Umbilical VIV Fatigue with Mode Number and Mode Amplitude Dependent Structural Damping

摘要

Subsea umbilicals exposed to ocean currents are susceptible to VortexInduced Vibrations (VIV). The possible fatigue damage resulting fromthis VIV requires careful calculation and prediction of the response. Akey characteristic of umbilicals is its large flexibility with respect tobending, in particular due to inter-layer sliding between helicallywound layers. This complex mechanical bending performance givesrise to a moment-curvature hysteresis of the cross section, also calledstick-slip hysteresis. The stick-slip hysteresis behaviour of an umbilicalresults in frictional energy dissipation during slipping, which can becharacterised by a viscous damping ratio. This stick-slip damping ismode number and mode amplitude dependent and can significantlyinfluence umbilical VIV fatigue life prediction. Methods to incorporatestick-slip damping in prediction methods has been the focus of recentindustry research as the predicted fatigue damage rate of deepwaterumbilicals without the inclusion of stick-slip damping effects havebeen prohibitively large. With stick-slip damping included, a greaterlevel of total damping will result and with it a reduction in the predictedamplitude of response and fatigue damage rate. This paper presents thefirst known implementation of mode number and mode amplitudedependent damping in a VIV prediction scheme.A beta version of a the industry’s most widely used VIV prediction toolhas been developed to allow for both mode number and modeamplitude dependent structural damping, enabling an accurateapplication of stick-slip damping. An iterative solver has also beenimplemented for convergence on the correct VIV solution. Todemonstrate the implementation of this new capability, a case studybased on a deepwater umbilical is presented.This paper also introduces a simplified screening method, to assist indetermining from the outset whether an umbilical will enter into anysignificant stick-slip regimes that would affect an increase in structuraldamping.The application of the methodology and tools presented in this papercan be extended to other helically wound flexible structures, such asunbonded flexible risers.