Patent Publication Number: US-7216724-B2

Title: Coupling for dual member pipe

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Application No. 60/483,151, filed on Jun. 27, 2003, the contents of which are incorporated herein fully by reference. 

   FIELD OF THE INVENTION 
   The present invention relates to the field of horizontal directional drilling, and more particularly but not by way of limitation, to a coupling for a dual member pipe to generate a torque output for transmission to a downhole tool of a horizontal directional drilling system. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a system of pipe sections comprising a plurality of pipe sections and a coupling assembly. The pipe sections are disposed in end-to-end engagement to form a drill string. Each pipe section has a rotatable outer member and a rotatable inner member. The rotatable inner member is situated within the outer member. Torque may be transmitted between the outer member of each pipe section and the outer member of an adjacent pipe section, and wherein torque may be transmitted between the inner member of each pipe section and the inner member of an adjacent pipe section. Finally, the coupling assembly is adapted to transmit torque between the outer member and the inner member of the pipe section at a downhole end of the drill string. 
   The present invention is further directed to a system for rotating a downhole tool comprising a plurality of pipe sections and a coupling assembly. The pipe sections are disposed in end-to-end torque transmitting engagement to form a drill string. Each pipe section has a rotatable outer member and a rotatable inner member situated within the outer member. The coupling assembly has a first end and a second end. The first end of the coupling assembly is operably connectable to a downhole tool, whereas the second end of the coupling assembly is operably connectable to the inner member and the outer member of the pipe section at a downhole end of the drill string. Additionally, the coupling assembly is adapted to receive a torque input from the inner member and a torque input from the outer member to generate a resultant torque output of the downhole tool. 
   In yet another aspect, the invention is directed to a horizontal boring system comprising a horizontal boring machine, a plurality of pipe sections, a coupling assembly, and a downhole tool. The horizontal boring machine has at least one drive system that is characterized by a first end and a second end. The first end of the drive system is connected to the horizontal boring machine. The plurality of pipe sections are disposed in end-to-end engagement forming a drill string such that the pipe section at an uphole end of the drill string is connected to the second end of the drive system. 
   Each pipe section in the drill string has a rotatable outer member and a rotatable inner member situated within the outer member. Torque may be transmitted between the outer member of each pipe section and the outer member of an adjacent pipe section. Additionally, torque may be transmitted between the inner member of each pipe section and the inner member of an adjacent pipe section. Further, the coupling assembly is adapted to transmit torque between an outer member and an inner member of the pipe section at a downhole end of the drill string, and the downhole tool is adapted to be in torque transmitting engagement with the coupling assembly. 
   In still another aspect, the present invention is directed to a horizontal boring system for use to drive a downhole tool. The horizontal boring system comprises a horizontal boring machine, a plurality of pipe sections, and a coupling assembly. The horizontal boring machine has at least one drive system that is characterized by a first end and a second end. The first end of the drive system is connected to the horizontal boring machine. 
   The plurality of pipe sections are disposed in end-to-end engagement to form a drill string such that the pipe section at an uphole end of the drill string is connected to the second end of the drive system. Each pipe section in the drill string has a rotatable outer member and a rotatable inner member situated within the outer member. Torque may be transmitted between the outer member of each pipe section and the outer member of an adjacent pipe section. Further, torque may be transmitted between the inner member of each pipe section and the inner member of an adjacent pipe section. Finally, the coupling assembly is adapted to receive a torque input from the inner member and a torque input from the outer member to generate a resultant torque output of the downhole tool. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagrammatic representation of a horizontal directional drilling system constructed in accordance with the present invention. 
       FIG. 2  is a fragmented, side elevational, partly sectional view of a pipe section used with a dual-member drill string. 
       FIG. 3  shows a fragmented, side elevational, cross-sectional view of the rotary drive system of the present invention. 
       FIG. 4  is a side view of a coupling assembly comprising a connector sub that may be removably attached to a downhole tool. 
       FIG. 5  is a cross-sectional view of the coupling assembly of  FIG. 4  constructed in accordance with the present invention. 
       FIG. 6  is a perspective view of a downhole tool with a connector sub formed as an integral part of the downhole tool. 
       FIG. 7  is a block diagram of a drive system constructed in accordance with the present invention. 
       FIG. 8  is a cross-sectional view of a coupling assembly comprising an over-running clutch constructed in accordance with the present invention. 
       FIG. 9  is a perspective view of the coupling assembly of  FIG. 8  comprising the over-running clutch constructed in accordance with the present invention, taken along cut-lines A—A in  FIG. 8 . 
       FIG. 10  is a cross-sectional view of a coupling assembly comprising a pawl clutch constructed in accordance with the present invention. The pawl clutch is arranged similarly to the over-running clutch of  FIGS. 8 and 9 . 
       FIG. 11  is a perspective view of a coupling assembly comprising a locking mechanism constructed in accordance with the present invention. A portion of wall of an outer drive shaft of the coupling assembly has been removed in order to better display other components of the locking mechanism. 
       FIG. 12  is a cross-sectional view of the hydraulic mechanism which drives axial movement of the inner member relative to the outer member as shown in  FIG. 11 . 
       FIG. 13(   a ) is a cross-sectional view of a coupling assembly connected to a dual member drill string and comprising a moveable spline connector constructed in accordance with the present invention. The moveable spline connector is shown in a first position in which only the inner drive shaft is connected to the downhole tool through a spline coupling. 
       FIG. 13(   b ) is a partial cross-sectional view of a coupling assembly comprising a moveable spline connector constructed in accordance with the present invention. The moveable spline connection is shown in a second position in which only the outer drive shaft is connected to the downhole tool through a spline coupling. 
       FIG. 13(   c ) is a partial cross-sectional view of a coupling assembly comprising a moveable spline connector constructed in accordance with the present invention. The moveable spline connector is shown in a third position in which both the inner drive shaft and the outer drive shaft are connected to the downhole tool through spline couplings. 
       FIG. 14  is a partial cross-sectional and partial side elevational view of a coupling assembly comprising a planetary gear system housed in a backreaming tool. 
       FIG. 15  is a depiction of a planetary drive housing assembly comprising a planetary gear system to drive a downhole boring tool. 
       FIG. 16  is a sectional view of the planetary drive housing assembly shown in  FIG. 15 . 
       FIG. 17  is a cross-sectional view of the planetary drive housing assembly of  FIG. 16  taken along cut-line A—A. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Turning now to the drawings in general, and to  FIG. 1  in particular, shown therein is a horizontal directional drilling system  10 , that is constructed in accordance with the present invention. The horizontal directional drilling system  10  generally comprises a drilling machine  12 , a control system  14 , a pipe handling assembly  16 , a drive system  18 , a dual member drill string  20 , and a downhole tool  22 . 
