Consistent VIV Fatigue Analysis Methodology of Umbilicals

The fatigue life assessment of umbilicals due to vortex induced vibrations (VIV) requires special attention to the structural properties of the umbilical due to the stick/slip behavior of helix elements in bending. Essential parameters for assessment of the VIV response are the structural damping in the stick regime (i.e. umbilical behaves as a solid cross section due to friction between the helix elements) as well as the additional damping introduced by the hysteretic damping due to the stick/slip behavior in bending. Furthermore, consistent fatigue stress recovery considering the stick/slip behavior in bending is essential for fatigue life predictions. The expected VIV response characteristics of different types of umbilicals (steel tube umbilicals, control umilicals and power umbilicals) are discussed with basis of typical mechanical properties of the different types. It is shown that these main types of umbilicals may experience distinctive different VIV response characteristics underlining the need for a consistent VIV fatigue analysis scheme. The overall objective of the paper is to establish consistent VIV analysis methodology for umbilicals in compliance with requirements given in ISO 13628-5 ‘Subsea Umbilicals’. A consistent VIV fatigue analysis scheme is outlined using commercially available state-of-the-art computer programs for global system modeling/eigenmode analysis (Riflex), VIV response analysis (Shear7) and cross sectional stress analysis (Helica). The performance of the calculation scheme is demonstrated by case studies of deepwater umbilicals in a complex long-term current loading environment. It is show that consistent treatment of the mechanical properties of umbilicals is essential for VIV fatigue life assessments. It is furthermore shown that the VIV fatigue damage in umbilicals may be significant underlining, the practical importance in design of deepwater dynamic umbilicals.