Out-of-Plane Bending Testing 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 or 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 full scale test frame was constructed that has the capacity of applying inter-link rotation to a pre-tensioned chain. Although the test frame limits the number of links that can be tested together as a chain, a significant amount of testing was performed for the following chain sizes: 1. 81 mm Studded Grade R3S. 2. 107 mm Studdless Grade RQ3. 3. 124 mm Studless Grade R4. 4. 146 mm Studless Grade RQ4. Various pretension levels were used, with instrumentation to extract link angles and chain link stresses. In this paper the OPB mechanism is explained, and the test frame and results are presented. An empirical relationship is found to predict the OPB stresses in the chain links as a function of pretension and inter-link rotation. The OPB stress relationship obtained was applied to the failed mooring chain of the off-loading buoy with reasonable agreement. 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.

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

“…The details of each scenario are listed in the Table 2. For Scenario 1, the outside of crown cross-section (C-C cross-section), the inner surface of the beginning of the bend area (B-B cross-section) and the inner surface of weld section (A-A crosssection), as shown in Figure 6, are treated herein as three potential locations that are prone to fatigue failure [6].…”

“…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.…”

“…Ter Brake et al [5] presented an S-N curve based fatigue analysis for mooring chains, in which the effects of the OPB on mooring chain stresses were determined by a FE analysis of chain links. 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].…”

“…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.…”

Deepwater Oil Export Systems: Past, Present, and Future

“…[1] and Ref. [2]). However, it was successfully repaired in May 2004 and has been operating normally since then.…”