   The drilling machine  12  preferably has a central frame  24  that supports the control system  14 , a rotary machine of the drive system  18 , and the pipe handling assembly  16 . The frame  24  also supports a spindle  26  mounted on a carriage  28  (shown in  FIG. 3 ). The carriage  28  can be advanced or retracted along the frame in a spindle connection area during drilling and backreaming operations. The pipe handling assembly  16  has a pipe storage device, such as a pipe rack  29 , for storage of a plurality of dual member pipe sections  30 . A pipe delivery system (not shown) may be located beneath the pipe rack  29  to transport pipe sections  30  between the pipe rack and the spindle connection area during drilling operations. Pipe sections  30  are added to or removed from the drill string  20  as needed, to extend or reduce the drill string  20  during drilling and backreaming operations. The drill string  20  thus comprises a plurality of dual member pipe sections  30  that are disposed in end-to-end engagement to form the drill string. 
   With reference now to  FIGS. 2 and 3 , each dual member pipe section  30  comprises a rotatable inner member  40  situated within a rotatable outer member  42 . Preferably, the positioning of the inner member  40  within the outer member  42  creates an annular space  44  to facilitate flow of drilling fluid in the pipe section  30  between an outer surface of the inner member and an inner surface of the outer member. Alternatively, the inner member  40  may comprise a pipe or tubular section so that drilling fluid may pass through the inner member. As a result, a string of connected inner members  40  and outer members  42  defines a passageway extending the length of the dual member drill string  20  that allows for the flow of fluid through the drill string to the downhole tool  22 . 
   In the preferred dual member pipe section  30 , the rotatable outer member  42  is elongate and tubular. The outer member  42  comprises a pin end  52 , a central body portion  53 , and a box end  54 . The pin end  52  and the box end  54  are threaded for connection to adjacent pipe sections  30 . Preferably, the pin end  52  is provided with tapered external threads, and the box end  54  is provided with tapered internal threads. Thus, the box end  54  of the outer member  42  of one pipe section  30  is connectable in torque transmitting engagement to the pin end  52  of an adjacent like pipe section  30 . The external diameters of the pin end  52  and the box end  54  of the outer member  42  may be larger than the external diameter of the central body portion  53  of the outer member. 
   The rotatable inner member  40  is preferably elongate and characterized by an external diameter less than the minimum internal diameter of the outer member  42 . In the preferred embodiment, the inner member  40  is integrally formed and comprises a solid rod. However, it will be appreciated that in some instances the inner member  40  may be tubular instead of being a solid rod. 
   The inner member  40  of the dual member pipe section  30  is preferably provided with a non-threaded geometrically shaped pin end  70  and a box end  72 . The box end  72  of the inner member  40  may be brazed, forged, welded or attached to the inner member  40  by any suitable means. The box end  72  has an internal contour which matches the geometric shape of the pin end  70 . As a result, the box end  72  matingly receives the inner member  40  of the pin end  70  of the adjacent inner member  40  in slip-fit torque transmitting engagement. A preferred geometric shape for the pin end  70  and box end  72  of the inner member  40  is a polygon such as a hexagon, octagon, pentagon, etc. 
   For purposes of this application, “geometrically shaped” denotes any configuration which permits the pin end  70  to be slideably received in the box end  72 , but which prevents rotation of the pin end relative to the box end when thus connected. Any geometric configuration that permits single action, connector-free, slip-fit engagement capable of transmitting torque between adjacent inner members  40  of the drill string  20  may be used. One such dual member pipe connection is described in U.S. Pat. No. 5,682,956, the contents of which are incorporated herein by reference. It will be understood that for purposes of this application, “geometrically shaped” does not include a perfectly circular shape as this would not allow torque transmission from one pipe section to the next. Additionally, as used herein, “connector-free” means the absence of any latch, pin, or other attaching device to retain the pin end  70  of the inner member  40  inside the box end  72  of an adjacent like inner member. 
   In the preferred dual member drill string  20 , the pin end  70  of the inner member  40  is recessed within the box end  54  of the outer member  42 , and the box end  72  of the inner member  40  protrudes beyond the pin end  52  of the outer member  42 . This pipe section  30  structure is used to form the drill string  20  with the outer member  42  in pin-up configuration and the inner member  40  in pin-down configuration. Other configurations for the pin and box ends of the outer and inner members are anticipated, such that, for example, the pin end of the inner member may protrude from the pin end of the outer member and the box end of the inner member may be recessed within the box end of the outer member. 
   With reference again to  FIG. 1 , the assembled dual member drill string  20 , is characterized by an uphole end  75  and a downhole end  76 . The uphole end  75  of the drill string  20  is operably connected to the drilling machine  12  that imparts driving forces, such as rotation and thrust forces, to the drill string to produce the subterranean borehole  32 . The downhole end  76  of the drill string  20  generally supports the downhole tool  22  that forms or finishes the borehole  32 . Preferably, the downhole tool  22  is operatively connected to the inner members  40  of the pipe section  30  at the downhole end  76  of the drill string  20 . The downhole tool  22  can thus be rotated by the inner members  40  and independently of the outer members  42 . A slant face drill bit and a tricone bit exemplify downhole tools  22  that are commonly attached to the downhole end  76  of the drill string  20  to form the borehole  32  in the subterranean earth. Alternatively, a backreaming tool is a downhole tool  22  that is commonly attached to the downhole end  76  of the drill string  20  to finish the borehole during withdrawal of the drill string  20  from the borehole  32  by cutting, expanding or packing, and thereby finally sizing the borehole. As used herein, “downhole tool” will be used to reference any tool operatively connected at the downhole end  76  of the drill string  20  and used in an application with a horizontal directional drilling machine  12 . 
   With reference now to  FIG. 3 , in the preferred embodiment for the dual member horizontal directional drilling system  10 , the assembled dual member drill string  20  and downhole tool  22  are driven by the drive system  18 . The drive system  18  preferably comprises a plurality of rotary devices, and more preferably two independent rotary drive devices  80  and  82 . Each rotary drive device  80  and  82  is operably connectable to the respective one of the members  40  and  42  of the pipe section  30  at the uphole end  75  of the drill string  20 . Each rotary drive device  80  and  82  independently drives either the plurality of assembled inner members  40  or the plurality of assembled outer members  42  of the drill string  20 . The rotary drive devices  80  and  82  may be, for example, hydraulic motors, variable speed motors, pneumatic motors, etc. One can also appreciate that in certain instances consistent with the invention, that at one instance the uphole drives  80  and  82  could be mechanically connected together or driven by a single motor. 
   As illustrated in  FIG. 3 , the drive system  18  preferably comprises the outer member drive group  80  for driving the plurality of outer members  42 , and the inner member drive group  82  for driving the plurality of inner members  40 . 
   The outer member drive group  80  is supported on the carriage  28  and comprises an outer drive motor  83 , an outer spindle  84  and a torque-transmitting member  86 . The outer drive motor  83  is operatively connected to the outer spindle  84  and transmits power and torque input to the outer spindle  84  through the torque-transmitting member  86 . Preferably, the torque-transmitting member  86  comprises a sprocket and chain assembly having upper and lower sprockets. The outer spindle  84  in turn is threadably connectable to the outer member  42  of the pipe section  30  at the uphole end  75  of the drill string  20 . In this manner, the outer spindle  84  transmits torque from the outer drive motor  83  to the plurality of outer members  42  comprising the drill string  20 . However, any means for transmitting power and torque input from the outer member drive motor  83  to the outer members  42  may be used. 
