Abstract:
A mechanism for adjusting the relative angular orientation of two coaxial components includes a mandrel having a cylindrical central section between upper and lower splined sections, a sleeve rotatably and slidably disposed around the mandrel&#39;s central section, and generally cylindrical upper and lower ratchet members positioned, respectively, about the mandrel&#39;s upper and lower splined sections. The ratchet members have internal grooves which receive the mandrel splines for torsional load transfer while permitting limited rotation relative to the mandrel, but their axial positions relative to the mandrel are fixed. The upper and lower ends of the sleeve have circumferentially-arrayed ratchet teeth engageable, respectively, with corresponding teeth on the upper and lower ratchet members. The central sleeve has torque-transferring external splines slidable within matching grooves on the inner surface of a cylindrical tool housing enclosing the mechanism. The mandrel is rotatable relative to the housing, but its axial position is fixed. The teeth of the sleeve and ratchet members are configured such that movement of the sleeve from a position engaging the upper ratchet member to a position engaging the lower ratchet member, or vice versa, will effect an incremental angular shift of the mandrel relative to the tool housing, while maintaining effective transfer of torsional loads therebetween.

Description:
[0001]    This application is the U.S. National Stage under 35 U.S.C. §371 of International Patent Application No. PCT/US2009/045490 filed May 28, 2009, which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/057,110 filed May 29, 2008, entitled “Mechanism For Providing Controllable Angular Orientation While Transmitting Torsional Load.” 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       FIELD OF THE INVENTION 
       [0003]    The present invention relates in general to mechanisms for providing controllable angular orientation between an outer tubular element and a coaxial inner tubular element while transmitting torsional load between the outer and inner tubular elements. More particularly, the invention is directed to such mechanisms which can be incorporated in a downhole tool coupled within a drill string in a wellbore to provide controllable angular orientation between the sections of the string above and below the tool, while the mechanism is subjected to torsional load. 
       BACKGROUND OF THE INVENTION 
       [0004]    In drilling a borehole (or wellbore) into the earth, such as for the recovery of hydrocarbons or minerals from a subsurface formation, it is conventional practice to connect a drill bit onto the lower end of a “drill string”, then rotate the drill string so that the drill bit progresses downward into the earth to create the desired borehole. A typical drill string is made up from an assembly of drill pipe sections connected end-to-end, plus a “bottomhole assembly” (“BHA”) disposed between the bottom of the drill pipe sections and the drill bit. The BHA is typically made up of sub-components such as drill collars, stabilizers, reamers and/or other drilling tools and accessories, selected to suit the particular requirements of the well being drilled. 
         [0005]    In conventional vertical borehole drilling operations, the drill string and bit are rotated by means of either a “rotary table” or a “top drive” associated with a drilling rig erected at the ground surface over the borehole (or in offshore drilling operations, on a seabed-supported drilling platform or suitably-adapted floating vessel). During the drilling process, a drilling fluid (commonly referred to as “drilling mud” or simply “mud”) is pumped under pressure downward from the surface through the drill string, out the drill bit into the wellbore, and then upward back to the surface through the annulus between the drill string and the wellbore. The drilling fluid carries borehole cuttings to the surface, cools the drill bit, and forms a protective cake on the borehole wall (to stabilize and seal the borehole wall), in addition to other beneficial functions. 
         [0006]    As an alternative to rotation by a rotary table or a top drive, a drill bit can also be rotated using a “mud motor” (alternatively referred to as a “downhole motor”) incorporated into the drill string immediately above the drill bit. The mud motor is powered by drilling mud pumped under pressure through the mud motor in accordance with well-known technologies. The technique of drilling by rotating the drill bit with a mud motor without rotating the drill string is commonly referred to as “slide” drilling, because the non-rotating drill string slides downward within the wellbore as the rotating drill bit cuts deeper into the formation. Torque loads from the mud motor are reacted by opposite torsional loadings transferred to the drill string. 
         [0007]    Directional drilling operations using a mud motor require means for controlling the orientation of the mud motor relative to earth while the motor is down hole, in order to control the resulting direction of the curved or deflected wellbore. When drilling with a conventional string of drill pipe, mud motor orientation control can be accomplished by rotating the entire pipe string from surface. However, when drilling with coiled tubing, which cannot easily be rotated from surface, orientation control must be accomplished using means capable of controlling the angular orientation of the mud motor relative to the coiled tubing. It is desirable for this relative orientation to be controllable while drilling operations are in progress, to avoid any unexpected and undesired changes in orientation due to the unwinding and recoiling of the coiled tubing that can occur when drilling is interrupted. 
