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Abstract Free spans of a subsea pipeline may undergo vortex-induced vibrations (VIVs) and could be damaged by fatigue failure. The free span length is a key parameter for determining the structural natural frequency, which would correspondingly influence the reduced velocity (Vr) for the VIV responses. The existing flume tests 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 free spanning pipeline could be up to ten-millions, and the very-high-cycle fatigue (VHCF) could be encountered during the engineering service. On the basis of the dimensionless vibration amplitude A/D-Vr curve and the recommended S-N curve for the high-strength steel pipelines with cathodic protection under seawater environments, a prediction method is proposed for the service life of a free spanning pipeline undergoing VIVs. A case study is then performed to evaluate the service life (in terms of vibration cycles) of the free-spans with the focus on VHCF. It is indicated that, if the characteristic flow velocity is given, the variation of the service life with the span length is generally nonlinear, which is attributed to being involved in various VIV branches of the A/D-Vr curve.
Song et al. (Sun,) studied this question.
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