Case study:

Pitting corrosion of mooring chains - a case of microbiologically influenced corrosion?

The customer: Global operator, West Africa, November 2020

NCIMB partners with ICR in the delivery of full microbiological and corrosion monitoring surveys of oil and gas production facilities, as well as investigations of unexpected failures. ICR is an industry leader in integrity monitoring solutions offering clients around the world independent and technical consultancy tailored to their specific needs.

This case study describes work undertaken partnership with ICR for a global operator in West Africa.


The Issue: Premature mooring chain failures

Over the past decade there have been several instances of premature mooring chain replacements due to unexpected failures. On retrieval of the mooring lines, it has been found that the upper chain sections have been significantly affected by the phenomena known as ‘mega pits’ (see Figure 1). This phenomena occurs in tropical waters, and several FPSOs off the coast of West Africa have been particularly badly affected. 
Two photographs side by side labelled A and B. Image A shows a piled up mooring chain. One link as a large pit. Image B is a close up of the large pit.
Figure1: Pitting corrosion "Mega pits" as seen on retrieved mooring chains

The Requirement: Microbial analysis of samples from an FPSO mooring chain

It was suspected that in these cases, the main corrosion mechanism was microbiologically influenced corrosion (MIC), which then influenced the rate of cracking. Previously, several investigations have been criticised for determining that the root cause was MIC with little, if any, microbiological data to confirm this conclusion. Although MIC is often associated with pitting and unexpected failures in marine environments, it is not the only possible explanation. If further unexpected failures were to be prevented, it was necessary to undertake a microbiological investigation to provide evidence in order to confirm the involvement of MIC. 
Figure 2: Section of retrieved mooring chain showing extensive pitting

The Service: Provision of sampling kit and work instruction

ICR Integrity and NCIMB were asked to undertake the analysis of samples from several mooring chains from an FPSO moored off the coast of West Africa (See Figure 2).

An important aspect of this service was the provision of a sampling kit and detailed work instruction that enabled a local crew member on site in West Africa to take samples using the correct procedure.  The kit provided ensured the samples were preserved during transportation to the oilfield microbiology laboratory at NCIMB, and arrived in good condition for the analysis.

The Service: Analysis of samples from retrieved mooring chains

The sampling kit and procedure were dispatched to the chain retrieval vessel and the samples were taken by a member of the crew from five different depths of a single mooring line once the chain was safely secured on the deck of the retrieval vessel.

The samples were analysed using both quantitative polymerase chain reaction (qPCR) and next generation sequencing (NGS), as combining these two analytical methods allows the generation of comprehensive data on the microbial populations present in the biofilm on the chains.

The Outcome:

The qPCR results indicated the presence of moderate to high numbers of bacteria and archaebacteria. The threat of MIC is associated with high numbers of microbes, but additional information is required to fully assess if MIC is involved in the corrosion process. Several samples were also analysed using NGS and it was determined that a high percentage of the organisms present in the communities were sulphate-reducing bacteria (SRB) and other groups of bacteria involved in the sulphur cycle. The results indicated that five samples were dominated by sulphate reducers but also highlighted the dominance of a single genera - Desulfobacter. The results are shown in Table 1.
As SRB are one of the main groups of organisms implicated in MIC, the predominance of SRB in the communities from the mooring chains give confidence to the theory that MIC was one of the mechanisms involved in the formation of the mega pits. High numbers of sulphate-reducing microorganisms such as SRB, sulphide oxidisers and thiosulphate-reducing bacteria were present in the samples from the chains. An example of the NGS results is given in Table 2. In contrast, numbers were low in samples of the seawater. Also, several samples contained high concentrations of sulphide (which is an indicator of SRB activity), whereas the seawater samples did not.

Table 1: qPCR, NGS and sulphide data from 5 depths and control sample

Locaton qPCR Total bacteria/cm2 qPCR SRB/cm2 NGS SRB/cm2 Sulphide mg/cm2
Link depth 1 2.7 x 105 1.9 x 105 1.8 x 105 <0.2
Link depth 2 3.7 x 104 6.5 x 103 1.1 x 104 5.06
Link depth 3 7.9 x 105 8.2 x 105 4.8 x 105 <0.2
Link depth 4 8.1 x 104 7.0 x 104 6.5 x 104 2.53
Shackle depth 5 1.3 x 105 6.0 x 104 8.8 x 104 4.23
Seawater (control) sample < 2.0 x 101 N/A <0.2

Table 2: NGS analysis results for Location C, chain link 370

Relative abundance % Identification Level of confidence
64.18 Desulfobacter curvatus Family
10.25 Pseudodeslfovibrio aespoeensis Family
2.35 Desulfobacter psychrotolerans Family
2.06 Parcubacteria Class
1.33 Desulfovibrio dechloracetivorans Family
1.25 Desulfotignum balticum Genus
1.12 Dethiosulfatibacter (uncultured organism) Genus
16.45 Other organisms<1% contribution N/A


These phenomena appear to occur in temperate waters, where there are high levels of nutrients in the water and actions to mitigate against the formation of these megapits are limited. However, with this information, the client can now increase their inspection procedures with a view to detecting issues before a failure occurs. It may be possible to clean the chains as removal of the marine growth may slow the rate of pitting corrosion.

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