         [0008]    Previous devices typically include an arrangement of lugs and spiral grooves, or an arrangement of lugs and circumferentially-spaced cam bodies, that convert axial motion of a piston into rotational motion of the lower string components. Such devices are generally very complicated in construction and operation, with large numbers of components. The devices also do not allow orientation to be controlled and adjusted while being subjected to torsional loads (such as under normal drilling conditions). 
         [0009]    Accordingly, there remains a need for improved and less complicated apparatus for controlling and adjusting the angular orientation between coaxial tubular elements, particularly while under torsional loading. The present invention is directed to this need. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    The present invention provides a mechanism which can be incorporated into a tool located between the end of a tubing string and a mud motor, whereby the angular orientation of the mud motor relative to the tubing string can be adjusted without interrupting well-drilling operations, while maintaining effective transfer of torsional loads from the mud motor to the tubing string. In preferred embodiments, the mechanism includes a generally cylindrical mandrel having a central bore throughout its length (for passage of drilling fluid), a cylindrical central section, an upper section above the cylindrical central section, and a lower section below the cylindrical central section. The mandrel is positioned coaxially within a cylindrical tool housing such that the mandrel is rotatable relative to the housing but its axial position relative to the housing is substantially fixed. In a typical well-drilling application of the mechanism, a mud motor will be coupled to the lower end of the mandrel (either directly or through intermediary components). 
         [0011]    A cylindrical central sleeve is disposed around the central cylindrical section of the mandrel, with the central sleeve having an internal diameter to provide a close but readily slidable fit with the central cylindrical section of the mandrel. The central sleeve is longitudinally slidable but substantially non-rotatable relative to the housing. In the preferred embodiment, this functionality is facilitated by forming the central sleeve with a plurality of longitudinally-oriented external splines slidingly received within complementary grooves formed in the inner surface of the housing. The upper and lower ends of the central sleeve each have a plurality of circumferentially-arrayed and equally-spaced ratchet teeth. In the preferred embodiment, each ratchet tooth has a first face that is parallel to the longitudinal axis of the mandrel, plus a second face that is angled relative to the first face (hereinafter these first and second faces will be referred to as “vertical faces” and “sloped faces” respectively). 
         [0012]    The mechanism also includes generally cylindrical upper and lower ratchet members disposed, respectively, about the upper and lower sections of the mandrel; i.e., on either side of the central sleeve. The upper and lower ratchet members are mounted such that their axial positions relative to the mandrel are substantially fixed, but also such that they are independently rotatable relative to the mandrel within a limited angular range. In the preferred embodiment of the mechanism, this limited rotational functionality is facilitated by providing the inner cylindrical surfaces of the upper and lower ratchet members with longitudinal grooves configured to receive complementary external splines formed on the upper and lower sections of the mandrel, but with the ratchet member grooves being wider than the corresponding mandrel splines. In preferred embodiments, biasing means (such as bow springs) will be provided to bias the mandrel splines against one side face of the corresponding ratchet member grooves to facilitate torque transfer during drilling. 
         [0013]    The lower end of the upper ratchet member has a plurality of circumferentially-arrayed and equally-spaced ratchet teeth configured for mating engagement with the ratchet teeth on the upper end of the central sleeve. Similarly, the upper end of the lower ratchet member has a plurality of circumferentially-arrayed and equally-spaced ratchet teeth configured for mating engagement with the ratchet teeth on the lower end of the central sleeve. The four pluralities of ratchet teeth have matching numbers of ratchet teeth, and, therefore, the same spacing (or angular interval) between adjacent ratchet teeth. 
         [0014]    The upper and lower ratchet members are axially spaced such that the central sleeve can slide along the mandrel between:
       an upper position in which the central sleeve&#39;s upper ratchet teeth are matingly engaged with the ratchet teeth of the upper ratchet member, with the central sleeve&#39;s lower ratchet teeth being clear of the ratchet teeth of the lower ratchet member; and   a lower position in which the central sleeve&#39;s lower ratchet teeth are matingly engaged with the ratchet teeth of the lower ratchet member, with the central sleeve&#39;s upper ratchet teeth being clear of the ratchet teeth of the upper ratchet member.       
