Drone tank inspection: the roof your monthly round can't see

The roof is the part of a storage tank nobody on the ground sees, and it's where a drone tank inspection does the most. In FlybyOps' count of PHMSA's breakout tank accident reports, 2010 to 2025, the roof was involved in 88 of 426 tank accidents. Fly it with the monthly round and send findings to the authorized inspector.

What does API 653 say about the monthly tank inspection?

API 653, the industry standard for inspecting aboveground storage tanks, gives the routine in-service check to the operator. As PHMSA quotes it in a 2015 enforcement notice, "The external condition of the tank shall be monitored by close visual inspection from the ground on a routine basis." Owner or operator staff may do it, and the interval "shall not exceed one month."

Earl Crochet, an API 653 inspector since 1993, writes on Inspenet that "This is the only inspection on ASTs per API 653 that can be performed by someone other than an API 653 authorized inspector." He also notes that "API 653 doesn't mention drones or robots or any other method for performing the visual inspections currently."

From the ground you see the shell, the foundation, the mixers and the valves. Fly a drone on the same round and it's still the operator's check, but now it covers what's above eye level: the roof and the upper shell.

Your operating staff are the right people to hand a camera to. In FlybyOps' count, local operating personnel, contractors included, were first to identify 440 of the 569 breakout tank accidents (77%). The reports don't say on what kind of visit.

Where do breakout tanks fail?

FlybyOps counted the 569 accidents operators reported to PHMSA on onshore breakout tanks, the tanks on regulated hazardous liquid pipeline systems, from 2010 to 2025. In 426 of them the tank itself was the item involved, rather than a valve, flange or pipe attached to it. By part of the tank, the bottom was involved in 118, the mixer in 90, the roof in 88, and all other parts together in 130.

The bottom sits on the foundation, and no camera outside the tank sees it. Mixers sit on the shell near the ground, where the walk-around already passes, so in our view a drone adds little there. That leaves the roof.

Most of the roof's 88 were in the roof drain system, which on a floating roof runs under the deck, out of a camera's sight. From above, the drone sees what a failing drain leaves on the roof: standing water, a blocked sump, a deck sitting low. It won't see the leaking line itself.

By barrels released, the roof leads on the strength of one accident. Roof accidents released 28,447 barrels in all, against 24,116 for the bottom, 19,694 for mixers and 26,425 for all other parts together. But one 10,009-barrel release is over a third of the roof's total. Set aside each part's largest release, and the roof and the bottom come out level at about 18,400 barrels each.

These reports include only accidents PHMSA requires operators to report, such as a release of 5 gallons or more, a fire or explosion, or property damage over $50,000. Refinery tanks and terminal tanks outside PHMSA's rules aren't in them, and these are counts of reports, not failure rates.

What to look at on a tank roof, part by part

Fly it with the monthly round, on the same path at the same angles each time, so this month's images sit beside last month's and a change shows.

On a floating roof, external or internal, look at:

• The rim seal, for gaps against the shell and torn or displaced seal material.
• The deck, for standing water or product, and for a deck that sits low or tilts.
• The drain sump and its screen, for blockage and water pooled around them.
• The pontoon hatch covers, for any that are open, damaged or missing.
• The grounding shunts between the roof and the shell.

Korial, which sells a drone system for this work, names the roof seal, rainwater on the roof and the grounding cables as what its flights check, in place of climbing tanks 25 meters tall.

On a fixed or cone roof, look at the coating and corrosion around the vents, the hatches and the joint where the roof meets the shell. On every tank, finish with the upper shell courses and the wind girder, which the ground sees only at a steep angle.

Flying at a tank farm

The site's own rules come first. One refinery contractor's drone checklist, published in AFPM's 2016 Q&A, calls for notifying the unit's Lead Operator and telling people in the unit "at least 30 minutes prior to flight." It also says, "As a drone is not intrinsically safe, Hot Work Permits will be required." Under that rule, a monthly roof flight needs a hot work permit every time.

OSHA's 1910.307 names "bulk storage plants for gasoline or other volatile flammable liquids" as an example of a hazardous location, where equipment "shall be intrinsically safe, approved for the hazardous (classified) location, or safe for the hazardous (classified) location." We read that as covering a drone in flight.

Plan for the aircraft losing its position near the steel. One firm that flies inside steel tanks said in 2019 that GPS and the compass don't work there, and ClassNK warned in 2018 that magnetic material may interfere with both in ship tanks. Close to the outside of a shell we'd expect some of the same, so brief the pilot to be ready to fly without position hold.

