Large Scale Guided Wave Testing (GWT) of Aboveground Tanks
InterSpec is the first company in the world to successfully utilize Long Range Ultrasonic Testing (LRUT) on an Aboveground Storage Tank (AST). The insulated stainless steel tank measured thirty-five feet in diameter and thirty-six feet tall. The inspection, completed in 2012, was able to quickly assess the condition of the tank and determine suitability for continued service.
The inspection procedure was developed with a focus on identifying environment-assisted cracking, particularly Chloride Stress Corrosion Cracking (CL-SCC).
The need for non-destructive testing (NDT) was initiated by the owner/operator after the discovery of a leak as a result of a large crack on the tank. This crack was located at fifteen-feet above the shell to bottom weld. At the time, it was assumed to have been localized or isolated to a small section of the tank. Rapidly identifying any other SCC was paramount to minimize down time and to determine fitness for continued service. The insulation jacket on the exterior of the tank and process residue on the interior provided limited inspection opportunity using conventional visual or penetrant testing methods. The size of the tank would have also required extensive non-destructive evaluation (NDE) time to identify the defects with these methods.
|Sensor installed||Data acquiring (vertical sensor)|
Guided Wave Testing (GWT) was used to perform the nondestructive evaluation of the tank and determine the extent of the stress corrosion cracking. The inspection procedure was tested using a small section of tank coupon with known defect sizes.
|Marking defect (Green Paint) orange marker is plate and weld marker||Close up of cracks|
Required cleanliness at the sensor location, downstream product residue, and crack geometry were shop tested with the coupon. The Magnetostrictive Sensor (MsS) based equipment was used to generate a non-dispersive guided wave for long-range ultrasonic inspection of the tank shell. Using fundamental shear-horizontal mode, sixty-five dual directional signals were generated and their reflections detected using a Magnetostrictive (Wiedemann) effect on the shell. A probe with with a predetermined center frequency was selected to generate the ultrasonic guided waves in the shell. Two vertical inspection areas along the interior of the shell were selected to inspect the tank with Long Range Ultrasonic Testing. The test areas for each scan were a two-inch wide section of probe, ten inches in length. The two vertical probe locations provided a 360 degree view of the shell for discontinuities. Low beam divergence angle of the probe provided a focused beam at targeted sections of the shell plates for defect detection. Each of the test locations were identified by the plate number and assigned a letter designation (A through E). Each letter signified a volumetric scan that extends in a positive and negative direction of the probe. Using the data from the GWT, cracks were located and marked on the interior of the shell using green paint.
Utilizing LRUT, Interspec’s team was able to identify numerous CL-SCC throughout the tank. A complete API-653 report was provided along with recommendations. Documentation, such as API 579/ASME FFS-1, was referenced for guidance in determining fitness-for-service based on qualitative engineering evaluation of the tank. The remaining life of unrepaired sections with SCC flaw mechanisms cannot be reasonably established with certainty. That is because reliable crack growth rates cannot be established for the tank when there is a dynamic process environment. Changing stresses due to product, other environmental loading such as wind and snow load and temperature change gradients in the tank are all factors that will affect crack growth. More significantly is the corrosive environment that is generated under the insulation in the presence of an aqueous solution. Another assessment methodology such as leak-before-break (LBB) analysis that uses a sign of a leak to address and prescribe repairs may not be appropriate because leaks cannot be readily identified with insulated structures. Additionally, the flaw may grow faster in the surface direction than in the depth direction. Catastrophic failure was also a concern.
Based on the inspection and the data acquired, it was determined the tank was not safe for continued use. The extent and severity of the cracks on the shell did not lend itself to selective replacement or repair of the affected areas.
The inspection plan did account for horizontal (X) and vertical (Y) sensor installation and screening of defects in both planes. Since circumferential stress or hoop stress has higher magnitude, vertically aligned cracks were anticipated and would be easily identified with vertically oriented sensors.
(Red line indicates sensor location. W1 – W6 indicate vertical shell butt-weld reflection. After screening out multiple reflections, trailing reflections and known geometrical features location of cracks are visible – circled in green)
InterSpec continues to adapt advanced technology to deliver fast decision-prompting results.
The project produced a cost effective method and procedure of identifying Chloride Stress Corrosion Cracking (CL-SCC) and other structural anomalies on tanks and pressure vessels.