 
         [0017]    When the central sleeve is in its upper position, its lower ratchet teeth will be offset relative to the ratchet teeth of the lower ratchet member, with the offset preferably being approximately one-half of the typical ratchet tooth spacing (or angular interval). In this configuration, torque from a mud motor connected to the bottom of the mandrel will be transferred from the mandrel to the upper ratchet member via the spline/groove connection therebetween, from the upper ratchet member to the central sleeve via the respective engaged ratchet teeth, and from the central sleeve to the tool housing via the spline/groove connection therebetween. 
         [0018]    Similarly, when the central sleeve is in its lower position, its upper ratchet teeth will be offset relative to the ratchet teeth of the upper ratchet member, with the offset preferably being approximately one-half of the typical ratchet tooth spacing (or angular interval). In this configuration, torque from a mud motor connected to the bottom of the mandrel will be transferred from the mandrel to the lower ratchet member via the spline/groove connection therebetween, from the lower ratchet member to the central sleeve via the respective engaged ratchet teeth, and from the central sleeve to the tool housing via the spline/groove connection therebetween. 
         [0019]    When the central sleeve is moved from its upper position toward its lower position, the central sleeve&#39;s upper ratchet teeth will begin disengaging from the ratchet teeth of the upper ratchet member, but torque transfer between the upper ratchet member and the central sleeve will remain effective until these two sets of ratchet teeth are fully disengaged, because their respective vertical faces will remain in load-transferring contact prior to full disengagement, and until such full disengagement there can be no rotation of the upper ratchet member relative to the sleeve. 
         [0020]    However, as the central sleeve is moved from its upper position toward its lower position, the central sleeve&#39;s lower ratchet teeth will begin engaging the ratchet teeth of the lower ratchet member before the central sleeve&#39;s upper ratchet teeth are fully disengaged from the upper ratchet member. As well, due to the previously-noted offset between the central sleeve&#39;s ratchet teeth and the ratchet teeth of the lower ratchet member, the continued downward movement of the central section&#39;s ratchet teeth into the ratchet teeth of the lower ratchet member will force the lower ratchet member to rotate approximately one-half of a ratchet tooth interval relative to the mandrel, due to the tips of the central sleeve&#39;s lower ratchet teeth bearing downward against the sloped faces of the ratchet teeth of the lower ratchet member. This limited rotational displacement of the lower ratchet member is possible because, as previously noted, the splines in the lower splined section of the mandrel are narrower than the corresponding grooves in the lower ratchet member. During this limited rotational displacement, any springs or other biasing means associated with the lower ratchet member will be compressed or otherwise stressed as the mandrel splines move in an arcuate path within the lower ratchet member grooves. 
         [0021]    As the central sleeve reaches its lower position, and as the central sleeve&#39;s upper ratchet teeth become fully disengaged from the upper ratchet member, torsional loads acting on the mandrel (e.g. from a mud motor) will cause a sudden angular displacement of the mandrel relative to the central sleeve, while concurrently relieving stresses induced in the biasing means (if present) during the movement of the central sleeve. The amount of this angular displacement will correspond to one-half of the ratchet tooth spacing. Because the central sleeve cannot rotate relative to the tool housing by virtue of the spline/groove connection therebetween, the effect of the angular displacement between the mandrel and the central sleeve is to create the same angular displacement between the tool housing and the mandrel—and therefore between the tool housing and any mud motor or other tool or appurtenance coupled to the mandrel. 
         [0022]    In a fashion similar to that described above, upward movement of the central sleeve back to its upper position will induce a similar and additional angular displacement of the mandrel relative to the tool housing. 
         [0023]    In alternative embodiments, the mechanism of the present invention may also be configured to internally drive the relative rotation that occurs during orientation in applications that are not subject to external torsional loads. 
         [0024]    Although the present invention has particularly beneficial applications in association with directional drilling with coiled tubing, persons skilled in the art will appreciate that it may be also be readily adapted for use in other applications where controlled angular orientation between two or more coaxial components is required, with or without the presence of applied torsional load. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    Embodiments of the invention will now be described with reference to the accompanying figures, in which numerical references denote like parts, and in which: 
           [0026]      FIG. 1  is a partial-cutaway elevation of a drill string incorporating an angular orientation mechanism in accordance with one embodiment of the present invention. 
