Field Studies of Microbiologically Influenced Corrosion of Mooring Chains

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 Љmicrobial baiting kitЉ, 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.

16S rRNA gene profiling of planktonic and biofilm microbial populations in the Gulf of Guinea using Illumina NGS

“…The Gulf of Guinea samples showed <0.005% zetaproteobacteria OTUs, suggesting that, at this particular site, ALWC was not a major concern. The low levels of nitrate in the water (Witt et al, 2016) provided further evidence for this suggestion. Recent work has indicated that steel structures positioned on the sediment layer may be more likely to develop biofilms containing iron-oxidizing bacteria (McBeth and Emerson, 2016).…”

“…In November 2014, coupons were exposed in the upper water column, 3 m below the surface of the sea, 113 km off the coast in the Gulf of Guinea. The pH of the water at sampling was 6.94, it contained <0.001 mg l À1 phosphate and total nitrate, 2960.3 mg l À1 sulfate and 28.45 mg l À1 ammonium ions (Witt et al, 2016). The system was originally designed to withstand exposure in the field for four to eight weeks (28e56 days).…”

“…Populations in the local seawater were compared with those attached to carbon steel coupons, including both those microbial cells removed from the surface by the traditional scraping method, and those remaining on the metal surface after scraping (denominated "outer" and "inner" biofilm, respectively). The corrosion associated with these coupons is presented and discussed in another document (Witt et al, 2016).…”

Microbially Influenced Corrosion of Steel in Marine Environments: A Review from Mechanisms to Prevention

“…This is due to their significant impact on the corrosion of marine structures, such as pipelines, ships, and oil rigs, which can result in economic losses and environmental hazards [ 50 ]. Methanogens and sulfur oxidizers are the primary archaeal groups involved in marine microbial corrosion, as studies have shown [ 18 , 51 , 52 , 53 , 54 ]. Methanogens produce methane by reducing carbon dioxide or organic matter in anoxic sediments or water [ 55 ].…”

pH Effect on Microbial Corrosion by <i>Pseudomonas aeruginosa Sp</i>. on Galvanized JIS3502 Steel

“…Near 40% of incidences of biocorrosion in oil reservoir souring, pipelines and fitting, wire ropes for mooring applications have been reported related to microbiologically influenced corrosion (MIC) [1,2]. This form of electrochemical corrosion in the form of crevices or localized pits were observed in many environments facilitated or accelerated by the presence of micro-organisms [3].…”