Study on Influence of Time-Varying Axial Tension on Vortex-Induced Vibration Characteristics of Deepwater Drilling Riser

Summary Vortex-induced vibration (VIV) poses a significant threat to the structural integrity of deepwater drilling risers operating in complex ocean environments. In this study, we propose a novel VIV prediction framework based on the discrete vortex method (DVM), which, for the first time, incorporates time-varying axial tension resulting from riser bending deformation into the hydrodynamic simulation. A key methodological advancement is the coupling of 2D vortex shedding with the actual lateral displacement of the riser, enabling simultaneous prediction of both in-line and crossflow VIV responses along the entire riser span. Unlike conventional models that assume constant axial tension or ignore bending-induced tension variation, the proposed approach captures the dynamic interaction between structural deformation and flow-induced forces. Validation is performed through comparison with existing numerical studies, confirming that the model can accurately reproduce the overall VIV response trends of the riser system. Comparative analysis demonstrates that the proposed model predicts a distinct lock-in region between 0.40 m/s and 0.75 m/s, with a critical transition at 0.75 m/s separating the upper and lower response branches. Moreover, a significant reduction in in-line vibration amplitude, up to 25%, is observed when time-varying axial tension is considered. In contrast, its impact on crossflow response remains limited. These findings underscore the importance of tension variation in realistic riser VIV prediction and provide a more robust basis for riser fatigue assessment in practical offshore applications.