           [0027]      FIG. 1   a  is an elevation of a mandrel suitable for use in accordance with one embodiment of the invention. 
           [0028]      FIG. 2  is a partial cutaway view of the orientation mechanism in  FIG. 1 , with the central sleeve in its upper position. 
           [0029]      FIG. 3  is a transverse cross-section through the tool housing, cylindrical piston, upper ratchet member, and mandrel of the orientation mechanism in  FIG. 2 . 
           [0030]      FIG. 4  is a transverse cross-section through the tool housing, central sleeve, and mandrel of the orientation mechanism in  FIG. 2 . 
           [0031]      FIG. 5  is a transverse cross-section through the tool housing, lower ratchet member, and mandrel of the orientation mechanism in  FIG. 2 . 
           [0032]      FIG. 6  is a partial cutaway view of the orientation mechanism in  FIG. 2 , with the central sleeve displaced slightly downward from its upper position, with its lower ratchet teeth beginning to engage the ratchet teeth of the lower ratchet member. 
           [0033]      FIG. 7  is similar to  FIG. 6  but with the central sleeve displaced further downward, with its lower ratchet teeth engaging the sloped faces of the ratchet teeth of the lower ratchet member so as to incrementally rotate the lower ratchet member in a counterclockwise direction. 
           [0034]      FIG. 8  is a partial cutaway view showing the central sleeve after full downward displacement to its lower position, with its lower ratchet teeth in full mating engagement with the ratchet teeth of the lower ratchet member, and with its upper ratchet teeth fully disengaged from the upper ratchet member. 
           [0035]      FIG. 9  is a transverse cross-section through the tool housing, lower ratchet member, and mandrel, as viewed during downward displacement of the central sleeve as in  FIG. 7 . 
           [0036]      FIG. 10  is a transverse cross-section through the tool housing, lower ratchet member, and mandrel, as viewed after full downward displacement of the central sleeve as in  FIG. 8 . 
           [0037]      FIG. 11  is a partial cutaway view of the orientation mechanism in  FIG. 2 , with the central sleeve displaced slightly upward from its lower position, and with its upper ratchet teeth beginning to engage the ratchet teeth of the upper ratchet member. 
           [0038]      FIG. 12  is similar to  FIG. 11  but with the central sleeve displaced further upward, with its upper ratchet teeth engaging the sloped faces of the ratchet teeth of the upper ratchet member so as to incrementally rotate the upper ratchet member in a counterclockwise direction. 
           [0039]      FIG. 13  is a partial cutaway view showing the central sleeve after full upward displacement back to its upper position, with its upper ratchet teeth in full mating engagement with the ratchet teeth of the upper ratchet member, and with its lower ratchet teeth fully disengaged from the lower ratchet member. 
           [0040]      FIG. 14  is a transverse cross-section through the tool housing, cylindrical piston, upper ratchet member, and mandrel, as viewed during upward displacement of the central sleeve as in  FIG. 12 . 
           [0041]      FIG. 15  is a transverse cross-section through the tool housing, cylindrical piston, upper ratchet member, and mandrel, as viewed after full upward displacement of the central sleeve as in  FIG. 13 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0042]      FIG. 1  illustrates an angular orientation mechanism  100  in accordance with one embodiment of the present invention, incorporated within a string of tubular elements constituting a downhole tool.  FIG. 1  depicts one possible orientation of the downhole tool relative to a wellbore, with the tool comprising a cylindrical tool housing  20  (typically made up from a plurality of tool housing members) having an upper end  20 U which may be coupled to the lower end of a pipe string or coiled tubing string (not shown), or to other tools or components that are coupled to the lower end of the string. For convenience, the adjectives “upper” and “lower” are used in this patent specification in reference to various components as if mechanism  100  were at all times vertically oriented as in  FIG. 1 . It will be appreciated, however, that these terms are used in a relative sense only, as the mechanism may be used in a variety of different orientations (such as during directional drilling operations). 
         [0043]    Mechanism  100  includes a generally cylindrical mandrel member  14  with a central bore  30  to permit passage of drilling fluid (mud).  FIG. 1   a  illustrates one embodiment of a mandrel  14  adapted for use in mechanism  100 . Mandrel  14  is axially and radially supported within housing members  20  such that it is coaxially rotatable relative to housing  20  but its axial position relative to housing  20  is substantially fixed. Persons skilled in the art will appreciate that specific means for supporting mandrel  14  within housing  20  as described above may be readily devised, and the present invention is not limited to any particular means of providing such support. 
