Requirements for both manufacture of chain links and in-service discardcriteria for inspection are well established by industry standards. Howeverstandards for discard criteria do not take into account one key element of themanufacturing process: the bending of round barstock around a form. Thiselement of manufacturing typically decreases the link diameter at the crown inthe axial direction, while potentially increasing the diameter in theperpendicular direction. When the axial crown diameter of in-service chains ismeasured, the discrepancy between the original manufacturing bar stock diameterand the in-service measurement values can lead operators to believe theirmooring systems are experiencing higher wear-corrosion rates than wereaccounted for in design.
To meet standards, the bar stock diameter of new chain links must be abovenominal diameter in all regions except the crown in the intergrip region. Aslong as the average of the two diameters within the link crown, axial andperpendicular, is at or above nominal diameter, the diameter at the crown maybe decreased by certain amounts, as defined by DNV and API.
Issues arise during time of in-service inspection because the axial diameter atthe crown is only accurately measured when there is tension on the chain. Theend-of-life discard criteria for chain links is based on nominal barstockdimensions, and does not take into account the axial crown diameter reductionthat occurs during manufacturing.
DNV and Lloyds Register both define chain discard criteria, for permanentlyinstalled mooring systems, as a 2% diameter reduction based on end of lifebreaking strength. The end of life breaking strength equates to the nominaldiameter (Dnom) minus the corrosion allowance (CA) built into the design. Basedon this definition, it is possible for newly manufactured chain to beapproaching in-service discard criteria before being used at all.
The discrepancy in nominal verses actual crown diameter should be determined bymeans of a baseline inspection prior to installation. Otherwise, following thefirst class survey at 5 years, asset owners/operators may believe that thecorrosion rate on the mooring system is in excess of 1mm/year, and that thechain will need to be replaced.
Welaptega has performed many in-service chain measurement surveys yieldingresults similar to the situation described above. As a result, Welaptega hasbeen informing the industry of the importance of pre/post-installation baselineinspection surveys to determine the actual chain dimensions, link length andaxial crown diameter and subsequently corrosion/wear rates present.
