug  254  through the D-shaped section  760  in the valve body  252 . The flow will continue into the annular area  249  between the sphere holder  260  and the lower cap  230 . Again, because the second valve  270  is closed, there is no straight flow path through the plug  274  of the second valve  270 , so flow will move through the body  272  via the D-shaped section. However, because there is an open flow path below the lower cap  230 , the fluid will flow into the throughbore  285  of the second valve  270 , through the transverse bore  287  of the second valve  270 , and downwardly into the drill string  108 . 
   When a valve  250 ,  270  is turned, the flow path through the manifold  200  changes. Referring to  FIG. 9 , the second valve  270  has been actuated by rotating the valve plug  274  by 90 degrees with respect to the valve body  272 , thereby opening the valve  270  and dropping the sphere  295 . In the rotated position, the transverse bore  287  of the valve  270  is disposed perpendicular to the longitudinal axis  205  of the manifold  200 , and the fouling mechanism  289  is no longer in the flow path. The throughbore  285  in the second valve plug  274  is aligned with the longitudinal axis  205  of the manifold  200 , thereby becoming open and providing an opening for the sphere  295  to drop down into the throughbore  234  of the lower cap  230 . 
   Thus, as shown in  FIG. 9 , once the sphere  295  has dropped, the second valve  270  will be in the dropping position with an open throughbore  285  aligned with the throughbores  264 ,  234  of the sphere canister  260  and the lower cap  230 , respectively, and the first valve  250  will remain in the holding position. In this configuration, as referenced by the flow arrows, the drilling fluid flows into the throughbore  214  of the upper cap  210 , through the flow slots  246  of the dart canister  240 , into the annular area  249  between the dart canister  240  and the housing  220 , and into the D-shaped section  760  of the first valve  250 . Because there is an open flow path below the first valve  250 , the fluid then flows into the throughbore  750  through end  752  of valve plug  252  and downwardly through the transverse bore  660 , the sphere canister  260 , the throughbore  285  of the second valve  270 , and downwardly into the drill string  108 . 
   Referring to  FIG. 10 , after the cement has been pumped through the manifold  200  in the position shown in  FIG. 9 , the valve plug  254  of the first valve  250  is rotated by 90 degrees with respect to the valve body  252  to open valve  250  and drop the dart  290 . In the rotated position, the transverse bore  660  is disposed perpendicular to the longitudinal axis  205  of the manifold  200  and the fouling mechanism  665  is no longer in the flow path. The throughbore  750  in the first valve plug  254  is aligned with the longitudinal axis  205  of the manifold  200 , thereby providing an opening for the dart  290  to drop down into the throughbore  264  of the sphere canister  260 , through the second valve  270  and lower cap  230 , and down into the drill string  108 . Thus, when the first valve  250  is rotated to drop the dart  290 , the throughbore  750  of the valve plug  254  is aligned to allow flow straight through the cementing manifold  200  and down into the drill string  108 . This position of the cementing manifold  200  is called the dropping position. 
   The single dart/single sphere manifold  200  shown in  FIG. 2  is reconfigurable to accommodate multi-darts or multi-spheres, such as, for example, the dual dart/single sphere manifold  300  as shown in  FIG. 3 . In many respects, the manifold  300  includes the same components as the manifold  200  of  FIG. 2 , but also includes an additional housing  320 , an additional dart holder  340 , and an additional dropping/holding valve  350  comprising a valve body  352 , a valve plug  354 , and a valve stem  356 . Thus, the housing  220  of the single dart/single sphere cementing manifold  200  is preferably modular in design to enable additional housings, such as housing  320 , to be stacked together and interconnected between the upper cap  210  and the lower cap  230 . Further, all of the valves  250 ,  270 ,  350  are preferably identical and interchangeable. This enables the operator to stack as many dart or sphere combinations as desired. 
   In contrast, the multi-dart or multi-sphere cementing manifolds of the prior art were either purpose-built or required the interconnection of single manifolds stacked together, creating a very long cementing manifold. In the multi-dart manifold  300  shown in  FIG. 3 , rather than adding approximately 8 feet by connecting two single dart manifolds together, only the length of the additional housing  320  is added, which is approximately 3½ feet long. 
   When only a single dart  290  is dropped from the manifold  200  of  FIG. 2 , some of the cement at the leading end mixes with the previously pumped drilling fluid to form a contaminated mixed fluid termed “rotten cement.” Thus, as previously described, the dual dart manifold  300  may be desired to prevent the cement from mixing with drilling fluid downhole, especially if only a small quantity of cement will be pumped. Thus, after the sphere  295  is dropped from the manifold  300  of  FIG. 3 , the first dart  390  is dropped immediately before the cement is flowed downhole, and the second dart  290  is dropped immediately following the flow of cement downhole to provide containment and prevent the cement from mixing with drilling fluid downhole. 
     FIG. 4  depicts a modified cementing manifold  400  containing only a large elastomeric sphere  495 . The cementing manifold  400  comprises the upper cap  210 , lower cap  230 , and a single valve  270  that acts as the sphere holding/dropping mechanism, which are the same components used in the manifolds  200 ,  300  of  FIGS. 2 and 3 , respectively. However, a specially designed larger sphere canister  460  is disposed above the valve  270  within the upper cap  210  and lower cap  230 . Canister  460  includes an upper enlarged bore  462  and a lower reduced diameter bore  464  forming a conical shaped transition  466  therebetween. The enlarged sphere  495  is received within enlarged bore  462  and then by means of transition  466  is forced into reduced diameter bore  464  for launching downhole. The elastomeric material of sphere  495  allows sphere  495  to compress to fit within reduced diameter bore  464 . 
   Thus, the preferred cementing manifolds  200 ,  300 ,  400  of the present invention comprise a number of advantages. In particular, the manifolds  200 ,  300 ,  400  are preferably easily assembled and disassembled, providing reloading capability in the field. The manifolds  200 ,  300 ,  400  preferably include dogs  280  that allow high torque transmission without requiring pre-torque at the threaded connections. Additionally, the manifolds  200 ,  300 ,  400  preferably include modular housings  220 ,  320  that can be stacked together and interconnected to add multi-dart or multi-sphere capability, as desired, thereby providing a high degree of flexibility. Further, the manifolds  200 ,  300 ,  400  preferably include identical, interchangeable valves  250 ,  270 ,  350  that require only a 90° turn to open or close. The valves  250 ,  270 ,  350  are preferably pressure balanced to minimize resistance to rotation, thereby enabling release of the darts  290 ,  390  and spheres  295 ,  495  while flowing. The valves  250 ,  270 ,  350  also preferably include large throughbores  750 ,  285 ,  385  to minimize flow erosion. Additionally, the manifolds  200 ,  300 ,  400  preferably provide internal bypass capability, internally loaded darts  290 ,  390  and spheres  295 ,  495 , and valve bodies  252 ,  272 ,  352  that install internally. Thus, only the small diameter valve stems  256 ,  276 ,  356  protrude externally from the pressure containing housings  220 ,  320  and lower cap  230 , thereby minimizing penetrations that act as stress concentration areas. Further, there are no externally mounted components that are welded or threaded. 
   While preferred embodiments of this invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit or teaching of this invention. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the apparatus and methods are possible and are within the scope of the invention. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims.

Summary:
Apparatus for cementing a string of tubulars in a borehole comprises an enclosure having a bore therethrough, an axially fixed sphere canister having a sphere aperture therethrough, a sphere valve member having a valve body disposed internally of said bore, and a sphere disposed in said sphere aperture, wherein said sphere valve member has a hold position closing said sphere aperture and a drop position opening said sphere aperture to release said sphere.