Abstract

One of the challenges facing the offshore oil and gas industry is the identification and quantification of internal corrosion for both topside and subsea process facilities. An increasing major cause of system failures and loss of production is that due to the presence of microorganisms that lead to the onset of Microbiologically Influenced Corrosion (MIC) in the offshore system [1]. Throughout the service life of the Otter oil production spool, internal corrosion was identified during the inspection programmes indicating severe metal loss of up to 1.7 mm/y for the Oil Topside Production spool. Internal corrosion and degradation were also detected in the subsea oil production pipeline during the 2010 and 2012 In-Line Inspection (ILI) programmes, where metal loss values of 12.5 mm depth were detected. Corrosion coupons and probes have confirmed that internal corrosion was active. The worst cumulative metal loss based on corrosion modelling indicated a corrosion rate of 0.22 mm/y. Furthermore, the microbial surveys performed using the most probable number (MPN) method has reported very low numbers of microorganism within the system.

Furthermore, the investigation of the spools when removed from topside, has confirmed severe internal corrosion aligned with inspection data. Samples of black deposits directly in contact with the carbon steel were taken for microbiological analysis and the novel DNA-based enumeration method quatitative Polymerase Chain Reaction (qPCR) has been used to assess whether or not MIC has occurred and to identify the key microorganisms responsible for this corrosion mechanism. High numbers of microorganisms were detected that could not be identified during service life indicating that MIC was in fact the primary mechanisms responsible for the damage found. Based on the results obtained, this paper discusses how the degree of internal corrosion was previously underestimated and how the microorganisms responsible for MIC that have been present were not properly identified and quantified. The paper also gives directions on how to strengthen MIC Threat Assesssment in the future.

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