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FIELD OF THE INVENTION 
       [0001]    The present invention relates to directional drilling tools for use in directionally drilling of oil wells, and more particularly to a retrievable directional orientation assembly. 
       BACKGROUND OF THE INVENTION 
       [0002]    For directional drilling of oil and gas wells, a drill string generally has, at its distal end, a drill bit which is energized by a hydraulically-powered “mud” motor. Such drill bit and hydraulically-powered motor is generally contained in a so-called “bent sub”. A universal bottom hole orienting sub assembly (“UBHO sub”), containing a directional measurement tool, is generally threadably coupled to the bent sub, uphole therefrom. The directional measurement tool within the UBHO sub contains electronic instrumentation for determining drill bit location and orientation, and together such form part of the drill string bottom hole assembly (“BHA”). 
         [0003]    The UBHO sub is adapted at its downhole end to be threadably connected to the bent sub, and at its uphole end to be threadably connected to various other bottom hole assembly components, such as mud pulser device/assembly for transmitting downhole drilling orientation information from the directional measurement tool to surface, gamma modules for determining geology of formations being drilled through, and battery-containing elements, all of which may be connected in series to each other, and to the UBHO sub at its uphole end. 
         [0004]    The UBHO orienting sub has a hollow cylindrical outer member (‘mule shoe’), typically having an arcuate camming surface on an interior surface thereof for landing against a mating arcuate camming surface on a directional measurement tool when such directional measurement tool is co-axially inserted and caused to nest within the outer mule shoe member. Specifically, the mule shoe member typically has an perpendicular aperture through the bore of the mule shoe, for receiving a key member which extends therethrough and into an axially extending channel in the directional measurement tool when the arcuate camming surfaces on the respective mule shoe member and inner directional measurement tool cause each of the aforesaid components to be rotated in a desired orientation one to another. The key member when inserted and pinned in such inserted position fixedly keeps such orientation of one to the other. A further set screw extending from the mule shoe member to the inner directional measurement tool in the prior art is then typically used to “lock” the directional measurement tool to the mule shoe and prevent the directional measurement tool from being removed from within the mule shoe member, and thereby allow the UBHO sub (containing the mule shoe and directional measurement tool locked therewithin) and the bent sub and drill bit attached thereto, to be lowered downhole so as to permit directional drilling. 
         [0005]    Disadvantageously, if the drill bit should become stuck for some reason in the wellbore during directional drilling, which sometimes occurs at long wellbores of narrow diameter with extensive directional changes, and it then becomes impossible to withdraw the drill string intact, and typically the drill string will become pulled apart at a location above the UBHO and above the gamma module, mud pulser, and batteries when the drill string is attempted to be raised to surface. Such results in not only loss of the bent sub and mud motor (which in any event are “stuck”), but further results in loss of the UBHO sub containing the directional measurement tool, as well as various associated equipment uphole thereof up to the point at which the drill string parted, which typically includes such components as the gamma module, batteries, and various connecting members. 
         [0006]    Loss of such UBHO sub and uphole equipment can be quite expensive. For example, in year 2011 (US) dollars, a directional module may cost in the range of $60,000(US), a mud pulser $36,000, batteries in the range of $8-10,000 each, and a gamma module in the range of $25,000, with various interconnects costing in the range of $4000, and thus h the entire assembly costing in the range of approximately $125,000. Accordingly, loss of such UBHO sub and related assemblies/components downhole is quite expensive. 
       SUMMARY OF THE INVENTION 
       [0007]    In order to avoid the problem of loss of expensive BHA equipment such as a UBHO sub downhole if and when a drill bit becomes stuck downhole, the present invention provides an improved UBHO sub assembly which is releasable and thus retrievable along with various uphole BHA components, should the drill bit become stuck within a wellbore. 
         [0008]    Such releasably coupling of the directional tool to the outer mule shoe of a UBHO sub is accomplished by providing a releasable locking pin member to releasably couple an outer mule shoe assembly to an inner mule shoe assembly which contains the directional measurement and orienting tool. In such fashion, the inner mule shoe assembly containing the directional measurement tool may, when the locking pin member is released in one of the two manners described below, from locking engagement with the inner mule shoe, withdrawn from within the outer mule shoe and brought to surface along with any other associated BHA tools, such as the mud pulser and gamma module. 
