Prediction and Mitigation of Torsional Vibrations in Drilling Systems

Abstract

Abstract Drilling systems are subject to torsional vibrations that are excited by bit-rock or by drillstring formation interaction forces. These torsional oscillations can be distinguished by mode shape and frequency. Well-known stick/slip oscillations are characterized by low frequencies (usually below 1 Hz) and affect the entire drill-string. High-frequency torsional oscillations (HFTO), in contrast, name the excitation of a high-order natural mode (reaching 400 Hz). In case of HFTO, the bottom-hole assembly (BHA) is exposed to high dynamics loads. Torsional vibrations compromise drilling efficiency and tool reliability. To address these challenges, we are proposing a method for automated BHA optimization based on mechanical drill string models. Through extensive analysis of high-frequency (1400 Hz sampling frequency) data from field measurements, an analytical, verified, and easy to use criterion for the prediction of the excited torsional mode and the corresponding loads was derived. The criterion is based on the comparison of the resulting excitation from cutting forces at the bit and the damping of a torsional mode. The criterion is unique for every torsional mode and can be used to rank the susceptibility of torsional modes for HFTO or stick/slip. A software application (Torsional Oscillation Advisor, TOA) has been developed for user-friendly interpretation of the underlying analytical method towards practical issues. The use of the TOA provides valuable input for drilling optimization: For stick/slip, the influence of various drill pipe sizes and the length of the drill pipe section on torsional stick/slip mode are analyzed. It is shown that the limit for stable drilling in case of bit induced stick/slip can be extended by stiffer drill pipes whereas the influence of the length of the drill pipe section is marginal. The material of the bit and its mass distribution is shown to have a considerable influence on the excitation of HFTO. The software also enables automated BHA optimization in both new product development and tool operation phases. A numerical optimization approach is used to minimize the susceptibility of the bottom-hole assembly for stick/slip and HFTO for given constraints of the geometry and material parameters. Herein, a significant increase of stable drilling conditions regarding weight on bit and bit rotational speed with respect to torsional oscillations is achieved. Even small changes in the drilling system design have a visible impact on the torsional stability. The ability to identify and predict modes of stick/slip and HFTO that are most likely to be excited while drilling, an extension of the stable drilling zone and the estimation of loads before field deployment will result in higher drilling efficiency, more reliable tools and lower non-productive time.