Bottomhole-Assembly Dynamics Software Helps Mitigate Twistoffs While Drilling
Authors: Carpenter Chris 
Published: January 2023

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This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 202174, “Mitigating Twistoffs While Drilling With the Help of Bottomhole-Assembly Dynamics Software,” by Mario A. Rivas, SPE, Andres A. Ramirez, and Bader S. Al‑Zahrani, SPE, Saudi Aramco, et al. The paper has not been peer reviewed.

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A major challenge during drilling operations is the occurrence of twistoffs on bottomhole assembly (BHA) components. To overcome this challenge, a study was performed of twistoffs experienced on BHA components. Based on the findings, drilling operations and recommendations were provided to reduce or eliminate twistoffs related to suboptimal drilling parameters. This analysis will help drilling engineers and personnel foresee vibration dysfunctions and act accordingly with the use of BHA dynamics software to optimize drilling parameters before and during drilling.

Twistoffs Caused by Drillstring Vibrations
Field experience has shown that unwanted drillstring vibrations lead to failures, especially stick/slip (torsional vibrations) that occurs approximately 50% of the time during drilling. Three main types of vibrations can be distinguished: torsional (stick/slip oscillations), axial (bit bouncing), and lateral (whirling motion caused by an imbalanced drillstring).
These three types of vibration can occur at any time during drilling. Induced axial vibrations at the bit can lead to lateral vibrations in the drillstring and BHA, and axial and torsional vibrations can be observed at surface. Sometimes the severe axial vibrations close to the bit may not be observed at surface, which may lead to a potential twistoff if drilling parameters are not changed and optimized.
Additionally, fatigue failure is intensified with high vibrational loads, specifically at curved well trajectories or in buckled drillstring sections as shown in Fig. 1, where high bending stresses occur. The most critical fatigue damage will happen when the drillstring is in resonance with one of its natural frequencies, resulting in twistoff events at a much lower drillstring component yield strength.
The objective of the study described in the complete paper is to pursue an integrated approach considering the use of vibration-analysis software in the design of a BHA to identify safe operating windows for rotary speed and weight on bit (WOB) to prevent buckling and analyze fatigue.

BHA Dynamics
Resonance occurs when the rotary speed is very close to one of the natural frequencies for torsional vibration (the primary cause of failure), and WOB natural frequency for flexural vibrations of the BHA. When the frequency of any excitation is one of the natural drillstring frequencies, then the drillstring will resonate and vibration levels will be highest. With very high amplitudes, vibrations will accelerate drillstring and BHA fatigue. To estimate the critical rotary speed (resonance frequency), the BHA can be simulated using available BHA dynamics and vibration software.
This software can analyze downhole vibrations; its simulations have matched several cases very closely to what the operator has experienced. Three different examples were analyzed and presented in the complete paper. The software uses nonlinear vibration theory to determine drillstring-resonance frequencies.