MIC and pitting corrosion on field recovered mooring chain links

Forensic investigations were performed on chain links recovered from two FSO mooring systems in West Africa and Indonesia. Although these links were in tropical seawater for only 7 years, they experienced extremely severe pitting corrosion not previously seen in any available records. The material presented has direct application to ensuring the integrity of mooring systems as it provides insight into the mechanical behaviour of highly corroded chain links, statistics of the observed corrosion rates in these locations and guidelines for examining chain links. Corrosion on the surface of the links was mapped using 3D photogrammetry. The observed long-term corrosion rate is significantly higher than the average corrosion rate of 0.4 mm/year given in API RP-2SK. Analysis of the elementary composition of the rusts indicates that the large pits observed were most likely associated with Microbiologically Influenced Corrosion (MIC). The obtained results represent a breakthrough in the field of corrosion of mooring chains deployed in tropical seawater, corroborating the latest models that have been developed taking into account MIC. This work was conducted under the scope of the SCORCH JIP which involves ExxonMobil, Chevron, BP, Total, Shell, BHP Billiton, ConocoPhillips, Petrobras, Statoil, BREE, DNV, ABS, Bureau Veritas, INPEX, SOFEC, InterMoor, KBR, SBM, Franklin Offshore, Vicinay Cadenas, Bridon, Arcelormittal, Arrium and Vryhof Anchors.