         [0044]    Mandrel  14  includes a central section  31  having a smooth cylindrical outer surface, an upper splined section  32  above central section  31 , and a lower splined section  33  below central section  31 . As shown in  FIG. 1   a , upper splined section  32  defines a plurality of longitudinally-oriented upper splines  141  spaced around the circumference of upper splined section  32  and projecting outward therefrom. Similarly, lower splined section  33  defines a plurality of longitudinally-oriented lower splines  142  spaced around the circumference of lower splined section  33  and projecting outward therefrom. 
         [0045]    The lower end  14 L of mandrel  14  may be coupled to a mud motor (not shown) or other tool or other additional lower tubular elements that require controllable angular orientation relative to housing  20  (and relative to a pipe string or tubing string supporting housing  20 ). Additional or auxiliary elements or appurtenances may be coupled above mandrel  14  (for example, components that provide axial or radial support to mandrel  14 , or components involved in controlling the actuation of the mechanism  100 ). However, such additional elements do not form part of the broadest embodiments of the present invention, and other embodiments of the invention could take alternative forms without departing from the scope of the invention. 
         [0046]    Mechanism  100  as illustrated is not limited to orientation relative to a wellbore as described above. In alternative embodiments, mechanism  100  may be inverted such that mandrel  14  is coupled to the lower end of the pipe string or coiled tubing string, or to other tools or components that are coupled to the lower end of the string, with housing  20  being coupled to a drilling tool or other additional lower tubular elements requiring angular orientation control. 
         [0047]    In the embodiment illustrated in the FIGS. (and as will be explained in greater detail), torque-transmitting components of mechanism  100  are configured to resist torsional loading applied in the clockwise direction when viewed from above. In alternative embodiments adapted to resist counterclockwise torsional loading, the configurations of torque-transmitting components would be essentially the reverse of the illustrated configurations. 
         [0048]      FIG. 2  is an enlarged detail illustrating the components of mechanism  100  in accordance with the embodiment of  FIG. 1 . As shown, mechanism  100  includes a generally cylindrical central sleeve  10  with longitudinal external splines  101 , plus a generally cylindrical outer housing  11  coupled to the lower end of tool housing  20 , and having longitudinal internal grooves  111  configured to receive splines  101  of sleeve  10  in closely-fitting fashion as shown in  FIG. 4 . The inner diameter of central sleeve  10  is slightly greater than the outer diameter of central section  31  of mandrel  14 , such that it may be coaxially disposed around central section  31  as shown in  FIG. 4 , and will be free to rotate relative to mandrel  14  and free to slide longitudinally relative to mandrel  14 . Splines  101  on central sleeve  10  and grooves  111  on housing  11  prevent relative rotation between sleeve  10  and housing  11 , while allowing sleeve  10  to travel axially relative to housing  11 . 
         [0049]    A generally cylindrical upper ratchet member  12  with internal grooves  122  is coaxially disposed around upper splined section  32  of mandrel  14 , such that splines  141  of mandrel  14  are received within grooves  122 . Grooves  122  are wider than splines  141  such that when a first vertical face  141   a  of a given spline  141  is bearing against a first vertical face  122   a  of the corresponding groove  122 , a vertical gap G- 1  will be formed between the second vertical face  122   b  of groove  122  and the second vertical face  141   b  of spline  141 , all as shown in  FIG. 3 . The axial position of upper ratchet member  12  is substantially fixed relative to mandrel  14 , but upper ratchet member  12  is free to rotate coaxially relative to mandrel  14 , to the extent allowed by gaps G- 1 . 
         [0050]    Preferred embodiments will include suitable biasing means such that when torque load is not present between upper ratchet member  12  and mandrel  14 , first vertical faces  141   a  of splines  141  will be biased toward and against the corresponding first vertical faces  122   a  of grooves  122 . As shown in  FIG. 3 , such biasing means may be in the form of bow springs  15  disposed within the gaps G- 1  between second vertical faces  122   b  and  141   b . However, the present invention is not limited to the use of this or any particular type of biasing means. Persons skilled in the art will appreciate that various functionally effective biasing means may be devised and provided in accordance with known technologies (e.g., torsion springs coupled between the mandrel and upper and lower ratchet members), without departing from the scope of the present invention, and the biasing means may be omitted in alternative embodiments. 
