Corrosion detection in the annular plate region of aboveground storage tanks is a critical need for tank farm operators in the oil and gas industry. The critical zone between the tank shell wall and the first few inches of the annular plate is difficult to inspect with conventional floor scanning methods due to the presence of coatings, uneven surfaces and a lack of access due to the presence of the weld toe.
This region, however, is prone to accelerated corrosion due to the additional stresses caused by the weight of the tank wall and the increased possibility of water entrapment under the annular plate. Repairs in this region demand replacement of the entire annular plate and this leads to long shutdown of the tank, and often failures occur without any warning.
Standard inspection techniques often include emptying the tanks for internal inspections, which implies large periods of operational unavailability, with expensive, time consuming and hazardous cleaning processes, involving safety issues to workers. For safety and environmental reasons, the Magnetic Flux Leakage (MFL)technique is one of the commonly used methods for carrying out this inspection.
Alternatively, Acoustic Emission (AE) has also been shown to indicate regions of corrosion activity. Accessibility to this zone of the annular plate (very close to and/or under the retainer wall) is highly restricted and hence poses difficulties when using the currently available MFL and similar floor scanning NDE methods.
A new concept for the improved inspection of corrosion under the annular plate region in large storage tanks using a medium-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. This technique employs the piezo-electric mode of generation of ultrasonic guided waves modes with appropriate wedges. Ultrasonic guided waves (axial mode waves, in this case) once generated will be reflected from corrosion and other features on the annular plate. Inspection can be carried out from the outside of the tank even when the tank is in service. Some of the advantageous features of the HOMC include the high frequencies, minimal displacement at the plate 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 & B-scan images were used to visualise and quantify the defect size and location.