Fatigue life estimate in metallic chains links of mooring system

Resumo

This contribution proposes the application of the Finite Element Method (FEM) for the study and
analysis of chain links of the Floating Production Storage and Offloading (FPSO) ship anchoring system, regarding
the level of stress at the hotspots. Besides that, cyclical flexion occurred outside the main plane of the links, known
as Out of Plane Bending (OPB) was considered. The loads increase the resulting friction allocated in the contact
between the links, making it behave like a bezel, so transverse forces result in bending out of the plane. The
loadings come due to the platforms operating under various factors of nature, such as variations in waves and sea
currents, in addition to the operating conditions of the platform. Thus, FPSO ships, designed to last more than
twenty years, recorded cases of failure in the anchoring system in less than two years. The material used in the
model of the links was offshore steel belonging to grade R4. The simulations were carried out with winding angles
of 60°, with loads in the range of 200 and 400 ton being applied. With the result of this analysis, it is possible to
validate the hotspots in the links of the moorings, identifying the influence of the winding angle. Thus, it is possible
to validate the equations that describe the behavior of the mooring-fairlead set taking in consideration the
transverse and axial loads. Finally, the fatigue life of the links was calculated using the Smith-Watson-Toper
(SWT) fatigue criterion, considering a loading with variable amplitude.