A Lyapunov Exponent Approach for Identifying Chaotic Behavior in a Finite Element Based Drillstring Vibration Model

The purpose of this work is to present a methodology to predict vibrations of drilllstrings for oil recovery service. The work extends a previous model of the drill collar between two stabilizers in the literature to include drill collar flexibility utilizing a modal coordinate condensed, finite element approach. The stiffness due to the gravitational forces along the drillstring axis is included. The model also includes the nonlinear effects of drillstring-wellbore contact, friction and quadratic damping. Bifurcation diagrams are presented to illustrate the effects of speed, friction at wellbore, stabilizer clearance and drill collar length on chaotic vibration response. Their effects shifts resonance peaks away from the linear natural frequency values and influences the onset speed for chaos. A study is conducted on factors for improving the accuracy of Lyapunov Exponents to predict the presence of chaos. This study considers the length of time to steady state, the number and duration of linearization sub-intervals, the presence of rigid body modes and the number of finite elements and modal coordinates. The Poincare map and frequency spectrum are utilized to confirm the prediction of Lyapunov exponent analysis. The results may be helpful for computing Lyapunov exponents of other types of nonlinear vibrating systems with many degrees of freedom. Vibration response predictions may assist drilling rig operators in changing a variety of controlled parameters to improve operation procedures and/or equipment.