Prediction and analysis of fatigue damage due to cross-flow and in-line VIV for marine risers in non-uniform current

• A practical prediction model for CF and IL VIV fatigue of risers is proposed.
• The proposed model is validated against two flexible riser experiments.
• Important variables are invested to discuss the effects on VIV fatigue damage.
• Aluminum alloy riser shows significant benefits in VIV fatigue.

Fatigue damage due to vortex induced vibrations (VIV) is one of the most significant failure modes for marine risers. Previous studies of fatigue prediction and analysis for marine risers typically focus on cross-flow VIV due to its larger response amplitude than in-line vibrations. In this study, a prediction model of VIV fatigue damage for riser accounting for both cross-flow and in-line vibrations is proposed based on the energy equilibrium theory and the forced vibration experimental data of a rigid cylinder. The coupling effect of CF- and IL-VIV is taken into account and the response amplitude is corrected by a weighted factor according to the input power within the overlap excitation region for each vibration mode. The experimental models of flexible riser under stepped and sheared current are simulated to validate the proposed approach and the results show reasonable agreement. Moreover, based on the prediction model, the top velocity of sheared current, top tension, internal fluid density and the material of structures are invested to discuss their effects on the fatigue damage. The study may be worthwhile for fatigue assessment and broaden the understanding of VIV fatigue damage characteristics for deepwater risers.