Guide to Non-Destructive Testing of Storage Tanks

Leading countries aim to establish energy autonomy, resilient supply chains, and carbon neutrality by 2050. Ensuring the safe storage of biofuels, oils, liquid gas, hydrogen, methanol, ammonia, and even carbon is a key element of these goals.

These materials, however, pose a substantial threat if they leak, vaporize, or explode. Thus, storage tanks must be regularly inspected for cracks, leaks, corrosion, erosion, and wear.

Non-destructive testing (NDT) is one of the fastest, most reliable ways to assess tank structural integrity and find the smallest defects at any depth. In this post, we cover common NDT methods for storage tank inspection and explain how drones simplify external and internal inspections of aboveground tanks.

The Importance of Regular Storage Tank Inspections

Without regular testing, storage tanks can become a compliance nuisance and even cause security incidents. Failure to spot defects may cause toxic substance leaks or explosions. Explosions and leaks pollute the environment, cause health problems, and may even lead to fatalities among workers.

Take it from SABIC, a global chemical manufacturing company. A naphtha leak, caused by undetected roof problems on a storage tank at its European facility, killed one worker and injured several more. After the investigation was concluded, SABIC had to pay 10 million euros in regulatory fines and bear ongoing reputational damage.

Companies, showing negligence, risk heavy regulatory penalties, time-consuming legal battles, and negative publicity.

The laws also get tighter. By 2026, individuals found damaging natural ecosystems, habitats, water, soil, or air quality may face jail time under a new EU law. The US authorities also plan to impose a $900 to $1,500 fine per ton of excess methane emissions, including those generated by oil and gas storage facilities.

To prevent leaks from occurring, storage tank inspections should be carried out regularly for signs of damage or wear.

Top Causes of Storage Tank Leaks

• Corrosion and erosion
• Environmental damage
• Extensive wear
• Lack of integrity in tank components (e.g., unsealed roof)
• Material incompatibility (e.g., carbon steel is not fit for acidic substances)
• Excessive operating pressure
• Subpar operating temperature (due to damaged heating/cooling systems or bad insulation)
• Human error and incorrect exploitation

Inspection of Storage Tanks: Key Requirements

The storage tank inspection guidelines depend on the asset manufacturer, local regulations, or the association’s rules (if you’re a registered member).

For reference, the main standards for aboveground tank inspections include:

• In the USA: API 653, Env-Or 300, EPA’s SPCC, STI SP001
• In Europe: EEMUA 159, EN 14015
• Internationally: ISO 16961:2015, ISO 24489:2024

The inspection type and frequency will vary with the tank size, configuration, design, contents, location, past inspection results, operating mode, and corrosion rates among other factors. A licensed storage tank inspector can recommend the optimal asset inspection schedule and guidelines.

External Inspections

External inspection of in-service storage tanks should occur monthly by the asset owner or their representative. You’ll need to verify the state of the storage tank’s roof, shells, heads, bottom, connections, pipes, ladders, drain valves, and the surrounding area (concrete or ground).

During the inspection, look for any abnormal conditions: leaks, stains, bulges, discoloring, wetting, mold, cracks, or corrosion on the walls. Sometimes, you may need to remove some insulation or coating to take measurements.

Be on the lookout for settlement irregularities, bulges, or deformations when examining shells and roofs. It’s also important to check the decked areas for oil accumulations.

Ultrasonic thickness measurements of storage tank walls should be done annually. Per STI SPP001, you should remove the tank out of service if at least 5% of any 12×12 inch (30x30cm) area has a remaining thickness of less than 50% of the original one. Inspection drones like Voliro T help perform thickness tests rapidly and reliably with a mounted dry film thickness gauge.

All storage tanks must be tested immediately in case of visible leaks or exposure to fire. And after a heavy storm, promptly examine the roof drainage system.

Internal Inspections

Prior to any internal inspection, asset owners must drain the tank from any liquids, solids, and by-products. Ensure there are sufficient oxygen levels, a safe entry point, and a non-explosive environment (for human-run tests).

Interior inspections help identify pitting, welding flaws, lap joint discontinuities, and corrosion. An inspector creates an up-to-date map of the defects, their depth, size, shape, and location. To avoid climbing, many use drones to take video footage of the whole surface and save it for future reference.

Specialized inspection drones like Voliro T also let you collect readings on wall thickness, and detect signs of corrosion, debonds, and porosity. One drone-led storage tank inspection can save up to 400 hours for scaffolding installation and dismantling (and the hefty associated costs).

Generally, internal tests take place at the following intervals:

• Initial inspection for tanks with ground contact after 10 years of operation
• Initial inspection for elevated (on racks or in cradles) after 20 years of operation

After the first internal testing, you will need to perform regular inspections every 5 years for gasoline tanks and every 10 years for tanks with other fuels and oils.

Non-Destructive Testing Methods for Storage Tanks

All standards specify non-destructive testing (NDT) as the preferred approach for determining the durability and integrity of a storage tank.

SP001 standard for inspecting aboveground storage tanks explicitly calls for visual, radiographic, ultrasonic, hydrostatic, and acoustic emissions tests, but it also allows other NDT techniques.

Do you need to perform all of these tests? No, it’s up to you and your inspector to decide on the best NDT technique based on the asset type and its operating requirements. Here’s an overview of all the options available.