         [0009]    Accordingly, in a broad aspect the present invention comprises a releasably lockable, retrievable mule shoe assembly for use in a bottom hole orienting assembly for use in directional drilling, comprising: 
         [0010]    (i) a substantially cylindrical hollow elongate outer mule shoe member, having:
       (a) an arcuate camming surface around an inner periphery of said hollow shoe member thereof;       
 
         [0012]    (ii) an inner cylindrical tool member, adapted to contain therewithin a directional measurement tool, and adapted for insertion within said hollow mule shoe member, having:
       (b) a mating arcuate camming surface adapted for mating engagement with said camming surface on said mule shoe member;   (c) (iii) an aperture in either of said inner cylindrical member or said outer cylindrical member, for receiving therewithin a locking pin member;       
 
         [0015]    (iii) said locking pin member moveable in a linear back and forth direction perpendicular to a longitudinal axis of said mule shoe member, and insertable in said aperture means for releasably coupling said mule shoe member to said directional tool member. 
         [0016]    In a preferred embodiment the arcuate camming surface on said outer mule shoe member and said mating arcuate camming surface on said inner tool member are adapted, when landed one upon the other, to angularly position the inner cylindrical member and directional tool therewithin in a desired, fixed angular position relative to said outer mule shoe member. 
         [0017]    The locking pin member is configured so as to releasably couple the inner tool member to the outer mule shoe member in one of three ways. 
         [0018]    In a first configuration, the locking pin member further comprises:
       a) spring means;   b) a detent member, biased by said spring means;       
 
         [0021]    wherein the spring means adapted to cause the detent member to releasably engage said aperture means on said directional measurement too, and thereby releasably couple the inner tool member (and directional tool therewithin) to the outer mule shoe member. 
         [0022]    Specifically, in such first configuration, the locking pin member has spring-biasing means and detent means so that when the inner tool member is inserted within the outer mule shoe member, the locking pin member is biased so as to be slidably inserted within the aperture means, which aperture means is located in the inner tool member if the locking pin member is located in the outer mule shoe, and in the outer mule shoe member if the locking pin is located in the inner tool member. The detent means on the locking pin member may comprise a pin having a hemispherical end, which when the pin is extended engages the aperture, preventing removal of the inner tool member from the outer mule shoe member unless sufficient parting force is applied to cause the spring-biased pin and rounded end thereof to be withdrawn from the aperture, thereby allowing the inner tool member (and directional tool therewithin) to be withdrawing from the outer mule shoe member. 
         [0023]    In a second configuration, pressure exerted by drilling mud on the exterior of the outer mule shoe when such assembly in inserted downhole causes or assists in causing the locking pin member to be biased in locking engagement with the aperture. In such configuration, if the drill bit becomes stuck, drilling stops and drilling mud is no longer in such circumstances forced downhole and against the exterior surface of the outer mule shoe member (and thus no longer against the locking pin member). In such circumstances the locking pin member, upon a separating force being applied as between the outer mule shoe member and the inner tool member when the inner tool member to attempted to be pulled to surface, can then be moved away from its locking position within the aperture, thereby freeing the inner tool member (and directional tool therewitihin) from locking engagement with the outer mule shoe, thereby then allowing the inner tool member to then be raised to surface. 
         [0024]    Accordingly, in such second embodiment, the locking pin member is adapted, when said assembly is placed downhole in a well and downhole fluid pressure is exerted on an exterior surface thereof, to lockingly engage said mule shoe member to said directional tool member, and when said downhole fluid pressure is lessened or removed, is further adapted disengage said directional tool member to allow said inner cylindrical tool member and said directional tool member to be withdrawn from within said mule shoe assembly. 
         [0025]    In a preferred configuration, the features of the first and second configurations are combined. 
         [0026]    Accordingly, in such preferred configuration, spring biasing means may further be provided, to bias the locking pin member in an engaged position within the aperture means, and the locking pin member further provided with detent means, so as to thereby allow the pin member to be biased in engagement with the aperture means and thus coupled engagement of the outer mule shoe member to the inner tool member, even when no pressure may be exerted against the side of the outer mule shoe member, such as when a drill string having such mule shoe assembly as a part thereof, is lowered downhole Again, however, due to the operation of the detent means (typically a hemispherical rounded edge supplied to the pin member), the inner tool member may when desired and when no pressure is exerted on the pin member by drilling fluid, be withdrawn from coupled engagement with the outer mule shoe member. 
