Analysis of a Drill-String Experimental Set-Up with Dry Friction-Induced Torsional Vibration

Abstract

The drilling process consists basically of a drive motor at the top end (surface) that provides torque to rotate a cut tool (drill-bit) at the bottom end. To connect these extremities there is a torque transmitting slender element so-called drill-string. Due to the slenderness, the borehole wall/drill-string, and, mainly, the drill-bit/rock interactions, the system undergoes axial, lateral and torsional vibrations. Among these modes, torsional vibration is present in most drilling processes and may reach an undesired severe phenomenon: stick-slip. In this work, we perform experiments on a torsional test rig, which executes dry friction-induced vibrations. The test bench consists in a DC-motor, a low-stiffness shaft and two discs. The motor provides rotation to the whole set-up: one disc ($R_1$) is placed on the opposite extremity of the motor, and the second one is intermediately placed ($R_2$). Resistive torques may be applied in both discs and the behaviour of the system is analysed. It is possible to observe torsional vibrations and the stick- slip when a friction torque is applied on $R_1$ and during this phenomenon, another friction torque is applied on $R_2$. The presence of the second frictional torque as strategy of mitigation has a major influence on the dynamics as it may change from a stable limit cycle to a stable equilibrium and then preventing stick-slip phenomenon.