Vortex-induced vibration features of a flexible pipe with a low mass ratio at subcritical Reynolds numbers

A new type of riser, such as the thermoplastic composite pipe, features a low mass ratio and is becoming increasingly common in marine engineering projects. However, vortex-induced vibration (VIV) poses a significant concern regarding fatigue damage in risers, and VIV understandings of risers with low mass ratios are still limited and require further investigation. The present study conducted experimental research on a flexible pipe with a critical mass ratio of 0.50 at subcritical Reynolds numbers (Re) through towing in still water. The displacement responses were reconstructed using modal superposition methods. The amplitude, frequency, and phase difference between in-line (IL) flow and cross-flow (CF) directions, motion trajectory, and fatigue damage are analyzed. The results indicate that the unexpectedly larger VIV amplitude with over 1.0 D appears in the IL direction, which is compared to that found in the CF direction. Such higher response amplitude in the IL direction leads to increased fatigue damage. Additionally, the VIVs in both IL and CF directions exhibit no apparent frequency lock-in phenomenon. The frequencies of the VIVs between the IL and CF directions maintain a twice relationship, and the motion trajectory follows a typical “8”-shaped pattern. The present work provides a basic understanding of VIVs in risers with critical mass ratios and offers helpful guidelines for the design of marine risers in engineering applications.