         [0027]    Lastly, in a third configuration of the mule shoe assembly of the present invention, the pin member is of a metal which is softer than that which comprises either the outer mule shoe and/or the inner tool. In such fashion, upon application of an upward separation force to said inner tool member and said mule shoe member, pin member may become sheared off, so as to allow separation of said inner tool member from said outer mule shoe. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    The accompanying drawings illustrate one or more exemplary embodiments of the present invention and are not to be construed as limiting the invention to these depicted embodiments: 
           [0029]      FIG. 1A  is an exploded perspective view of a mule shoe assembly of the prior art, which uses a threaded set screw to lockably (non-releasably) secure an inner tool to an outer mule shoe; 
           [0030]      FIG. 1B  is a non-exploded view of the prior art mule shoe assembly of  FIG. 1A ; 
           [0031]      FIG. 2  is cross-sectional view through the prior art mule shoe assembly of  FIG. 1B  along section A-A thereof; 
           [0032]      FIG. 3  is enlarged view of encircled area “B” of  FIG. 2 ; 
           [0033]      FIG. 4  is an exploded perspective view of a first embodiment of a mule shoe assembly of the present invention, which uses a spring and a detent member to lockably and releasably secure the inner tool to the outer mule shoe; 
           [0034]      FIG. 5  is a non-exploded view of the mule shoe assembly of  FIG. 4 ; 
           [0035]      FIG. 6  is is cross-sectional view through the mule shoe assembly of  FIG. 5 , along section A-A thereof; 
           [0036]      FIG. 7  is enlarged view of encircled area “B” of  FIG. 6 ; 
           [0037]      FIG. 8  is an exploded perspective view of a second embodiment of a mule shoe assembly of the present invention, which uses a modified spring and a detent member to lockably and releasably secure the inner tool to the outer mule shoe; 
           [0038]      FIG. 9  is a non-exploded view of the mule shoe assembly of  FIG. 8 ; 
           [0039]      FIG. 10  is is cross-sectional view through the mule shoe assembly of  FIG. 9 , along section A-A thereof; 
           [0040]      FIG. 11  is enlarged view of encircled area “B” of  FIG. 10 ; 
           [0041]      FIG. 12  is an enlarged view of encircled area “C” of  FIG. 11 ; 
           [0042]      FIG. 13  is a view of the threaded retention nut of  FIG. 11 ; 
           [0043]      FIG. 14  is a view of the detent member of  FIG. 11 ; 
           [0044]      FIG.15  is a detailed perspective view of the thin curved cylindrical spring members of  FIG. 11 , shown spaced apart for clarity; 
           [0045]      FIG. 16  is an exploded perspective view of a third preferred embodiment of a mule shoe assembly of the present invention, which uses both a modified spring and a detent member to lockably and releasably secure the inner tool to the outer mule shoe; 
           [0046]      FIG. 17  is a non-exploded view of the mule shoe assembly of  FIG. 16 ; 
           [0047]      FIG. 18  is is cross-sectional view through the mule shoe assembly of  FIG. 17 , along section A-A thereof; 
           [0048]      FIG. 19  is enlarged view of encircled area “B” of  FIG. 18 ; 
           [0049]      FIG. 20  is a detailed bottom perspective view of the detent member shown in  FIG. 19 . 
           [0050]      FIG. 21  is is cross-sectional view through the mule shoe assembly 
           [0051]      FIG. 22  is enlarged view of encircled area “B” of  FIG. 21 ; and 
           [0052]      FIG. 23  is a detailed bottom perspective view of the detent member shown in  FIG. 22 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0053]      FIGS. 1A ,  1 B,  2 , &amp;  3  show various views of a mule shoe assembly  10  of the prior art. 
         [0054]    Mule shoe assemblies  10  of the prior art typically comprise a substantially cylindrical mule shoe member  12 , which contains a hollow bore  28 . Mule shoe  12  is adapted at a downhole end  13  thereof to be threadably coupled to a mud motor and associated drill bit (not shown), and at an uphole end  17  thereof adapted to be secured to additional BHA components, such as a mud pulser (not shown). 
