Design and In Situ Validation of a Guided Wave System for Corrosion Monitoring in Coated Buried Steel Pipes

Ultrasonic guided waves allow long-range screening of the full cross-section of pipelines and are thus ideal for corrosion detection. Systems using removable arrays of sensors for one-off inspections are nowadays widely-available. Guided wave pipeline monitoring, on the other hand, consists of using cheap, robust and usually permanently-mounted sensors to compare the present state of the structure with a previous or undamaged condition. Current application of available monitoring systems shows that robustness and defect detection capability remain a challenge, and some major oil and gas companies therefore consider monitoring to be less well-established. This paper deals with the design and testing of a system designed for monitoring buried and coated pipelines, focusing especially on the development of an array (or collar) of sensors. It starts by showing in detail the characterization of single piezoelectric transducers. Finite element modelling was then used to establish the dimensions of an array of these transducers, and to describe its wave-field generation characteristics. These results were then validated experimentally, initially in laboratory conditions; subsequently, a prototype was built and blind tested in near-operational conditions, which consisted of a buried full-scale pipe segment coated with Stopaq and polyethylene. The pipe segment was subjected to electrolytically accelerated corrosion at uncertain locations and at a rate which was unknown to the authors at LAMEF–UFRGS who were involved in data analysis. Results show that after 2 years of periodic data gathering, the collar showed good stability and robustness, and that it was able to detect corrosion defects found in the dissection of the pipe.