Coating is crucial for protecting pipelines in- and outside from negative up to destructive impacts. Predominantly, the coating is associated with an external coating consisting of coal tar or concrete in older pipelines or since the 1990ies by fusion-bonded epoxy (FBE). However, when these protective layers become porous or disbonded, due to improper installation or aging, corrosion can occur manifesting as pitting or larger corrosion pockets, around the bottom position. These defects are well detectable through Inline Inspection (ILI) data, particularly from Ultrasonic Testing (UT) compression wave methods, and field verifications.

However, this paper focuses on internal coating, which is hidden and whose damage is not easily detected in the inspection data or when examining the outside of the pipeline.

Different from the external coating, the inner coating has the function of protecting the metal from direct contact with a challenging and/or aggressive medium, equalizing surface roughness, or working as an internal repair, e.g. for pitting corrosion. Key indicators of internal coating health include bonding and disbonding, coating blistering, and the degree of fading in both length and thickness, all of which signify the condition of the internal protection layer.

Although the basic principle of extracting coating information from ultrasonic wall signals is well-established, recent years have seen significant advancements in the quality of coating analysis and assessment. This paper will present these new insights, along with emerging approaches for quantifying and reporting internal coating conditions.