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FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to a connector assembly. More particularly, this invention relates to a connector assembly for connecting a sleeve to a rotatable shaft. Exemplary embodiments of this invention may advantageously increase the torsional and/or bending load bearing capacity of the shaft as compared to the prior art.  
       BACKGROUND OF THE INVENTION  
       [0002]     In downhole drilling applications, torque is often transferred from a power source at the surface to a drill bit. In such applications, the torque is conventionally transferred through the shafts and/or cylindrical tubes that make up the drill string. In other downhole drilling applications, torque may be transferred from a downhole drilling motor to the drill bit. In such applications, the torque is transferred through the shafts and/or cylindrical tubes deployed between the drilling motor and the drill bit.  
         [0003]     In either of these downhole drilling arrangements, the shafts and/or cylindrical tubes deployed between the power source and the drill bit must also typically accommodate significant bending loads, such as are commonly encountered in deviated boreholes. Such torsional and bending loads must typically be transferred through joints (e.g., threaded box and pin connections) at which adjacent components (e.g., drill string tubes) are connected.  
         [0004]     In certain downhole applications, torque is transferred from a cylindrical tube (such as the tubes that make up the drill string) to a shaft (such as a rotatable shaft in a downhole tool). One common way to accommodate torsional and bending loads in such applications is to thread a mating sleeve to the shaft and tighten it (with sufficient torque to withstand anticipated torsional and bending loads) against a shoulder on the outer diameter of the shaft. Alternatively, another common approach includes using a second threaded sleeve as a shoulder. The mating sleeve is then tightened against the second sleeve (e.g., similar to a conventional locking nut). While such methods have been utilized to connect shafts and sleeves in various drilling operations, one drawback is that they typically require the sleeves to be tightened with a make up torque equal to about one-half the torsion yield of the connector (e.g., to about 45,000 foot pounds in common downhole applications). The use of such a high make up torque increases the stresses in the shaft and sleeves and thus tends to limit the bending and torsion loads that they may safely withstand.  
         [0005]     Locking devices for downhole box and pin connectors are also known in the prior art.  FIG. 1  shows one such prior art locking device that includes three cylindrical sleeves  75 ,  90 , and  95  deployed coaxially about the box  80  and pin  70 . Torque sleeve  75  includes a plurality of dogs  73  formed on an inner surface thereof and is configured to engage corresponding slots  72  formed on an outer surface of the pin member  70 . Slide sleeve  95  includes a plurality of dogs  83  formed on an inner surface thereof is configured to engage slots  82  formed on an outer surface of the box member  80 . Alignment sleeve  90  includes first and second sets of radial splines  79  and  91  formed on an internal surface thereof. The first set  79  includes  41  splines and is configured to engage splines  78  formed on an outer surface of torque ring  75 . The second set  91  includes  40  splines and is configured to engage splines  92  formed on an outer surface of the slide ring  95 . As such, the pin member  70  is rotationally coupled to the box member  80  when spines  78  and  79  and splines  91  and  92  are engaged.  
         [0006]     With continue reference to  FIG. 1 , the locking device is made up about the connection by deploying the torque sleeve  75  about pin member  70  with dogs  73  engaged with slots  72 . Alignment sleeve  90  and slide sleeve  95  are deployed about box member  80  with dogs  83  engaged with slots  82 . The pin member  70  is then threaded to box member  80  (with threads  71  engaging threads  81 ) and tightened to a predetermined torque. A spacer (not shown) is translated into place and holds torque sleeve  75  firmly in place against shoulder  85  of box member  80 . The alignment sleeve  90  is then rotated between the torque ring  75  and slide sleeve  95  until splines  78  and  79  and splines  91  and  92  are aligned. The slide sleeve is then translated axially towards the torque ring such that splines  78  and  79  and splines  91  and  92  engage. One or more soft nails are then driven into grooves  98  to hold sleeves  75 ,  90 , and  95  together.  