         [0051]    A generally cylindrical lower ratchet member  13  with internal grooves  132  is coaxially disposed around lower splined section  33  of mandrel  14 , such that splines  142  of mandrel  14  are received within grooves  132 . Grooves  132  are wider than splines  142  such that when a first vertical face  142   a  of a given spline  142  is bearing against a first vertical face  132   a  of the corresponding groove  132 , a vertical gap G- 2  will be formed between the second vertical face  132   b  of groove  132  and the second vertical face  142   b  of spline  142 , all as shown in  FIG. 5 . The axial position of lower ratchet member  13  is substantially fixed relative to mandrel  14 , but lower ratchet member  13  is free to rotate coaxially relative to mandrel  14 , to the extent allowed by gaps G- 2 . Preferred embodiments will include suitable biasing means such that when torque load is not present between lower ratchet member  13  and mandrel  14 , first vertical faces  142   a  of splines  142  will be biased toward and against the corresponding first vertical faces  132   a  of grooves  132 . As shown in  FIG. 5 , such biasing means may be in the form of bow springs  21  disposed within the gaps G- 2  between second vertical faces  132   b  and  142   b.    
         [0052]    The lower end of upper ratchet member  12  has a circumferentially-arrayed plurality of ratchet teeth  121 , each having a vertical face  121   a  and a sloped face  121   b . The upper end of lower ratchet member  13  has a similar plurality of ratchet teeth  131 , each having a vertical face  131   a  and a sloped face  131   b . The upper end of central sleeve  10  has a plurality of ratchet teeth  102 , each having a vertical face  102   a  and a sloped face  102   b , and configured to mate with ratchet teeth  121  on upper ratchet member  12 . Similarly, the lower end of central sleeve  10  has a plurality of ratchet teeth  103 , each having a vertical face  103   a  and a sloped face  103   b , and configured to mate with ratchet teeth  131  on lower ratchet member  13 . 
         [0053]    Upper ratchet member  12  and lower ratchet member  13  are positioned on mandrel  14  to permit a certain amount of axial movement of central sleeve  10  along mandrel  14 , such that when ratchet teeth  102  of central sleeve  10  are matingly engaged with ratchet teeth  121  of upper ratchet member  12 , ratchet teeth  103  of central sleeve  10  will be clear of ratchet teeth  131  of lower ratchet member  13 . Torque may thus be transmitted between central sleeve  10  and upper ratchet member  12  (i.e., by engagement of ratchet teeth  102  and  121 ) or between central sleeve  10  and lower ratchet member  13  (i.e., by engagement of ratchet teeth  103  and  131 ), depending on the axial position of central sleeve  10  during operation of mechanism  100 , as will be further explained below. 
         [0054]    The incremental angular displacement that occurs during one index cycle is determined by the angular spacing between adjacent ratchet teeth, which is determined by the total number of ratchet teeth of each plurality of ratchet teeth. The tool may be configured with the required number of ratchet teeth per ratchet plurality to achieve a selected incremental angular displacement for each cycle. For example, a ratchet plurality comprising  24  teeth would result in an incremental angular rotation of 15 per index cycle. 
         [0055]    The operation and function of mechanism  100  may be clearly understood with reference to the FIGS. and the foregoing description.  FIG. 2  illustrates an embodiment of mechanism  100  with central sleeve  10  in its upper position (as previously defined), with ratchet teeth  102  of central sleeve  10  in mating engagement with ratchet teeth  121  of upper ratchet member  12 , and with ratchet teeth  103  of central sleeve  10  axially separated from ratchet teeth  131  of lower ratchet member  13 . Any torsional load (for example, due to drilling using a mud motor coupled to mandrel  14 ) is transmitted from mandrel  14  to housing  11  through splines  141  and grooves  122 , ratchet teeth  102  and  121 , and splines  101  and grooves  111 . 
