Tank bottom inspections are essential for ensuring operational and environmental safety and for the creation of reliable maintenance and repair strategies.

However, if we wish to optimize maintenance and repair strategies as well as maximize inspection intervals, it is pivotal that the data received from a tank floor inspection is complete, accurate, and reliable. To achieve such actionable data sets, it is essential that the equipment used has exceptional detection capabilities - down to 12% wall loss - and exhibits minimal dead zones, especially in the all-important critical zone.

Critical Zone inspection with FloormapX

Figure 1: Critical Zone inspection with FloormapX

Of particular interest during a tank floor inspection is the region that exists between the tank shell and the bottom plates, often referred to as the Critical Zone (CZ). This region represents a pivotal aspect of any tank floor inspection. Damage mechanisms that affect the integrity of the tank are often located in the CZ.

Critical Zone Corrosion detected with FloormapX

Figure 2Critical Zone Corrosion detected with FloormapX

Determining if immediate repairs are required in the CZ of a tank floor is a fundamental requirement for any tank floor inspection. Today, however, we need to ask for far more of our equipment than previously required. We need to identify, especially in the critical areas, the beginnings and the earliest on-set of flaws/damage mechanisms so that the best maintenance and repair strategies can be devised and thus avoid expensive and unnecessary downtime.

To provide this accurate and actionable data, it is essential that the critical zone is inspected with equipment capable of:

  • High probability of detection (PoD): when following API 653 to achieve a 20-year in-service interval 12% metal loss needs to be actioned.
  • Minimizing dead zones: if equipment dead zones are reduced to a minimum, PoD increases and confidence in the inspection significantly improves.

These requirements can be met. Today it is possible to inspect the CZ with state-of-the-art powerful technologies that deliver ‘clean’ wall loss information down to 10% and within 12mm (or 0.5 of an inch) of the shell wall. This is achieved via advanced signal processing, the ability to perform a curved scan next to the shell wall, for plate thicknesses up to 18mm and market leading PoD.


There is a direct correlation between inspection report quality and equipment capability. The more able the equipment is - in terms of efficient data gathering, signal to noise ratio, positioning, and coverage - the more actionable and reliable the report is.

Furthermore, when considering tank floor storage inspection, it is essential that asset owners have total confidence in inspection findings and the ensuing reports. Tank floor inspection equipment must not only be efficient and highly capable, but data sets should be traceable and integrity maintained throughout.

Tank Inspection Reporting Software

Figure 3: Tank Inspection Reporting Software

How do we achieve these standards? Modern, independently verified, Magnetic Flux Leakage (MFL) tank bottom scanners meet these standards. Certain tank floor scanners are now able to deliver a corrosion map to within 12mm (Xin) of the shell wall with detection capability down to 10% wall loss.

As we know, there are many benefits of using modern MFL technology to generate actionable reports. MFL based technologies are eminently capable of delivering extremely efficient, highly accurate and reliable reports that a recipient can have confidence in when generating optimal maintenance and repair strategies. These reports are generated via rapid, easy-to-use, and accurate tank floor inspection equipment capable of critical zone coverage, delivering information regarding defect location, severity, surface origin and the extent of coverage. Moreover, report recipients are now able to fully interrogate data sets, determine rates of corrosion, understand reporting decisions, and have complete reporting clarity and thus gain confidence to formulate key repair or remaining life decisions.

Aboveground storage tanks tend to be circular.

Typical Aboveground Storage Tanks

Figure 4: Typical Aboveground Storage Tanks

This circular nature has traditionally presented a challenge for accurate, efficient, and reliable inspection of the CZ. When considering an MFL based tank floor inspection, due to the curved nature of the tank shell, traditional methods of inspecting the CZ often entail secondary equipment such as extensive manual UT (often referred to as a UT scrub), which in itself is time-expensive, subject to the human factor and often does not provide a map of the area which is required for future inspections to determine corrosion rates and repair strategies based on threshold variation.

Alternatively, complementary MFL technology may be employed that can sacrifice performance and detection capability, especially for thick plates, to reduce the dead zone between the shell and tank floor and minimize the manual UT scrub. Often, a combination of secondary methods is required to ensure complete annular and CZ coverage. This process is time-consuming, can result in differing data sets, fallible, often not mapped, and may rely heavily on the integrity of the inspector and the inherent dead zones of the inspection equipment employed to inspect the tank floor.

Today, it is possible to efficiently inspect and map the CZ to within 12mm (0.5in) of the shell wall via a single, powerful, MFL tank floor scanner that can provide consistent wall loss information to optimize inspection intervals – even for thick floor plates up to 18mm (Xin). This result is possible by utilizing the latest generation of tank floor scanners which offer curved scanning. Curved scanning in the CZ ensures the scrub is focused on the region recommended in the regulations.

Often a tank floor inspection report of the critical zone will offer something akin to the image in Figure 3.

Tank floor coverage achieved with primary MFL equipment without curved scan add custom scan capability.

Figure 5: Tank floor coverage achieved with primary MFL equipment without curved scan add custom scan capability.

Figure 5 may present a notable confidence issue for the formulation of reliable maintenance and repair strategies – in the unmapped (white regions) the report recipient has lost data interrogation capability and is solely reliant upon the inspection integrity, technician training and experience and other human factors such as fatigue; there are questions when presented with Figure 5. The questions that naturally arise from Figure 5 include:

  • Was the critical zone inspected?
  • If yes, what method(s) was used?
  • Is that method(s) reliable? How is it known if coverage was total?
  • PoD: Is bottom surface microbial induced corrosion (MIC) a consideration, which can have a small surface area and be very difficult to detect without a powerful detection platform?
  • How do I interrogate the data via threshold analysis to determine repair needs today and in the future?

Figure 6 removes all the questions and greatly increases confidence.

Tank floor coverage possible with primary equipment cqpqble of curved and angled scanning.

Figure 6: Tank floor coverage possible with primary equipment cqpqble of curved and angled scanning.

From Figure 6 the report recipient understands coverage, understands that highly powerful equipment with excellent detection capability was employed and can have a threshold to any level desired.

Clearly tank inspection is a multi-faceted process. There are many considerations when developing an optimal maintenance and repair strategy. Continuing technological advancements in accordance with market demands, research-led development and evolving regulations have necessitated and compelled the continual evolution of all factors associated with tank inspection.

Inspection tools available today for magnetic flux leakage focused devices are now at such a state where optimal maintenance and repair strategies are eminently possible. Moreover, such strategies can be delivered in a hitherto efficient and accurate manner. Get with our team to discuss your next inspection campaign and stay Beyond Current.

Authors: Andrew Simpson, Matthew Boat, Farzaneh Mayamey and Thomas Laprise.