Very High Cycle Fatigue Life of Free-Spanning Subsea Pipeline Subjected to Vortex-Induced Vibrations

Free-spanning subsea pipelines subjected to vortex-induced vibrations (VIVs) are particularly prone to fatigue failure. Existing flume observations indicated that the VIVs of a near-bed cylinder may be triggered effectively in moderate shear flows. This may imply that the vibration cycles of a spanned pipeline could be up to tens of millions. As such, very high cycle fatigue (VHCF) can occur during engineering service. The free span length is a key parameter for determining the structural natural frequency and the corresponding reduced velocity (Vr). On the basis of the dimensionless vibration amplitude A/D–Vr curve and the recommended S-N curves for high-strength steel pipelines with cathodic protection under seawater environments, a prediction method is proposed for the fatigue life of a free span undergoing VIVs. A parametric study is then performed to evaluate the fatigue life of the spanned pipelines with a focus on VHCF. It is indicated that the minimum fatigue life emerges at certain flow with a moderate velocity for a given span length. With a further decrease or increase in the flow velocity, the fatigue life would be enhanced correspondingly, which could be within the VHCF regime. Such nonlinear variation of the fatigue life with the span length and the flow velocity is attributed to being involved in various VIV branches of the A/D–Vr curve.