Stick-slip whirl interaction in drillstring dynamics

This paper attempts to explain the complicated behavior of oilwell drillstring motionwhen both torsional stick-slip and lateral whirl vibration are involved. It is demonstratedthat the observed phenomena in experimental drillstring data could be due to the fluidforces of the drilling mud. A Stick-slip Whirl Model is presented which consists of asubmodel for the whirling motion and a submodel for the stick-slip motion, both as simpleas possible. The Stick-slip Whirl Model is a simplification of a drillstring confined in aborehole wall with drilling mud. The model is as simple as possible to expose only thebasic phenomena but is discontinuous. Bifurcation diagrams of this discontinuous modelfor varying rotation speeds reveal discontinuous bifurcations. The disappearance of stickslipvibration when whirl vibration appears is explained by bifurcation theory. The numericalresults are compared with the experimental data from a full-scale drilling rig.