Fatigue life estimate of metallic chain links of mooring systems assuming out of plane bending: From constant amplitude to random loading

Abstract

In this work, a numerical and analytical study of the fatigue life of chains in the mooring system of FPSO platforms is carried out. Initially, analytical equations are proposed to calculate the stress at the hotspot of the critical chain link, considering axial stresses generated by axial loads and out-of-plane bending (OPB). The analytical equations consider the operating angle of the mooring systems as well as the interlink angle of the links. Thus, the validation of the equations is done by comparing the stress values obtained through numerical simulations of the mooring-fairlead system, where a satisfactory correlation was observed. In addition, loads with constant and variable amplitude are generated as well as random load histories, always between the maximum and minimum operating load of the system, which represent 1/3 and 1/6 of the minimum breaking load (MBL) of the chain link. Based on the Smith-Watson-Topper (SWT) approach, the fatigue life of the chain was estimated, considering the analytical value of the stress at the hotspot based only on the axial contribution and later, due to the axial and OPB contributions. Finally, it was verified that the analytical equations were able to determine the stress values at the hotspot and that the effect of normal stress at the hotspot due to OPB can cause a severe loss in the fatigue life of the mechanical component.