Monitoring pipelines for microbial corrosion can be challenging because obtaining samples is difficult. This is particularly true of subsea pipelines where the only available samples are oil samples and pig returns. Oil samples do not provide reliable data regarding microbial concentrations in biofilms on internal pipe surfaces and the heterogeneous nature of pig returns further complicates the situation by making it difficult to obtain representative samples. Moreover, there is no consensus regarding the best testing method to be used. Microbial growth tests have the longest history of use in the industry, but have more recently been questioned for being too slow, and for not providing comprehensive microbiological data. Test methods other than microbial growth tests, such as ATP (adenosine triphosphate) quantification and genetic testing (quantitative polymerase chain reaction or qPCR), can be used to monitor microbial populations in oilfield samples. This manuscript provides case study data from subsea crude oil pipelines that addresses the questions of how to obtain the best quality samples from pig returns for microbiological testing, and what are the relative merits of different test methodologies.
Microbiologically influenced corrosion (MIC) can cause pitting corrosion on the internal surfaces of pipes and can pose a serious corrosion concern. 1,2,3 Monitoring pipelines for internal corrosion and MIC is an important aspect of a corrosion monitoring program. Since MIC results from microorganisms growing as biofilm attached to pipe surfaces, biofilm samples provide the best data for monitoring MIC in pipelines. Biofilm samples can be obtained from corrosion coupons and from pipeline pigging operations. Subsea pipelines present corrosion monitoring challenges because some of the monitoring data available with on-shore pipelines are simply not available with subsea pipelines.4
Corrosion coupons are small pieces of metal that are inserted into pipelines and left in place for 3-to-6 months and any loss of weight of the metal coupons is used to calculate corrosion rates in a pipeline.