Abstract
Ultrasound is a standard method in non-destructive testing to measure the wall thickness of steel pipelines. By placing a measurement probe on the outside of the pipeline, the wall thickness can be measured while the pipeline is in operation. Permanently mounted ultrasound probes can be used for highly sensitive wall thickness monitoring, being able to detect wall thickness changes down to the micrometer range. The high sensitivity allows for rapid determination of corrosion rates, which can give feedback on the effect of corrosion inhibitors.
For subsea applications, steel pipelines are often covered by an insulating coating. Such a coating makes thickness measurements with ultrasound more challenging, as the coating usually is of a material that absorbs ultrasound, leading to a lower signal to noise level in the measurement. In this study, we have demonstrated that ultrasound can be used for internal corrosion monitoring of steel pipelines coated by an insulating polymer layer, without damaging or removing the coating. A coated steel pipeline identical to the pipeline used at the Ormen Lange field was immersed in saltwater, and the decrease of wall thickness due to corrosion was monitored for 6 weeks. The sensitivity of wall thickness measurements of a coated and uncoated steel pipeline is compared.
