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Volume 3, Issue 2, 2025

Abstract

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To investigate the dynamic response and potential structural degradation of carbon fiber sucker rod strings during operation, a torsional vibration model incorporating helical buckling-induced torque excitation has been developed. In this model, the upper suspension boundary condition is idealized as a torsional spring, whose stiffness is determined as a function of both axial displacement and applied load at the suspension point. The torsional stiffness is categorized into time-dependent and mean (average) components, both of which are examined through numerical simulation using the finite difference method. The results reveal pronounced torsional oscillations at the upper section of the rod string, indicating significant torsional deformation of the suspension assembly. A non-monotonic relationship is observed between stroke length and vibration amplitude, wherein torsional vibration initially intensifies with increasing stroke before attenuating, suggesting the presence of resonance phenomena within specific operational ranges. The simulations further demonstrate that time-varying and average torsional stiffnesses yield comparable influences on the overall torsional response. Helical buckling deformation is shown to play a critical role in amplifying torsional stress, with the induced torque predominantly localized in the mid-to-lower segments of the wellbore. The presented model provides an essential theoretical framework for understanding the complex interaction between axial deformation and torsional instability, offering new insights into the mechanisms that may precipitate longitudinal splitting or fatigue failure in carbon fiber sucker rod strings. These findings are expected to support the optimization of rod string design and operational strategies in advanced artificial lift systems.
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