Study Explores Interaction Between Bit Whirl and High-Frequency Torsional Oscillations
Authors: Carpenter Chris 
Published: December 2024

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This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 23989, “The Nature of the Interaction Between Bit Whirl and High-Frequency Torsional Oscillations,” by Andreas Hohl, SPE, and Armin Kueck, Baker Hughes, and Vincent Kulke, TU Braunschweig, et al. The paper has not been peer reviewed. Copyright 2024 International Petroleum Technology Conference.

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Understanding the interaction of high-frequency torsional oscillations (HFTO) with other vibrational phenomena is essential to develop effective HFTO mitigation strategies. While coupling with axial vibrations and stick/slip already have been studied extensively, the interaction of HFTO with lateral vibrations has received less attention. The complete paper analyzes this interaction based on a bit/rock interaction model that accounts for the superimposed movement of whirl and HFTO at the bit. The new bit model provides a physics-based explanation of why bit backward whirl and torsional vibrations cannot be observed simultaneously.

Introduction
The torsional movement of HFTO is localized to the lower part of the bottomhole assembly (BHA). The amplitudes along the distance from the bit are scaled by complex mode shapes with a comparably small wavelength. This deflection of the BHA leads to high tangential acceleration and dynamic torsional torque loads. The self-excitation mechanism of HFTO can be modeled as a drilling torque that decreases when the bit rotary speed increases. The nonlinear drilling torque characteristic can be identified from field data. Because both stick/slip and HFTO correspond to a torsional movement of the drilling system, and both are excited by the rock-cutting process, they interact strongly. The mitigation strategies and optimal operational parameters to manage HFTO are, thus, strongly linked to the understanding of this interaction.
In this work, the interaction of HFTO and bit backward whirl was analyzed and modeled to further interpret this observation. Bit backward whirl is a complex lateral movement of the bit. The understanding of the interaction between HFTO and backward whirl drives improvements of vibration-mitigation strategies and bit design to mitigate HFTO or bit backward whirl.

Modeling of the Cutting Forces
To study the interaction of whirl and HFTO in more detail, a simplified bit cutting-force model is needed. For a common polycrystalline-diamond-compact (PDC) bit, the cutting torque of the bit results from the rock-cutting process. The cutting process results in cutting forces on each individual cutter. For simpler modeling, the cutting forces are assumed to be proportional to the normal force on each cutter. The proportionality factor between the normal force and the cutting force at a cutter is called cutter aggressiveness and is dependent on the velocity of the cutter. The radial distance between the geometric center of the bit and a cutter, multiplied by the cutting force, results in the portion of the cutting torques of each cutter. These add up to the overall bit torque. The three central variables in the torque calculations are the normal force, cutting speed, and cutter aggressiveness.