         [0007]     Another prior art locking device for a downhole box and pin connector includes two sleeves deployed coaxially about the threaded box member. The first sleeve includes four large axial dogs on one axial face disposed to engage four corresponding radial slots disposed on the outer surface of the box member. The second sleeve includes a plurality of axial splines configured to engage with corresponding axial splines disposed on a shoulder portion of the threaded pin member. The sleeves are further disposed to engage one another via a plurality of interlocking axial splines. As such, the box member may be rotationally coupled to the pin member when the first and second sleeves are engaged. The second sleeve includes  19  axial splines on one axial face and  20  axial splines on the opposing axial face. The locking device further includes a covering member deployed about the first and second sleeves. The covering member is intended to provide axial support for the first and second sleeves and is held in place by a soft nail pressed into a circumferential groove in the shoulder portion of the pin member.  
         [0008]     The above-described connector assemblies, while potentially serviceable, are complex (e.g., including three splined sleeves nailed together and held in place by a spacer or including two splined sleeves with an encircling cover nailed in place). Moreover, the above described connector assemblies do not efficiently utilize the available diametrical space. For example, in both assemblies the sleeves are deployed about the box and pin members, thereby reducing the maximum outer diameter of the box and pin members. Such a reduced outer diameter tends to reduce the strength of the connection. Therefore, there exists a need for improved connector assemblies for downhole drilling tools. In particular, there exists a need for connector assemblies for connecting a mating sleeve to a rotatable shaft that are suitable to support large torsional and/or bending loads, often cyclic in nature, and that efficiently utilize the diametrical space available in the downhole tool.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention addresses one or more of the above-described drawbacks of the prior art. Aspects of this invention include generally an apparatus for connecting a mating sleeve to an end of a threaded shaft. The shaft includes a plurality of splines formed on an outer surface thereof. The mating sleeve may be threadably coupled to the shaft and includes a plurality of splines formed on an inner surface thereof. The apparatus further includes a center sleeve deployed between the shaft and the mating sleeve. The center sleeve includes a plurality of splines formed on an inner surface thereof sized and shaped for engagement with the splines on the shaft. The center sleeve further includes a plurality of splines formed on the outer surface thereof sized and shaped for engagement with the splines on the inner surface of the mating sleeve. In certain exemplary embodiments of this invention, the center sleeve may include a different number of splines on the inner and outer surfaces (e.g., 30 splines on the inner surface and 29 splines on the outer surface).  
         [0010]     Exemplary embodiments of the present invention advantageously provide several technical advantages. Various embodiments of this invention reduce the make-up torque requirements of the mating sleeve and therefore may advantageously increase the cyclic torsional and/or bending load capacity of the shaft. Various embodiments also increase the total torsional load capacity of the connection. Furthermore, embodiments of this invention more efficiently utilize available diametrical space. As such, the diameter of the torquing shoulder and the box and pin members may be greater than that in the prior art, thereby enabling them to support greater torsional and bending loads. Moreover, as described more detail below, certain embodiments of this invention may advantageously be utilized on opposing ends of a shaft on a downhole tool. As such, tool maintenance may be simplified since the tool may be assembled and disassembled from either end.  
         [0011]     In one aspect the present invention includes a connector assembly for a downhole tool. The connector assembly connects to a rotatable shaft in which the shaft includes a plurality of radial splines formed on an outer surface thereof. The connector assembly includes a mating sleeve having a plurality of radial splines formed on an inner surface thereof. The mating sleeve is further threadably coupleable about the shaft. The connector assembly also includes a center sleeve sized and shaped for deployment between the shaft and the mating sleeve. The center sleeve includes a first plurality of radial splines formed on an inner surface thereof. The first plurality of radial splines are sized and shaped for engagement with the plurality of splines on the shaft. The center sleeve further includes a second plurality of radial splines formed on an outer surface thereof. The second plurality of radial splines are sized and shaped for engagement with the plurality of splines on the mating sleeve.  
