Inspecting above-ground atmospheric storage tanks is not only required by various standards and associations, it’s also essential for security and environmental reasons.
Despite their apparent simplicity, a great number of tank components must be inspected. Some inspections, however, require shutting down the asset and pre-cleaning. Not all tank inspections require this, such as visual inspection, subsidence monitoring, and 3D deformation analysis (performed via laser scanning), but many NDT inspections do. If so, before any inspection work can take place, bottom cleanliness and preparation must be addressed, because it impacts the ability of inspectors to achieve the desired result quality. Cleaning is, however, usually not too extensive. Failing to perform NDT inspections can lead to loss in revenue and, at worst, costly accidents.
Tank inspection hot spots
- Bottom plates and their welding joints
- Shell plates and their welding joints
- Annular rings and their welding joints
- Roof plates and their welding joints
Of course, there are many more elements to be inspected, but they fall outside the purview of this article.
Inspecting the tank bottom
When inspecting above-ground tank bottoms, you are mainly looking at:
- Bottom plates and annular rings for corrosion and deformation
- Bottom plate and annular ring welding joints for cracking
Tank bottom plates
The most common technology used to inspect bottom plates for corrosion and cracking is magnetic-flux leakage (MFL) because it’s sensitive to volumetric variations. First, MFL uses a strong magnet to induce a magnetic field in the bottom plate. Then, when it encounters corrosion of a certain size, the magnetic field leaks, so to speak—the more important the proportional volumetric rate, the greater the leakage. Finally, the leakage is detected by a combination of several types of sensors, but MFL technology is incapable of detecting whether defects are on the top side or bottom side of plates. For this reason, some manufacturers combine MFL with surface topology air-gap reluctance sensors (STARS). This enables completely mapping storage tank floors and discriminate between top and bottom-side defects. This has the effect of giving more value to the repair plan created when defects are found.
A single inspection can contain several datasets, therefore inspection data management (IDM) software offers the capacity to visualize same-source datasets in different views and on different layers—essential to elaborating tank floor repair requirements. Certain IDM software packages enable you to select whether to only show top-side defect data (using the aforementioned STARS data) or combine all the data into a single view, for example. The IDM software stitches all the data together into one layout for a complete overview of the asset. This can be laid over computer-assisted design (CAD) drawings, precisely positioning scans inside the asset. Further, specifying positioning data during inspections enables performing this task automatically, which saves considerable time and enables precise condition monitoring, as defects can easily be located spatially, nearly eliminating the human factor inherent in the manual operation.
Ultimately, this makes data quicker to analyze, making the decision process associated to the analysis smoother and easier to prioritize.
After performing an MFL screening, suspicious areas are inspected using ultrasounds (UT) for a more detailed analysis.
Tank bottom plate welding joints
Bottom plate welding joints (whether lap welds or shell-to-annular ring welds) are prone to cracking. So, for this specific application, inspection companies use UT, eddy current array (ECA), or ACFM to inspect welding joints.
Several ECA probes are capable of addressing the inspection needs of sectors relying heavily on carbon steel welds. The onshore and offshore oil and gas, wind power, and structural industries are good examples of these sectors. ECA probes can detect and position axial and transverse surface-breaking cracks common in welds. Furthermore, some probes also measure the length and depth of cracks, as deep as 7 mm (0.28 in)—without the need to remove paint or protective coatings. Spring-loaded fingers or a padded, flexible interface enable simultaneously scanning weld caps, toe areas, and heat-affected zones (HAZ) at speeds up to 200 mm/s (8 in/s).
If warranted, bubble leak testing can also be performed with a vacuum box to make sure the bottom plates are not leaking, preventing precious fuel from being lost and environmental contamination.
Tank annular rings
UT and pulsed eddy current (PEC) can be reliably used to detect corrosion in annular rings from outside storage tanks. More specifically, it’s possible to perform this with PEC technology while the tank is in service. PEC sensors are designed to tolerate liftoff between the surface under test such as air, soil, water, concrete, asphalt, and corrosion products. Specifically designed probes exist for this type of inspection. One such probe has a thin 4.8 mm (0.2 in) titanium blade that can slide up to 400 mm (16 in) under above-ground storage tank annular rings, enabling it to assess the remaining wall thickness of this critical component, exposed to corrosion. The probe sensor can tolerate up to 13 mm (0.5 in) of liftoff and software features optimize thickness measurements, which ensure the best performance and repeatability.
Inspecting the tank shell
Tank shell wall thickness measurement
The most common method used to assess the wall thickness of above-ground atmospheric storage tank shells is the remote-access UT crawler. It is usually designed to perform cost-effective UT thickness measurements on above-ground ferromagnetic structures without scaffolding or rope access. UT crawlers can be operated automatically or manually. They are used to perform line scans on the tank shell surface or scan specific areas including the roof (involving more sophisticated XY scanners). In most cases, the tank shell is divided into 8, 16, or more equal sections according to the inspection regulation followed. Each section has B-scan UT data recorded from bottom to top, showing the condition of each course of that tank’s wall section. The speed of the UT crawler and the data acquisition rate of its inspection system play a great role in the efficiency of tank wall surface inspections. Some crawlers can travel and inspect at up to 180 mm/s (7 in/s) with a dry-coupled wheel probe, which eliminates the need for the complex water supply system usually necessary in UT inspections.
The UT crawler acquisition and analysis software usually enables displaying ultrasonic A-scans, C-scans, thickness measurements, and position in real time, which can be saved with the scans. The software also incorporates C-scan and B-scan layers, which enables operators to quickly switch between scan views. A-scan gates can also be added post inspection to measure between several parts of A-scan traces, making simultaneously measuring the signal amplitude, part thickness, internal surface profile, and external surface profile possible. This minimizes the need for rescans caused by variations in surface conditions and minor setup errors.
Furthermore, C-scans offer an effective method of viewing general wall thinning and larger defects. Small pits and inclusions are, however, often difficult to see because of their small area. Reviewing acquisitions in B-scan mode enables identifying and sizing potentially critical indications.
Tank shell plate welding joints
UT and ECA can both be used to establish the presence and extent of cracking in the circumferential and longitudinal plate welds, as described above.
Inspecting the tank roof
Just like the other components of above-ground atmospheric storage tanks, roof plates must be inspected for corrosion and deformation, and welding joints for cracking. MFL and remote UT crawlers used on shell plates can also be used on the roof. ECA and UT can be used to test all the welds joints.
The majority of above-ground atmospheric storage tank components can be reliably inspected through a wise combination of UT, ECA, MFL, and PEC for maximum efficiency, minimum down time, and optimal asset management strategies. Hardware like Reddy®, Lyft®, Scorpion2, Floormap, and Sharck™, as well as software like CMAP and Magnifi® can easily help you reach this very important goal.
Corrosion Cracking Eddy current array (ECA) Electromagnetic testing Information Liquid penetrant testing (PT) Magnetic flux leakage (MFL) Magnetic particle testing (MT) Oil & Gas Problem-Solution Tangential eddy current array (TECA) Ultrasonic testing (UT) Ultrasonic thickness measurement (UTM) Visual inspection (VI)