Abstract: Under real marine environmental conditions, vortex-induced vibration (VIV) constitutes a critical factor contributing to fatigue damage in low mass ratio marine flexible pipes. Current industrial and academic research on low mass ratio flexible pipes remains relatively limited, resulting in substantial empirical approaches and safety risks in the design of related structures. In this study, a flexible pipe with a mass ratio of 0.50 is investigated by hydrostatic towing experiments. The modal superposition method was employed to reconstruct displacement responses, with subsequent analysis of amplitude characteristics, frequency features and motion trajectories in both IL and CF direction. Results demonstrate that the low mass ratio flexible pipe exhibits super upper branch amplitudes in the IL direction comparable to those observed in the CF direction. Notably, no frequency lock-in phenomenon was detected in IL or CF direction. The frequency ratio between IL and CF direction shows a two-fold relationship, and the trajectories show a typical “8” shape. These findings contribute to a better understanding of the VIV characteristics of flexible pipes with low mass ratios and provide useful guidance for the design of related structures.

Key words: low mass ratio, flexible pipe, uniform flow, vortex-induced vibration (VIV), model test