Patent Publication Number: US-9890592-B2

Title: Drive shaft for steerable earth boring assembly

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and the benefit of U.S. Provisional Application Ser. No. 62/188,071, filed Jul. 2, 2015 the full disclosure of which is hereby incorporated by reference herein in its entirety for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present disclosure relates to a system for controlling a path of a drill bit in a subterranean formation. More specifically, the present disclosure relates to a steerable drilling assembly having a static seal and a tapered bore. 
     2. Description of Prior Art 
     Earth boring drilling systems are typically used to form wellbores that intersect subterranean formations having hydrocarbons so that the hydrocarbons can be extracted from the formations. The drilling systems usually include a rotatable drill string having a drill bit on its lower end for excavating through the formation. The drill string and drill bit are typically rotated by either a top drive or rotary table provided on surface. The types of drill bits are usually either roller cone bits or drag bits; and where cutting elements are generally formed on the bits. The combination of axial pressure on the drill string, combined with drill string rotation, causes the cutting elements to excavate through the formation and form cuttings that are circulated back uphole with drilling fluid. 
     Non-vertical or deviated wellbores are sometimes formed by whipstocks that are disposed in the wellbore and deflect the bit and drill string along a designated path in the formation. Deviated wellbores are often formed using mud motors mounted onto the drill string, which have fixed or adjustable angle bent sub housings and, when used in a sliding only mode are selectively oriented to direct the bit along a chosen direction. Deviated wellbores are otherwise formed using rotary steerable systems, which provide a means of steerable drilling while also permitting most or all of the drill string to rotate during steering operations. 
     SUMMARY OF THE INVENTION 
     Disclosed herein is an example of a steerable earth boring assembly. One example of the steerable earth boring assembly includes an annular collar, a drive shaft circumscribed by the collar and that swivels with respect to the collar, a bore through the drive shaft having a downstream end that selectively receives a drill bit, a flow tube having an upstream end in communication with drilling fluid and a downstream end in communication with the bore, and a shroud on a portion of the drive shaft distal from the bit and that circumscribes the flow tube, and which is defined where an inside diameter of the bore exceeds an outer diameter of flow tube to accommodate swiveling of the drive shaft. In one example, the inside diameter of the bore tapers radially outward with distance away from the drill bit. The steerable earth boring assembly can further include a static seal formed between the downstream end of the flow tube and the bore. In this example, the drive shaft pivots about a plane in which the static seal is disposed. In an embodiment, the collar and drive shaft are rotationally coupled by spline gears. The spline gears can be male splines coupled to the drive shaft, female splines coupled to the collar, and wherein the male splines can be crown splines. Further optionally included with the steerable earth boring assembly is an orientation device coupled with the drive shaft for selectively swiveling the drive shaft with respect to the collar. 
     Also disclosed herein is an example of a steerable earth boring assembly that is made up of an annular collar, an elongate drive shaft having a portion circumscribed by the collar and that selectively swivels about a pivot point in a precession like motion, and a bore in the drive shaft having a downstream end that selectively receives a drill bit and an upstream end, and an upstream end distal from the downstream end, the bore having a diameter that tapers radially outward proximate the upstream end. In one alternative, the steerable earth boring assembly further includes a flow tube having an upstream end in fluid communication with drilling fluid in a drill string that that selectively couples to a drill string and a downstream end in fluid communication with the bore in the drive shaft. The bore in the drive shaft can be strategically dimensioned so that sidewalls of the bore and the flow tube remain out of interfering contact with one another. Further optionally included is a static seal between an outer surface of the flow tube and an inner surface of the bore in the drive shaft that blocks fluid flow between the flow tube and the inner surface of the bore in the drive shaft. The pivot point and the static seal can lie in the same plane. Male spline gears can optionally be included that are coupled to the drive shaft, and that mesh with female spline gears coupled to the collar, so that rotating the collar in turn rotates the drive shaft and the drill bit, and wherein the male spline gears can be on a crown portion. 
