Dual concentric pipe joint

摘要

Apparatus is disclosed for drilling a pilot hole and subsequently enlarging the pilot hole in earth dormation. A dual concentric pipe string is used for circulating air downwardly through the outer pipe, a cross-over sub and through a pilot bit and upwardly through the bore hole outside the pipe string to bail cuttings, during drilling of the pilot hole. The air pressure expands the cutters of an expandible bit while a limited portion of the air supplied cools the cutters. The dual concentric pipe is made up by joints having pin and box structures which provide inner and outer flow passages and shoulders which abut between the outer members and which are resiliently sealed between the inner members.