         [0012]     In another aspect this invention includes a downhole steering tool. The steering tool comprises a rotatable shaft rotatably deployed within a substantially non rotatable steering tool body, the shaft having longitudinally opposed first and second ends, each of said ends providing a plurality of shaft splines extending radially outward from an outer surface of the shaft. The steering tool further comprises first and second connector assemblies, one each of the connector assemblies connected to a corresponding one of the ends of the shaft. Each connector assembly comprises a shaft sleeve threadably coupled to the end of the shaft, a mating sleeve also threadably coupled the end of the shaft and tightened against the shaft sleeve, the mating sleeve further including a plurality of mating sleeve splines extending radially inward from an inner surface thereof. Each connector assembly further comprises a center sleeve also received on the end of the shaft via splined engagement with the shaft splines. The center sleeve includes a plurality of inner radial splines formed on an inner surface thereof, the inner radial splines sized and shaped for engagement with the shaft splines. The center sleeve further includes a plurality of outer radial splines formed on an outer surface thereof, the outer radial splines sized and shaped for engagement with the mating sleeve splines. The mating sleeve is further received on the center sleeve via splined engagement between the mating sleeve splines and the outer radial splines of the center sleeve.  
         [0013]     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:  
         [0015]      FIG. 1  depicts an exploded view of a prior art locking device for a box and pin connection.  
         [0016]      FIG. 2  depicts an offshore oil and/or gas drilling platform utilizing an exemplary embodiment of the present invention.  
         [0017]      FIG. 3  depicts a one exemplary embodiment of a connector assembly according to the present invention.  
         [0018]      FIG. 4  is a cross sectional view as shown on  FIG. 3 .  
     
    
     DETAILED DESCRIPTION  
       [0019]      FIG. 2  schematically illustrates one exemplary embodiment of a connector assembly  100  according to this invention in use in an offshore oil and/or gas drilling assembly, generally denoted  10 . In  FIG. 2 , a semisubmersible drilling platform  12  is positioned over an oil or gas formation (not shown) disposed below the sea floor  16 . A subsea conduit  18  extends from deck  20  of platform  12  to a wellhead installation  22 . The platform may include a derrick  26  and a hoisting apparatus  28  for raising and lowering the drill string  30 , which, as shown, extends into borehole  40  and includes a drill bit assembly  32  and a downhole steering tool  200  including one or more force application members  232 . Steering tool  200  is shown coupled to a downhole sub  50  (e.g., a cross over sub or a section of drill pipe) via connector assembly  100 . It will be appreciated that connector assembly  100  may be utilized to couple substantially any downhole components (such as measurement tools, steering tools, stabilizers, subs, and various sections of drill pipe) and therefore many be deployed at substantially any location in the drill string  30 . As shown in  FIGS. 2 and 3 , in certain applications the connector assembly  100  may be advantageously utilized to couple a downhole steering tool to one or more downhole subs. It will also be appreciated that connector assembly  100  may be utilized on opposing ends of a downhole tool (e.g., to connect a sub to an upper end of steering tool  200  and a drill bit assembly  32  to a lower end of steering tool  200 ). Drill string  30  may further include a downhole drill motor, a mud pulse telemetry system, and one or more sensors, such as LWD and/or MWD tools for sensing downhole characteristics of the borehole and the surrounding formation.  
         [0020]     It will be understood by those of ordinary skill in the art that the deployment illustrated on  FIG. 1  is merely exemplary for purposes of the invention set forth herein. It will be further understood that the connector assembly  100  of the present invention is not limited to use with a semisubmersible platform  12  as illustrated on  FIG. 1 . Connector assembly  100  is equally well suited for use with any kind of subterranean drilling operation, either offshore or onshore.  
         [0021]     It will be further understood that the present invention is also not limited to subterranean drilling applications. Embodiments of the invention include a connector assembly that facilitates increased torsion and bending capacity in any shaft comprising connected joints.  
