Simulation and Testing of an Isolator Tool for High-Frequency Torsional Oscillation

Abstract

Abstract During downhole drilling, severe vibration loads can occur that affect the reliability and durability of tools in bottom-hole assemblies (BHA). These high-frequency torsional oscillations (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. This isolation increases the performance and reliability of BHA components through reduction of the vibration load. The paper provides a brief summary of the theoretical background for predicting the critical torsional eigenfrequencies, mode shapes, and susceptibility of BHA design to HFTO. Finite-element (FE) models were used to simulate the effect and efficiency of the isolation. The isolator tool was tested in a full-scale laboratory test setup that emulated the critical mode shapes to analyze the torsional dynamics and the expected isolation effect. A BHA with the isolator tool positioned between a rotary steerable system (RSS) and measurement while drilling (MWD) tools was excited in the critical HFTO mode shapes with an electrodynamic shaker. Vibration response was measured using triaxial accelerometers. Later, the tool was tested in the field in a BHA configured to log the vibration below and above the tool at sufficiently high sample rates to validate the isolation effect of the tool. The laboratory results showed the isolating effect between BHA components above and below the isolator tool, in particular for the critical HFTO-frequencies and mode shapes as predicted by the finite-element simulation. The torsional deflection shapes from testing using distributed triaxial accelerometers showed a high correlation with the predicted mode shapes from the FE model. The measurements from the field test also identified the isolation effect between the BHA sections. The measurement data showed that as predicted only the lower part of the BHA below the isolator tool oscillated at non-critical torsional eigenfrequency. The isolator tool reduced torsional vibrations and improved tool reliability, tool lifetime and service delivery, especially while drilling in formations that are susceptible to excite HFTO.