COUPLED TORSIONAL AND BENDING VIBRATIONS OF DRILLSTRINGS SUBJECT TO IMPACT WITH FRICTION

A dynamic model for coupled torsional and bending vibrations of drills trings is presented. The bit/formation interactions are assumed to be related to the bit motion, leading to a set of highly non-linear equat ions with a potential for self-excited behaviour. Impacts with the bor ehole wall are accounted for by a consistent contact model which is ca pable of capturing rolling both with and without slip of the drill col lars along the borehole wall. Simulation results show that the non-lin ear coupling significantly affects the response. Due to the combined p arametric and forcing excitations and the effect of strong non-lineari ties, the response is not simple or easily predictable. At certain fre quencies, there is significant energy transfer between the two modes o f vibrations. The simulation results agree well with laboratory and fi eld observations when the so called stick-slip vibrations occur. This study suggests a new mechanism for this behaviour which was shown to b e due to static friction or speed dependent torque on bit. (C) 1998 Ac ademic Press.