     A yet another example of a steerable earth boring assembly includes an annular collar, a drive shaft in the collar that includes an axial bore, a receptacle on one end that selectively receives a drill bit, a shroud portion on an end distal from the receptacle, and a profile on an inner surface of the bore between the receptacle and the shroud portion defined where the diameter of the bore changes along a path oblique to an axis of the bore and along a designated axial distance. Also included in this example of the steerable earth boring assembly is a flow tube in selective fluid communication with drilling fluid, and having an end that inserts into the shroud portion and into sealing contact with the profile, and an annular space between an outer surface of the flow tube and inner surface of the bore in the shroud portion having a radius that increases with distance away from the receptacle. In one embodiment, an end of the collar distal from the drive shaft couples to a rotating drill string for rotating the collar, drive shaft, and drill bit for excavating a wellbore. Selective pivoting of the drive shaft in a designated orientation, in combination with rotation of the drive shaft in a subterranean formation, can form a deviated wellbore. Further optionally included are female splines coupled with an inner circumference of the collar and that mesh with corresponding male splines coupled with the drive shaft, and wherein a mid-portion of at least one of the male splines or female splines comprises a radial projection that defines a crown, so that when the drive shaft pivots with respect to the collar, the male and female splines remain in coupling engagement. In one example, also included with the steerable earth boring assembly is an orientation sleeve having a generally cylindrical outer surface and an axial bore in which the shroud portion is inserted, wherein the axial bore extends along a path that is oblique with an axis of the orientation sleeve. An axis of the axial bore can be spaced radially away from the axis of the orientation sleeve at an end of the orientation sleeve. In one alternative, the drive shaft pivots with respect to the collar, an amount of clearance remains between the flow tube and shroud portion. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Some of the features and benefits of the present invention having been stated, others will become apparent as the description proceeds when taken in conjunction with the accompanying drawings, in which: 
         FIGS. 1A-C  are side partial sectional views of an example of a steerable earth boring assembly forming a wellbore. 
         FIG. 2  is a side sectional view of an example of steering unit assembly for use with the earth boring assembly of  FIGS. 1A-C . 
         FIG. 3  is a side view of an example of a flow tube for use with the steering unit assembly of  FIG. 2 . 
         FIG. 4  is a side sectional perspective view of an example of an orientation sleeve collar for use with the steering unit assembly of  FIG. 2 . 
         FIG. 5  is a side sectional perspective view of an example of a drive shaft for use with the steering unit assembly of  FIG. 2 . 
         FIG. 6  is a perspective view of an example of a female spline for use with the steering unit assembly of  FIG. 2 . 
         FIG. 7  is a perspective view of an example of a male spline for use with the steering unit assembly of  FIG. 2 . 
         FIG. 8  is a side view of an example of a steering collar for use with the steering unit assembly of  FIG. 2 . 
         FIGS. 9A and 9B  are side sectional views of examples of a drive shaft for use with the steering unit assembly of  FIG. 2  respectively pivoted into different orientations. 
         FIGS. 10A and 11A  are side sectional views of the drive shaft of  FIGS. 9A and 9B  respectively with an example of an associated flow tube. 
         FIGS. 10B and 11B  are side sectional and enlarged views of portions of  FIGS. 10A and 11A  respectively, and where an O-ring is disposed between the flow tube and drive shalt. 
         FIG. 12  is a side sectional view of an example of a control unit assembly that selectively mounts to an upstream end of the steering unit assembly of  FIG. 2 . 
     
    
    
     While the invention will be described in connection with the preferred embodiments, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF INVENTION 
     The method and system of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments are shown. The method and system of the present disclosure may be in many different forms and should not be construed as limited to the illustrated embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey its scope to those skilled in the art. Like numbers refer to like elements throughout. In an embodiment, usage of the term “about” includes +/−5% of the cited magnitude. In an embodiment, usage of the term “substantially” includes +/−5% of the cited magnitude. 
     It is to be further understood that the scope of the present disclosure is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. In the drawings and specification, there have been disclosed illustrative embodiments and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation. 