         [0056]    When adjustment is required with respect to the angular orientation of mandrel  14  relative to housing  11 , an index cycle is initiated by forcing central sleeve  10  downward toward its lower position (previously defined) using suitable central sleeve actuation means capable of providing sufficient force to overcome the friction between sliding or otherwise mechanically-engaged components (e.g., spline/groove arrangements; mating ratchet teeth) during indexing. In the illustrated embodiment, the central sleeve actuation means comprises:
       a generally cylindrical piston  19  which is disposed above central sleeve  10  and is axially movable within an annular space between housing  11  and upper ratchet member  12 ;   a cylindrical drive sleeve  17  which is disposed below central sleeve  10  and is axially movable within an annular space between housing  11  and lower ratchet member  13 ; and   a helical return spring  16  disposed below and reacting against drive sleeve  17  in association with a drive sleeve retention ring  18 .       
 
         [0060]    In this embodiment, piston  19  is actuated by exposure to fluid pressure (either liquid or gaseous) sufficient to force central sleeve  10  downward against drive sleeve  17  so as to compress return spring  16 . As return spring  16  is compressed, central sleeve  10  begins to travel axially along central section  31  of mandrel  14 , while ratchet teeth  102  of central sleeve  10  begin to move downward relative to ratchet teeth  121  of upper ratchet member  12 . During this phase of the indexing operation, however, vertical faces  102   a  of ratchet teeth  102  remain in sliding contact with opposing vertical faces  121   a  of ratchet teeth  121  (as may be seen in  FIGS. 6 and 7 ), and thus remain capable of transmitting torsional load. 
         [0061]    As illustrated in  FIG. 6 , representative ratchet tooth  102 - 1  is initially located between adjacent ratchet teeth  121 - 1  and  121 - 2 . As central sleeve  10  continues to travel downward, sloped faces  103   b  of ratchet teeth  103  begin to contact sloped faces  131   b  of ratchet teeth  131 , as shown in  FIG. 7 . Due to the angular inclination of sloped faces  103   b  and  131   b , lower ratchet member  13  is thus forced to rotate relative to mandrel  14  opposite to the direction of torsional load (i.e., counterclockwise in the illustrated embodiment), while bow springs  21  compress and vertical faces  132   a  of grooves  132  separate from vertical faces  142   a  of splines  142 , as shown in  FIG. 9 . Ratchet teeth  102  continue to separate from ratchet teeth  121  until they fully disengage. At this point, there is a sudden relative rotation between mandrel  14  and central sleeve  10  in the direction of torsional load. Concurrently, ratchet teeth  103  become fully engaged with ratchet teeth  131  as central sleeve  10  reaches its lower position, as shown in  FIG. 8 . Rotation between mandrel  14  and central sleeve  10  continues until vertical faces  142   a  of splines  142  contact vertical faces  132   a  of grooves  132 , as shown in  FIG. 10 . At this point of the index cycle, angular displacement between mandrel  14  and central sleeve  10  is approximately one-half of the total angular displacement of one full index cycle. In this position, ratchet teeth  102  and  121  are separated, and torsional load is transmitted from mandrel  14  to housing  11  through splines  142  and grooves  132 , ratchet teeth  103  and  131 , and splines  101  and grooves  111 . 
         [0062]    To complete the index cycle, fluid pressure acting on piston  19  is sufficiently decreased such that return spring  16  forces central sleeve  10  to travel axially along mandrel  14  to return to its upper position. Ratchet teeth  103  begin to separate from ratchet teeth  131  while remaining torsionally engaged and capable of transmitting torsional load, with vertical faces  103   a  of ratchet teeth  103  remaining in sliding contact with opposing vertical faces  131   a  of ratchet teeth  131  as seen in  FIGS. 11 and 12 . Because of the angular displacement between central sleeve  10  and mandrel  14 , as ratchet teeth  102  and  121  begin to reengage, ratchet tooth  102 - 1  is now located between ratchet teeth  121 - 2  and  121 - 3 . Contact between sloped faces  102   b  of ratchet teeth  102  and sloped faces  121   b  of ratchet teeth  121 , as shown in  FIG. 12 , causes upper ratchet member  12  to rotate relative to mandrel  14  opposite to the direction of torsional load, while bow springs  15  compress and vertical faces  122   a  of grooves  122  separate from vertical faces  141   a  of splines  141 , as shown in  FIG. 14 . Travel of central sleeve  10  continues until ratchet teeth  103  disengage from ratchet teeth  131 , and torsional load causes mandrel  14  to rotate relative to central sleeve  10 . Vertical faces  102   a  of ratchet teeth  102  engage with vertical faces  121   a  of ratchet teeth  121 , and vertical faces  141   a  of splines  141  contact faces  122   a  of grooves  122 , as shown in  FIGS. 13 and 15 . Mechanism  100  has now returned to the initial position shown in  FIG. 2 , but with ratchet teeth  102  and  121  having indexed one incremental amount, determined by the angular distance between adjacent teeth, and with mandrel  14  having rotated by this same amount relative to housing  11 . The index cycle is repeated until the desired orientation between elements above and below the tool is achieved. 
