Patent ID: 8061450

Claim:
A percussion drilling assembly for drilling through earthen formations and forming a borehole, the assembly comprising: a percussion drill bit; a retainer sleeve having an upper end with an outer diameter; and a tubular casing engaging the retainer sleeve, the tubular casing comprising: a first tubular portion engaging the upper end of the retainer sleeve at a first end having an outer diameter substantially equal to the outer diameter of the retainer sleeve; a second tubular portion connected to the first tubular portion at a first end and having a second end with an outer diameter that differs from the outer diameter of the retainer sleeve; a third tubular portion coupled to the second tubular portion, wherein the first tubular portion and the third tubular portion each have a length configured to enable gripping of the tubular casing over at least one of the first tubular portion and the third tubular portion using tongs; and a plurality of dimples arranged on an outer surface of at least one of the first, second, and third tubular portions of the tubular casing, wherein the dimples comprise a hemispherical surface and alter a flow behavior of fluid passing along the dimples during drilling.