     Shown in a side partial sectional view in  FIGS. 1A through 1C  is one example of a drilling assembly  10  forming a wellbore  12 . Wellbore  12  intersects a formation  14  and wherein drilling assembly  10  includes a rotating drill string  16  for delivering rotational power to form the wellbore  12 . A steering unit assembly (“SUA”)  18  is shown mounted on the lower end of drill string and which provides the cutting action to excavate the wellbore  12 . Included within SUA  18  is a steering sub  20  which has an articulated sub  22  projecting from its downstream end. A drill bit  24  mounts on a lowermost end of articulated sub  22 . As illustrated in  FIG. 1B , articulated sub  22  can be pivoted so that it is oriented at an angle that is oblique with steering sub  20 . Referring now to  FIG. 1C , the selective pivoting of the articulated sub  22  redirects the path SUA  18  so that it forms a bend  26  in wellbore  12 . Downhole of the bend  26 , the SUA  18  can be guided along a generally horizontal path as shown to thereby form a deviated portion  27  of the wellbore  12 . However, deviated portion  27  can also be at an angle that is generally oblique with the vertical section of wellbore  12  shown uphole of bend  26 . 
     An optional controller  28  shown on surface, which can downlink to the SUA  18 , and in an example provide control signals or commands from surface to SUA  18 , which the SUA  18  is configured to decode and perform a function in response to the control signal or command. Downlinking can be performed mechanically to generate the signals downhole, such as by varying drill string rotation, varying mud flow rate, mud pulse telemetry, to name a few. In an alternative, a control line  29  is shown providing communication between controller  28  and SUA  18 . Embodiments exist wherein control signals and feedback may be transferred via control line  29 . Alternatively, information regarding downhole conditions or operational parameters of the SUA  18  can be transmitted to the controller  28 . 
       FIG. 2  shows in a side sectional view one example of the SUA  18  and which includes a collar  30  on its outer surface. Collar  30  as shown in the illustrated example is an elongate annular member, provides a protective outer layer for components of the SUA  18 , and whose structure as well as a means for coupling and structurally securing these components. A port  32  is shown formed radially through the housing of collar  30 . As will be described in more detail below, collar  30  is a generally annular member, which is elongate, and includes selective profiles on its inner surface for the coupling of the components within SUA  18 . An annular and elongate housing  34  is shown inserted within the annular space of collar  30  and having an end that projects axially out from an upstream end of collar  30 . Grooves  36  circumscribe an outer surface of housing  34  at its upstream end, i.e. the end closer to the opening of wellbore  12  ( FIGS. 1A-1C ) when the SUA  18  is inserted in the wellbore  12 . In an example grooves  36  provide coupling to drill string  16  ( FIGS. 1A through 1C ); and the annular space  37  inside of housing  34  may selectively receive drilling fluid (not shown) therein which is circulated within drill string  16 . 
     A flange-like ledge  38  is depicted formed on a downstream end of housing  34  that is disposed within collar  30 . Ledge  38  projects radially outward a distance from the lower terminal end of housing  34 . A projection  39  is illustrated adjacent a lower end of ledge  38 . Projection  39  is formed where an inner diameter of collar  30  reduces along a portion of its axial length. An upstream radial surface of ledge  38  abuts a downward-facing radial surface of a projection  39 , so that projection  39  provides an axial stop thereby preventing relative upward movement of housing  34  with respect to collar  30 . Axially formed through a sidewall of housing  34  is a passage  40 , which extends the length of housing  34 . Sealed feed through connectors  42 ,  43  are provided respectively at the downstream and upstream ends of passage  40 . As will be described in more detail below, passage  40  allows for the wired communication between connector  42  and  43 . Connector  42  prevents ingress of dielectric fluid contained in collar  30 . 
     Still referring to  FIG. 2 , as shown the outer diameter of housing  34  is spaced radially inward from an inner diameter of the inner surface of collar  30 , an annulus  44  is formed between these members that extends along a portion of the axis of the housing  34 . A ring-like piston  46  is shown inserted within annulus  44  and which is axially moveable within annulus  44 . An annular chamber  48  is defined in the annulus  44  on a side of piston  46  distal from grooves  36 . An annular nut  50  is shown in chamber  48  and landed on an upstream radial surface of projection  39 . Nut  50  of  FIG. 2  is coupled to an outer surface of housing  34 . 
