Vertical vibrations in rotary drilling systems

Abstract

This study deals with vertical drill bit vibrations (chatter) in the deep rotary drilling. The vertical vibrations may play negative role in this type of drilling when they cause detachment of a drill bit from the rock and subsequent impacts (bit-bounce) decreasing the cutting efficiency and producing additional wear on the drill bit. On the other hand, if accurately controlled, this kind of vibrations can enhance the drilling performance by reducing friction and improving bit cleaning. The main mechanism of the chatter is the bilinear nature of stiffness in the drilling system because the long drill string has low stiffness as compared to high stiffness of the rock. In this study, we assume the rock as infinitely rigid and model the system as an impact oscillator. This oscillator is excited by the vertical momentum created by the rock asperities encountered by the rotating drill bit in the process of drilling. The oscillations are governed by the combined mass of the drill bit, the collar, the mud trapped there as well as the rotational speed, the viscosity of the mud and the fraction of the energy consumed in rock fragmentation. We demonstrate that the high rotational speeds lead to the vertical chatter and discuss the ways to control these oscillations.