MEC Technique for Challenging Tank Floor Inspection

The MEC (Magnetic Eddy Current) technique applied by the PETROBOT robotic inspection solutions has demonstrated superior defect detection capability in two successful tests performed for a major petroleum and petrochemical operator.

The first analysis focused on the simulated inspection of a tank floor plate installed on top of an existing floor with varying distances in between. The ability of the MEC technique to inspect reliably the “New” plate on top of the “Old” plate with varying distances to each other (no gap, 3mm and 5mm gap underneath) as well as the reliability of defect detection on either side of the top layer plate was tested. The target was to analyse the influence of the “Old” bottom plate below the “New” bottom plate on the inspection results as well as the capability of the MEC technique to detect flaws in a distanced second steel layer.

Results showed that the defect indications and magnetic field in the top layer plate are not affected by the direct contact or increased gap distance of the bottom plate. Defects on the upper and underside of the top plate layer can be confidently detected and analysed without the influence from the defects in the bottom plate unless they are of significant size. This is possible because the MEC technique operates its magnetic field strength of at the retentivity point of the hysteresis curve which requires lesser field strength. This is a major difference to the MFL (Magnetic flux leakage) technique operating at the saturation level of the hysteresis curve where the close or in contact neighbouring plate layer will affect the field strength absorption.

The second analysis focused on the ability of the MEC technique to defect corrosion on the underside of the tank floor alongside the weld near to the tank shell.

Corrosion formed on the underside of the tank floor, extending alongside the double fillet weld at the area approximately 75mm from the shell, is a critical tank integrity issue experienced by operators, particularly for their storage tanks in Asia. As a result, operators requires an inspection technique that has a higher accuracy with targeted depth sizing of at least ±10% whilst the tank is out of operation and that also has the ability to penetrate the GRP (Glass-fibre Reinforced Polyester) lining and provide details of the defect profile.

Corrosional Attack

Corrosion defect on underside of tank floor next to tank shell

The TÜV certified MEC-F15 Floorscanner was used to perform the tests on the sample with a side wall mock-up as shell simulation. Despite the 10mm clearance due to the obliqueness of the shell, pitting defects such as 20mm diameter RBH (round bottom holes) underneath the shell were well detected. The same defect detection results were produced with the addition of a 4mm thick plastic layer mimicking the GRP lining.

F15 on wall                                        F15 on wall 2

The picture on the left shows the setup of the MEC-F15 Floorscanner near a side wall mock-up, while the picure on the right illustrates the 10mm clearance between scanner and shell due to obliqueness of side wall.