     An annular flow tube  54  is shown disposed within collar  30  and having an upstream end  55  ( FIG. 3 ) that inserts into a lower portion of the annular space  37  that extends through housing  34 . A diameter of the annular space  37  projects radially outward proximate ledge  38  to accommodate insertion of the upstream end  55 . A passage  56  is shown extending axially through the sidewall of housing  34  adjacent upstream end  55 . An upstream end of passage  56  projects radially outward and into fluid communication with chamber  48 . Optionally, a port  57  projects radially outward from passage  56  through housing  34  to its outer surface. A downstream end of passage  56  opens into a chamber  58  that is in an annular space between flow tube  55  and an inner surface of collar  30 . Accordingly, piston  46  in combination with chambers  48 ,  58  and passage  56  provide a pressure compensation means for pressurizing the space within chamber  58  to that of ambient. In the illustrated embodiment, piston  46  will move within annulus  44  in response to changing ambient pressures. More specifically, when ambient pressures exceed pressure in chamber  58 , piston  46  is urged downward thereby pressurizing fluid in chambers  48 ,  58  and passage  56 , until pressure in chambers  48 ,  58  and passage  56  is substantially equal to ambient pressure. Similarly, when ambient pressure is less than that in chambers  48 ,  58  and passage  56 , piston  46  is urged upward in annulus  44  to relieve pressure in chambers  48 ,  58  and passage  56  until equal to ambient. In one example, port  57  communicates fluid between passage  56  and inside of nut  50  thereby equalizing pressure on a lower end of nut  50  to that within chamber  48 . 
     Included within chamber  58  is a motor assembly  59  which includes a ring-like rotor  60  set on an outer radial portion of chamber  58  and extending along an axial portion of chamber  58 . Set radially within rotor  60  is a stator  62 , which also is a ring-like member and within chamber  58 . A magnet rotor  64 , which in the example shown is an elongate ring-like array of permanent magnets, is disposed between rotor  60  and stator  62  and coupled to the inner radial surface of rotor  60 . In an example of operation, the motor assembly  59  operates when a control signal is supplied from a control unit, such as within controller  28  ( FIG. 1A /B), through the connectors  42 , 43  to the stator  62 . In this example, the control signal energizes a set of coils (not shown) integral to the stator  62 , which then imparts a rotational motive force on the magnet rotor  64 . The resulting rotational movement of the magnet rotor  64  in turn results in rotational movement of the rotor  60  and an orientation sleeve  72 . Below motor assembly  59  is a ring-like retaining nut  66  which axially threads to an inner surface of a collar-like flow tube positioner  68 , and which provides an axial stop for flow tube  54 . As shown in  FIG. 2 , bearings  70  are provided between flow tube  54  and flow tube positioner  68 . In the illustrated example, bearings  70  are shown as roller-type bearings and provide relative rotation between flow tube positioner  68  and flow tube  54 . However, other types of bearings can be used in this application, including journal bearings, as well as a thin film of lubricant. Optionally included with SUA  18 , and disposable downhole, is a turbine and controller (not shown), wherein turbine is rotatable in response to drilling fluid flowing down drill string  16  and selectively generates electrical power for operating motor assembly  59 . 
     Still referring to  FIG. 2 , an orientation sleeve  72  is shown mounted to a downstream end of flow tube positioner  68 . Orientation sleeve  72  is a generally annular member that has a substantially cylindrical outer surface and projects axially away from motor assembly  59  and within collar  30 . Rotor  60  is coupled to flow tube positioner  68 , thus energizing motor assembly  59  causes rotation of rotor  60 , that in turn produces selective rotation of flow tube positioner  68  and orientation sleeve  72 . 
     Referring now to  FIG. 4 , orientation sleeve  72  is shown in a side perspective cut away view. In the illustrated, a bore  74  that extends axially through orientation sleeve  72 . Bore  74  is not coaxially disposed within sleeve  72 , but instead an axis A 74  of bore  74  is shown projecting along a path that is at an angle θ which is oblique to an axis A 72  of orientation sleeve  72 . In one example the positioning of bore  74  is offset within orientation sleeve  72 , so that not only is axis A 74  oblique to axis A 72 , axes A 72 , A 74  are set radially apart from one another at opposing ends of orientation sleeve  72 . To better illustrate the radially set apart axes A 72 , A 74 , a sidewall thickness t 1  of sleeve  72  at one azimuthal location is less than a sidewall thickness t 2  at an angularly spaced apart location. 
