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