Tank roof corrosion is seldom uniform. Both internal and external corrosion usually take the form of isolated attacks governed by roof geometries, construction methods, plate flatness, and coating conditions. Visual inspection is essential, but ultrasonic gauging can yield useful information.
Care must be taken when working on fixed and floating storage tank roofs at all times. Extra care is essential when the roof is visibly corroded. The internal compartments of floating roofs should only be gauged when the tank is empty and free of gas. Ultrasonic thickness measurements of roof plates may be performed along the radial grid lines (extension of vertical grid lines) at 200 mm (7.9 in) from the shell, at 2/3 radius and at 1/3 radius. In addition, thickness should be measured where there is significant visible external corrosion.
Ultrasonic gauging/screening is performed with remote-access crawlers such as Scorpion2 or corrosion mapping systems such as RMS2. Handscan can also be used on fixed roofs to improve the PoD when conditions allow it safe use.