     Referring back to  FIG. 2 , a downstream end of flow tube  54  is shown inserted into a bore  76  that projects axially through a drive shaft  78 . As will be described in more detail below, strategic axial positioning of the flow tube  54  can create a static seal on an end of the flow tube  54  and drive shaft  78 .  FIG. 5  shows in a side sectional view one example of drive shaft  78 . In this example, the diameter of bore  76  increases proximate the downstream end of drive shaft  78  to define a receptacle  79 , that as shown in  FIG. 1  can receive drill bit  24  for excavating wellbore  12 . A portion of the drive shaft  78  having the receptacle defines a base portion  80 , wherein an outer diameter of base portion  80  projects radially outward above the upstream end of receptacle  79 . A portion of drive shaft  78  distal from receptacle  79  defines a shroud portion  81 ; the diameter of bore  76  adjacent shroud portion  81  increases with proximity to its upstream end. As described below, drive shaft  78  is pivotable about its mid-portion, thus the strategic dimensioning of the diameter of bore  76  within shroud portion  81  allows a pivoting action around flow tube  54  so that the inner surface of bore  76  remains out of interfering contact with the outer surface of flow tube  54  as the drive shaft  78  is being pivoted. Further shown in FIG.  5  are a series of profiled sections  82   1 - 82   3  in bore  76  that are formed where the diameter of bore  76  changes to form these profiles  82   1 - 82   3 . Profile  82   2  is strategically formed to be in contact with an O-ring  84  that is set in a recess  85  circumscribing a portion of flow tube  54  proximate its lower end  83  ( FIG. 3 ). The O-ring  84  defines a static seal between the flow tube  54  and drive shaft  78 . Thus when the drive shaft  78  pivots along the path represented by curved arrow A, a static seal is maintained between O-ring  84  and profile  82   2 . It should be pointed out that the pivoting motion of drive shaft  78  relative to collar  30  is not limited to motion in a single plane, but can include swiveling where the relative movement between drive shaft  78  and collar  30  occurs across more than one plane. For example, swiveling motion can resemble a precession type motion. An advantage of the static seal along O-ring  84  is that the need for a seal that rotates or is otherwise dynamic is eliminated, as the static interface between the lower end  83  and profile  82   2  defines a flow barrier that blocks fluid flow passage from within flow tube  54  and bore  76  to outside of drive shaft  78 . Accordingly, any fluid flowing within flow tube  54  from drill string  16  ( FIGS. 1A through 1C ) will not make its way between flow tube  54  and the inner surface of bore  76 , but instead will continue within bore  76  downstream of profile  82   3  and towards receptacle  79 . 
     Referring back to  FIG. 2 , a bearing assembly  86  is shown provided on an inner surface of collar  30 , radially adjacent an outer surface of orientation sleeve  72 , and axially proximate the lower end of orientation sleeve  72 . Bearing assembly  86  reduces rotational friction as orientation sleeve  72  rotates within collar  30 . Bearing assembly  86  is shown as a roller-type bearing assembly, but can instead be a journal type, as well as a thin floating film-type. A ring-like bearing shoulder ring  87  is shown just below bearing assembly  86  and generally coaxial with bearing assembly  86 . Thus the outer surface of bearing shoulder ring  87  is in close contact with an inner surface of collar  30 , and wherein ring  87  provides axial support for bearing assembly  86 . Ring  87  has a wedge-like cross-section whose thickness increases with distance away from bearing assembly  86 . The respective lower ends of ring  87  and orientation sleeve  72  are positioned at roughly the same axial location within collar  30 . A ring-like spherical bearing outer race  88 , which is also in the annular space between collar  30  and drive shall  78 , is set on a lower end of ring  87 . Outer race  88  is in selective rotating contact with a spherical bearing inner race  90  shown mounted on an outer circumference of drive shaft  78 . The contact surfaces between races  88 ,  90  run along a path that is oblique to an axis A X  of collar  30  and project radially outward with distance away from a lower end of orientation sleeve  72 . 
