An Out-of-Plane Bending Fatigue Assessment Approach for Offshore Mooring Chains Considering the Real-Time Updating of Interlink Bending Stiffness

Abstract

Fatigue failure caused by frequent tension and bending loads is a crucial safety concern for mooring chains used on floating structures in the oil and gas industry. The bending effect for a chain’s fatigue is usually not considered by existing fatigue analysis methods, and even if it is considered, existing commercial software can only calculate the constant interlink bending stiffness without consideration of the stiffness variations due to tension and friction. To address this issue, this study develops an in-plane bending (IPB)/out-of-plane bending (OPB) fatigue assessment approach for offshore mooring chains considering the time-varying nonlinear interlink bending stiffness. Initially, the mooring tension and IPB/OPB angles are calculated by an in-house code MeCAP (multi-element coupled analysis program), considering the time-varying interlink stiffness between the chain links. The fatigue life of mooring chains for pure tension–tension (TT) and OPB combined fatigue at different hotspot locations (A, B, and C) is calculated by MeCAP-fatigue, a newly-developed module in MeCAP. Based on the analysis model, a comparative study is implemented to investigate the effects of interlink stiffness on fatigue damage. The differences and advantages of the combined fatigue calculation method over the pure TT fatigue assessment method are discussed. The results illustrate that combined fatigue would be underestimated significantly if zero interlink stiffness (hinge joint assumption) is applied or even constant interlink stiffness (calculated by mooring pretension), while pure TT fatigue will be largely unaffected. Moreover, compared to pure TT fatigue, the OPB combined fatigue assessment method, considering the effects of OPB/IPB and the time-varying properties of the connection between chain links, could evaluate fatigue life more comprehensively.