Dynamics modeling of roller cone bit axial vibration

Abstract

It has been reported in previous studies that dominant peaks of a power spectrum for roller cone bit axial vibration moved to higher frequencies as the bit tooth wear progressed. This experimental result implies that monitoring and analyzing roller cone bit vibration could be a new real-time diagnosis method of downhole bit wear conditions.
In this paper, dynamics of roller cone bit axial vibration was modeled to make a quantitative evaluation of bit vibration characteristics. The model includes kinematics of roller cone bit motion, and a model for bit-tooth/rock interaction in consideration of worn tooth geometry and inclination of tooth penetration. Sequentially solving the equation of motion with increments of cone rotation angles under some constraints on cone rotations, time history of axial bit vibration was simulated.
The developed model was validated by comparing the power spectral densities for simulated time histories of bit displacement and weight on bit with experimental results in full-scale drilling tests. Simulation study revealed that the characteristics of roller cone bit axial vibration was very sensitive to a small variation of cone rotation speed, and that bit tooth tracking on bottomhole craters could constrain the cone rotation speeds depending on the situation.