         [0055]    Mule shoe  12  has a camming member  24 , which is insertable within bore  28  and which has an arcuate camming surface  14  thereon. In the prior art, camming member  24  is welded within interior bore  28  of mule shoe  12 , in a desired location and angular orientation. 
         [0056]    An inner cylindrical tool member  16  is adapted to be inserted within mule shoe  12 , as shown in  FIGS. 1A &amp; 1B . Inner tool member  16  is adapted to contain within its bore  40  a directional measurement tool (not shown), and is kept in such bore  40  by threaded sleeve  42  and associated “O” ring  44 . 
         [0057]    Inner tool member  16  possesses an exterior arcuate camming surface  18  on an outer periphery  19  thereof, which shape is complementary to arcuate camming surface  14 , and which is adapted for landed engagement with camming surface  18  upon insertion of inner tool member  16  within outer mule shoe  12 . 
         [0058]    In prior art mule shoe assembly  10 , a plurality of threaded set screws  30  are provided, which are adapted to be threadably inserted within apertures  32  in mule shoe  12 . When inner tool  16  is inserted in bore  28  of outer mule shoe  12 , such causes mutual engagement of each of arcuate camming surfaces  14  and  18  causing relative rotation of the inner tool member  16  relative to outer mule shoe  12 , so that apertures  32  in mule shoe  12  become aligned with, and positioned directly above, respective apertures  34  in inner tool member  16 . 
         [0059]    Upon mutual alignment of apertures  32  with apertures  34  in inner tool member  16 , set screws  30  may further be tightened to thereby extend into apertures  34  in inner tool member  16 , thereby fixedly securing inner tool member  16  (with a directional tool inserted within a bore  40  thereof) to outer mule shoe  12 . 
         [0060]    Typically in the prior art, for further maintaining the angular position of inner tool member  16  to outer mule shoe  12 , a channel  41  is further provided on outer periphery  19  thereof. Upon landed engagement of the respective accurate camming surfaces  14 ,  18 , channel  41  becomes aligned with slot  60  in mule shoe  12 . A key member  70  is then inserted in slot  60  which simultaneously engages channel  41  in inner tool member  16 , thereby preventing any further insertion of inner tool  16  within bore  28  of mule shoe  12  and further preventing any relative angular rotation of inner tool  16  (and directional tool member contained therein) relative to mule shoe  12 . Key member  70  may be fixedly secured in slot  60  in mule shoe  12  by means of friction fit pin members  80 . 
         [0061]    Disadvantageously, as explained above, with commencement of drilling, should the drill bit become stuck in a wellbore, due to set screws  30  fixedly securing inner tool  16  and the directional measurement tool therein to mule shoe  12 , it is impossible when such mule shoe assembly is downhole to separate the inner tool  16  and the directional measurement tool from the outer mule shoe  12 , which remains fixedly secured to the mud motor and drill bit. Accordingly, such mule shoe assembly, including the inner tool member  16  and directional orienting tool therewithin, as well as typically other uphole BHA components, will be lost upon forceable removal of the drill string, which typically separates further uphole from the BHA assembly. 
         [0062]    As well, the welding of camming member  24  to interior bore  28  of mule shoe  12  is awkward, to provide camming surface  14  within mule shoe  12  is time consuming, and difficult. 
         [0063]    Accordingly, in a broad aspect of the present invention, a mule shoe assembly  100  is provided as shown in  FIGS. 4-19  inclusive, which releasably secures the inner tool  160  and associated directional measurement and/or orienting tool to the outer mule shoe  120 . Advantageously, in the event a drill bit becomes stuck downhole, the inner tool  160  and associated directional measurement tool may be separated from the outer mule shoe  120  and thereby recovered to surface. 
         [0064]    In a first embodiment of the improved mule shoe assembly  100  of the present invention shown in  FIGS. 4-7 , a cylindrical mule shoe member  120  is provided, which contains a hollow bore  280 . Mule shoe  120  is adapted at a downhole end  130  thereof to be threadably coupled to a mud motor and associated drill bit (not shown), and at an upstream end  170  thereof adapted to be secured to additional BHA components, such as a mud pulser (not shown). 
