Nations are striving to achieve energy independence, robust supply chains, and carbon neutrality by 2050. A critical component of these objectives is ensuring the secure storage of biofuels, oils, liquid gas, hydrogen, methanol, ammonia, and carbon.
However, these materials present significant risks if they leak, vaporize, or explode. Consequently, storage tanks must undergo regular inspections for signs of cracks, leaks, corrosion, erosion, and wear.
Non-destructive testing (NDT) is among the most efficient and dependable methods to evaluate tank structural integrity and detect even the smallest defects at any depth.
Drones like the Voliro T can enhance NDT inspections by saving up to 400 hours on scaffolding setup and removal. Outlined below are common NDT techniques for inspecting storage tanks and highlights how Voliro drones facilitate both external and internal examinations of aboveground tanks.
The Significance of Regular Storage Tank Inspections
Without routine inspections, storage tanks can become compliance liabilities and pose safety risks. Undetected defects may lead to toxic leaks or catastrophic explosions, which can harm the environment, create health issues, and result in worker fatalities.
For instance, SABIC, a global chemical manufacturer, experienced a naphtha leak at its European facility due to undetected roof damage on a storage tank. The incident caused one fatality and injured several workers. Following an investigation, SABIC faced a €10 million fine and ongoing reputational harm.
Negligence in this area can lead to substantial regulatory penalties, lengthy legal proceedings, and adverse publicity for companies.
Regulatory frameworks are also becoming more stringent. By 2026, individuals responsible for harming ecosystems, water, soil, or air quality may face imprisonment under a new EU law. In the U.S., authorities are planning fines ranging from $900 to $1,500 per ton of excess methane emissions, including those from oil and gas storage facilities.
To minimize the risk of leaks, storage tank inspections must be conducted regularly to detect damage or wear.
Common causes of storage tank leaks:
- Corrosion and erosion
- Environmental factors
- Significant wear
- Component failure (e.g., unsealed roofs)
- Material incompatibility (e.g., carbon steel reacting with acidic substances)
- Excessive operating pressure
- Suboptimal temperatures (caused by faulty heating/cooling systems or poor insulation)
- Human error or improper use
Storage Tank Inspection Requirements
Inspection guidelines depend on the manufacturer’s specifications, local regulations, or association standards (for registered members).
Primary standards for aboveground tank inspections include:
- USA: API 653, Env-Or 300, EPA’s SPCC, STI SP001
- Europe: EEMUA 159, EN 14015
- Internationally: ISO 16961:2015, ISO 24489:2024
The type and frequency of inspections vary based on tank size, design, contents, location, past inspection outcomes, operational mode, and corrosion rates. Licensed inspectors can recommend optimal schedules and protocols.
External Inspections
In-service storage tanks should undergo monthly external inspections by the owner or their representative. This includes assessing the condition of the roof, walls, bottom, connections, pipes, ladders, drain valves, and the surrounding area.
Inspect for abnormalities such as leaks, stains, discoloration, mold, cracks, or corrosion. Some insulation or coatings may need to be removed for measurements. Check for irregularities like bulges, deformations, or settlement issues on shells and roofs. Inspect decked areas for oil accumulation.
Ultrasonic wall thickness measurements should be performed annually. According to STI SP001, tanks must be taken out of service if at least 5% of any 12×12-inch (30×30 cm) area has less than 50% of its original thickness remaining. Drones like Voliro T can expedite thickness tests with precision using a mounted dry film thickness gauge.
After significant events like visible leaks, fires, or severe storms, immediate inspections are necessary, particularly for roof drainage systems.
Outside wall thickness measurement of a large storage tank using the Voliro T drone
Internal Inspections
Before internal tank inspections, tanks must be drained of all contents and residues, ensuring adequate oxygen levels, safe entry points, and non-explosive conditions. Internal inspections detect defects such as pitting, welding flaws, lap joint issues, and corrosion. Inspectors map out defect locations, sizes, and depths.
Drones are increasingly used to capture video footage of tank interiors, eliminating the need for climbing. Devices like the Voliro T can collect thickness readings, identify corrosion, and detect defects like porosity and debonding.
Internal inspection intervals:
- Ground-contact tanks: initial inspection after 10 years
- Elevated tanks: initial inspection after 20 years
- Subsequent inspections: every 5 years for gasoline tanks, every 10 years for tanks storing other fuels and oils
Non-Destructive Testing Methods for Storage Tanks
NDT is the preferred approach for evaluating storage tank durability and integrity. Standards like SP001 recommend visual, radiographic, ultrasonic, hydrostatic, and acoustic emission tests, though the choice depends on the tank and its requirements.
Visual Inspection
This basic method relies on direct observation to identify surface-level issues, such as cracks, bulges, or corrosion. Drones can access elevated areas more efficiently than traditional methods. While visual inspections are limited to apparent defects, they often guide the use of more advanced NDT techniques.
Ultrasonic testing
Ultrasonic testing (UT) uses high-frequency sound waves to measure wall and coating thicknesses. It detects flaws as small as 0.05 mm and is effective on various materials, including metals, plastics, and polymers.
Drones equipped with UT probes, like Voliro T, can take up to 200 readings per hour, inspecting walls, roofs, and floors without direct access.
The Voliro inspection drone is equipped 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, ideal for inspecting corroded, dirty, and uneven surfaces such as in-use storage tanks.
Eddy Current Testing
Eddy current testing identifies near-surface flaws in conductive materials like steel. Pulsed eddy currents (PEC) can detect corrosion hidden under insulation without direct contact. Voliro plans to introduce a drone-mounted PEC payload in late 2024, enhancing inspection capabilities for insulated tanks.
Voliro T with the PEC payload
Dry Film Thickness Measurement
This technique evaluates protective coatings, such as epoxy or zinc. Voliro drones with DFT probes enable rapid measurements, ensuring coating integrity even immediately after application.
Acoustic Emission Testing
Acoustic emission testing (AET) detects active defects by translating noise vibrations into data. It is highly sensitive but can be affected by external noise interference.
Other methods like liquid penetrant testing, electromagnetic particle testing, and radiographic testing are also available, though they are more suited for smaller tanks or lab environments.