   The inner member drive group  82  is adapted to be supported on the carriage  28  and comprises an inner drive motor  90  and an inner spindle  92 . The inner drive motor  90  drives the inner spindle  92 . The inner spindle  92  is connectable to the inner member  40  of the pipe section  30  at the uphole end  75  of the drill string  20 . Preferably, the spindle  92  connects to the inner member  40  in a torque transmitting hexagonal slip-fit connection. However, any other type of connection that permits the transmission of torque between the inner spindle  92  and the inner member  40  at the uphole end  75  of the drill string  20  may be used. Thus, the inner drive motor  90  transmits torque to inner members  40  and, ultimately, to the downhole tool  22  during boring and backreaming operations. 
   Therefore, it will be appreciated that in accordance with the present invention, the drill string  20  torque is transmitted independently across both the outer members  42  and the inner members  40  of the dual member drill string. However, if the inner members  40  and the outer members  42  are made to rotate together, the amount of torque, which may be transmitted by the drill string  20  to the downhole tool  22  would include the torque input capabilities of both the inner members and the outer members. 
   The present invention provides for a coupling assembly  100  capable of interconnecting the inner members  40  with the outer members  42  so as to enable the inner members and the outer members to rotate together. Additionally, the coupling assembly  100  is configured to receive torque input from both the outer members  42  and the inner members  40  to generate a resultant torque output for transmission to the downhole tool  22 . The coupling assembly  100  may transmit the torque output to the downhole tool  22  on an ongoing basis. Alternatively, the coupling assembly  100  may selectively transmit torque output to the downhole tool  22  as dictated by operating requirements. 
   With reference now to  FIGS. 4 and 5 , there is illustrated therein a coupling assembly  100  such as a connector sub  102  capable of coupling the outer member  42  with the inner member  40 . The outer member  42  and the inner member  40  are coupled in such a manner so as to transmit torque input on an ongoing basis between the outer member and the inner member of the pipe section  30  at the downhole end  76  ( FIG. 1 ) of the drill string  20 . A resultant torque output of the downhole tool  22  such as the backreamer shown in  FIG. 4  is thus generated. Preferably, the connector sub  102  is made of steel. However, other materials, such as alloys or composite materials may be used. 
   The connector sub  102  preferably comprises an uphole portion  104  and a downhole portion  106 . Preferably, the uphole portion  104  of the connector sub  102  is connected to the drill string  20  and the downhole portion  106  of the connector sub is connected to the downhole tool  22 . The uphole portion  104  of the connector sub  102  preferably comprises an inner drive connector  112  and an outer drive connector  114 . The connector sub  102  also is preferably characterized by a plurality of fluid passages  115  (shown in  FIG. 5 ) which permit fluid to pass through the connector sub. 
   The outer drive connector  114  is adapted to connect with the outer member  42  of the pipe section  30  at the downhole end  76  of the drill string  20 . In the preferred embodiment, the outer drive connector  114  comprises a threaded pin end. The outer drive connector  114  is connectable in torque transmitting engagement to the box end  52  of the outer member  42  of the pipe section  30 . 
   The inner drive connector  112  extends from the outer drive connector  114 . The inner drive connector  112  is adapted to connect with the inner member  40  of the pipe section  30  at the downhole end  76  of the drill string  20 . Preferably, the inner drive connector  112  comprises a box end for connection to the pin end  70  of the inner member  40 . More preferably, the inner drive connector  112  is geometrically shaped to receive the pin end  70  in a torque transmitting arrangement. 
   In the preferred embodiment, the inner drive connector  112  and outer drive connector  114  are torsionally fixed with respect to each other. More preferably, the connectors  112  and  114  are integrally formed as part of the connector sub  102 . However, the connectors  112  and  114  may also be torsionally connected in other ways, such as by being welded or pinned. 
   With continued reference to  FIGS. 4 and 5 , the downhole portion  106  of the connector sub  102  is adapted to be connected to the downhole tool  22 . Preferably, the downhole portion  106  comprises a box end  124  with tapered internal threads. The downhole tool  22  will preferably comprise a pin end  125  with tapered external threads. Thus, the box end  124  of the downhole portion  106  of the connector sub  102  may be threadably connected to the pin end  125  of the downhole tool  22 , and may thereby be removably attached to the downhole tool. 
   Alternative arrangements are anticipated for torsionally fixing the downhole portion  106  of the connector sub  102  to the downhole tool  22 . For example, the downhole tool  22  may be secured to the connector sub  102  with a pinning arrangement. In another embodiment, the downhole portion  106  of the connector sub  102  may be non-removably attached to the downhole tool  22 . For example, the connector sub  102  may be formed as an integral part of the downhole tool  22  as illustrated in  FIG. 6 . 
   With reference now to  FIG. 7 , there is shown therein a schematic representation of the drive system  18  for use with the present invention. The drive system  18  preferably will have the capability of driving the outer members  42  and the inner members  40  of the drill string  20  at the same general rotational speed. As discussed above in reference to  FIG. 3 , the drive system  18  comprises the outer drive motor  80  and the inner drive motor  82 . As shown in  FIG. 7 , the drive system  18  may also comprise a feedback control system  128 , an outer drive motor load sense pump  130 , and an inner drive motor load sense pump  131 . Preferably, the outer drive motor  80  and the inner drive motor  82  are driven in response to the pressure feedback from the inner drive motor and rotational speed feedback from the outer drive motor. 
   The inner drive motor  80  and the outer drive motor  82  drive rotation of the inner members  40  and outer members  42  respectively. The outer load sense pump  130  and the inner load sense pump  131  are operatively connected to the motors  80  and  82 , respectively, and regulate power input to the motors. Preferably, the motors  80  and  82  are hydraulic motors and the pumps  130  and  131  regulate flow of hydraulic fluid to the motors. This results in the hydraulic motors being driven at generally the same speed. Should one motor increase in speed slightly, the pressure in that loop will increase and the pump will react by reducing flow to that motor, thus regulating the motors to the same speed. 
   Alternatively, an automatic electronic control system  128  is operatively connected to the motors  80  and  82  and to the pumps  130  and  131 . The control system  128  receives information about the operation of the motors  80  and  82  and communicates control information to the pumps  130  and  131 . In the preferred embodiment, pressure sensors (not shown) on the inner drive motor  82  and outer drive motor  80  sense the rotation pressure of the inner and outer drive motors and sends corresponding pressure signals to the control system  128 . Speed pickup sensors (not shown) on the outer drive motor  80  and the inner drive motors  82  senses the rotational speed of the outer drive motor and sends a corresponding speed signal to the control system  128 . Alternatively, other sensors can be used to sense information about the operation of the motors  80  and  82 . 
   The control system  128  receives the signals from the sensors on the motors  80  and  82  to control operation of the motors based on predetermined operating characteristics for the output of the drive motors. The control system  128  can then send control signals to the pumps  130  and  131  to produce the desired operating characteristics. For example, it may be desirable for the outer drive motor  82  to function as the primary source of torque to the drill string  20  and downhole tool  22 . When the outer drive motor  82  reaches a maximum torque output, the inner drive motor  80  can provide additional torque output to the drill string  20  and downhole tool  22 . 
