Field studies of microbiologically influenced corrosion of mooring chains
Journal
Proceedings of the Annual Offshore Technology Conference
Journal Volume
4
Pages
2910-2927
Date Issued
2016
Author(s)
Witt, D., Ma, K.-T., Lee, T., Gaylarde, C., Celikkol, S., Makama, Z., Beech, I.
Abstract
Severe corrosion found on steel mooring components (CSMC) at several sites worldwide has caused concern in recent years as to whether the components can safely meet their design life. A pilot study was initiated to understand the underlying corrosion causes with the aim of developing successful CSMC mitigation methods. In 2014, a field test was conducted offshore at two different locations in West Africa in order to confirm the contribution of microbiologically influenced corrosion (MIC) to CSMC. The study provided evidence that MIC is a root cause of the observed severe corrosion in the form of mega-pits at one of the two test sites. The tests consisted of deploying carbon steel coupons on a fiber rope, herein referred to as a "microbial baiting kit", at facilities near the mooring systems to capture the biofilm forming microorganisms. The kit was submerged approximately three meters below the water surface for an extended period of time allowing for free swimming microorganisms to colonize the coupons. The kit was the first of its kind to be used in the industry for investigating MIC of mooring systems. Upon recovery of the coupons, pitting damage was revealed underneath the fouling deposits. Following DNA extraction, subsequent analysis of sequences representing fragments of the bacterial 16S rRNA gene demonstrated that, regardless of the test location, the outer part of the biofilm formed on coupon surfaces had significantly different microbial community structure when compared to the surrounding seawater. In both test sites, biofilm DNA analysis confirmed that obtained bacterial sequences represented the initial colonizers of submerged structures in marine environments. Sequences identified as belonging to sulfate-reducing bacteria (SRB), which are considered major contributors to MIC in suboxic/anoxic aquatic environments, were more abundant in biofilms but scarce in water samples. A higher number of SRB sequences were associated with coupons retrieved from the test location where pitting attacks were prominent. Sequences indicative of acetic acid-producers and non- SRB hydrogen sulfide-producing microorganisms, that are also likely MIC contributors, were identified; however, further work is required to prove the involvement of these prokaryotes in steel deterioration. The results and finding from this pilot work set the stage for a comprehensive Joint Industry Project (JIP) launched by DeepStar® and is entitled DeepStar® CTR12402 Integrity Management of Mooring Systems Against Corrosion JIP. The aim of the DeepStar® JIP is to determine possible measures of MIC mitigation.
SDGs
Type
conference paper