Free span is a risk of security of submarine pipelines. Fatigue caused by vortex-induced vibration (VIV) is a main failure mode of free spans. The height of free span which influences the VIV fatigue load is an important factor for the fatigue life assessment. In this paper, taking an in-service submarine pipeline as an example, the relation between the height and the fluctuating lift coefficient was firstly investigated by the method of computational fluid dynamics, and the critical height which can neglect its effect on the coefficient was obtained. The VIV structural response of free span with different height and length was analyzed with the finite element method. Furthermore, considering the in-service environment of the submarine pipeline, fatigue life of free span was evaluated numerically with reference to the measured data of flow velocity and its variation with time. Those results provide technical support for the maintenance of free span for the submarine pipeline.

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