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Cladded Pipeline Inspections for Metal Loss: Why UT Matters
Authors: Suji Kurungodan, Project Manager, UAE | Ibrahim Abu-Aisha, Key Account Manager, UAE
The question "Why ultrasonic technology for Corrosion Resistant Alloy (CRA) pipelines?" is often brought up when I speak with operators.
As industry demand increases for the transportation of corrosive products under extreme operating conditions, CRA pipelines have become the most prevalent choice for new construction pipelines.
Moreover, as part of a fully-fledged integrity management program, we are also noticing an increased awareness by operators that operate CRA pipelines for the need to not only inspect the existing carbon steel base material but also the internal alloy and austenitic layers on the inner surface of the pipeline. Potential corrosion and cracking within the stainless steel clad or carbon steel, preferential pitting features in the corrosion resistant alloy and disbonding between the stainless steel clad and carbon steel, all present integrity threats that often need a comprehensive inspection solution.
As far as Magnetic Flux Leakage (MFL) technology is concerned, CRA pipelines are unable to be inspected due to the material being non-magnetic. Therefore, many operators turn to Ultrasonic Technology (UT) for a precise, complex inspection.
The principles of UT enable a direct measurement of the pipe wall thickness, which can inspect both CRA layers and base metal of these pipelines, provided that CRA pipe is metallurgicallly-cladded.
By applying UT direct measurement principles, operators are sure to get an absolute and not a relative measurement of their wall thickness, which serves as a "blue-print" reference for the pipeline either immediately post construction or during its operation cycle. Furthermore, as UT relies on sound wave propagations through metals, it can be utilized on higher wall thickness pipelines, irrespective of the internal CRA.
There is also a practical awareness within the industry that the improved corrosion resistance often offered by CRA does not extend to the girth welds or heat affected areas (HAZ) because of their sensitization due to the formation of microstructural gradients and intermetallic phase recrystallizations. Higher-resolution and detection capabilities maintained by UT in the heat affected zones by enabling non-contact direct measurement enhances the value of these inspections by addressing pitting threats in girth weld / seam weld HAZ.
In conclusion, the added accuracy and sizing capabilities of UT provides an extremely important factor for implementing a robust integrity management program and preforming remaining life assessment calculations.