SCORCH JIP: Examination and Testing of Severely-Corroded Mooring Chains From West Africa

The paper describes a forensic investigation performed on severely corroded(pitted) chains recovered from a FSO mooring system in West Africa. During theinvestigation, it became apparent that a similar phenomenon had beenexperienced by JIP participants operating at other locations in West Africa, indicating that it may be a common problem deserving attention. The tentativeconclusion of the present investigation is that the large pits most likely canbe attributed to Microbiologically Influenced Corrosion (MIC). Subsequent pulltests of the chains to determine their residual strength gave surprisingly goodresults. Despite the large reduction in cross-sectional area, the effectivebreaking loads of the tested samples were found to be around 80-90% of thecatalogue minimum breaking load (MBL). The results also showed the chain linksto be resilient in strength. Introduction This paper summarizes some of the on-going work by the SCORCH JIP oncorroded/pitted chain links that were recovered from a Floating StorageOffloading (FSO) system based in West Africa. Although these links were inservice for only seven years, they experienced severe pitting corrosion (Figure 1) not noted previously in any available records. In some cases the pittingcaused a reduction in cross-sectional area of 35%. The chain links, whichexhibited strong signs of corrosion, were donated by the operator to the JIPfor research. They were shipped to Vicinay Cadenas in Spain for examination andtesting. This was performed according to the SCORCH JIP examination procedure. It is designed to gather all information required to perform a scientificanalysis in order to explain the observed corrosion. The FSO and its mooring system were installed in shallow water offshore WestAfrica in 1997. The external turret mooring system used a 6-leg all-chaindesign. The mooring chains were not isolated from the vessel hull, and thusmight have had some limited coverage by the Impressed Current CathodicProtection (ICCP) system on the vessel. The mooring legs consist of foursections, provided by three different vendors (numbered 1 to 3), as illustratedin Figure 2. Initially, there were three segments, a top and a bottom sectionprovided by the same vendor, with a catenary inflection weight (CIW) insertedto increase clearance and avoid clashing with the bow. Section 2 hangs from theCIW, serving as a clump weight. Section 3 was inserted between the CIW and theground chain when the FSO was relocated to a slightly deeper water depth. Allthree types of chain are 76mm in diameter. An early hypothesis theorized thatthe dissimilarity of the three materials was the root cause of the observedsevere pitting corrosion. The present investigation disproves thishypothesis During the annual chain survey in 2005, some visible corrosion on the chainsimmediately above waterline was noted. To determine the extent of metal loss, rope men were sent down to measure those chain links in air using a caliper. The survey showed that the in-air links had lost some cross-sectional area, apparently due to the well-known splash-zone effect [5]. Their remaining areaswere still within allowable limits at that time. During inspection in 2007, pitting corrosion was observed for the first time during a diver inspectionafter heavy marine growth was removed (see Figure 3). However, the extent andseverity of the problem was not evident at the time. In 2009, a decision wasmade to change out the mooring system. When all chains were recovered to terrafirma and marine growth was removed, it was found that the condition of thesubmerged chains was as poor as the splash-zone chains, although the damage wasof a different form.