   Other combinations for the output of the drive motors may be desired. However, the overall average torque output of both the inner drive motor  82  and the outer drive motor  80  would still be maintained. The overall average torque output is maintained because the second drive functioning as the additional source of torque output of the downhole tool  22  will be a factor only when the drive motor functioning as the primary source of torque output of the downhole tool reaches its maximum torque output. 
   Alternatively, the fluid streams to the inner drive motor  82  and the outer drive motor  80  may be supplied from a common pressurized source in order to rotate both the inner drive motor and the outer drive motor at the same rotational speed. As a result, the outer members  42  and the inner members  40  would both rotate at the same rotational speed and the resultant torque output transmitted by the connector sub  102  is the combined torque input of the inner drive motor  82  and the outer drive motor  80 . It should be noted that the same rotational speed of both members  40  and  42  simply refers to an overall average rotational speed of both members. As a result, the rotational speed of the inner members  40  at any given instant may be different from the rotational speed of the outer members  42 . 
   In addition to transmitting torque from the inner members  40  and the outer members  42  to the downhole tool  22 , the connector sub  102  also prevents relative rotary motion between the outer members  42  and the inner members  40  of the drill string  20  at the downhole end  76  of the dual member drill string  20 . Therefore, use of the connector sub  102  has a positive effect on the performance of the downhole tool  22  as the downhole tool now has the combined torque to advance through the formation without “hanging up”. Additionally, wear on the pipe sections  30  is limited because as the downhole tool  22  has the torque needed to advance through rock, the “wind-up” of pipe sections in a long bore does not occur. 
   With reference now to  FIG. 8 , there is shown therein an alternative embodiment for the coupling assembly of the present invention. The coupling assembly  132  is adapted to selectively couple the inner member  40  and the outer member  42  of a dual member pipe section  30  at the downhole end  76  of the drill string  20 . The downhole end of the coupling assembly  132  is operatively connected to a downhole tool  22   a . As shown in  FIGS. 8 and 9 , the downhole tool  22   a  illustrated is a tricone bit used for drilling the borehole in rock. As with other embodiments of the present invention, the coupling assembly  132  may be used with any downhole tool  22  for use during boring or backreaming operations. 
   The coupling assembly  132  comprises an outer drive  134 , an inner drive  136 , and a clutch mechanism  138 . As will be discussed, when the clutch mechanism  138  is not engaged, the outer drive  134  and the inner drive  136  are disposed to rotate independently of one another. When the clutch mechanism  138  is engaged, the outer drive  134  and the inner drive  136  become coupled and rotate at the same speed. 
   Preferably, the outer drive  134  is generally cylindrical and defines a tubular open interior section  139 . The outer drive  134  comprises an outer member connector  140  at an uphole end of the drive. The outer member connector  140  is adapted to be connected to the outer member  42  of the pipe section  30  at the downhole end  76  of the drill string  20 . As shown in  FIG. 8 , the outer member connector  140  is a threaded pin end connector for mating with the threaded box end  52  of the outer member  42 . More preferably, the outside diameter of the outer member connector  140  is substantially similar to the outside diameter of the box end of the outer member  42 . 
   The inner drive  136  comprises a center shaft  142 , an inner member connector  144 , and a tool connector  146 . The center shaft  142  passes through, and is disposed generally coaxially within, the interior section  139  of the outer drive  134 . The center shaft  142  is retained independently rotatable within the interior section  139  by sets of seals  148  and bearings  150 . The center shaft  142  may also have one or more fluid passages  151  that permit fluid passed through the drill string  20  to be transmitted to the downhole tool  22   a.    
   The inner member connector  144  is attached at an uphole end of the center shaft  142  and extends beyond the outer member connector  140 . The inner member connector  144  is preferably attached to the center shaft  142  by pinning or welding. Alternatively, the connector  144  may be torsionally fixed to the center shaft  142  by other means, or by integrally forming the connector with the shaft. The inner member connector  144  is adapted to connect to the inner member  40  of the pipe section  30  at the downhole end  76  of the drill string  20 . As shown in  FIG. 8 , the inner member connector  144  is a box end connector adapted to receive the geometrically shaped pin end  70  of the inner member  40 . 
   The tool connector  146  is attached at the downhole end of the center shaft  142  and is adapted to connect to the downhole tool  22   a . As with the inner member connector  144 , the tool connector  146  may be attached in any manner that permits transmission of torque from the shaft  142  to the tool connector. In the preferred embodiment shown in  FIG. 8 , the tool connector  146  includes a cylindrical uphole end  152  sized to receive the center shaft  142 . A fastening nut  154  is threaded onto the shaft  142 , securing the tool connector  146  to the shaft. A spline connection #? torsionally connects  146  to inner shaft  142 . Alternatively, the tool connector  146  may be integrally formed with the central shaft  142 , or otherwise fixed to the shaft. 
   The tool connector  146  is connected to the downhole tool  22   a  in a torque transmitting arrangement. Preferably, the tool connector  146  comprises a threaded box end for receiving a threaded pin end connector  156  of the downhole tool  22   a . More preferably, the outer diameter of the tool connector  146  is substantially the same as the diameter of the outer drive  134 . However, other arrangements for securing the downhole tool  22   a  to the tool connector  146  are anticipated. For example, the downhole tool  22   a  could be integrally formed with the inner drive  136  or with the coupling assembly  132 . 
   The structure of the coupling assembly  132  and the relationship between the outer drive  134  and the inner drive  136  sometimes permit the drives to rotate independently of each other. Thus, as the inner members  40  and outer members  42  of the drill string  20  are rotated, the inner drive  136  and the outer drive  134  of the coupling assembly  132  rotate correspondingly. As will now be described, the clutch mechanism  138  of the coupling assembly  132  allows for the selective coupling of drives  134  and  136 . The selective coupling of the drives  134  and  136  permits transmission of torque from both the drives and the drill string  20  to the downhole tool  22   a.    
   With continued reference to  FIG. 8 , the clutch mechanism  138  is preferably positioned within the interior section  139  of the outer drive  134  and around the center shaft  142  of the inner drive  136 . In the preferred embodiment, the clutch mechanism  138  is an over-running clutch, but any mechanism permitting selective coupling of the outer drive  134  and the inner drive  136  would also be suitable for use with the coupling assembly  132 . 
   Preferably, the clutch mechanism  138  is press fit or otherwise secured to the wall of the interior section  139  such that no rotational slip is allowed between the outer drive  134  and the clutch mechanism. One skilled in the art will appreciate radial forces will keep the clutch  138  from rotating relative to the outer drive  134 . Other matingly engaging mechanisms, such as splines or keyed arrangements, may also be used to prevent rotational slip between the clutch mechanism  138  and the drives  134  and  136 . 
   As previously discussed, the structure of the coupling assembly  132  permits the outer drive  134  and the inner drive  136  to rotate independently of each other, and the clutch mechanism  138  permits the selective coupling of the drives. When the rotational speed of the outer drive  134  is less than the rotational speed of the inner drive  136 , the clutch  138  does not engage and the inner drive  136  and the outer drive  134  rotate independently of each other. As a result, the downhole tool  22   a  receives torque input from inner drive  136 , and, therefore, only from the inner members  40  of the drill string  20 . 
