Pitting corrosion of offshore water injection steel pipelines

The internal corrosion and pitting of water injection pipelines (WIP) is of much interest in the oil and gas industry. Water injection is used to aid the recovery of oil or gas from reservoirs that are nearing depletion. Most pipelines are low alloy high strength steels, many kilometres in length, not cathodically protected and placed on the seafloor. In some cases after some years service very severe internal corrosion, mainly pitting and channeling corrosion is observed, typically at the 6 o'clock position and sometimes at transverse welds. Various causes have been proposed. However, a critical issue is the depth of corrosion pitting that is likely to occur as the pipeline continues in use. Data extracted from intelligent pigging records for a number of water injection pipelines is presented. It is examined in the framework of conventional Extreme Value analysis. The data do not follow a Gumbel EV distribution as would be expected from fundamental theory and from (some, shortterm) past experience. All the pipelines considered show similar trends for departure from the ideal, theoretical Gumbel distribution. As a result, extrapolation to longer pipelines or longer exposure periods is problematic. The reasons for these observations are considered and recommendation made as to how the data can be used in the most productive way for estimation of likely future maximum pit depth