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NDT.net Issue - 2019-03 - NEWS
2019-03 NDT.net Journal,

PAUT: Corrosion detection/survey under shell/tank bottom repairs using lap-welded patch plates

Sonotron NDT65, Rehovot [Israel]
NDT.net Journal
Issue: 2019-03
Since API-653 permitted shell/tank bottom repairs using lap-welded patch plates the inspection monitoring of the corrosion under the said repairs became a desired inspection task along with the inspection of the bottom areas under the column supports, and the like. The dimensions of the patch may reach more than a meter in both measurements so the use of short range guided wave inspection (SRUT GW) is applicable practically for the whole area under the patch beside the 50…120 mm width zones adjacent to the lap welds: the defects in the said zones are not so well resolvable for SRUT GW testing due to multiple reflections from the weld structure masking the defects echoes; on the other hand the probability of appearing new pitting and cracking under the patch has been increased for these zones due to the welding process applied.

Corrosion detection and survey for the said areas is possible using new inspection software application ISONIC EXPERT Patch developed for the portable phased array (PA) instrument of ISONIC Series, either ISONIC 3510 or ISONIC 2010 or ISONIC 2009 from Sonotron NDT and the appropriate probes and accessories. ISONIC EXPERT Patch provides Multi-skip Shear Wave Volume Corrected (True-to-Geometry, True-to-Shape) sectorial scan coverage and imaging. Thus the complete solution for the detection/survey of the corrosion under lap-welded patch-plates is based on the of multi-skip shear wave volume corrected sectorial scan and SRUT GW coverage, both have been implemented using corresponding wedged PA probes through scanning along the lap joint. Use of PA modality for the SRUT GW inspection allows significantly better flexibility vs conventional thanks to the ability of performing several modes of pulsing-receiving cycles of guided wave signals (focal laws) in one pass, each covering either whole region of interest of the selected part of it.

The self-explaining video below illustrates the trials performed at the qualification trials by one of the users. The samples of the tank bottom plate with small artificial and natural defects covered by the lap-welded patch

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