Application of stability approach to torsional and lateral bit dynamics

This paper addresses the dynamic stability of polycrystalline diamond compact (PDC) bits under induced torsional and lateral vibrations. Dynamic stability is investigated by considering three simple mechanical systems representing string-torsional vibration, bit-lateral dynamics, and coupled torsional-lateral vibration of the bit-string assembly. For each system, the rotary-speed (rev/min)/ weight-on-bit (WOB) domain within which the system is dynamically stable is determined. The influence of various drillstring and bit parameters on the stability of bit motion is studied through their effect on the size of the stability zone on (rev/min)/WOB plots. These illustrate the power of the stability approach in providing valuable qualitative information on the main parameters influencing bit stability.