Isolator Tool for High-Frequency Torsional Oscillation Proves Effective
Authors: Carpenter Chris 
Published: November 2019

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 193313, “Simulation and Testing of an Isolator Tool for High-Frequency Torsional Oscillation,” by Dennis Heinisch, SPE, Baker Hughes, a GE Company; Vincent Kulke, Braunschweig Technical University; Volker Peters, Andreas Hohl, Cord Schepelmann, and Hanno Reckmann, Baker Hughes, a GE Company; and Georg-Peter Ostermeyer, SPE, Braunschweig Technical University, prepared for the 2018 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 12–15 November. The paper has not been peer reviewed.
During downhole drilling, severe vibration loads can occur that affect the reliability and durability of tools in bottomhole assemblies (BHAs). Such high-frequency torsional oscillation (HFTO) can cause•premature damage to tools or their subcomponents. This paper presents dynamic simulations, prototype testing, and field-test results of a BHA component that isolates the upper part of the BHA from HFTO. The isolator tool reduced torsional vibrations and improved tool reliability, lifetime, and service delivery, especially while drilling in formations that are liable to excite HFTO.
Introduction
HFTO is a self-excited torsional vibration of the drillstring caused by the interaction of the drill bit and the rock. The latest downhole measurement devices can measure and record torsional accelerations at frequencies greater than 100 Hz and enable observation of HFTO in the field. Consequently, this phenomenon has been well-described and analyzed.
In contrast to stick/slip, which occurs at the first torsional eigenfrequency of the drillstring, HFTO occurs at a higher natural torsional frequency, typically between 50 and 500 Hz. In most cases, the mode exhibiting the highest excitability is the stick/slip mode. However, the mode with the second-highest excitability has a frequency that is typically between 50–500 Hz with modal amplitudes concentrated at the BHA. Whereas the frequency of the stick/slip mode varies with the length of the overall drillstring, the BHA-concentrated HFTO modes remain constant in frequency and amplitude, independent of drillstring length. An HFTO mode and the stick/slip mode can occur simultaneously.
Isolation of HFTO
Previous laboratory tests have shown that the critical HFTO mode shapes of a BHA predicted by finite-element simulation can be reproduced accurately by torsional harmonic excitation using an electrodynamic shaker. The concept of isolation is based on restricting potential damaging HFTO vibrations to a part of the BHA that is by design strong enough to survive HFTO. On the basis of modeling research and experimental analysis of HFTO, a tool for isolating HFTO has been developed. This new isolator tool is part of the BHA and isolates HFTO from the lower to the upper part of the BHA. By the design of the isolator tool, the torsional dynamics of the BHA are modified so that critical HFTO mode shapes only have significant amplitudes in the section below the isolator tool, where the BHA is designed to cope with HFTO. Above the tool, the amplitudes are comparatively low. This effect significantly improves the performance and reliability of measurement-while-drilling (MWD) and logging-while-drilling (LWD) tools and reduces damages and consequent failures caused by HFTO. In addition, the effect opens design freedom for BHA components no longer subjected to these•vibrations.