Keep crews out from under the flight path. The same AFPM Q&A tells of a drone hit by a bird at a 100-foot hover that fell "straight down 100 feet" and "crash-landed only feet from an active laser scan crew." Put both points on your preflight checklist.

Who does each API 653 inspection, and where the drone fits

The tank may stay in service for the external. RCA/4N is the standard's formula built on the gap between the shell's measured thickness and its minimum required thickness, and on its corrosion rate. Crochet's view is that "For the 5-year visual, a drone can certainly take video of an AST to be reviewed by an authorized API 653 inspector." Agree with the inspector before the flight on the views they need. One inspection company warns that drone data collected without inspection expertise can turn out incomplete or unusable, forcing a second inspection under qualified supervision.

Ultrasonic thickness readings on the shell are due within 5 years when the corrosion rate is unknown, and at the smaller of RCA/2N or 15 years when it's known.

The internal inspection takes the tank out of service. Crochet calls it "the most important inspection": it checks that the bottom isn't "severely corroded or leaking," and gathers the minimum bottom and shell thickness and the bottom's settlement. With the tank in service, he says, "drones are obviously not practical due to the AST being full of liquid." That inspection comes once in 10 to 20 years; for breakout tanks under the 2001 edition, PHMSA puts the cap at "20 years, or 10 years when the corrosion growth rates are not known." The roof you can fly on every monthly round.

For breakout tanks, PHMSA's rule (49 CFR 195.432(b)) still points to API 653's third edition of December 2001, and doesn't allow its risk-based interval for internal inspections. In June 2026 PHMSA proposed moving to the 5th edition, with risk-based intervals, "a maximum timeline of 20 years for performing an initial inspection and a maximum reinspection interval of 25 years." Comments closed on August 3, 2026, no final rule had been issued as of November 2, 2026, and the proposal doesn't mention drones. (For the pipe between the tanks, see our guide to drone pipeline patrols.)

Tanks under EPA's oil spill rule (SPCC) have a separate requirement. 40 CFR 112.8(c)(6) says "you must frequently inspect the outside of the container for signs of deterioration, discharges, or accumulation of oil inside diked areas," and names no method or tool.

Writing up roof findings for the authorized inspector

API 653 says what the round's findings are for. Evidence of leaks, shell distortions, signs of settlement, corrosion, and the condition of the foundation, coatings, insulation and appurtenances "should be documented for follow-up action by an authorized inspector." A roof image becomes that kind of finding when the inspector can tell what it shows, where and when. Each one needs:

• The tank ID and the part of the roof, named the same way every month.
• The flight date, the pilot and the aircraft.
• The images, from last month's path.
• Who reviewed the images, and when.
• The finding, written for follow-up.
• When the inspector closed it.

For tanks under the spill rule, 40 CFR 112.7(e) asks for "a record of the inspections and tests, signed by the appropriate supervisor or inspector," kept with the SPCC Plan for three years. If a release is ever traced back to the roof, those images show what was seen and when, so keep the footage's chain of custody from the day it's flown.

FlybyOps keeps the flight side of that hand-off. Each flight log is connected to its project, pilot, equipment and compliance documentation, and can carry permits, images, reports and risk assessments, so the hot work permit and the roof images sit on the flight they came from. Custom fields hold the tank ID and the roof zone. Fly a supported DJI aircraft on DJI Pilot 2 and Flight Sync files the flight for you, matched to the drone in your equipment registry by serial number. The site's drone procedures can live as documents with version history. A drone that comes down in the tank farm goes on the same record as an incident, and every plan has an append-only audit log of who changed what.

Frequently asked questions

Can a drone do the API 653 external inspection?

No. The external belongs to the authorized inspector, and the drone's video only supports it. Crochet adds that "API 653 does not currently address an authorized inspector who does not perform the visual inspection themselves."

Can a drone inspect inside a storage tank?

Only once the tank is out of service, emptied and ventilated, under the site's permits. In one inspection firm's account of a caged drone flight inside a Texas refinery tank, ventilation fans were running at 50% speed. OSHA names tanks among spaces that may be confined spaces, and entry happens "as soon as any part of the entrant's body breaks the plane of an opening into the space" (29 CFR 1910.146). By our reading, flying a drone through a manway isn't entry by a person. Flyability's distributor says the Elios 3 is not ATEX or IECEx certified, should fly in potentially explosive areas only with a valid hot work permit, and can carry a gas sensor that's a warning, not a certified detector.