Eassom, A., Marcollo, H., Potts, A.E., Boustead, N. and Kilner, A.A. (2016). Paper 2016-TPC-0877 published in proceedings of ISOPE Conference, Rhodes, Greece. 24 June - 2 July 2016.

Abstract

Subsea umbilicals exposed to ocean currents are susceptible to Vortex Induced Vibrations (VIV). The possible fatigue damage resulting from this VIV requires careful calculation and prediction of the response. A key characteristic of umbilicals is its large flexibility with respect to bending, in particular due to inter-layer sliding between helically wound layers. This complex mechanical bending performance gives rise to a moment-curvature hysteresis of the cross section, also called stick-slip hysteresis. The stick-slip hysteresis behaviour of an umbilical results in frictional energy dissipation during slipping, which can be characterised by a viscous damping ratio. This stick-slip damping is mode number and mode amplitude dependent and can significantly influence umbilical VIV fatigue life prediction. Methods to incorporate stick-slip damping in prediction methods has been the focus of recent industry research as the predicted fatigue damage rate of deepwater umbilicals without the inclusion of stick-slip damping effects have been prohibitively large. With stick-slip damping included, a greater level of total damping will result and with it a reduction in the predicted amplitude of response and fatigue damage rate. This paper presents the first known implementation of mode number and mode amplitude dependent damping in a VIV prediction scheme.

A beta version of a the industry’s most widely used VIV prediction tool has been developed to allow for both mode number and mode amplitude dependent structural damping, enabling an accurate application of stick-slip damping. An iterative solver has also been implemented for convergence on the correct VIV solution. To demonstrate the implementation of this new capability, a case study based on a deepwater umbilical is presented.

This paper also introduces a simplified screening method, to assist in determining from the outset whether an umbilical will enter into any significant stick-slip regimes that would affect an increase in structural damping.

The application of the methodology and tools presented in this paper can be extended to other helically wound flexible structures, such as unbonded flexible risers.