Numerical Study on the Out-of-Plane Bending (OPB) Behaviors of Studless Mooring Chain Links in Fairlead Structures Available to Purchase

Mooring chains are Safety Critical Elements (SCE) of offshore Floating Production Systems (FPS). Out-of-Plane Bending (OPB) fatigue has recently been identified as one of the failure mechanisms of mooring chains (Jean et al, 2005), and has become the bottleneck of the mooring fatigue life in many deepwater projects (Brinkhuis et al, 2015; Lassen et al, 2009). In the past decade, extensive experimental and numerical studies have been carried out in the offshore industry to understand and evaluate chain OPB fatigue. However, the majority of these efforts only focus on the interlink behaviors of two adjacent chain links, whereas the presence of the fairlead structures and their interactions with the chain links pertaining to OPB are largely neglected. In the present paper, a numerical study is carried out leveraging non-linear Finite Element Analysis (FEA) to examine the OPB behaviors of mooring chain links in real fairlead structures, on the example of a 7-pocket fairlead wheel (FW). In this study, a comprehensive FEA procedure is established with four 168mm R4S studless chain links and a 7-pocket FW to examine the chain OPB mechanism on a fairlead structure. The model is shown to be able to capture correctly both the interlink mechanics and the contact/interaction between the chain links and the FW. The interlink stiffness curves and the hotspot stress evolutions on both the first and second OPB link are carefully investigated under the full OPB loading cycle. The results reveal that the presence of the 7-pocket FW deteriorates the OPB fatigue due to the additional contact and friction between the chain link and the fairlead structure. In addition, it is demonstrated that the chain link is subjected to significant OPB stresses/damage even if it is fully supported and rested in the pocket of the FW. The implications of these findings on deepwater mooring system design and the optimization are also discussed.