Visual inspection

Visual inspection is a standard method for routine external inspections. It relies on direct observation of surfaces to verify the storage tank’s characteristics (size, shape, wear) and identify any noticeable changes in dimensions/color or visible leaks.

Visual inspections of aboveground storage tanks are most often used to check the bottom, base edges, and shell connections — anything easily accessible. Scaffolding, cherry-picker, or a drone will be needed to inspect elevated components. Inspection drones are the fastest and most cost-effective option.

On the downside, visual storage tank inspections can only locate apparent defects such as pits of at least 2mm and deeper, large ripples or bulges, and any noticeable corrosion. But they often hint at the placement of deeper nested defects, allowing the inspector to apply another NDT method with precision.

Ultrasonic Testing

Ultrasonic testing (UT) uses high-frequency sound waves to penetrate the material and measure the storage tank’s wall and coating thicknesses. By analyzing the pattern and timing of ultrasound pulses, an NDT inspector can detect flaws of as little as 0.05mm. Comparing the current wall thickness to the original one indicates the degree of corrosion and the remaining asset lifespan.

Ultrasound testing is a highly effective NDT method. It’s sufficient to inspect only one side of a storage tank to get reliable data. It’s also suitable for all types of containers, as sound waves can penetrate any metallic, plastic, or polymeric wall, coating, and insulation.

Using inspection drones, you can collect up to 200 readings per hour from the walls, roof, and floor, even without physically accessing the tank. Using a Voliro T drone with a UT probe, inspection providers can now perform 5-10 tank inspections per day, while providing their customers with more accurate data.

The Voliro inspection drone comes with two types of UT inspection tools:

• Voliro UT probes have echo-to-echo and pulsed echo test modes and offer a thickness range of 4-150 mm. The high-temp UT probe can perform high-precision scanning even at an operating temperature of 0 – 260 °C / 32 – 500 °F.

• Voliro EMAT probe delivers sound impulses without the need to touch the materials. It is perfect for inspecting corroded, dirty, and uneven surfaces, such as in-use storage tanks.

Our subscription also includes a 3-day operator training package, teaching the piloting best practices, annual hardware rotation, and sensor recertification.

Eddy Current Testing

Eddy current testing is another effective method for testing storage tanks made of conductive materials (e.g., steel or carbon steel). This NDT method uses electromagnetic induction to identify near-surface flaws, such as corrosion, breaks, and thinning. By using high-frequency eddy currents, you can verify the integrity of very thin protective linings (like zinc or aluminum for storage tanks).

Pulsed eddy currents (PEC) also help locate micro-cracks, metal loss, and corrosion hidden under insulation and coatings. PEC testing requires no direct contact with the inspected surface and covers a large area in a single pass. Usually, though, the inspector still has to remain nearby to control the current penetration. But this will soon change.

Voliro is about to launch the first drone-mounted eddy current payload in Q4 2024. The probe penetrates insulation thicknesses of up to 100 mm and can detect undercoating corrosion before it has a chance to spread. With the drone’s 360-degree mobility and specialized algorithms, you will receive high-precision results regardless of the tested area altitude.

Dry Film Thickness Measurement

Dry film thickness (DFT) measurement is the process of inspecting the storage tank’s protective coatings, usually epoxy, vinyl ester, polypropylene, zinc, and paints). DFT tests confirm that sufficient protective coating was applied and detect further thinning as the lining wears off.

Voliro has a high-precision DFT probe for drones, allowing you to measure coating thickness even within minutes after application. With the available magnetic induction and eddy current modes, it can take measurements on ferrous and nonferrous materials with a range of 0 to 1500 um / 700 um.

Acoustic Emission Testing

Acoustic emission testing (AET) is an NDT method that translates transient elastic waves (in other words, noise vibrations) caused by defect formation into eclectic signals, showing the defect severity and expansion.

In storage tank inspections, AET is mostly used to detect active leaks and corrosion proliferation that lead to tank floor degradation, wall thinning, or content loss. The advantage of AET is that it is highly sensitive and capable of immediately detecting developing defects.However, AET may not provide reliable data since external noises or other vibrations can have an impact on the results.

Other NDT Methods

Other NDT methods can be used to inspect smaller storage tanks, where direct surface access can be ensured, or in-lab testing can be accommodated.

• Liquid penetrant testing (LPT) relies on the application of colored or fluorescent penetrant substances on a surface to detect cracks, discontinuities, pores, or holes. LPT works for most materials and can direct toward leak sources in storage tank floors and walls. But it can be difficult to apply for large-surface tanks, plus the test requires parallel to the ground positioning of the test object.

• Electromagnetic particle testing uses magnetic force and colored or fluorescent magnetic particles for detecting surface and sub-surface defects. This technique helps visualize cracks, pores, holes, and inclusions and is particularly useful for storage tank weld inspections.

• Industrial radiographic testing uses X-rays (gamma rays) to detect material flaws deep beneath the surface. Because radiographic testing is usually performed on-premises, it only applies to smaller, portable storage tanks.

• Thermography measures and images heat radiation differences between tested surfaces and flaws. The method is highly sensitive and reliable, but it requires sophisticated and expensive equipment.