Water injection is one of the methods for maximizing oil production and improving oil recovery by way of maintaining reservoir pressure. It entails injection of water in sufficient quantities with acceptable quality into the appropriate zones of the reservoir in a cost efficient manner. Treated water is transmitted from treatment to well generally through carbon steel pipelines; as there are flexible pipelines, internally coated pipelines being used for water injection also. And this paper is solely for carbon steel pipelines. The corrosion of these water injection pipelines is a major issue for the Oil & Gas industry with a view to preserve the reservoir permeability and integrity of the pipelines.

A few case studies pertaining to failure of API 5L carbon steel subsea water injection pipelines with their root cause analysis are presented in this paper. In order to identify the cause and mechanism of failure, detailed laboratory and analytical investigations were carried out with a view to characterize the material property as well as to evaluate the corrosivity of operating environment. Studies like visual inspection, non destructive testing, corrosion product analysis, elemental compositional analysis, metallographic investigations, inclusion content test, hardness testing, tensile strength studies, impact test, stereomicrscopy, scanning electron microscopic (SEM) coupled with energy dispersive spectroscopy (EDS) studies, have been carried out. Process parameters, operating & maintenance practices, chemical treatment program with quality parameters etc. have also been critically examined.

Pipeline material was found to conform to the specification API 5L. The analysis showed that the most probable cause of the failure of the pipeline was under-deposit/grooving corrosion, which started with oxygen and Microbiologically Influenced Corrosion due to Sulphate Reducing Bacteria and was subsequently supported by very low flow velocities in those pipelines. The stagnant water and the formation of thick deposit consisting of corrosion product, sludge or suspended solids and biomass as a result of proliferation of bacterial colonies were the conditions for attack to set off. Galvanic corrosion further accelerated the metal loss process as grooving/channeling resulting into failure at 6 O'clock position. Presence of large number of inclusions in some of pipeline materials was an additional cause of accelerated corrosion process.

On the basis of root cause failure analysis, the major integrity threats to the water injection pipelines resulting from process control were identified. Accordingly, preventive and remedial measures essential for preserving integrity of water injection pipelines have been suggested. Implementation of good operating and maintenance practices, robust quality control measures, continuous monitoring of corrosion trends, prompt corrective actions are the key requisites to abate failures due corrosion.

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