5 Drillstring Design and Operation

Drillstrings transmit mechanical rotary power from the surface to drill bits, serve as a conduit for drilling fluid, apply force to drill bits and affect hole direction. A major portion of drillstrings is made up of drill pipe while the bottom portion (roughly 700 ft) is a heavier pipe called drill collars. The bottom portion (∼150 ft) of the drill collar section contains tools (drill bit, PDM, Turbine, MWD and stabilizers) called the bottom hole assembly (BHA). Even though the total length of a drillstring may be several thousand feet long, the BHA affects everything: rate of penetration, footage cost and well bore direction.