Monitoring of Microbiologically Influenced Corrosion (MIC) in the oil industry has in the past been conducted mainly on sulfate-reducing bacteria (SRB) using cultivation based techniques. However, with the introduction of novel DNA-based methods for enumeration of microbes, more accurate and fast methods are now available which offer better determination of MIC on a routine basis.

The content of produced water in the Halfdan oil system during production is relatively low (BS&W < 1–2%) and MIC has for this reason not been considered to be a threat to the system integrity. Initial results from wall thickness inspection of oil export spools have indicated on-going corrosion. As a consequence, spools have been replaced, enabling sampling of cut-out sections of the spools. These cut-out sections have been sent onshore to DTI Oil & Gas for further investigation of general corrosion, MIC and scale composition.

The investigation demonstrated that the analysed Halfdan oil export spool sections were highly corroded. This conclusion was based on visual inspection of the metal surfaces of the cut-out sections that revealed presence of under-deposit and pitting corrosion underneath thick layers of solid scale material. In support of the visual indications of corrosion, it was demonstrated that MIC related microorganisms were present in the solids in close contact with the metal surface together with corrosion products. In particular, high numbers of the hydrogen consuming methanogens (108–109 cells g−1) were situated in the scale directly in contact with the carbon steel pipe wall. These important findings could only be achieved when results obtained from X-Ray analysis (WDXRF) and visual inspections of the oil export spools were combined with DNA-based enumeration methods (qPCR).

Based on the obtained results, this paper discusses how microbial numbers obtained from DNA-based enumeration methods are evaluated and interpreted in the best way with respect to general risk assessment and system integrity measures of aging offshore assets in the Danish Sector of the North Sea.

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