   When the outer drive  134  rotates at a speed substantially equal to or greater than the rotational speed of the inner drive  136 , the clutch mechanism  138  engages. When engaged, the clutch  138  will cause the inner drive  136  to rotate at substantially the same speed as the outer drive  134 . Consequently, the clutch  138  effectively couples the inner members  40  and the outer members  42  of the drill string  20 . As a result, the clutch  138  and the coupling assembly  132  permit torque input from the outer members  42  through the outer drive  134  to be transmitted to the downhole tool  22   a.    
   If the speed of the outer drive  134  is decreased to be less than the rotational speed of the inner drive  136 , or if inner drive speed is increased to be greater than that of the outer drive, the clutch mechanism  138  disengages. At this point, the clutch  138  slips or disengages the inner drive  136  from the outer drive  134 . The inner drive  136 , and the inner members  40  of the drill string  20 , then, are again the sole source of torque input for the downhole tool  22   a.    
   One skilled in the art will appreciate that when boring on a straight path use of the coupling assembly  132  with the clutch  138  in the engaged mode can provide significant advantages. Thus, with use of the coupling assembly  132  of the present invention, bores of longer distance may be bored as wind-up of the inner members  40  of the drill string  20  is limited in comparison to drill strings with uncoupled inner and outer members. This is because both the outer drive  134  and the inner drive  136  provide increased torque output to the downhole tool  22   a . Use of the coupling assembly  132  also allows for use of large downhole tools because of the added torque that can be employed with the larger diameter outer members  42  of the drill string  20 . 
   With reference now to  FIG. 10 , there is shown therein an alternative embodiment for a clutch mechanism  160  for use with the coupling assembly  132  shown in  FIG. 8 . Preferably, the clutch mechanism shown in  FIG. 10  comprises a pawl clutch  160 . As with the clutch mechanism  132  of  FIG. 8 , the pawl clutch  160  is positioned between the inner drive  136  and the outer drive  134 . As shown in  FIG. 10 , the outer drive  134  may include a first key way  162 , formed in the wall of the interior section  139  of the outer drive  134 . The first key way  162  matingly receives a first key pin  164  on an outer wall of the clutch  160 . Similarly, a second key way  166  is formed in the center shaft  142  of the inner drive  136 . The second key way  166  matingly receives a second key pin  168  on an inner wall of the clutch  160 . The key way  166  and key pin  168  prevent relative rotation between the inner drive  136  and the inner wall of the clutch  160 . The key way  162  and key pin  164  prevent relative rotation between the outer drive  134  and the outer wall of the clutch  160 . The pawl clutch  160  of  FIG. 10  will operate in an engaged mode and a disengaged mode similar to the clutch mechanism  138  shown in  FIGS. 8 and 9 . 
   Turning now to  FIGS. 11 and 12 , there is shown therein another embodiment for a coupling assembly  200  and drive system  250  built in accordance with the present invention. The coupling assembly  200  will again serve to allow the inner members  40  and outer members  42  of the drill string  20  to be selectively coupled together. 
   The coupling assembly  200  comprises an outer drive or housing  202 , an inner drive  204 , and a locking mechanism  206 . Preferably, the housing  202  is cylindrical and defines an interior chamber  208 . An uphole end  210  of the housing  202  is adapted to be connected to the outer member  42  of the pipe section  30  at the downhole end  76  of the drill string  20 . As shown in  FIG. 11 , the uphole end  210  of the housing  202  comprises a threaded pin end connection  212  for connecting with the box end  54  of the outer member  42 . A downhole end  214  of the housing  202  is open ended to allow the inner drive  204  to pass through the housing, as yet to be described. 
   The inner drive  204  comprises a center shaft  220 , an inner member connector  222 , and a tool connector  224 . The center shaft  220  passes through, and is disposed generally coaxially within, the housing  202 . Preferably, the shaft  220  is cylindrical in shape. The center shaft  220  may be retained independently rotatable within the housing  202  by sets of seal and bearing arrangements (not shown). 
   The inner member connector  222  is attached at an uphole end of the center shaft  220  and extends beyond the pin end connection  212  of the housing  202 . The inner member connector  222  is preferably attached to the center shaft  220  by pinning or welding. Alternatively, the connector  222  may be torsionally fixed to the center shaft  220  by other means, or by integrally forming the connector with the shaft. The inner member connector  222  is adapted to connect to the inner member  40  of the pipe section  30  at the downhole end  76  of the drill string  20 . As shown in  FIG. 11 , the inner member connector  222  is a box end connector adapted to receive the geometrically shaped pin end  70  of the inner member  40 . 
   The tool connector  224  is attached at the downhole end of the center shaft  220  and is adapted to connect to the downhole tool  22   a . As with the inner member connector  222 , the tool connector  224  may be connectable in any manner that permits transmission of torque from the shaft  220  to the tool connector. Preferably, the connection between the shaft  220  and the connector  224  will be such that permits axial movement of the shaft relative to the connector, for purposes yet to be described. As shown in  FIG. 11 , the tool connector  224  includes a cylindrical uphole end  225  sized to receive a downhole end  226  of the center shaft  220 . More preferably, the center shaft  220  and the uphole end  225  of the connector  224  are geometrically shaped for connection in a pin/box torque transmitting arrangement. As illustrated, the downhole end  226  of the center shaft  220  comprises a pin end with splines for receipt in a matingly splined box end  225  of the connector  224 . 
   More preferably, the tool connector  224  comprises a biasing spring  227  for urging the center shaft  220  axially uphole relative to the connector. Other biasing mechanisms are anticipated for use with the coupling assembly  200 , provided the center shaft  220  and the tool connector  224  remain torsionally fixed regardless of the axial movement of the shaft. 
   A downhole end  228  of the tool connector  244  is adapted to be connected to the downhole tool  22   a . The downhole end  228  of the tool connector  224  may be connected to the downhole tool  22   a  in any torque transmitting arrangement. Preferably, the downhole end  228  comprises a threaded box end  230  for receiving a threaded pin end connector of the downhole tool  22   a . More preferably, the outer diameter of the downhole end  228  is substantially the same as the diameter of the housing  202 . However, other arrangements for securing the downhole tool  22   a  to the tool connector  224  are anticipated. For example, the downhole tool  22   a  could be integrally formed with the inner drive  204  or with the coupling assembly  200 . Preferably, the tool connector  224  also comprises fluid ports  229  to allow drilling fluid to pass through the connector to the downhole tool  22   a.    
   The structure of the coupling assembly  200  and the relationship between the housing  202  and the inner drive  204  permit the drive to rotate independently of housing. Thus, as the inner members  40  and outer members  42  are rotated, the inner drive  204  and the housing  202  of the coupling assembly  200  rotate independently. As will now be described, the locking mechanism  206  of the coupling assembly  200  allows for the selective coupling of drive  204  and the housing  202 . The selective coupling of the drive  204  and the housing  202  permits transmission of torque from the inner members  40  and the outer members  42  of the drill string  20  to the downhole tool  22   a.    
