An exploration and production company in Nigeria operates an extensive crude oil trunkline system that connects into an export terminal. One of these pipelines, which extends for 37km, suffered severe internal corrosion soon after being commissioned in 1994. An intelligent pig survey conducted in 1996 revealed a maximum wall loss of approximately 5 mm, equivalent to an average corrosion rate > 4mm/yr. A field campaign was conducted to determine the cause of corrosion, and propose solutions. The main defects were verified with ultrasonics, whereas detailed analyses of the aqueous phase, including on-line measurements were used to investigate the corrosion mechanism. Corrosion was predominantly located at the bottom of the pipeline, at places where the oil/water emulsion breaks down and a continuous water phase can accumulate. Only very small amounts of oxygen were found in the pipeline system and oxygen corrosion was ruled out as the main corrosion mechanism, although oxygen may have played a secondary role through the formation of corrosive species which could stimulate under deposit corrosion. The measured dissolved carbon dioxide content of the water in the pipeline is very low and could not cause significant corrosion in the pipeline. Significant quantities of sulfate reducing bacteria were found in water sampled at all locations. In view of the presence of deposits in the lines, bacterial corrosion is extremely likely to be the main cause of the corrosion in the pipeline. A combination of pigging and a batch biocide treatment has been adopted to control the corrosion.
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