     A ring-like load spacer bearing  92  is shown on a lower end of race  90 . Set axially downward from load spacer bearing  92  is a ring-like female spline  94  that couples to an inner surface of collar  30 . Shown in perspective view in  FIG. 6  is one example of the female spline  94 , and which can be made up of multiple sections that are mounted within collar  30 . Spline members  96  or elements project from, and axially across, a radially inward facing surface of the female spline  94 . Spline members  96  are generally raised members at spaced apart locations that resemble gear teeth. Referring back to  FIG. 2 , a male spline  98  is shown that is in selective engagement with female spline  94 . Male spline  98  is also a ring like member, and as shown in  FIG. 7  includes corresponding spline members  100  that project radially outward, and extend axially along its outer radial surface. Spline members  100  selectively mesh into recesses between adjacent spline members  96  of female spline  94  ( FIG. 6 ). Optionally, spline members  100  are involute having widths greater at their mid portions than at their ends. Rotation of one of the female or male splines  94 ,  98  necessarily causes rotation of the other spline  94 ,  98  and in the same rotational direction. In this fashion, rotation of the collar  30  via the drill string  26  (FIGS.  1 A through  1 C) causes corresponding rotation of the drive shaft  78 . In the cutaway view of  FIG. 2 , a dowel  102 , which is a pin-like member, extends axially within an opening  104  ( FIG. 7 ) shown formed along an inner surface of the male spline  98 . As dowel  102  is coupled with the outer surface of drive shaft  78 , the presence of dowel  102  thus rotationally attaches drive shaft  78  and male spline  98 . Therefore any rotation of male spline  98  correspondingly induces rotation of drive shaft  78 . One or more threaded fasteners  105  may be used to attach female spline  94  to collar  30  so that when collar  30  is rotated, female spline  94  also rotates and in the same direction. Another dowel (not shown), similar to dowel  102 , retains female spline  94  to collar  30 . 
     A thrust ring  106  is shown set in a lower end of male spline  98  and which circumscribes drive shaft  78 . Just below ring  106  are inner and outer races  108 ,  110  which contact one another along an oblique interface and which are similar in construction with races  88 ,  90 . Thus, the combination of races  88 ,  90 ,  108 ,  110  allow for relative pivoting of drive shaft  78  to collar  30 . Additionally, in an example, the interface between races  88 ,  90  and races  108 ,  110  are along an outer surface of a sphere S, wherein sphere S is bisected by a plane P in which O-ring  84  is disposed. A retention ring  112  coaxially threads to an inner surface of a lower end of the collar  30 . While a portion of retention ring  112  is circumscribed by the collar  30 , a lower portion projects axially downward from the lower terminal end of collar  30 . Axially set lower from races  108 ,  110  is a seal sleeve  114  that provides a lower seal between collar  30  and drive shaft  78 . Seal sleeve  114  circumscribes the portion of the retention ring  112  that extends past the lower end of collar  30 . Circumscribed by retention ring  112  is an annular bellows assembly  116 , which is made up of a bellows  118 . In the illustrated example bellows  118 , is a thin-walled member with walls that are undulating along its length to thereby allow for axial movement as well as pivoting and yet can still maintain a seal between the drive shaft  78  and collar  30 . Also included with the bellows assembly  116  is a bellows nut  119  that couples to a lower end of bellows  118 . 
       FIG. 8  shows in a side view one example of collar  30  and wherein drive shaft  78  projects axially from one end and wherein housing  34  extends axially outward from an opposite end. In this example, a stabilizer  120  is shown on the outer surface of collar  30  which is made up of some raised portions that are spaced circumferentially apart and wherein each portion follows a generally, helical pattern along the outer surface of collar  30 . The presence of stabilizer  120  can provide a spacing between the collar  30  and inner surface of wellbore to thereby provide protective separation between the two. 
     In one example of operation, as shown in  FIGS. 1A-1C  and  FIG. 2 , drill string  16  has an upstream end depending from drilling rig  122 . A top drive or rotary table  124  provides a rotational force onto the drill string that in turn rotates SUA  18 . Rotating SUA  18  provides a rotating force onto the outer surface of collar  30  that via splines  94 ,  98  and drive shaft  78  causes rotation of drill bit  24 , that in one embodiment mounts into receptacle. To form the bend  26  of  FIG. 1C , motor assembly  59  is selectively activated to cause rotation of rotor  60  that as described above rotates orientation sleeve  72 . The obliqueness of bore  74  then causes a precession-type movement of drive shaft  78  to move drive shaft in the precession-like motion with respect to drill string  16  and collar  30 . Rotating the orientation sleeve  72  at a designated rotational velocity, can keep the drive shaft  78  in a constant azimuthal orientation with respect to a vertical axis, even though the drill string  16  and collar  30  continues to rotate. Knowing a designated azimuthal position, the bend  26 , and thus deviated wellbore  27 , can be formed as described above. An advantage of the crown in the splines allows continued rotational motion transfer between collar  30  and drive shaft  78  even though drive shaft  78  can pivot, thereby causing the respective spline members  96 ,  100  to move axially with respect to one another. In an example of operation, to obliquely orient the drive shaft  78  (and bit  24 ) with respect to collar  30 , orientation sleeve  72  is rotated in a circular direction opposite the rotational direction of drill string  16 , but at the same angular rotational rate as drill string  16 . Changing direction, or directing the drill bit  24  along a straight non-deviating path, can be accomplished by rotating the orientation sleeve  72  in a direction opposite the drill string  16 , but at a rate of rotation that is different from that of the drill string  16 . 