   With continued reference to  FIG. 11 , the locking mechanism  206  is preferably cylindrical in shape, positioned within the housing  202  and around the center shaft  220 . The locking mechanism  206  may have fluid passages  231  that allow drilling fluid to pass through the coupling assembly  200 . Preferably, the locking mechanism  206  is press fit or otherwise secured to the wall of the housing  202  such that no rotational slip is allowed between the housing and the locking mechanism. Other matingly engaging mechanisms, such as splines or keyed arrangements, may be used to prevent rotational slip between the locking mechanism  206  and the housing  202 . 
   The locking mechanism  206  defines an axial opening  232  through which the center shaft  220  passes and may include one or more fluid passages  233  to allow drilling fluid to flow through and past the locking mechanism. Preferably, the opening  232  defines an inner surface  234  adapted to engage the center shaft  220  in a locked and an unlocked mode. More preferably, the surface  234  of the opening  232  defines a spline arrangement for engaging a corresponding set of splines  236  fixed around the circumference of a portion of the center shaft  220 . The splines  236  on the center shaft  220  will engage the surface  234  of the opening  232  when the shaft is axially advanced downhole in a manner yet to be described. Alternatively, the splines  236  can be arranged so that axial movement in either the uphole or downhole direction will result in a locked mode for the center shaft  220 . 
   When the splines  236  on the center shaft  220  are not engaging the surface  234 , the coupling assembly  200  operates in the unlocked mode and the housing  202  and the inner drive  204  can rotate independently. In the unlocked mode, only the inner drive  204 , and consequently the inner members  40  of the drill string  20 , drive rotation of the downhole tool  22   a . When the splines  236  engage the surface  234 , the coupling assembly  200  operates in the locked mode and the housing  202  and the inner drive  204  rotate together. In the locked mode, then, the rotational torque from both the inner members  40  and the outer members  42  of the drill string  20  will be transmitted to the downhole tool  22   a.    
     FIG. 12  illustrates the drive system  250  modified for use with the coupling assembly  200 . The drive system  250  of the present embodiment is preferably a dual rotary drive system, similar to the drive system shown in  FIGS. 1 ,  3 , and  7 , for driving the inner members  40  and outer members  42  of the drill string  20 . The drive system  250  comprises an inner member drive motor  252  and an outer member drive motor  254 . The inner member drive motor  252  drives movement of an inner drive shaft  256 . The outer member drive motor  254  drives movement of the outer drive shaft  258 . 
   The drive system  250  also comprises an axial translating assembly  260  operatively connected to the inner drive motor  252 . The translating assembly  260  preferably is adapted to axially or longitudinally move the inner drive motor  252  and the inner drive shaft  256  relative to the outer drive shaft  258 . The translating assembly  260  preferably comprises a hydraulic piston and cylinder assembly  262 . More preferably, the piston and cylinder assembly  262  is operatively attached to the inner drive motor  252  and secured to the carriage  28  of the drilling machine  12  (see  FIGS. 1 and 3 ). As shown in  FIG. 12 , the cylinder assembly  262  is disposed to move the inner drive motor  252  and the inner drive shaft  256  relative to the carriage  28  and, consequently, the outer drive shaft  258 . 
   The disposition of and connections for the piston and cylinder assembly  262  are intended to be exemplary only. Other structures and operations are anticipated for the translating assembly  260 . For example, the piston and cylinder assembly  262  could be operatively connected to the inner drive shaft  256  or could comprise a gear and chain mechanism. Any structure permitting axial movement of the inner drive shaft  256  relative to the outer drive shaft  258  would be appropriate for use with the translating assembly  260  of the present embodiment. It is also anticipated that the translating assembly  260  may be actuated by the control system  14  or by an operator as needed. 
   The assembly  262  will preferably operate between a standard position and a forward position. Alternatively, the translating assembly  262  may operate between a plurality of positions such that the inner drive shaft  256  and the outer drive shaft  258  can be axially moved relative to each other. In the standard position of the present embodiment, the piston and cylinder assembly  262  is not extended and the drill string  20  is used in the conventional manner, with the inner members  40  and the outer members  42  of the drill string rotating independently of each other. 
   In the forward position as depicted in  FIG. 12 , the piston and cylinder assembly  262  extends, advancing the inner drive motor  252  and the inner drive shaft  256  forward. The forward movement of the inner drive shaft  256  results in the inner members  40  also moving forward, relative to the outer members  42 . The movement of the inner members  40  results in a forward movement of the center shaft  220  of the coupling assembly  200 . Forward movement of the center shaft  220  permits the splines  236  on the shaft to engage the inner surface  234  of the locking mechanism  206 . It will be appreciated that the center shaft  220 , and thus the inner members  40  of the drill string  20 , may be rotated slightly to permit the splines  236  to matingly align with the inner surface  234 . As previously discussed, when the center shaft  220  engages the locking mechanism  206 , the inner members  40  and the outer members  42  of the drill string  20  will be coupled and the torque from both the inner drive shaft  256  and the outer drive shaft  258  can be transmitted to the downhole tool  22   a.    
   Another preferred coupling assembly  300  for use with the drive system  250  (shown in  FIG. 12 ) is illustrated in  FIG. 13   a . The coupling assembly  300  is adapted to be connected, in a manner yet to be described, to the inner member  40  and the outer member  42  of the pipe section  30  at the downhole end  76  of the drill string  20  (see, for example,  FIGS. 5 and 8 ). Preferably, an opposing end of the coupling assembly  300  is connected to a downhole tool  22   b . The coupling assembly  300  of the embodiment shown in  FIG. 13   a  will allow the rotation of the downhole tool  22   b  to be controlled by only the inner members  40  of the drill string  20 , or by only the outer members  42  of the drill string, or by both the inner members and the outer members. 
   Preferably, the coupling assembly  300  comprises a housing  302 , an inner drive shaft  304 , a tool adapter  306  and a locking assembly  308 . The housing  302  is preferably cylindrical and has an uphole end adapted to be connected to the outer member  42  at the downhole end  76  of the drill string  20 . As with previous embodiments, the housing  302  preferably comprises a threaded pin end connection  310  for connection to the outer member  42  of the drill string  20 . The housing  302  further defines an interior chamber, preferably comprising a collar  312 . More preferably, the collar  312  comprises an interior set of splines  313  for purposes yet to be described. 
   The inner drive shaft  304  is preferably cylindrical and disposed generally coaxially within the housing  302 . An uphole end of the inner drive shaft  304  is adapted to be connected to the inner member  40  at the downhole end  76  of the drill string  20 . As with previous embodiments, the drive shaft  304  comprises a box end connection  314  for receiving the geometrically shaped pin end  70  of the inner member  40 . The drive shaft  304  preferably extends through the housing  302  to or just beyond the collar  312 . The drive shaft  304  may also comprise fluid ports (not shown) to facilitate the flow of drilling fluid from the drill string  20  to the downhole tool  22   b.    
   The tool adapter  306  is disposed at an open downhole end  315  of the housing  302  and comprises a cylindrical open ended chamber  316  and a tool connector  318 . Preferably, the cylindrical open ended chamber  316  is sized to be received in the open end  315  of the housing. More preferably, the chamber  316  of the adapter  306  is rotatably supported in the housing  302  by a seal and bearing arrangement  320 . The chamber  316  preferably comprises a collared end  321  that is aligned with the collar  312  of the housing  302 . More preferably, the collared end  321  of the chamber  316  comprises an interior set of splines  323  for a purpose yet to be described. 
