Dual rod directional drilling system

1. A dual rod drill rod for a dual rod drilling system having a drill string extending from an above ground drilling machine to a downhole drilling tool, the drill string being formed by a plurality of dual rod drill rods joined end-to-end, the dual rod drill rod comprising:

a tubular outer rod including internal threads on a first end having a first shoulder, and external threads on a second, opposite end having a second shoulder;

an inner rod within the tubular outer rod, the inner rod including

a first end having

a torque-carrying section; and

a flow collar having an outer rod interfacing surface; and

a second end having

a torque-carrying section; and

a groove;

an inner rod coupling having an inner bore and defining a longitudinal axis, the inner rod coupling including

a first end in which the inner bore has a non-circular profile receiving the torque-carrying section of the second end of the inner rod;

a second end in which the inner bore has a non-circular profile configured to mate with a torque-carrying section of a first end of an adjacent inner rod of the drill string;

a cross aperture perpendicular to the longitudinal axis and offset to a side of the longitudinal axis; and

a retention pin held within the cross aperture of the inner rod coupling and positioned within the groove in the second end of the inner rod; and

a flow sleeve on the second end of the inner rod and having an outer rod interfacing surface;

wherein the inner rod is retained within the outer rod by the flow collar interfacing with the first shoulder of the outer rod, and by the flow sleeve interfacing with the second shoulder of the outer rod.

2. The dual rod drill rod of claim 1 , wherein the groove has a width larger than a width of the retention pin, and wherein the inner rod coupling and the inner rod are axially movable with respect to one another.

3. The dual rod drill rod of claim 1 , wherein the flow sleeve engages the inner rod coupling, and includes at least one fluid flow passage.

4. The dual rod drill rod of claim 3 , wherein the flow sleeve is positioned around an exterior surface of the inner rod coupling.

5. The dual rod drill rod of claim 3 , wherein the flow sleeve includes a plurality of fluid flow passages positioned circumferentially about an exterior surface of the flow sleeve.

6. The dual rod drill rod of claim 1 , wherein the inner rod includes an inner bore aligned with the inner bore of the inner rod coupling, and wherein the retention pin does not extend into the inner bore of the inner rod.

7. The dual rod drill rod of claim 1 , wherein an annular fluid flow passage is defined between the inner rod and the outer rod.

8. The dual rod drill rod of claim 7 , wherein the flow collar includes at least one peripheral fluid passage that allows fluid flow therethrough, wherein the at least one peripheral fluid passage of the flow collar is positioned within the annular fluid flow passage between the inner rod and the outer rod.

9. The dual rod drill rod of claim 1 , wherein the outer rod interfacing surface of the flow collar is a continuous annular surface configured to contact the first shoulder, and wherein the outer rod interfacing surface of the flow sleeve is a continuous annular surface configured to contact the second shoulder.

10. The dual rod drill rod of claim 1 , wherein an axis of the cross aperture is nonintersecting with the longitudinal axis.

11. The dual rod drill rod of claim 1 , wherein the groove extends circumferentially about an entire circumference of the inner rod.

12. The dual rod drill rod of claim 1 , wherein the cross aperture is a first cross aperture, the inner rod coupling further comprising a second cross aperture that is perpendicular to the longitudinal axis and spaced on an opposite side of the longitudinal axis from the first cross aperture; and

wherein the retention pin is a first retention pin positioned within the first cross aperture, the dual rod drill rod further comprising a second retention pin positioned within the second cross aperture of the inner rod coupling and within the groove of the inner rod.

13. The dual rod drill rod of claim 1 , wherein the second end of the inner rod further comprises a non-torque-carrying section having a cross-sectional width smaller than a cross-sectional width of the inner bore at the first end of the inner rod coupling, and wherein the groove is positioned between the non-torque-carrying section and the torque-carrying section of second end of the inner rod.

14. The dual rod drill rod of claim 1 , wherein the groove is spaced from an end face of the inner rod and adjacent the torque-carrying section of the second end.

15. The dual rod drill rod of claim 1 , wherein the groove is defined in part by a radially-innermost groove surface that is cylindrical and co-axial with the longitudinal axis of the inner rod coupling.

16. A dual rod drill rod for a dual rod drilling system having a drill string extending from an above ground drilling machine to a downhole drilling tool, the drill string being formed by a plurality of dual rod drill rods joined end-to-end, the dual rod drill rod comprising:

a tubular outer rod including internal threads on a first end having a first shoulder, and external threads on a second, opposite end having a second shoulder;

an inner rod within the tubular outer rod such that an annular fluid flow passage is defined between the inner rod and the outer rod, the inner rod including

a first end having

a torque-carrying section; and

a flow collar having an outer rod interfacing surface, and having and at least one fluid flow passage; and

a second end having

a torque-carrying section; and

a groove that extends circumferentially about an entire circumference of the inner rod, the groove being spaced from an end face of the inner rod and adjacent the torque-carrying section of the second end;

an inner rod coupling having an inner bore and defining a longitudinal axis, the inner rod coupling including

a first end in which the inner bore has a non-circular profile receiving the torque-carrying section of the second end of the inner rod;

a second end in which the inner bore has a non-circular profile configured to mate with a torque-carrying section of a first end of an adjacent inner rod of the drill string;

a first cross aperture perpendicular to the longitudinal axis and offset to a first side of the longitudinal axis;

a second cross aperture perpendicular to the longitudinal axis and offset to a second side of the longitudinal axis;

a first retention pin held within the first cross aperture of the inner rod coupling and positioned within the groove in the second end of the inner rod; and

a second retention pin held within the second cross aperture of the inner rod coupling and positioned within the groove in the second end of the inner rod; and

a flow sleeve on the second end of the inner rod, the flow sleeve having an outer rod interfacing surface and at least one fluid flow passage;

wherein the inner rod is retained within the outer rod by the flow collar interfacing with the first shoulder of the outer rod and by the flow sleeve interfacing with the second shoulder of the outer rod.

17. The dual rod drill rod of claim 16 , wherein the groove is defined in part by a radially-innermost groove surface that is cylindrical and co-axial with the longitudinal axis of the inner rod coupling.

18. The dual rod drill rod of claim 16 , wherein the outer rod interfacing surface of the flow collar is a continuous annular surface configured to contact the first shoulder, and wherein the outer rod interfacing surface of the flow sleeve is a continuous annular surface configured to contact the second shoulder.

19. The dual rod drill rod of claim 16 , wherein the flow sleeve engages the inner rod coupling.

20. The dual rod drill rod of claim 16 , wherein the flow sleeve is positioned around an exterior surface of the inner rod coupling.