         [0065]    Mule shoe  120  has a camming member  240 , which is insertable within bore  280  and which has an arcuate camming surface  140  thereon. Friction-fit dowel members  200 , each adapted for friction fit within respective apertures  220  in camming member  240 , are provided so as to make it possible for camming member  240  to be fixedly coupled to bore  280  of mule shoe  120  without time-consuming welding. Due to dowel member  200  and associated pre-drilled apertures  220 , the positioning of camming member  240  within bore  280  is already pre-determined, thus saving time in positioning and securing such camming member  240  within bore  280 . Alternatively, as will now occur to persons of skill in the art, instead of friction-fit dowel pin members  200 , threaded screws (not shown) may alternatively be used instead of friction fit dowel pins  200 . Alternatively, or other similar pinning members employed, such as flush mounted rivets (not shown). 
         [0066]    An inner cylindrical tool member  160  is adapted to be inserted within mule shoe  120 , as shown in  FIGS. 4 and 5 . Inner tool member  160  is adapted to contain within its bore  400  a directional measurement tool (not shown), and is kept in such bore  400  by threaded sleeve  42  and associated “O” ring  44 . 
         [0067]    Inner tool member  160  possesses an exterior arcuate camming surface  180  on an outer periphery  190  thereof, which shape is complementary to arcuate camming surface  140 , and which is adapted for landed engagement with camming surface  180  upon insertion of inner tool member  160  within outer mule shoe  120 . 
         [0068]    Advantageously, instead of the set screws  30  being provided as in the case of the prior art mule shoe assemblies  10  shown in  FIG. 1A ,  1 B, one or more novel locking pin members  500  are provided, as shown in  FIGS. 4-7 . Specifically, in such first embodiment, as best seen in  FIG. 4  and  FIG. 7 , a detent member  502 , having a hemispherical end portion  506  adapted for mating engagement with apertures  340 , are provided. Accordingly, when inner tool member  160  is inserted within bore  280  of mule shoe  120 , and arcuate camming surfaces  140 ,  180  become landed and the inner tool  160  caused to rotate relative to mule shoe  120 , apertures  340  become aligned with pin members  500 , and hemispherical end portions  506  may matingly engage apertures  340 , thereby releasably securing inner tool  160  to mule shoe  120 . 
         [0069]    Specifically, to ensure mating engagement of respective hemispherical portions  506  with respective apertures  340 , a linear spring member  510  may be provided, as best seen in  FIG. 7 . Spring member  510  biases hemispherical portion  506  of detent member  502  into mating engagement with aperture  340 . A threaded retention screw  512  is further threadably inserted in threaded orifice  514  in outer mule shoe  120 , to retain spring  510  is a biased state where hemispherical portion  506  is biased in mating engagement with aperture  340 . 
         [0070]    In such manner, in accordance with the above configuration, Inner tool member  160  may thereafter be releasably separated from mule shoe  120  upon sufficient separating force being applied to inner tool  160  to cause hemispherical portion  506  to cause it to move upwardly and outwardly from aperture  340 , thereby allowing detent member  502  to become disengaged from aperture  340 , and thus inner tool member  160  thereby permitted to become removed from mule shoe  120 . 
         [0071]    In a preferred embodiment hemispherical portion  506  of detent  502  is heat treated to a lesser Rockwell hardness than that of aperture  340  in inner tool member  160 , so that detent  502  wears sooner than aperture  340  in inner tool member  160 . 
         [0072]    If desired, for further maintaining the angular position of inner tool member  160  relative to outer mule shoe  120 , a channel  410  may be further provided on outer periphery  190  of inner tool member  160 . Upon insertion of inner tool  160  into bore  280  of mule shoe  120  and resultant landed engagement of the respective accurate camming surfaces  140 ,  180 , channel  410  becomes aligned with slot  600  in mule shoe  120 . Key member  700  may then be inserted in slot  600  so as to engage channel  410  in inner tool member  160 , thereby preventing any further insertion of inner tool  160  within bore  280  of mule shoe  120 , and further preventing any relative angular rotation of inner tool  160  (and directional tool member contained therein) relative to mule shoe  120 . Key member  700  may be fixedly secured in slot  60  in mule shoe  120  by means of friction fit pin members  800 . Notably, however, due to channel  410  being open at an end thereof proximate camming surface  180 , inner tool  160  may be withdrawn from mule shoe  120  without interference from key member  700  and without having to remove key member  120 . 
