High-Frequency Torsional Oscillation Laboratory Testing of an Entire Bottom Hole Assembly

Abstract

Abstract During the downhole drilling process, severe vibration loads can occur that affect the reliability and durability of tools in bottom hole assemblies (BHA) and may cause premature damage to the tools or their subcomponents. This paper presents a laboratory test setup for high-frequency torsional oscillations (HFTO) of a 23-meter BHA. Laboratory testing is highly important for investigating this phenomenon and developing appropriate mitigation strategies. The paper provides a summary of the theoretical background for predicting the critical torsional eigenfrequencies, mode shapes, and the susceptibility of the BHA design to HFTO. In addition, the hardware and signal processing requirements for measuring HFTO and enabling differentiation of lateral and torsional vibrations are discussed. A laboratory test setup that emulates the critical mode shapes of the BHA is essential to thoroughly investigate HFTO, and to enable the development and testing of mitigation strategies. The paper presents an appropriate test setup that has been optimized in advance by finite-element simulations. Additionally, the influence of boundary conditions (i.e., clamping the BHA) is discussed. A 240-kilowatt electromagnetic shaker system is used as an excitation source, and lateral and torsional vibration are measured by multiple triaxial accelerometers along the BHA. From the measurements, torsional vibration amplitudes and operational deflection shapes are derived and compared to results from simulation models. The results from the presented laboratory HFTO test show an excellent correlation of the predicted critical frequencies and mode shapes from the simulation model to the corresponding operational deflection shapes of the tested BHA. The sensitivity of the BHA’s torsional dynamics to the boundary conditions is demonstrated by experimental variation of the string side boundary condition. Moreover, the test setup emulated the dynamic BHA’s characteristics and field-like vibration amplitudes were achieved during the test. Additionally, a method is proposed and applied to optimize correlation by variation of the excitation frequency. Using this method, a correlation of the critical HFTO mode shapes to the operational deflection shapes reproduced by the laboratory test of more than 96% can be obtained. The method demonstrates that the dynamic load profile to which the BHA is subjected during drilling operation due to HFTO can precisely be reproduced by the developed and presented laboratory setup. The ability to test entire BHAs with field-like vibration loads enables the development of fit-for-purpose drilling tools that can withstand extreme drilling conditions.