Mooring Chain Wear Experiments and Findings

Abstract

Whilst mooring chain design practice has traditionally considered wear and corrosion together under a single allowance for material loss, the two phenomena are influenced by different factors whereby they may act synergistically to accelerate degradation rates or act completely independently. As chain wear has been attributed to both the acceleration of corrosion-related degradation and the cause of chain failures and early mooring repairs by itself, it was investigated as part of the SCORCH JIP. An extensive literature review and meta-analysis of wear research was carried out in order to determine a framework for assessing the wear of mooring chains. The literature review found that the phenomenon of wear in general and mooring chain wear in particularly was not well understood, with little agreement as to a general analytical model for wear prediction. The particular wear regimes under which mooring chain wear occurs was identified and a critical analysis of available data on mooring chain wear was carried out where it was determined that experimental conditions were likely to have been outside of the bounds of validity for what would be experienced in the field. A generic formulation for chain wear was presented, where the amount of wear is proportional to the work done at the interlink contact due to the relative sliding of the chain links. Supplementary to the literature review, a series of wear tests of full-scale mooring chain links were carried out under varying load conditions for different grades of chain. These tests were used to determine empirical coefficients for the prediction of chain wear. A model for wear prediction was presented based on the position of the chain in the mooring line, the tension in the line and the relative motion of the chain links. The evolution of the contact surface between chain links was modelled using 3D geometries of idealized chain links in order to determine a relationship between the volume of worn metallic area and the depth of wear, which can be more easily measured in-service.

Modified Method for Determination of Wear Coefficient of Reciprocating Sliding Wear and Experimental Comparative Study

“…The service environment of the anchor chain includes the atmospheric area, splash zone, and full immersion area, and this study mainly focuses on the dry-wear behavior, which is representative of the wear behavior in the atmosphere zone. The cycle of the reciprocating motion of chains is close to a wave cycle, which is approximately 10-12 s [5], and the actual movement angle of the chain link is approximately 1-2° [4,10,14]; however, considering the test cost and test period, the actual wear test cycle is much shorter, and the amplitude is much higher. In Yaghin's study [10], the test cycle was approximately 5 s (0.2 Hz), whereas, in the full-scale test conducted by Pauw [6,16], the test cycle was even shorter (1 Hz).…”

“…For floating structures (e.g., platforms and buoys) in the ocean, six-degreeof-freedom movement is inevitable due to the actions of wind, wave, and current, which then drives a periodic movement of the mooring system (such as mooring chain and mooring cable). For mooring chains exposed to the atmosphere, the relative movement between links results in wear, and for chains immersed in seawater, the wear-corrosion is inevitable [4]. The typical wear morphology of chain links recovered from a mooring chain of a buoy is shown in Figure 1, where severe wear can be observed in the contact region of two adjacent chain links.…”

“…This value is the time-history statistical value of the dynamic tension response of the mooring line under typical sea conditions [14]. Jayasinghe [4] showed that the average wave period was approximately 12 s, and the range of motion between the chains under the actions of a floating body and ocean current was between 1 • and 2 • [10]. Liu [5] indicated that the fretting sliding distance between the contact surfaces of a chain link was approximately 10-50 µm.…”

Time-dependent reliability and optimal design of scraper chains based on fretting wear process

“…According to the wear measurement data of most metal material friction pairs under unlubricated conditions, the law of interlink wear is obtained, that is, the wear volume is proportional to the average tension F c t ð Þ and unit sliding distance DL s (t) in the time interval Dt, and inversely proportional to the hardness of the wear surface H (Zhu et al, 2018;Jayasinghe et al, 2018). As the initial wear is limited to small, only some portions of the surface are removed and a smooth contact wear surface is produced.…”