STABILITY ANALYSIS OF DRILLSTRING ROTATION

The stability of oilwell drillstring rotation is analyzed. The drillpi pe is represented by a one-dimensional continuum in torsion, while the bottom-hole-assembly is considered to be a rigid body. Conditions of asymptotical stability of the drillstring rotation are obtained in clo sed form. The obtained conditions introduce the critical length and th e critical rotational speed. The uniform rotation is unstable if the d rillstring is longer than the critical length or if it rotates below t he critical rotational speed. Recommendations for avoidance of self-ex cited torsional oscillations are offered and discussed.