Abstract
Under-deposit corrosion represents one of the most damaging forms of corrosion to the piping system in oil and gas pipelines. It is one of the most critical phenomena leading to pipeline failures, and can occur in sub-sea water injection pipelines, well-fluid pipelines, and large diameter transmission lines. Under-deposit corrosion is typically very aggressive and localized, causing deep penetration of the metal surface with lesser general corrosion in the surrounding areas, due to surface deposits, electrical imbalances or some other initiating mechanisms. In some cases of under-deposit corrosion, pitting is extended throughout the entire metal surface giving it an irregular or very rough surface profile. In other instances, pits are concentrated in specific areas leaving the majority or the metal surface in like-new condition. Microbiologically influenced corrosion is also associated with under-deposit corrosion and may be associated with the development of internal deposits within the piping system. Presence of microbial bacterial population can produce an acidic environment helping to corrode the metal piping at highly accelerated rates exceeding 1.9 mm/year [75 mils/year].
This paper pertains to a study of an API 5L X-60, Carbon steel, injection water pipeline, which failed prematurely in 4 years after commissioning due to under-deposit corrosion. A detailed study was carried out to investigate the failure causes; this included pipeline material constitution, injection water parameters such as suspended solids, turbidity, microbial counts of sulphate reducing bacteria, dissolved oxygen levels, iron contents, pigging operation, chemical addition (ammonium bi-sulphite – oxygen scavenger) and use of water injection corrosion Inhibitor. An event sequence leading to the pipeline failure was proposed.