         [0073]    A second embodiment of the mule shoe assembly  100  and locking pin members  500  of the present invention is shown in  FIGS. 8-15 . Such second embodiment operates identically to the first embodiment except as noted below. 
         [0074]    As best shown in  FIG. 8  and  FIGS. 10-12 , a modified spring  600  is utilized in place of spring  510 , such spring  600  comprising pairs of respectively convex curved “washer” like elements  602  and concave washer elements  604 , in a “Belleville disk” arrangement wherein each of convex/concave washer elements  602 ,  604  are placed in respective mutually juxtaposed position with each other, as shown in  FIGS. 11 &amp; 12 . Washer elements  602 ,  604  may have an optional cylindrical aperture  606  therein, and may be of the type provided by Gardner Spring Co, of Tulsa, Okla., part no. MB0500-025-S, and may be stacked as shown in  FIG. 12 , or may each be doubled so as to effectively provide double the spring force on detent member  502 , if desired. In all cases, however, washer elements  602 ,  604  function as modified spring  600  when biased into compression by threaded set screw  605  when set screw is threaded into orifice  514  in outer mule shoe  120 , and serves to bias detent member  502  in mating engagement with aperture  340 . 
         [0075]    In operation, similar to the operation of the first embodiment, locking pin member  500 , comprising detent member  502 , washer spring members  602 ,  604 , and threaded set screw  604  are placed in juxtaposed position as shown in  FIGS. 11 &amp; 12 , and inserted within orifice  514  in outer mule shoe  120 , and set screw turned so as to engage threads  700  on outer mule shoe  120 , and thereby apply force against spring members  602 ,  604 , thereby compressing them so that they in turn apply a biasing force against detent member  502 , forcing same into aperture  340  in inner tool  160 , thereby retaining same in bore  480  of outer mule show  120 . When desired, such as in a situation when a drill bit becomes lodged downhole, inner tool member  160  may be releasably separated from mule shoe  120  upon sufficient separating force being applied to inner tool  160  to cause hemispherical portion  506  of detent member  502  to move upwardly and outwardly from aperture  340 , thereby allowing detent member  502  to become disengaged from aperture  340 , and thus allow inner tool member  160  to be removed from bore  480  of mule shoe  120 . 
         [0076]    A third embodiment of the mule shoe assembly  100  and locking pin members  500  of the present invention is shown in  FIGS. 16-20 . Such third embodiment is preferable to that of the first and second embodiments, and operates identically to the second embodiment except as noted below. As best shown in  FIG. 16  and  FIGS. 18 &amp; 19 , a modified spring  600  is utilized, comprising pairs of respectively convex curved “washer” like elements  602  and concave washer elements  604 . Each of washer elements  602 ,  604  is provided with a cylindrical aperture  606  therein to drilling mud pressure may be supplied to upper surface  714  of detent member  502 . Washer elements  602 ,  604  may be of the type provided by Gardner Spring Co, of Tulsa, Okla., part no. MB0500-025-S. Washer elements  602 ,  604 , when biased into compression by threaded set screw  604 , serve to bias detent member  502  in mating engagement with apertures  340  in outer mule shoe  120 . 
         [0077]    Importantly in this embodiment, set screw  605  having external threads  607 , has an aperture  610  therein which may be hexagonal in shape to allow insertion of an allen key to allow tightening of such set screw  605  within orifice  514  in outer mule shoe  120 . Aperture  610  allows drilling mud pressure to be exerted on upper surface  714  of detent member  502 , thereby (along with washer elements  602 , 604 ) biasing such detent member  502  downwardly and into aperture  340  when such apertures  340  become aligned with orifice  514  upon landing of the arcuate camming surfaces  140 ,  180 . Detent member  502  may possess a hemispherical end portion  506 , or simply an arcuate rounded portion  599 , as shown in  FIG. 20 . 
