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External Corrosion Behavior of Oil Pipelines With Polyethylene Coating Under Pressure Fluctuations

Picture for External Corrosion Behavior of Oil Pipelines With Polyethylene Coating Under Pressure Fluctuations
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External corrosion on buried pipe steels has been demonstrated to be a major threat to pipeline integrity. Although it has been extensively investigated and well understood electrochemically there is no particular model that is widely accepted by industry due to the complex nature of external corrosion. This paper demonstrates that the severity of external corrosion on polyethylene (PE) tape coated oil pipe is exacerbated by proximity downstream of pump stations and suggests mechanistic explanations related to pressure fluctuations. There are very limited studies available on this topic for buried liquid pipelines.External corrosion features reported by the most recent In-Line Inspection (ILI) ultrasonic wall measurement (UTWM) tools for two oil pipelines were correlated with the distance to upstream (U/S) pump stations. ILI data with UTWM is used because the data includes features that are under sleeve. These two oil pipelines with PE coating were investigated for a total length of about 2500 km. The pipelines have been in service for more than 40 years.Analysis of the results shows that external corrosion features with a depth <20% wall thickness (WT) reported by UTWM tools are distributed uniformly along the pipelines. However a clear increase in the frequency of corrosion features with depth >=20%WT is seen within 10 km from the U/S pump station. There is a much more pronounced increase in the density of deeper feature classes in the same region. This trend is further validated by the distribution of field nondestructive examination (NDE) excavations driven by external corrosion. Based on these data it is undeniable that the frequency and severity of external corrosion on these tape coated pipelines is positively correlated to U/S pump station proximity.Possible mechanisms may include mechano-electrochemistry and pressure fluctuations that facilitate electrochemical processes and/or PE tape coating damage: 1) Pressure fluctuations increase dislocation density and thus electrochemical potential ahead of corrosion tip due to stress concentration. 2) Pressure fluctuations may refresh the electrochemical environments inside corrosion and thus accelerating corrosion. 3) The PE coating damage accelerated by pressure fluctuations could contribute to the external corrosion distribution. One of the benefits of the findings in this paper is to help External Corrosion Direct Assessment (ECDA) to locate severe corrosion features especially for the lines that are non-piggable. Also the discoveries can provide guidance for future research and pipeline integrity management with external corrosion.

Product Number: 51319-13199-SG
Author: Yanping Li
Publication Date: 2019
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