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Internal Axial Corrosion in Offshore Pipeline: Inspection and Assessment

Internal long axial corrosion is the most common corrosion type in offshore crude oil and water injection pipelines. It has frequently a complex shape that ranges from a smooth uniform wall thickness reduction to a rugged surface with varying depths. Long axial corrosion anomalies can be reliably detected and sized by means of ultrasonic inline inspection (UT ILI). The rough surface of the corrosion can lead to outliers in the gathered ILI data. Accordingly, an elaborated filtering and re-processing of the inspection data is crucial for a reliable data assessment. The inspection report usually provides maximum anomaly dimensions (total length, peak depth, etc.) and does not sufficiently describe the complex shape of corrosion anomalies. Therefore, methods based on corrosion depth profile (river-bottom profile, RBP) have to be applied for pressure capacity assessment. In addition, corrosion growth rates are ideally obtained by comparing RBPs of consecutive inspections.

This paper outlines the main results of a recent joint industry project that provides guidance to the assessment of long axial corrosion based on UT ILI results. It involves the determination of RBPs, the calculation of the safe operating pressure, the determination of corrosion growth rates and the extrapolation of the future pressure capacity. Compared to other assessment methods which are also based on RBP, the presented assessment approach accounts for a higher probability of failure associated with a higher number of corroded sections in a pipeline.

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