Failure of crude oil pipeline due to microbiologically induced corrosion

A 25·5 km long and 28 in. diameter underground pipeline used for transporting wet sour Arab Light grade crude oil failed after 3 years of service. The pipeline was operated at 40% of permissible flow velocity and capacity and protected under a corrosion inhibitor programme. The failure occurred due to corrosion of the lower surface of the pipeline resulting in pinholes within a period of one month. Eight corrosion damaged areas were detected over the entire length of the pipeline. Ultrasonic non-destructive inspection and close potential surveys were conducted on the failed pipeline. Various sections of the corrosion products were metallurgically evaluated using scanning electron microscopy coupled with energy dispersive X-ray spectroscopy. The corrosion product was identified using photometric and potentiometric analysis combined with X-ray diffraction. Crude oil and water samples obtained from the pipeline were also analysed. The material of the pipeline was identified as AISI grade C1018. Ultrasound testing revealed localised pitting at failed locations, while close potential surveys ruled out external corrosion. Experimental results indicated that the failure was caused by extensive microbiologically induced corrosion at the internal surface of the pipeline. Sulphate reducing bacteria present in the crude oil, along with below standard flow velocity of the latter, is thought to be responsible for the observed microbiologically induced corrosion. Replacement with a 20 in. diameter pipeline was suggested as a permanent solution to the corrosion problem, whereas a number of short-term measures were also recommended.