FEA of Out-of-Plane Fatigue Mechanism of Chain Links

Abstract

Several mooring chains of an off-loading buoy failed after only 8 months of service. These chains were designed according to conventional fatigue assessment using API RP 2SK T-N curves to a fatigue life of 20 years with a factor of safety equal to 3 on life. Of particular interest is that the mooring chain failure underwent significant mooring chain motions that caused interlink rotations. Although traditionally neglected, these interlink rotations, when combined with significant chain tensions can cause bending stresses in the chain links. In this paper we identify a mechanism, here identified as Out-of-Plane Bending (OPB) that explains the extensive fatigue damage causing the mooring chains of the off-loading buoy to fail. A previous paper [4] presented experimental results of applying inter-link rotation to a pre-tensioned chain. Various pretension levels were used, with instrumentation to extract link angles and chain link stresses. In this paper, the physics of the OPB mechanism is examined through finite element models of the 124mm chain link tests. The various modes of interlink rotation are examined. The proof loading procedure that the chain undergoes at manufacture is identified as a likely cause for creating a tightly mated surface that is conducive to activating the OPB mechanism. To comply with Single Buoy Moorings (SBM) requirements addressing publication of internal research, many of the graphs included in this paper have had the stress values removed from the y-axis. However, with SBM’s management approval, some numerical references to stress amplitudes remain in the text. Overall, this limitation does not detract from the study, trends are evident and relevant comparisons can be made.

Failure of Chains by Bending on Deepwater Mooring Systems

“…OPB stress vs interlink angle curves presented in the present paper are confirmed by FEA, see Ref [1]. Sufficient data must be gathered to improve our understanding of stresses in chains under high pretensions.…”

“…[1]) of two links has shown that, under a tension equal to proof load, the surface of contact between the two links is permanently deformed, see Figure 6. Plastic elastic FEA (see Ref.…”

Fracture mechanics analysis for a mooring system subjected to tension and out-of-plane bending

“…For Scenarios 2 and 3, the most critical location of mooring chain links prone to fatigue damage moves to the middle of crown section (D-D cross-section) as shown in Figure 8 [6][7][8]. Figure 9 shows a typical normal stress distribution over the cross-section described in Figure 8.…”

“…Melis et al [6] presented experimental results of mooring chain stresses induced by OPB. Vargas and Jean [7] compared mooring chain stresses calculated by FE method with experimental results of Melis et al [6]. Lassen et al [8] and Lassen et al [9] performed a mooring chain fatigue life using an S-N curves based approach, and the behaviour of chain links subjected to tension and OPB was investigated by both experimental and FE methods.…”

Crack initiation characteristics of ring chain of heavy-duty scraper conveyor under time-varying loads

“…Considering the fatigue damage of ring chain, Li 4 summarized the fatigue damage mechanism of ring chain in coal mine and its influencing factors. Melis et al 5 and Vargas and Jean 6 analyzed the fatigue damage mechanism of marine mooring anchor chain. Dong et al 7 proposed the empirical prediction method of fatigue life of ring chain of scraper conveyor.…”