Design and rigid body dynamics analysis of a novel retractable drill bit used in casing-while-drilling

Utilizing the Geneva drive mechanism, an inventive retractable drill bit tailored for casing-while-drilling (CwD) applications was developed. This novel design allows the lower segment of the drill bit to be swung outward through rotation, facilitating to drill a larger hole and retrieved effortlessly by raising the drill tools. An exploration into the dynamic behavior of this retractable drill bit under varying drilling conditions was undertaken, focusing on parameters such as weight-on-bit, rotation speed, friction coefficient between the drill bit and rock, and lifting force. Findings from the investigation revealed that, under conditions of 20 kN weight-on-bit, a friction coefficient of 0.3, and a rotation speed of 30 rpm, the drill bit achieved full extension within 3.8 s. Notably, the movement speed of the lower drill bit was notably influenced by weight-on-bit, the friction coefficient, and lifting force, demonstrating accelerated movement with increased values of these factors. Rotation speed, however, exhibited negligible impact on the movement of the drill bit. An indoor experimental examination mirrored the outcomes obtained through simulations. Subsequent to numerical simulations and indoor testing, a large-diameter drill bit was produced and field tested, showcasing its capability to flexibly extend and retract during drilling, achieving an average drilling speed of 1.7 m/h.