SUMMARY

The present invention is directed to a pipe assembly comprising a hollow outer member and a hollow inner member. The inner member has a longitudinal internal bore and is at least partially nested within the outer member and cooperates with the outer member to define boundaries of an annular space. The inner member is characterized by a pin end having a polygonal outer profile and an opposed box end having a polygonal inner profile. An endless groove is formed in the inner member and surrounds the internal bore. A fluid seal is received within the endless groove.

The present invention is also directed to a drill string having a first end situated in an underground borehole and an opposed second end situated above ground. The drill string comprises a plurality of pipe assemblies arranged in end-to-end and torque-transmitting relationship. Each pipe assembly comprises a hollow outer member and a hollow inner member. The inner member has a longitudinal internal bore and is at least partially nested within the outer member and cooperates with the outer member to define boundaries of an annular space. The annular spaces of the plurality of pipe assemblies comprise segments of a first fluid path within the drill string. The internal bores of the plurality of pipe assemblies comprise segments of a second fluid path within the drill string. The drill string further comprises a plurality of fluid seals. Each fluid seal is interposed between adjacent pipe assemblies of the drill string and surrounds the second fluid path.

The present invention is further directed to an elongate drill string having a first end situated in an underground borehole, an opposed second end situated above ground, and mutually exclusive first and second fluid paths. Each fluid path extends between the first and second ends. The drill string comprises a plurality of pipe assemblies arranged in end-to-end and torque-transmitting relationship, each pipe assembly includes segments of the first fluid and second fluid paths. The first fluid path surrounds the second fluid path. The drill string further comprises a plurality of fluid seals. Each fluid seal is interposed between adjacent pipe assemblies of the drill string and surrounds the second fluid path.

