The fatigue life assessment of umbilicals and flexible risers due to vortex-induced vibrations (VIV) require special attention to the structural properties of the flexible bundle due to stick/slip behavior of helical elements in bending. The VIV response on umbilicals and flexible risers is complex, and it is controlled by several parameters. Structural damping is one of these parameters, but this is typically calculated based on the material damping of the structure. Similar to flexible risers and umbilicals, hysteresis damping due to the stick/slip behavior in the helical layers may be significant for cable structures. The purpose of this study is to evaluate the effect of the hysteresis damping on VIV response, and to demonstrate a procedure for consistent VIV fatigue analysis of flexible risers and umbilicals.
The stick/slip hysteresis varies with varying contact force, and the contact force is dependent on internal pressure and effective tension. The effect of varying these parameters is included in the study. Furthermore, the study includes different riser system configurations and current profiles.
In this paper, the stick/slip hysteresis has been established for two umbilicals and two flexible risers, with varying internal pressure and effective tension. Combined with five different configurations (both shallow and deep water) and 16 different current profiles, the VIV modes have been calculated and the corresponding stick/slip modal damping ratio has been established.
Finally, using fatigue current profiles, fatigue life estimates have been made based on different damping ratios, including the estimated stick/slip damping ratio.
The study presented in this paper has been carried out as part of the Norwegian Deepwater Programme (NDP).