Abstract
Umbilicals are multi-functional cables essential for offshore operations, yet their high aspect ratio and flexible nature make them susceptible to Vortex-Induced Vibrations (VIV), which can lead to severe fatigue damage. This study presents a comprehensive experimental investigation of VIV in a 30-meter-long full-scale umbilical (𝐷 =129 mm) subjected to uniform flow with Reynolds numbers ranging from 3.85×104 to 2.57×105. Conducted in the SINTEF Ocean Basin under project and supervision of UFSC and PETROBRAS, the experiments utilized a custom rig to evaluate the influence of two axial pre-tension levels (35 kN and 45 kN) on the hydroelastic response. The decay tests revealed a typical structural damping (𝜁 ≈ 4%), attributed to the internal interlayer friction of the helical construction. The VIV results captured a sequential progression of dominant modes (up to the fifth order) and revealed a deterministic modal jump at 𝑈 = 0.8 m∕s, characterized by a sudden non-linear redistribution of energy between modes. Maximum non-dimensional amplitudes reached 1.3𝐷 at 𝑉𝑟 ≈ 9, exceeding standard rigid-cylinder predictions and providing new evidence for expanded synchronization bandwidths in complex umbilicals. Furthermore, mean drag coefficients showed dynamic amplification, peaking at 0.7 before following the downward trend of the critical flow regime (drag crisis). Numerical simulations using VIVANA-TD were validated against the experimental data. It was found that standard industrial frequency parameters underpredict the response; however, an optimized calibration of the synchronization envelope and amplitude-dependent excitation coefficients achieved a high correlation with experimental peaks. These findings offer a robust benchmark for the design of subsea cables, demonstrating that the accounting for inherent damping and precise modal synchronization is vital for accurate interference and fatigue life assessments in offshore engineering.