Analysis of internal corrosion in subsea oil pipeline

• Oil leakages of corroded subsea pipeline occurred after 27 years in service.
• The presence of chloride ions in the water phase promoted flow-induced corrosion.
• The corrosion rate could be reduced by controlling flow patterns.

Failure of a subsea crude oil API 5L X52 steel pipeline which led to oil leakage has been reported to occur after 27 years in service. Some leaks were found to form at the bottom of the horizontal API 5L X52 steel pipeline near an elbow section which connected the pipeline to a riser. The present investigation aims to analyze the main cause of failure by conducting standard failure analysis methods including visual examination, chemical and mechanical characterizations, metallurgical examinations using optical microscopy in combination with scanning electron microscopy (SEM) equipped with energy dispersive X-ray (EDX) analysis and corrosion test using a three-electrode potential technique. Results of this investigation suggest that the cause of failure is electrochemical corrosion combined with mechanical process known as flow-induced corrosion. The failure mechanism is discussed with specific attentions are paid to fluid flow rate and chloride-containing water phase.