         [0063]    Persons skilled in the art will appreciate that any of various means or mechanisms could be used to actuate piston  19 , and the present invention is not limited or restricted to the use of any particular means of actuating piston  19 . In alternative embodiments, piston  19  could be actuated by functionally effective means other than fluid pressure, without departing from the scope of the present invention. Furthermore, the invention is not limited or restricted to use of the central sleeve actuation means described and illustrated herein, or any other particular central sleeve actuation means. Persons skilled in the art will recognize that other functionally effective central sleeve actuation means can be readily devised and provided in accordance with known technologies, without departing from the scope of the invention. 
         [0064]    In accordance with embodiments of the present invention as described above, applied torsional load drives the relative angular rotation that occurs during an index cycle. Mechanism  100  could alternatively be configured such that the relative angular rotation is internally driven. One way to achieve this would be to have strong enough biasing means between upper ratchet member  12  and mandrel  14 , and between lower ratchet member  13  and mandrel  14 , to induce enough torque to effect the relative rotation of mandrel  14  during the index cycle. 
         [0065]    Another method would be to have upper ratchet member  12  and lower ratchet member  13  rotationally fixed to mandrel  14 . In that configuration, as central sleeve  10  translates axially on the downstroke or upstroke, contact between sloped faces  103   b  and sloped faces  131   b , or between sloped faces  102   b  and sloped faces  121   b , would provide the driving force to rotate mandrel  14  relative to housing  11 , so that indexing could be accomplished in the absence of an applied torsional load. 
         [0066]    It will be readily appreciated by those skilled in the art that various modifications of the present invention may be devised without departing from the essential concept of the invention, and all such modifications are intended to come within the scope of the present invention. It is to be especially understood that the invention is not intended to be limited to illustrated embodiments, and that the substitution of a variant of a claimed element or feature, without any substantial resultant change in the working of the invention, will not constitute a departure from the scope of the invention. To provide one particular non-limiting example, the central sleeve actuation means could be provided in a variety of alternative forms, such as upper and lower gas-actuated or hydraulically-actuated pistons above and below the central sleeve, without a return spring being required. 
         [0067]    In this patent document, the term “ratchet teeth” is not to be interpreted as being limited solely to ratchet teeth of form or configuration specifically as described and illustrated herein, but is also intended to encompass alternative means of torque-transferring engagement between the central sleeve and the upper and lower ratchet members in accordance with the described operative principles of the present invention. Similarly, the term “ratchet member” is to be understood as referring to a member incorporating means for torque-transferring engagement with the central sleeve, and such engagement means may but will not necessarily comprise ratchet teeth as such. Persons skilled in the art will recognize that alternative torque-transfer engagement means may be devised using known technologies without departing from the scope of the invention. To provide only one non-limiting example, the torque-transfer engagement means in an alternative embodiment of the present invention could comprise a series of circumferentially-spaced lugs on either end of the central sleeve, with each lug being operatively engageable with a ratchet-shaped slot along the circumference each of the upper and lower ratchet members. 
         [0068]    In this patent document, any form of the word “comprise” is to be understood in its non-limiting sense to mean that any item following such word is included, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article “a” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one such element. 
         [0069]    Any use of any form of the terms “connect”, “engage”, “couple”, “attach”, or any other term describing an interaction between elements is not meant to limit the interaction to direct interaction between the subject elements, and may also include indirect interaction between the elements such as through secondary or intermediary structure. Relational terms (such as but not limited to) “parallel”, “perpendicular”, “coaxial”, “coincident”, “intersecting”, and “equidistant” are not intended to denote or require absolute mathematical or geometrical precision. Accordingly, such terms are to be understood as denoting or requiring substantial precision (e.g., “substantially parallel”) unless the context clearly requires otherwise.