Pipelines are supported at intermediate distances by pipe supports. Corrosion is more prone to occur at these support regions of pipelines due to the presence of all the key ingredients needed to accelerate corrosion like water, minerals, and the stress concentration in the presence of a crack. The common types of corrosion at the supports include corrosion spread along the circumferential or longitudinal direction of the pipe. In most cases the corrosion defect at the support locations is localized in nature with smaller pitting corrosion.
In order to mitigate the corrosion to pipes at these locations, the industry often resorts to providing a sacrificial plate (often called sacrificial welded pad) that is placed in between the pipe and the support. The plate is tack welded around the boundaries of the plate. However, it has been observed that while this sacrificial pad reduces corrosion, it does not completely prevent it. The inspection of the pipe in this region with the welded pad is more difficult in comparison to the scenario without the pad.
Among the various techniques used for the detection of corrosion like defect in pipelines, ultrasonic NDE plays a major role. To perform a conventional ultrasonic inspection in such inaccessible region (i.e. at support locations), the pipes have to be lifted out of the supports, which involves complete shutdown of the flow lines and the risk of stressing a pipe that would have been already weakened by corrosion.
Hence, it is necessary to come up with an alternate method of NDE inspection where the hidden portion of the pipelines is inspected without disturbing the structural arrangement.
A new concept for the improved inspection of corrosion at the support region of pipelines using a short-range ultrasonic guided wave technique that uses a collection of Higher Order Modes Clusters, called here as HOMC has been developed and Patented by Dhvani. Ultrasonic guided waves (circumferential or axial modes), once generated will be reflected from corrosion and other features on the pipe. Inspection can be carried out from the accessible portion of the pipe .
Circumferential guided wave modes are used for most pipe sizes, however, in larger diameter pipe where the support pad welded extends over nearly one half of the circumference, the axial guided wave mode will be more advantageous to deploy.
Some of the advantageous features of the HOMC include the high frequencies, minimal displacement at the pipe surfaces, and its non-dispersive nature, which will allow the user to inspect inaccessible regions that are normally difficult using conventional guided wave techniques that employ single modes.
Experimental data obtained from the calibration sample with programmed defects and on field tests show that the size and location of the defects correlates well with the time-of flight and amplitude ratio of the reflected signals from the defects. A and B-scan images were used to visualise and quantify the defect size and location.
A Semi-Automated Scanning system called CUPS (Corrosion Under Pipe-support Scanner) has been developed and tested to ensure high speeds of inspection in line with industry requirements .