         [0022]     Referring now to  FIG. 3 , a longitudinal cross section of one exemplary embodiment of a connector assembly  100  according to the present invention is shown. As shown in  FIG. 3  (and described above with respect to  FIG. 2 ), one exemplary application is depicted in which a downhole steering tool  200  is coupled to a downhole sub  50  via connector assembly  100 . In the exemplary embodiment shown, connector assembly  100  includes a mating sleeve  110  deployed about cylindrical shaft  220  of steering tool  200  and threadably coupled with downhole sub  50  via a conventional box  114  and pin  52  connection. In the embodiment shown, mating sleeve  110  includes a threaded inner surface  109  for threadably coupling with a threaded outer surface  219  of cylindrical shaft  220 . Connector assembly  100  further includes a center sleeve  130  deployed between the cylindrical shaft  220  and the mating sleeve  110 . The center sleeve  130  is rotationally engaged with cylindrical shaft  220  and mating sleeve  110  via radial splines  136 ,  138  (shown on and described in more detail below with respect to  FIG. 4 ) on its inner  133  and outer  134  surfaces, respectively.  
         [0023]     With continued reference to  FIG. 3 , cylindrical shaft  220  is disposed to rotate about axis  60  in the substantially non-rotating housing  210  of steering tool  200 . In the embodiment shown, the steering tool  200  further includes a bearing assembly  212  and a sealing assembly  214  deployed about shaft  220 . A shaft sleeve  120  is threadably engaged with cylindrical shaft  220  via threads  119  and  224 . In the embodiment shown, shaft sleeve  120  is tightened against bearing spacer  216  (e.g., to a torque of about 5,000 foot pounds) to tighten bearings  212  against shoulder  222  of the cylindrical shaft  220 . Mating sleeve  110  is then threadably engaged with cylindrical shaft  220  (as described above) and tightened against shaft sleeve  120 . In the embodiment shown, a portion  112  of the mating sleeve  110  having a reduced outer diameter is received in an enlarged counter bore  122  of the shaft sleeve. One or more compression sleeves  115  may be optionally deployed between a lower end  117  of the mating sleeve  110  and an interior shoulder  127  of shaft sleeve  120 . Such compression sleeve(s)  115  may be advantageously fabricated, for example, from a beryllium copper alloy. The mating sleeve  110  is typically tightened sufficiently such that a predetermined maximum bending load (e.g., determined from a maximum dogleg severity of a borehole) does not cause separation of the lower end  117  of the mating sleeve  110  and the interior shoulder  127  of the shaft sleeve  120 . For example, in one exemplary embodiment, mating sleeve  110  may be tightened with a torque of about 26,000 foot-pounds (although the invention is not limited in this regard). After mating sleeve  110  is tightened to shaft sleeve  120 , center sleeve  130  (along with one or more optional sealing sleeves  132 ) is deployed between and rotational engaged with cylindrical shaft  220  and mating sleeve  110  as described in more detail with respect to  FIG. 4 .  
         [0024]     Turning now to  FIG. 4 , the engagement between cylindrical shaft  220 , center sleeve  130 , and mating sleeve  110  is described in more detail. As shown in  FIG. 4  (and as described briefly above), center sleeve  130  includes a plurality of radial splines  136  on an inner surface  133  thereof sized and shaped to engage radial splines  226  on an outer surface of cylindrical shaft  220 . Center sleeve  130  further includes a plurality of radial splines  138  on an outer surface  134  thereof sized and shaped to engage radial splines  116  on an inner surface of mating sleeve  110 . As shown, center sleeve  130  rotationally couples mating sleeve  110  to cylindrical shaft  220 . As such, use of center sleeve  130  advantageously reduces make up torque requirements, since torsional loads are supported by the radial splines  136 ,  226 ,  138 , and  116  rather than frictional forces on threads  109  and  219 . As described above, the make up torque is intended simply to support bending loads. Thus, as described above, the torque requirement is typically less than that for a threaded connection required to withstand both torsional and bending loads through the threads and shoulder.  