   The tool connector  318  is configured to connect to the downhole tool  22   b . Preferably, the downhole tool  22   b  is threaded onto the connector  318 . However, alternative arrangements are anticipated, such as using pins or screws to secure the tool  22   b  to the connector  318 . Alternatively, the downhole tool  22   b  may be integrally formed with the tool adapter  306  and the coupling assembly  300 . 
   The locking assembly  308  functions to couple the tool adapter  306  and the downhole tool  22   b  to the inner drive shaft  304 , to the housing  302 , or to both. The assembly preferably comprises a forward spline arrangement  322 , an aft spline arrangement  324 , and a biasing member  326 . The forward spline arrangement  322  is secured to the inner drive shaft  304 , preferably at a downhole end of the shaft. The aft spline arrangement  324  is disposed around and rotatably supported by the inner drive shaft  304 , adjacent to the forward spline arrangement  322 . Preferably, the aft spline arrangement  324  is supported around the inner drive shaft  304  by a bearing arrangement  328 . 
   In the preferred embodiment the biasing member  326  is a spring, although other biasing mechanisms could also be used. The biasing member  326  is positioned in the chamber  316  of the tool adapter  306 , and exerts pressure on the forward spline arrangement  322  and the inner drive shaft  304  in an uphole direction. The biasing member  326 , in conjunction with the drive system  250 , serves to position the spline arrangements  322  and  324  in preferably three operating positions within the coupling assembly  300 . 
   In a first position, shown in  FIG. 13   a , the piston and cylinder assembly  262  of the drive system  250  (see  FIG. 12 ) is retracted, allowing the spring of the biasing member  326  to extend and force the spline arrangements  322  and  324  to move in the uphole direction. In this first position, the forward spline arrangement  322  is aligned and in operative contact with the splines  313  on both the collar  312  of the housing  302  and the splines  323  on the collared end  321  of the tool adapter  306 . The housing  302 , then, is torsionally locked first to the forward spline  322  and the inner drive shaft  304 , and, as a result, to the tool adapter  306  and the downhole tool  22   b . The coupling assembly  300  thus allows the drilling machine  12  to operate such that the torque from both the inner drive shaft  256  and the outer drive shaft  258  of the drive system  250  are transferred to the downhole tool  22   b.    
   In a second position, shown in  FIG. 13   b , the piston and cylinder assembly  262  (see  FIG. 12 ) would be partially extended, forcing the inner members  40  of the drill string  20  to axially advance with respect to the outer members  42 . The movement of the inner members  40  also causes the inner drive shaft  304  and the spline arrangements  322  and  324  to advance, partially compressing the biasing member  326 . In this second position, the forward spline arrangement  322  is aligned with the splines  323  on the collared end  321  of the tool adapter  306 . The aft spline arrangement  324  is aligned with the splines  313  on the collar  312  of the housing  302 . Only the inner drive shaft  304 , and consequently the inner members  40  of the drill string  20 , is operatively connected in torque transmitting engagement to the downhole tool  22   b . Rotation of the housing  302  does not result in any torque being transmitted to the downhole tool  22   b  because the bearing arrangement  328  with the aft spline  324  and the bearing arrangement  320  around the tool adapter  306  permit the housing to rotate independently of the downhole tool  22   b . The coupling assembly  300  thus allows the drilling machine  12  to operate in a conventional mode where rotation of the downhole tool  22   b  is controlled exclusively by the inner drive shaft  256  of the drive system  250 . 
   In a third position, shown in  FIG. 13   c , the piston and cylinder assembly  262  (see  FIG. 12 ) is fully extended, forcing the inner members  40  of the drill string  20  to axially advance with respect to the outer members  42 . The movement of the inner members  40  also causes the inner drive shaft  304  and the spline arrangements  322  and  324  to axially advance, further compressing the biasing member  326 . In this third position, the aft spline arrangement  324  is aligned with the splines  323  on the collared end  321  of the tool adapter  306  and with the splines  313  on the collar  312  of the housing  302 . This arrangement permits the housing  302 , and consequently the outer members  40  of the drill string  20 , to be operatively connected in torque transmitting engagement to the downhole tool  22   b . Rotation of the inner drive shaft  304  does not result in any torque being transmitted to the downhole tool  22   b  because the forward spline arrangement  322  is not engaged with either spline  313  or  323 , and bearing arrangement  328  with the aft spline  324  permits the inner drive shaft to rotate independently of the downhole tool  22   b . The coupling assembly  300  thus allows the drilling machine  12  to operate such that rotation of the downhole tool  22   b  is controlled exclusively by the outer drive shaft  256  of the drive system  250 . 
   With reference now to  FIG. 14 , there is shown therein an alternative embodiment for a coupling assembly  440  constructed in accordance with the present invention. The coupling assembly  440  is connected to the inner member  40  and the outer member  42  of the pipe section  30  at the downhole end  76  of the drill string  20 . The coupling assembly  440  is preferably housed within a downhole tool  450 , shown in  FIG. 14  as a backreamer for use during a backreaming operation. The downhole tool  450  comprises an outer wall  452  defining an interior tool chamber  454 . The outer wall  452  may be configured to have a plurality of cutting elements  456  such as carbide cutters, cutting teeth, etc. interspersed on its surface. The interior tool chamber  454  houses the coupling assembly  440  and facilitates connection to the drill string  20 . 
   The coupling assembly  440  comprises an outer drive shaft  462 , an inner drive shaft  464  and a gear mechanism  466 . Preferably, the gear mechanism  466  is a planetary gear system and is adapted to operatively connect the outer drive shaft  462  and the inner drive shaft  464  to the downhole tool  450  individually or in combination. The planetary gear system  466  provides a gear reduction for the downhole tool  450 . With gear reduction, the output speed of the downhole tool  450  may be relatively low compared to the output speed of the dual member drill string  20 . However, high torque may be transmitted to the downhole tool  450  by the drill string  20  with a planetary gear system  466 . 
   The outer drive shaft  462  has an uphole end adapted to be connected to the outer member  42  at the downhole end  76  of the drill string  20 . Preferably, the uphole end comprises a threaded pin end connector  468  for connection to the box end of the outer member  42 . The outer drive shaft  462  also comprises a gear connector  470  adapted to connect with the planetary gear system  466 . 
   The inner drive shaft  464  is preferably cylindrical and is disposed generally coaxially within the outer drive shaft  462 . An uphole end of the inner drive shaft  464  is adapted to be connected to the inner member  40  at the downhole end  76  of the drill string  20 . Preferably, the inner drive shaft  464  comprises a box end connector  472  extending beyond the pin end connector  468  of the outer drive shaft  462  for receiving the pin end of the inner member  40  in torque transmitting engagement. The inner drive shaft  464  is preferably retained within the outer drive shaft  462  by bearings  474  and extends into the interior tool chamber  454 . The bearings  474  permit the inner drive shaft  464  to rotate independently of the outer drive shaft  462 . 
