Quantification and Validation of Fatigue Life Differences in Push-the-Bit Rotary Steerable Systems via Vibration Pattern Measurement

Abstract

The downhole high-frequency measurement is critical for preventing costly failures of push-the-bit rotary steerable systems (PTB-RSSs). However, the differences in the fatigue life under multimode vibrations have not been clearly identified using existing measurement and interpretation methods. This study establishes a multiboundary coupled dynamics model of PTB-RSS, validated with field measurements, which incorporates the steering force friction, bit–rock interaction, and bottom hole assembly (BHA)–borehole contact mechanisms. Validation using high-frequency field data showed a Pearson correlation coefficient of 0.933 for rotational speed, close agreement in acceleration spectral characteristics, and an average error of only 6.13% in steering displacement compared with theoretical values, confirming the model’s high fidelity in reproducing key downhole vibration patterns. Stress signals obtained from the validated model were processed using the rainflow counting method. The results indicate that stick–slip vibration reduces the fatigue life of BHA by at least 68.38%, while high-frequency whirl and torsional vibrations reduce it by more than 90%. Moreover, when the steering force approaches 30 kN, it readily excites the high-frequency vibration in BHA, significantly shortening its service life. The core methodological innovation of this study lies in establishing a validated dynamics model that serves as a virtual sensor. This model enables the first translation of measurable vibration patterns into quantified fatigue life differences, thereby providing an operable, measurement-driven technical framework for predicting BHA fatigue life based on high-frequency dynamic signal measurements.