Isolator Tool for High-Frequency Torsional Oscillation Proves Effective
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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.
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.
In developing and validating the effect of the isolator tool, the following steps were taken:
- Simulation and prediction of the critical torsional eigenfrequencies of a typical BHA with and without the isolator tool, and finding optimal mechanical parameters for the tool design
- Creating a mechanical concept and design to manufacture a prototype tool
- Full-scale laboratory tests of the prototype using a typical BHA configuration
- Field test of the prototype isolator tool
The main task of the isolator tool is to isolate HFTO from the section of the BHA below the tool up to the part of the BHA above it. Therefore, a good compromise for the position was found above the rotary steerable system (RSS). The principle of isolating HFTO is based on mechanical low-pass filtering of the torsional dynamic vibration occurring at the bit. This can be accomplished by using a component in the BHA with a sufficiently low torsional stiffness and is discussed in detail in the complete paper. Fig. 1 shows a setup of a typical field BHA comprising an isolator tool.
The results for the BHA with the isolator tool reveal only torsional vibration amplitudes at the bit and the RSS. Above the isolator tool, the vibration amplitudes are significantly lower, in particular in comparison with the amplitudes of the modes of the reference BHA. This demonstrates the isolation efficiency for significantly reducing HFTO-vibration amplitudes of the tools located farther uphole. MWD and LWD tools benefit from this vibration reduction with improved quality of data measured and recorded as well as increased reliability as a result of drastically decreased torsional dynamic loads.
Field Test of Isolator Tool
After a successful laboratory test, the isolator tool was tested in the field. The formation where the tool had been deployed was typical for scenarios causing HFTO. The isolator tool was used in a typical 4¾-in. BHA as shown in Fig. 2. This enabled direct comparison of two similar BHAs with regard to HFTO using simulation results and high-frequency rotational acceleration measurements obtained while drilling.
Compared with the BHA without the isolator tool, the same BHA with the tool had only one HFTO mode, with a significant susceptibility in the critical frequency range that was calculated at 299.5 Hz. In analogy to the dual-mass flywheel, the system below the isolator had its own first torsional mode at approximately 30 Hz. The system had several more modes around 30 Hz, but with a very similar mode shape and amplitude below the isolator.
These lower-frequency modes were not critical owing to their low amplitudes and frequency, which indicated significantly lower local loads (acceleration and torque) as well as fewer load cycles compared with typical HFTO modes. Nevertheless, the mode that was most likely excited was at 299.5 Hz. From the laboratory testing, it could be reasoned that no higher amplitudes can be expected at the BHA compared with the HFTO modes of the BHA without the isolator tool, even for the components below. The expected mode at 299.5 Hz showed the maximum amplitude at the bit. A second local maximum was located within the torsionally flexible portion of the isolator tool, but with only a fraction of the modal amplitude at the bit.
Spectra of angular acceleration and dynamic torque recorded during the field test with a high-speed MWD measurement tool above the isolator tool at a bit rotary speed of 80 rev/min show the predicted 299.5-Hz mode occurring at 300 Hz as well as the amplitudes at 30 Hz, which indicate additional vibration of the BHA at the predicted lower-frequency modes. The occurring frequencies validate the dynamic modeling of the BHA and the prediction of HFTO frequencies.
Examination of amplitudes of angular acceleration and dynamic torque expected for the bit rotary speed of 80 rev/min along the BHA and at the MWD tool shows that the loads are within the design limits of affected components and should not compromise their function or cause premature damage, even at higher rotational speeds.
Although HFTO can be measured and its effect modeled and predicted, little can be done to mitigate HFTO while still maintaining efficient drilling. Analysis of simulated BHA configurations showed that a string component with certain mass and stiffness properties could be used to isolate a portion of the BHA from HFTO. These properties are that of a mechanical low-pass filter that prevents the propagation of higher-frequency torsional vibrations upward the BHA and string. Extensive finite-element modeling and simulation showed the effectiveness of this concept. A prototype tool with these isolation properties was built and tested in a laboratory environment. In laboratory tests with external forced torsional excitation, the tool showed the isolation effect by reducing the vibration amplitude to less than a fifth for the most-critical mode. A BHA with the isolator tool on top of the RSS was run in a well with formations known to excite HFTO. The downhole measurements below and above the isolator tool confirmed the isolation effect of the tool in this harsh drilling environment. The application of these isolator tools in wells with high amounts of excited HFTO will lead to an improvement in BHA reliability and performance, resulting consistently in longer runs, reduced nonproductive time, and minimized cost per foot.
Isolator Tool for High-Frequency Torsional Oscillation Proves Effective
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