Analysis of Whirl Characteristics of Drill String Under Multidirectional Coupling Conditions

Abstract

Summary The nonlinear contact between the bottomhole assembly (BHA) and the borehole can cause backward whirl. During an actual drilling process, the backward whirl will increase wear of the downhole drilling tool and lead to bending or even bulking failure. Therefore, analyzing the drillstring whirl characteristics considering multidirectional coupling effects, combined with field conditions, is a key issue and research hotspot. In this paper, considering the relationships between axial, torsional, and lateral vibrations of the drillstring system, the whirl model is established with finite element method. Research results indicate that with higher driving speed and larger friction coefficient between drillstring and borehole wall, the drillstring lateral vibration changes from forward to backward whirl. On the contrary, the increasing of drilling fluid density can effectively suppress the occurrence of backward whirl. As drilling fluid density increases, forward and backward whirl coexist, and only the forward whirl occurs at last. The research results can provide a basis for formulating whirl control strategy, and promoting the efficient and safe operations of drilling engineering.