An Innovative Deepwater Umbilical VIV Analysis Method Incorporating Current Directionality

Abstract Vortex-induced vibrations (VIV) in deep-water umbilicals result from the interaction between ocean currents and the umbilical structure. In deepwater areas with strong and complex currents, VIV can substantially contribute to fatigue damage and drive the fatigue design of dynamic umbilicals. Consequently, accurate assessment of VIV response and damage is essential for the optimal design of deep-water umbilicals. This study presents a comprehensive global-local VIV analysis methodology. The global model is developed in OrcaFlex, where the natural frequencies and mode shapes of the umbilical system are generated. These are then incorporated into the Shear7 VIV analysis. The VIV-induced stress and fatigue in the umbilical are highly sensitive to structural damping, including material damping and hysteretic damping due to friction between components in the umbilical cross-section. This complex damping is accounted for in the local VIV fatigue damage calculation by utilizing stress-curvature relationships at various tension levels derived from detailed umbilical cross-section analysis performed in UFlex2D. The magnitude and frequency of vortex-induced vibrations (VIV) are influenced by various factors, including current speed, the global configuration of the umbilical, cross-sectional design, and tension distribution along the umbilical. Conventionally, VIV damage is assessed using nominal mode shapes, with analysis conducted for two current directions – parallel and perpendicular to the umbilical plane—to estimate in-plane and out-of-plane VIV damage, respectively. This method is often conservative, as it does not account for the impacts of current profile amplitude and directionality on the umbilical configuration and resultant natural modes, relying instead on one nominal set of mode shapes. This is due to the impracticality of performing modal analysis for each current case among hundreds of VIV current profiles. Additionally, it conservatively assumes all current profiles are applied in the same direction, which causes the most severe VIV response, and that the excited VIV motion occurs all in the same plane, leading to high bending stress (and fatigue damage) at the same position around the umbilical cross-section circumference. These factors contribute to the over-conservatism inherent in conventional analysis methods. This study introduces an innovative VIV analysis approach that automatically accounts for the amplitude and directionality of each current profile, utilizing the current-specific modes, configuration and tension distribution to calculate VIV fatigue damage. The new approach provides better accuracy through sound use of the VIV current data and reduces the VIV damage concentration inherent in the conventional VIV analysis approach. The effectiveness of this approach is demonstrated through a case study of a lazy wave umbilical system in a deep-water application. The lazy wave configuration, commonly used for deep-water umbilicals hung off FPSOs, is susceptible to VIV design challenges in regions with strong and complex ocean currents. The study shows that this new method significantly improves the system’s fatigue life compared to the conventional VIV analysis approach. This new VIV analysis approach eliminates the need for additional VIV mitigation solutions and can result in substantial cost savings.