Assessing Buried Aviation Fuel Tanks for Fitness For Purpose

Assess a semi-buried fuel storage tank’s condition and fitness-for-purpose within a tight timeline and without any former inspection records.

Semi-buried bulk fuel storage tanks

In this particular use case, regular fuel sampling indicated an increase in water content over the years, and the tank owner/operator had concerns about the integrity of the tank’s roof and internal roof lining. No formal inspection records were available for the tank; its condition was unknown. An internal survey of the semi-buried fuel storage tank was required to determine its current condition and deem its fitness-for-purpose. The tank owner was required to take the asset offline, evaluate the inspection results, make any repairs required, and return the tank to service all within a very aggressive timeline.

Synergistic non-destructive testing inspection techniques provide actionable data enabling more efficient tank repair operations.

Underside of floor cutouts showing underfloor corrosion
Characteristic circular staining around weld highlights pinhole type leak

With access provided to the cleaned tank, the inspection was performed utilizing advanced non-destructive testing instruments. Visually, the floor plates appeared to be in excellent condition considering the age of the large fuel storage tank. However, a magnetic flux leakage or MFL survey identified multiple areas of the tank floor that had been attacked by underfloor corrosion. The most severe corrosion was recorded at 75 percent loss of the nominal plate thickness. Several coupons were selected to be cut from the floor, and physical measurements verified the accuracy of the data results. Despite the severity of corrosion, non-destructive testing inspection results showed that most of the corroded regions were limited to isolated or localized areas, so the storage tank was deemed repairable.

Ultrasonic testing was employed to survey the tank shell with no significant thinning detected. A leak was noted on a shell plate weld during this inspection. Although the perforation in the weld was extremely small, it was easily identified by a characteristic circular damp area caused by leaked product being drawn back into the tank due to capillary action. This was remedied using a “chemical metal” compound.

Visual inspection of the roof lining did not reveal any suspect areas; however, a continuously damp area on a floor plate led to the detection of a roof leak. The area was excavated and repaired externally.

Inspection results showed that, except for the floor plates, the tank was in relatively good condition considering its age.

The tank owner was able to leverage the reporting software to make several cost calculations based on different repair scenarios by using the corrosion display filters. The software was set to display defects with more than 40, 50, or 60 percent loss. At each display threshold, the owner calculated the time, material, and labour costs required to repair the corroded areas, able to make informed decisions based on repair costs versus expected service life. Due to the semi-buried nature of this storage tank, the asset owner was able to take more cost-effective measures than replacement offered. All corrosion with more than 60 percent loss was repaired, and a five-year internal inspection regime was introduced for all storage tanks onsite.

A comparison of results later showed that the underfloor corrosion had not grown significantly between inspections. The tank owner was able to use software reporting tools for tank comparison and patch plate design to make calculations within a matter of hours rather than a matter of days, as was the case during the initial inspection.