Sonomatic, a provider of automated ultrasonic inspection services, has developed a tool for the internal inspection of caissons. The integrity of caissons is critical to safe and efficient operation of offshore structures. As offshore facilities age, there is a growing need for inspection of caissons, with knowledge of their condition providing a basis for justification of ongoing operation or highlighting the need for repairs or replacements before in-service failures occur. The development was carried out internally by Sonomatic’s research and development group and brings together their experience in mechanical engineering, automation, electronics, software and signal processing.
In the first field application of the new Internal Caisson Tool (ICT), Sonomatic was recently tasked to perform critical inspections on three caissons for a North Sea operator. The work was completed in collaboration with Cape Specialist Services in a single campaign with a combined offshore team. Cape Specialist Services deployed their environmentally safe caisson cleaning system capable of operating below sea level to remove all scale and marine growth from the caisson interior back to deck level for disposal. This prevents debris from dropping to the seabed or being sucked into the lift pumps on adjacent caissons.
In response to operators wishing to reduce POB (personnel on board) where possible, Sonomatic and Cape Specialist Services have assembled a team where cross training of the disciplines has been undertaken. This led to a three or four-member team to conduct HP water jet cleaning, digital image surveying and non-destructive testing inspection from a single mobilization, throughout which the ICT performed reliably.
Recognizing the needs of industry, the focus of Sonomatic’s inspection service is on providing accurate and consistent data, this allowing clients to make effective integrity decisions that contribute to operational safety and efficiency. The ICT was developed with this objective in mind.
The inspection was performed from the internal surface of the caissons, using 0° Corrosion Mapping. Data scan files were collected at 500 mm axial intervals, with the data collected in a helical scan pattern. All of the ultrasonic data was processed in making several million thickness measurements for each caisson. This data was used to create corrosion maps that provide a colored graphic representation of areas of wall loss.
Caissons often have internal corrosion, this means the inspection is from the corroded side which represents a challenge to maintaining accuracy of corrosion mapping. Sonomatic has optimized its approach to the inspection set up and has developed a range of new signal processing algorithms specifically to enhance the accuracy and reliability of measurements made from the corroded surfaces.
“Obtaining reliable inspection data from the inside of caissons has long been recognized as a challenge in the industry”, says Sonomatic’s Topside Project Manager, Scott Bulloch. “This was one of the main drivers for development of the internal caisson inspection tool. In addition, Sonomatic is also advancing the methods of data presentation to ensure clients get the most from the data. This includes 3D presentation of corrosion maps, statistical summaries and comprehensive datasets for efficient creation of finite element models used in fitness for service assessments."
Read more:
Assessing caisson inspection techniques
Image: Internal Caisson Tool/Sonomatic
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