Investigation on leakage cause of oil pipeline in the west oilfield of China

摘要

A crude oil pipeline used in an oilfield in the west oilfield of China leaked many times in a short time. Serious corrosion was found near the leaking position of the failed pipe, the wall thickness of pipeline also decreased near the failure section. To determine the reason for this, the failed pipeline was investigated and analysed by macroscopic analysis, chemical composition tests, metallurgical analysis, mechanical property analysis, scanning electronic microscopy (SEM) analysis, X-ray diffraction (XRD) analysis and corrosion simulation tests. Based on the systematic analysis, it can be concluded that the mechanical properties of the failed pipeline met the related standards. The crude oil conveyed in the pipeline contained a large amount of formation water; because the formation water was acidic (the pH value was 5.0-6.5) and had high salinity, the materials of the pipeline had a very high corrosion rate when exposed to the crude oil, which was confirmed by the corrosion simulation test. At the same time, because the fluid flow changed suddenly in the pipeline lifting location, fouling deposition occurred inside the pipeline, which led to serious localized corrosion caused by the small anode/big cathode corrosion galvanic cell reaction, and the wall thickness decreased quickly in a short time. Finally, the pipeline leaked due to fluid erosion and the high speed impact of solid particles and grit in the crude oil. Improvement suggestions are put forward to the pipeline user at the end of this paper, such as improving the pipeline design, reducing the salinity in the conveying fluid, adding a corrosion inhibitor and replacing the pipeline lifting segment with an anticorrosive coating inside the tube; the crude oil pipeline did not leak again after following these suggestions.