VIV-induced fatigue damage study of helical wires in catenary unbonded flexible riser in time domain

Abstract

Helical wire is a key component of unbonded flexible riser, and is vulnerable to fatigue failure. The present study simplifies the flexible riser into a beam element with constant axial and bending stiffness, and then investigates the vortex-induced vibration (VIV)-induced fatigue damage of the helical wire in a catenary flexible riser using a time domain VIV approach. The simplification of the flexible riser is based on the equivalence of axial and bending stiffness. The former can be iteratively calculated based on the integrated axisymmetric formulation in which the interlayer contact and separation are taken into account, while the latter is simply taken as the combination of all layers’ bending stiffness and keeps constant since helical wire may not slide (i.e. full-sticking) under intact outer sheath due to high external pressure. Based on the simplification, this study compares the VIV characteristics of the flexible riser under full-sticking and full-sliding conditions of helical wires since the latter associated with conservative results is often applied, and then parametrically investigates the fatigue damage of the helical wire. The results indicate that the critical position is located near touchdown point, and seabed stiffness, helical wire lay angle and the top end position have a significant effect on the fatigue damage.