OPB and IPB phenomena on mooring chain observed by scaled dynamic tank experiment

The high failure rate of mooring systems due to fatigue damage remains a critical industry concern, necessitating a deeper understanding of Out-of-Plane Bending (OPB) and In-Plane Bending (IPB) on mooring chains. While prior studies have investigated OPB/IPB mechanisms in large-diameter chains, their test benches applied only static displacement, without replicating the realistic dynamic catenary response induced by floater motion. Consequently, the direct contribution of floater dynamics to chain bending remains unclear. To address this limitation, the authors conducted a scaled tank experiment successfully measuring OPB and IPB strain under dynamic catenary motion. The experiment utilized 4 mm and 6 mm diameter chains instrumented with micro-sized strain gauges with waterproof treatment to investigate: (i) OPB and IPB behavior, (ii) OPB strain distribution within a link, (iii) strain decay along downstream links and (iv) scale effects between 4 and 6 mm diameter. The results provided new insights into OPB/IPB phenomena and verified the correlation of bending strain with mooring tension, acceleration, and displacement. Based on these findings, the OPB/IPB mechanisms were identified, and new formulas were proposed for estimating both OPB and IPB stresses directly from floater motion, without relying on Interlink angle. This paper details the experimental setup, key measurement results, the stress calculation equations, and a discussion of the limitations in scaled experiment.