         [0078]    ‘O’ rings  802  are provided in inner tool member  160 , and optionally in groove  811  in detent member  502 , to ensure a pressure differential exists between an exterior surface  722  of mule shoe  120 , and aperture  340  in inner tool  160 . 
         [0079]    In operation, similar to the operation of the second first embodiment, locking pin member  500 , comprising detent member  502 , washer spring members  602 ,  604 , and threaded set screw  605  are placed in juxtaposed position as shown in  FIG. 19 , and inserted within orifice  514  in outer mule shoe  120 , and set screw  605  turned using an allen key inserted in hexagonal aperture  610  so as to engage threads  700  on outer mule shoe  120 , and thereby applying force against spring members  602 ,  604 , thereby compressing them so that they in turn apply a biasing force against detent member  502 , forcing same into aperture  340  in inner tool  160 , thereby retaining same in bore  280  of outer mule show  120 . When mule shoe assembly  100  is placed downhole, drilling mud pressure on exterior surface  722  exerts a further pressure on detent member  502 , further biasing detent member  502  into engagement with aperture/slot  340 . 
         [0080]    In a situation when a drill bit becomes lodged downhole, drilling mud pressure being supplied downhole to a mud motor is stopped, and pressure is thereby removed from upper surface  714  of detent member  502  Inner tool member  160  may then be releasably separated from mule shoe  120  upon sufficient separating force being applied to inner tool  160  to cause rounded portion  599  of detent member  502  to move upwardly and outwardly from aperture  340 , thereby allowing detent member  502  to become disengaged from aperture  340 , and thus allow inner tool member  160  to be removed from bore  480  of mule shoe  120 . 
         [0081]    In a fourth embodiment shown in  FIGS. 21-23 , the locking pin member  500  is configured and functions similar to the second embodiment, save and except for different configuration and operation of detent member  502  forming part of locking member  500 . Specifically, detent member  502  does not possess a hemispherical portion or rounded portion, but instead possess a milled slot portion  778  for lockingly engaging a mating slot  777  on inner tool member  120 . Importantly, detent member  502 , or at least such milled slot portion  778  thereof, is of a metal which has considerably weaker shear strength than that of mating slot  777  of inner tool member  120 , or of mule shoe  120  in the region of aperture  514  therein, and is adapted to be sheared upon application of sufficient shear stress to such component. 
         [0082]    In operation, similar to the operation of the second embodiment, locking pin member  500 , comprising detent member  502 , washer spring members  602 ,  604 , and threaded set screw  605  are placed in juxtaposed position as shown in  FIG. 22 , and inserted within orifice  514  in outer mule shoe  120 , and set screw  605  turned so as to engage threads  700  on outer mule shoe  120 , and thereby applying force against spring members  602 ,  604 , thereby compressing them so that they in turn apply a biasing force against detent member  502 , forcing same into aperture  340  in inner tool  160 , thereby retaining same in bore  280  of outer mule shoe  120 . However, for desired removal of inner tool member  160 , such may be releasably separated from mule shoe  120  upon sufficient separating force being applied to inner tool  160  to cause milled slot  778  of detent member  502 , being of a weaker material than mule shoe  120  and inner tool member  160 , to shear, thereby allowing detent member  502  to become disengaged from aperture/slot  340 , and thus allowing inner tool member  160  to be removed from bore  280  of mule shoe  120 . 
         [0083]    The scope of the claims should not be limited by the preferred embodiments set forth in the foregoing examples, but should be given the broadest interpretation consistent with the description as a whole, and the claims are not to be limited to the preferred or exemplified embodiments of the invention.

Summary:
A mule shoe assembly for use in directional drilling which advantageously has releasable locking means to free the directional orienting tool from engagement with the outer mule shoe in situations where the drill bit may become stuck. The releasable locking means comprises a moveable pin which engages an aperture. A first configuration utilizes detent means and spring biasing of the pin. In a second configuration, the pin is adapted for locking engagement by pressurized downhole drilling mud, and may be disengaged from the aperture when pressure from drilling mud ceases. In a preferred embodiment the pin both has detent means and is spring biased, and also relies on drilling mud pressure to remain in locking engagement. The pin may be disengaged upon removal of drilling mud pressure and application of a positive separation force between the directional measurement tool and associated inner tool member, and outer mule shoe member.