The present paper addresses fatigue crack-growth for free-spanning pipelines. The main sources of cyclic stresses which cause the crack-growth are vortex-induced vibrations (VIV) of the pipeline in the cross-flow and in-line directions. In the presence of initial weld defects, such cyclic stresses may lead to leakage and sudden fracture. The crack-growth process is modelled using so-called line-spring elements. These are matched with shell elements which are applied for modelling the pipe itself. The crack-growth is simulated by performing several simulations with different crack sizes. The shape of the crack also allowed to vary during the growth (i.e a/c-ratio). The static equilibrium position of the pipeline for a specific free span is first established by the non-linear Finite Element program ABAQUS. The line-spring elements are matched to interface with the shell elements which represent the pipe outside the region where the crack is located. Based on such simulations, the stress intensity factors at the crack-tip are computed. These calculations are performed for several different crack-sizes. Finally, the remaining fatigue life is estimated by means of fracture mechanics in terms of analytical and semi-empirical approaches.

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