Justification for Reducing In-Service Weld Inspection Delay Times for Liquid Pipelines

The time to peak hydrogen concentration was the same time that the post weld heating time was applied, which inferred that once the post-weld heating time was completed that the weld would be ready for final inspection. The results of the analysis indicate that it would be justified to reduce the inspection delay time to 30 minutes or less depending on the post-weld heating temperature and pipeline wall thickness.

This work focused solely on the inspection delay time and not the risk of hydrogen cracking and not the probability of a crack occurring. It is important to note that many of the maximum times to peak hydrogen concentration reported were associated with relatively low amounts of it. There was no attempt made to determine what the minimum threshold of concentration that is required to form a crack. It is also important to note that the minimum threshold of hydrogen would be affected by the weld microstructure. The more martensite that is present in the weld HAZ the less hydrogen that would be needed to form cracks.

The authors would like to acknowledge BMT Fleet for allowing access to their diffusion model as well as an anonymous pipeline operator for funding this work towards the goal of reducing the NDT inspection delay time.

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