   The planetary gear system  466  is disposed within the interior tool chamber  454  and is operatively connected to the outer drive shaft  462  and the inner drive shaft  464 . Preferably, the planetary gear system  466  comprises four major elements: an outer ring gear  500 , a center sun gear  502 , a carrier  504  and one or more planet gears  506 . The planet gears  506  are situated between the sun gear  502  and the ring gear  500  and are held by the planet carrier  504 . Additionally, the sun gear  502  and the planet gears  506  may be held together by a connecting member such as a band (not shown) that is connected to a central axis of the sun gear and a central axis of the ring gear. The connecting member functions to keep the sun gear  502  and the planet gear  506  in the same plane. 
   The sun gear  502  preferably is connected to the inner drive shaft  462  in torque-transmitting engagement. More preferably, the sun gear  502  is disposed around the inner drive shaft  462  at the end of drive shaft extending into the tool chamber  454 . The sun gear  502  will rotate with the inner drive shaft  462  at the same rotational speed as the inner drive shaft. 
   The ring gear  500  is operatively connected to the gear connector  470  of the outer drive shaft  462 . Preferably, the gear connector  470  comprises geared teeth on an inner surface of the connector. A corresponding arrangement of teeth on an outer circumference of the ring gear  500  permits the ring gear to rotate with the outer drive shaft  462 . 
   The carrier  504  is secured to the downhole tool  450 . Preferably, the carrier  504  is bolted or screwed to a plate  508  in the interior tool chamber  454  of the downhole tool  450 . An opposing end of the planet carrier  504  forms a recess  513  that receives the front portion  482  of the inner drive shaft  462  supported on the bearings  480 . The carrier  504  is connected to a central axis of each planet gear  506 . Preferably, a plurality of shafts  516  are secured to the carrier  504  and pass through the central axis of the planet gears  506 . The shafts  516  are supported in each of the planet gears  506  by bearings (not shown), permitting independent rotation of the planet gears relative to the carrier. 
   With continued reference to  FIG. 14 , the planetary gear system  466  operates with the sun gear  502  functioning as the driver or input gear of the planetary gear system. Each planet gear  506  rotates about its central axis and is driven by the sun gear  502 . Additionally, the ring gear  500  intermeshes with the planet gears  506  through the set of gear teeth on the inner circumference of the ring gear and an outer circumference of the planet gear to cause the planet gears to travel around the sun gear  502 . The carrier  504 , by its attachment to the axis of the planet gears  506 , will rotate with the planet gears as they rotate around the sun gear  502 . In the preferred configuration of the planetary gear mechanism  466  shown in  FIG. 14 , the carrier  504  is the output device for taking power out of the planetary gear system  466 . 
   In operation, the planetary gear system  466  preferably provides increased torque output to the downhole tool  450  as the speed of the inner members  40  and the outer members  42  of the drill string  20  are varied. As the inner members  40  are rotated, the sun gear  502 , connected to the inner drive shaft  464 , will also rotate with the inner members. The rotation of the sun gear  502  drives the planet gears  506 . The planet gears  506  will be driven around the sun gear  502 , traveling on the intermeshing gear teeth  508  between the ring gear  500  and planet gears  506 . The rotation of the planet gears  506  will drive the carrier  504  to rotate in a direction opposite to that of the sun gear  502 . The operation of the planetary gear system  466  will cause the downhole tool  450  to rotate at a reduced speed and increased torque in comparison to the inner drive shaft  462 . Thus the downhole tool  450  attached to the carrier  504  is able to excavate the soil at an increased torque output as compared to a downhole tool that is not connected to a planetary gear system. 
   One skilled in the art will appreciate the resultant torque output can be affected by changing the number of teeth in the planetary gear system  466  and by altering the relative rotation rate of both the drive shafts  462  and  464 . Additionally, different gear ratios using the planetary gear system  466  may also be produced by varying which gear is used as the input, which gear is used as the output and which gear is held still. 
   Further, it may be noted that while the above discussion describes the utilization of the planetary gear system  466  for providing increased torque output for transmission to downhole tool  450  such as a backreamer during backreaming operations, the same process may be adapted to be utilized for other downhole tools, such as tricone bits during drilling operations. Additionally, other gear systems or sets, such as bevel gear drives, planocentric drive, mutating gears, harmonic drives, and spur gears, among others, may be used to operate similarly to the planetary gear system  466 . 
   With reference now to  FIGS. 15 and 16 , there is shown therein an alternative for the coupling assembly  540  of the present invention for use with a downhole tool, such as a boring tool  542 . Preferably, the coupling assembly  540  comprises an outer drive  544 , an inner drive  546 , and a planetary gear system  548 . More preferably, the gear system  548  is similar to the gear mechanism  466  described for use with a backreamer tool in  FIG. 14 , and is adapted to operatively connect the outer drive  544  and the inner drive  546  to the boring tool  542  individually or in combination. 
   The outer drive  544  has an uphole end  550  adapted to be connected to the outer member  42  of the drill string  20 . The outer drive  544  also comprises a gear connector  552  adapted to connect with the gear system  548 . The inner drive  546  is preferably disposed generally coaxially within the outer drive  544  and has an uphole end adapted to be connected to the inner member  40  of the drill string  20 . A series of bearings and seals  554  are preferably used to retain the inner drive  546  within the outer drive  544 , such that the inner drive may rotate independently of the outer drive. 
   With continued reference to  FIG. 16  and with reference to  FIG. 17 , the gear system  548  of the present embodiment comprises an outer ring gear  556 , a center sun gear  558 , a carrier  560 , and at least one planet gear  562 . Preferably, the ring gear  556  is connected to the gear connector  552  of the outer drive  544  for rotational movement with the outer drive. The sun gear  558  is preferably connected to the inner drive  546  in torque-transmitting engagement. More preferably, the sun gear  558  is disposed around the inner drive  546  and will rotate with the inner drive. 
   The planet gears  562  are held by the carrier  560  and are preferably situated between the ring gear  556  and the sun gear  558 . Additional sets of seals and bearings  564  permit the carrier  560  to be retained and rotate with respect to the outer drive  544  and the inner drive  546  The carrier  560  is also adapted to be connected to the boring tool  542 . As shown in  FIG. 16 , the carrier  560  has a downhole end  566  for threaded connection to the boring tool  542 . Alternatively, the carrier  560  and the boring tool  542  could be integrally formed or operatively connected by other means. 
   The present invention thus provides a mechanism to control and improve performance of the downhole tool, when employing a dual member drill string. The inner and outer members of the dual member drill string are connected to the downhole tool through a coupling assembly at a downhole end of the drill string. The coupling assembly is adapted to receive torque input from both the inner members and the outer members of the dual member drill string to generate a resultant increased torque output of the downhole tool. This enables the downhole tool to advance through the rock face without “hanging up” against the rock race. Additionally, wear on the dual member drill string is limited because as the downhole tool has the torque needed to advance through the rock face, “wind-up” of the pipe sections in a long bore does not occur. 
   It is clear that the present invention is well adapted to attain the ends and advantages mentioned as well as those inherent therein. While the presently preferred embodiments of the invention have been described for purposes of this disclosure, it will be understood that numerous changes may be made in the combination and arrangement of the various parts, elements and procedures described herein without departing from the spirit and scope of the invention as defined in the following claims.