One early morning of January, 2018 the technical supervisor of some oil storage field realized that the annular ring plate of one storage tank in service has been cracked suddenly: about 3.5…4 inch length crack appeared in the radial weld along the fusion line propagating from the plate end towards the shell of reservoir and it is not clear whether it penetrates inside already or not… Due to the risk of product leak out of the tank the urgent evaluation of the crack length was required in order to settle the needful repair schedule.
The overnight-found solution is based on the VLFS PA inspection SW application being a standard option of Sonotrron NDT’s portable ISONIC series PA instruments ISONIC 3510, ISONIC 2010, ISONIC 2009 UPA Scope (VLFS – Vertical Line Focusing Scanning). Among other features the use of the said inspection SW package allows precise sizing of vertical cracks either surface breaking or internal based on the analysis of tip diffraction echoes obtained through sectorial scan coverage of the region of interest using either longitudinal or shear waves as it is illustrated by the videos below
The solution was implemented trough the placement of the wedged 32-elements 1 mm pitch PA probe (part # 214381) onto the end side surface of the annular ring plate; with use of the said probe the sharp focusing and significant SNR (signal to noise ratio) are achievable for wall thickness range from 50 to 200 mm whilst performing shear wave half- and full skip S-Scan coverage.
The solution was checked on the quickly prepared sample manufactured out of the specimen of planar butt weld having the thickness of parent material equal to the thickness of the annular ring plate. Upon obtaining positive results on the trials sample the urgent evaluation of the crack was performed backing the decision on the repair schedule. The video illustrating the pre-inspection trials and inspection and explaining the results is shown below.