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.