         [0025]     In certain advantageous embodiments, center sleeve  130  includes a different number of inner  136  and outer  138  splines. For example, there may be one or two more inner splines  136  than outer splines  138  (or conversely one or two more outer splines  138  than inner splines  136 ). In such exemplary embodiments, the inner  136  and outer  138  splines are circumferentially offset from one another with the degree of circumferential offset varying about the circumference of the tool. During assembly of the connection, the center sleeve  130  is typically rotated (about axis  60 ) until the inner  136  and outer  138  splines line up with splines  226  and  116  respectively. The center sleeve may then be inserted between mating sleeve  110  and shaft  220  such that splines  136  engage splines  226  and splines  138  engage splines  116 . It will be appreciated that for embodiments in which the difference between the number of splines on the inner  133  and outer  134  surfaces of the center sleeve  130  is a single spline, there is one unique rotary position at which the center sleeve may be inserted between the connection sleeve  110  and the shaft  220 . For embodiments in which the difference is two splines, the center sleeve may be inserted at two rotary positions offset from one another by 180 degrees.  
         [0026]     In the exemplary embodiment shown on  FIG. 4 , center sleeve  130  includes  30  radial splines  136  on inner surface  133  and  29  radial splines  138  on outer surface  134  (although it will be appreciated that the invention is not limited to any particular number of splines  136  and  138 ). As such, it will be appreciated the center sleeve  130  may possibly be engaged between the mating sleeve  110  and the shaft  220  at one of  870  (29 times 30) unique rotary positions. Since there is a discrete number of circumferential positions ( 870  in the embodiment shown on  FIG. 4 , which corresponds to a circumferential spacing of 0.41 degrees, i.e., 360 divided by 870) it may be advantageous in certain embodiments to fabricate splines  136 ,  226 ,  138 , and  116  such that they engage one another with a predetermined clearance. Such clearance is intended to assure that there is at least one rotary position at which the center sleeve  130  may be engaged between the mating sleeve  110  and the shaft  220 . For example a connection in which center sleeve  130  has a 4¾ inch outer diameter and a clearance of 0.020 inches between each pair of mating splines (splines  136  and  226  and splines  138  and  116 ) allows for about 0.96 degrees of circumferential movement. Such clearance is greater than the spacing of the circumferential positions (0.41 degrees) and is therefore sufficient to enable the center sleeve  130  to engage with the mating sleeve  110  and cylindrical shaft  220  in at least one and as many as three sequential circumferential positions. It will be appreciated that this invention is not limited to any particular number of radial splines, nor to any particular clearance between the various mating splines when such clearance is provided.  
         [0027]     It will be appreciated that embodiments of this invention may be utilized on opposing ends of a shaft on a downhole tool. In particular, embodiments of this invention may be advantageously utilized on opposing ends of a rotatable shaft in a downhole steering tool (such as steering tool  200  shown on  FIG. 3 ). It will be appreciated that embodiments of this invention obviate the need for upsets at one end of the shaft for increasing torque and bending moment load capacity. As such, the tool may be advantageously assembled and disassembled from either end so that the seals, seal housings, bearings, and other such components may be replaced without requiring complete disassembly of the tool, thereby simplifying maintenance procedures. Further, as described above, the make up torque requirements are reduced at both ends of the shaft (as compared to the prior art), thereby advantageously increasing the torsional and bending load capacity of the shaft.  
         [0028]     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alternations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.

Summary:
A connector assembly for connecting with a rotatable shaft is provided. The connector assembly includes a mating sleeve threadably coupleable about the shaft and a center sleeve sized and shaped for deployment between the shaft and the mating sleeve. The center sleeve includes pluralities of splines formed on inner and outer surfaces thereof. The splines on the inner surface of the center sleeve are engageable with splines on the outer surface of the shaft and the splines on the outer surface of the center sleeve are engageable with splines on the inner surface of the mating sleeve. Exemplary embodiments of this invention are useful in downhole tools to reduce make-up torque requirements of the mating sleeve, and therefore may advantageously increase the cyclic torsional and/or bending load capacity of the shaft, while also increasing the total torsional load capacity of the connection.