Dynamic Stability Analysis of Pinned-Pinned Underwater Pipelines Conveying Fluids: Effects of Internal Flow Velocity on Natural Frequencies and Critical Thresholds

Abstract

Underwater pipelines carry oil and gas all over the world. Their functioning in aggressive marine environments, however, raises fundamental engineering challenges. This study presents the dynamic performance of fluid-conveying underwater pipelines. It addresses the role of fluid-structure interaction (FSI) and hydrodynamic loading on external pipeline behaviour. We establish a pinned-pinned pipeline mathematical model based on Euler-Bernoulli beam theory. It provides closed-form solutions of natural frequency and mode shape. Analytical and numerical analysis prove that internal fluid velocity lowers natural frequency considerably. This increase in internal fluid velocity enhances dynamic instability danger, with critical velocities determined for each vibration mode. Sensitivity study proves that pipe length negatively affects natural frequency, while wall thickness and Young's modulus increase structural stiffness. These findings highlight the importance of incorporating FSI and material properties during pipeline design to avoid resonance and ensure safety. This study enriches our knowledge of underwater pipeline dynamics and encourages further research on nonlinear effects, multiphase flows, and real-time monitoring using machine learning.