A Case Study in Vault Examinations using Short Range Guided Wave

Problem

Traditional NDT methods do not allow easy inspection of pipe directly adjacent to, or on the inside of concrete vault walls. Corrosion and wear goes undetected in these environments. Of particular concern is the non-cased section of pipe (approx 6” to 10”) within the concrete wall and on the soil side of the vault. Removal of concrete is expensive and labor intensive as is exposing the pipe on the exterior sides of the vaults.

Traditionally, Guided Waves are highly attenuated by concrete and as such limits the test length within concrete to less than one foot in thickness.

Qualification

The client fabricated a mocked up specimen with known flaws for a blind test for prequalification by several NDT service providers. The premise was to prequalify the vendor’s tool, the technology and the technician (“the three T’s). Long Range Guided Wave-LRUT (GUL 3, Teletest MK3 , the Magnetostrictive SWRI), and Short Range Guided Wave-SRUT (ISONICS2006) were used in the prequalification test. Coast NDE was successful using the Teletest-MK3 LRUT technology and was the most accurate of the 3 companies but this was still not precise enough for the client. Coast NDE’s ASNT level III recommended utilizing the ISONIC’s 2006 SRUT floor scanning tool. The ISONIC tool utilizes SRUT to send a signal under the annular ring of a tank floor to inspect the floor to shell weld inside. This scenario was exactly what was required for the examination of the concrete vaults. Coast was prequalified a 2nd time with the ISONIC 2006 and rendered a near perfect result in detecting, sizing and classifying the known defects.

Result

Upon the award of this project to Coast NDE, we were invited to write a technical procedure for examining, classifying defects and recommending remediation to the client. Coast provided technical guidance for the client to fabricate a mobile test mock up that would be used to train all technicians that would be performing the vault examinations.

Coast NDE performed several vault inspections on over forty pipe sections with varying diameters and wall thickness. The program consisted of 2006 prequalification on the mock up, 2007 initial examination’s from inside the vault with a 2008 post verification program. The exterior walls of the vaults were exposed and further scans were shot from the exterior to the interior. The concrete surrounding the pipe was removed in some cases for a definitive validation of the tool scans.

Several areas were identified as minor to moderate classifications as per the procedure and recommended for 2008 validation. Upon validation, all minor and moderate anomalies reported by the “tool” were visually confirmed, measured and downgraded to non relevant.

The program is ongoing as the client has a significant number of vaults.