     Shown in side sectional views in  FIGS. 9A and 9B  are examples of the drive shaft  78  pivoting between different orientations. Pivoting drive shaft  78  in a clockwise direction, as illustrated by arrow A CW , changes the orientation of the drive shaft  78  of  FIG. 9A  to that of  FIG. 9B . Similarly, pivoting drive shaft  78  in a counter-clockwise direction, as illustrated by arrow A CCW , changes the orientation of the drive shaft  78  of  FIG. 9B  to that of  FIG. 9A . In each of  FIGS. 9A and 9B , axis A 76  of bore  76  is oblique with axis A 18  of steering unit assembly  18  ( FIG. 2 ). In the examples of  FIGS. 9A and 9B , axes A 76 , A 18  are radially offset from one another at the opening of the shroud  81 , and proximate the receptacle  79 . However, the radial order of axes A 76 , A 18  changes between the pivoted orientations illustrated in  FIGS. 9A and 9B . For example, axis A 18  is closer than axis A 76  to the Y-axis of the Cartesian coordinates of  FIG. 9A  proximate the opening of bore  76 ; but axis A 18  is spaced farther away from the Y-axis than axis A 76  proximate the opening of bore  76 . Depicted in  FIGS. 9A and 9B  the axes A 76 , A 18  intersect one another at pivot point P P ; thereby indicating a point or axis about which drive shaft  78  rotates while being pivoted. Pivot point P P  is at the center of sphere S (and in plane P); as described above the outer surface of sphere S is coincident with interfaces between races  88 ,  110  and races  90 ,  108 . 
       FIG. 10A  is a side sectional view of an example of the drive shaft  78  having substantially the same orientation as that of  FIG. 9A  and so that axis A 76  of bore  76  is lower on the Y-axis than axis A 18  of the steering unit assembly  18  ( FIG. 2 ). Also shown in  FIG. 10A  is flow tube  54  inserted into bore  76  and in sealing contact with an inner surface of bore  76 . In this example, flow tube  54  remains substantially aligned with axis A 18 , and thus drive shaft  78  is pivotable with respect to flow tube  54 . As indicated above, the diameter of bore  76  increases with distance from end  83  so that the sidewalls of the bore  76  remain clear of the flow tube  54  as the drive shaft  78  pivots in response to rotation of sleeve  72  ( FIG. 2 ). Thus the presence of flow tube  54  inside bore  76  does not interfere with drive shaft  78  pivoting. 
       FIG. 10B  illustrates in side sectional and enlarged view a portion of an example of flow tube  54  proximate its end  83  and inserted into drive shaft  78 . As depicted in the example of  FIG. 10B , while the outer surface of flow tube  54  remains clear of drive shaft  78 , O-ring  84  is shown in sealing contact with flow tube  54  inside of recess  85  extending across a gap G between flow tube  54  and drive shaft  78 , and into sealing contact with the profile  82   2  formed along bore  76  in drive shaft  78 . As shown, the outer surface of flow tube  54  upstream of O-ring  84  is closer to the sidewalls of bore  76  than that downstream of O-ring  84 . In the illustrated embodiment, because O-ring  84  (and recess  85 ) is strategically located proximate end  83 , the sealing interface formed by O-ring  84  between flow tube  54  and drive shaft  78  operates as a “static seal.” In an example a static seal provides a flow and a pressure barrier between surfaces that have little to no movement relative to one another. As illustrated in the example of  FIGS. 11A and 11B , drive shaft  78  has swiveled, so that when viewed in cross section, the drive shaft  78  appears to have pivoted in a clockwise direction so that the relative vertical location of axes A 18 , A 76  has changed over that of  FIGS. 10A and 10B , thereby bringing the surface of flow tube  54  that is downstream of O-ring  84  closer to the inner surface of bore  76  than the surface of flow tube  54  upstream of O-ring  84 . Referring now to  FIGS. 10B and 11B , in the illustrated example of operation,  FIG. 10B  depicts the drive shaft  78  in its farthest counter-clockwise pivot, and in  FIG. 11B , the drive shaft  78  is shown in its farthest clockwise pivot; thus comparing  FIGS. 10B and 11B  the drive shaft  78  is shown in orientations describing its full range of pivoting motion. Further illustrated is how there is little to no axial movement between O-ring  84  and recess  85  or between O-ring  84  and profile  82   2 . Further an annular gap G is shown between the outer surface of flow tube  54  and profile  82   2 , where the thickness of gap G on opposite sides of recess  85  changes between the counter-clockwise and clockwise pivot positions of the drive shaft  78  illustrated in  FIGS. 10B and 11B . Example thicknesses of gap G range from about 0.005 inches to about 0.015 inches. 
