Storage tank inspection: seeing both sides of the steel wall without sending people in

Most deterioration of steel storage tanks is corrosion. The floor is attacked from the soil side and from the water and sludge that settle on top. The shell weathers on the outside, and where water gets under insulation, corrosion under insulation (CUI) grows out of sight. Weld cracking, distortion from uneven foundation settlement and floating-roof problems are the other main routes to serious failure.

Earthquakes matter too. In the 2003 Tokachi-oki earthquake, long-period shaking caused sloshing of the stored oil and led to floating-roof tank fires in Tomakomai. The condition of floating roofs and rim seals is exactly what routine inspection should watch.

There are two kinds of inspection: external inspection while the tank stays in service, and out-of-service (internal) inspection after it is emptied. For internal work the product is drawn off, sludge removed, the vapour purged (gas-freeing), and oxygen and flammable gas measured before anyone can enter. For a large tank the whole cycle can take months.

The heaviest costs are lost service time and the danger of entry. Taking one tank out of service disrupts storage and shipping plans. Oxygen-deficiency and toxic-gas accidents in confined spaces keep recurring in Japan and abroad. Shortening the time people spend inside is the central problem of tank inspection.

The drone’s job is to carry eyes to high places and into the open volume instead of people. Outside, it flies while the tank is in service; inside, it flies after opening and before anyone enters. Each job needs its own aircraft and sensors.

For a cylindrical tank, the basic pattern is orbiting in overlapping horizontal bands at different heights. The images can be unrolled into a shell development drawing, so corrosion is recorded by course and bearing. Inside, the flow becomes “survey everything by drone first, then scaffold only where needed” — which cuts the amount of scaffolding itself.

The inside of a tank is a tough place for a drone. Steel walls block GNSS, steel distorts the magnetic field so the compass cannot be trusted, and without lights it is pitch dark. Even after gas-freeing, residual vapour and oxygen deficiency remain concerns. Outside, the tank farm itself may be a hazardous area where flammable vapour can be present.

• Confined space: use a caged, collision-tolerant drone sized to pass through the manway. Confined spaces
• Darkness: fly with onboard lighting and fixed exposure; change angle where the walls bounce glare back. Darkness
• GNSS-denied: choose aircraft that localise from cameras or LiDAR, and expect featureless walls to degrade the estimate. GNSS-denied
• Magnetic disturbance: use flight modes that do not depend on magnetic heading and check sensors before take-off. Magnetic disturbance
• Explosive atmospheres: ordinary drones are not explosion-protected. Measure gas and build the flight into the work-permit procedure. Explosive atmospheres
• Oxygen deficiency & toxic gas: some teams send a drone carrying a gas detector in first, as a check before people enter. Oxygen deficiency & toxic gas

Tank inspection always ends in thickness and corrosion rate: how much metal is gone, how fast it is going, and when the tank must next be opened. Images and 3D data gather that evidence faster; the pass/fail decision and the next interval belong to the responsible inspector.

In Japan, tanks storing petroleum and other hazardous materials are regulated under the Fire Service Act as outdoor tank storage facilities and must be inspected periodically. Specified outdoor tank storage facilities of 1,000 kL or more are subject, depending on size, to internal inspection after opening and to safety inspections by the local authority, with intervals that depend on the protective measures applied (for example, internal inspection of 1,000–10,000 kL tanks within 13 years in principle, or 15 years with notified storage-management measures). Spherical LPG tanks and similar fall under the High Pressure Gas Safety Act.

For flying inside plants, METI, the Fire and Disaster Management Agency and MHLW have issued guidelines for the safe operation of drones in plants. They address flying near hazardous areas, gas detection and abort decisions, and serve as the basis for site rules.

Abroad, API 653 (inspection, repair, alteration and reconstruction of aboveground tanks) is widely used and requires, among other things, external visual inspection by an authorised inspector at intervals not exceeding five years. In the UK EEMUA 159 plays a similar role. Both define what is inspected, by whom and how often; the drone is a tool used within that framework. Compare flight rules in world regulations.

• Underside of the floor: soil-side corrosion is invisible to cameras. It needs magnetic or ultrasonic scanning after opening, or methods such as acoustic emission while in service.
• Under insulation: you must strip insulation to see. Thermal imaging can hint where water has entered, but cannot measure metal loss.
• Internal weld flaws: crack depth and internal defects need ultrasonic or other NDT.
• Flights where explosion protection is required: ordinary aircraft cannot fly in or right next to an in-service tank.
• Cleaning and gas-freeing: the tank must still be emptied and cleaned before flying inside. Drones do not remove that work.
• Pass/fail decisions: evaluating thickness, deciding repairs and signing the statutory record stay with people.

At the edge of the tank farm sits a drone hangar. Once a week a drone flies a fixed route around every tank, photographing roof tilt, seal condition and shell coatings from the same angles as last time. After an earthquake, every floating roof has been checked before anyone climbs a stair.

On opening day, the first thing to enter is a caged drone. It measures gas, images every inner surface and leaves a 3D record. People go in only to gauge thickness and repair. That is our editorial view — and we expect the floor will still depend on other technologies and human hands even then.