     Illustrated in side sectional view in  FIG. 12  is an example of a control unit assembly  126  that can optionally be included with the steering unit assembly  18 . Control unit assembly  126  includes an annular control collar  128  has an end shown coupled with an end of collar  30  of steering unit assembly  18 . In the illustrated example, collar  128  provides an outer covering for components within the control unit assembly  126 . Further, threads T are provided on an end of collar  128  distal from where it is coupled with collar  30 . In an embodiment, an end of drill string  16  distal from drilling rig  122  ( FIG. 1 ) couples with threads T. As such, in the example of  FIG. 12 , rotational energy from drill string  16  rotates control collar  128 , which in turn rotates collar  30 . As discussed above, rotating collar  30  ultimately produces rotation of drill bit  24  ( FIGS. 1A-1C ). An optional stabilizer  130  is shown mounted on an outer surface control collar  128  for use in stabilizing assembly  126  during drilling operations. A bore  132  is formed within control collar  128  and in which a generator assembly  134  is disposed. In the example of  FIG. 12 , electricity is generated by generator assembly  134 , which is used to power components within and associated with drilling assembly  10  ( FIG. 1 ). An upstream end of generator assembly  134  is equipped with a frusto-conically shaped bullnose  136  for diverting fluid (such as drilling mud) flowing through bore  132  towards blades of an impeller assembly  138  disposed downstream of bullnose  136 . In one example of operation, directing fluid flow past the impeller assembly  138 , rotates impellers and an associated shaft in the assembly  138 , that in turn rotates a rotor  140  disposed in a magnetic field thereby generating electricity. An elongate annular pressure housing  142  is shown downstream of generator assembly  134 ; and having an end distal from generator assembly  134  that terminates at an upstream end of a flow diverter  144 . A bore  146  is shown formed axially through a downstream portion of flow diverter  144 . Bore  146  is in communication with an upstream end of annular space  37 , so that fluid flowing in annulus  147  between collar  128  and pressure housing  142  is directed through bore  146  and into annular space  37 . 
     Electricity generated within generator assembly  138  is directed to power and control electronics  148  via line  150 . In an example, electricity from generator assembly  138  is conditioned by power and control electronics  148  so that the electricity is usable by components within the drilling assembly  10  ( FIG. 1 ). In an embodiment, conditioning of the generated electricity includes rectifying the current, and/or adjusting values of voltage/current to match operational specifications of the user components. Line  152  transmits the conditioned electricity from power and control electronics  148  to an electrical connector  154 , that in an example is rotatable. Power and control electronics  148  and lines  150 ,  152  are disposed within pressure housing  142 , whereas connector  154  is housed in cavity  156  formed in an upstream portion of flow diverter  144 . An optional antenna  158  is shown formed on an outer surface of collar  128 , wherein antenna  158  can be used for communicating signals uphole or to surface, where the signals can include data from sensors disposed downhole, or control commands for directing operation of the drilling assembly  10 . 
     The present invention described herein, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While a presently preferred embodiment of the invention has been given for purposes of disclosure, numerous changes exist in the details of procedures for accomplishing the desired results. These and other similar modifications will readily suggest themselves to those skilled in the art, and are intended to be encompassed within the spirit of the present invention disclosed herein and the scope of the appended claims.