What Is Drone Ultrasonic Thickness Testing?
Drone ultrasonic thickness testing is the measurement of the remaining wall thickness of an aboveground storage tank (AST) shell using an ultrasonic probe carried and pressed against the steel by a contact-inspection drone. The technique — often written UTM or UTG for ultrasonic thickness measurement or gauging — works on a simple physical principle: a transducer sends a short pulse of high-frequency sound into the plate, the pulse reflects off the far wall, and the instrument times how long the echo takes to return. Because the speed of sound in steel is known, that travel time converts directly into a thickness reading, typically to a hundredth of a millimetre.
What is new is the carrier. Instead of a technician holding the probe by hand from scaffolding or a rope-access seat, a wall-sticking UAV — a contact drone of the Voliro class, whose rotors tilt to push a probe head against a vertical surface — flies to the shell, applies acoustic couplant, and holds the transducer steady against the plate long enough to capture a valid reading. Repeating this at a planned grid of points up each shell course builds a thickness map of the tank wall. This is a fundamentally different operation from a visual drone survey: here the aircraft must make and maintain firm mechanical contact with the structure, not merely photograph it.
The result is quantitative data, not just imagery. Where a visual tank inspection tells you what the outside of the shell looks like, ultrasonic thickness testing tells you how much sound steel is actually left — the number that governs whether the tank is fit to keep in service.
Why Storage Tank Shell Thickness Matters
The tank farms of Jurong Island and Singapore's wider petrochemical cluster hold thousands of ASTs storing crude, refined product and chemicals. Every one of them is a steel shell under constant attack — external corrosion from the marine tropical atmosphere, internal corrosion from the stored product and its water bottoms, and erosion at turbulent zones. Over years of service the wall thins, course by course, and the loss is rarely uniform. If it goes unmeasured, the first sign can be a weeping seam or, in the worst case, a shell failure and product release.
This is exactly the risk that API 653, the industry standard for AST inspection, repair and reconstruction, exists to manage. API 653 sets the minimum acceptable thickness for each shell course as a function of the tank's dimensions and the hydrostatic head the plate must carry, and it requires shell thickness to be monitored periodically so that a corrosion rate and a remaining life can be computed. The inspection interval itself is driven by that remaining-life figure. Miss the wall loss and you lose the ability to predict failure; measure it well and the tank's continued service is defensible on data.
Storage tanks form one of the highest-value asset classes across Singapore's oil and gas and refinery facilities, and shell integrity is central to keeping them running safely between turnarounds.
Drone UT vs Traditional Thickness Surveys
Traditionally, a shell thickness survey means getting a technician to every reading location by hand. That is either scaffolding erected around the tank, a man-lift working the perimeter, or a rope-access team abseiling the shell — each of which takes days to set up, exposes people to work-at-height, and in practice limits how many points get read, especially on the upper courses that are hardest to reach.
A contact drone changes the economics of coverage. It flies straight to any point on the shell, from the base course to the top angle, and captures gridded readings across the full height in a fraction of the time, without a single anchor point or scaffold clip. Modern multi-echo ultrasonic probes read the true metal thickness through paint and protective coatings, so the shell does not need to be blast-cleaned back to bare steel before measuring. Every reading is logged against its position, and the completed grid feeds directly into the corrosion-rate and remaining-life calculation.
| Factor | Rope Access / Scaffold | Contact Drone UT |
|---|---|---|
| Access setup | Days to erect scaffold or rig rope-access anchors | Fly on arrival — no access structure required |
| Work-at-height | Technicians suspended or elevated on the shell | Crew stays on the ground; only the drone is aloft |
| Upper-course reach | Slow and physically demanding near the top | Same effort at any height on the shell |
| Grid density | Often limited by access time and fatigue | Dense, repeatable grid captured quickly |
| Coatings | May require removal at each point | Multi-echo probe reads through coating |
How a Drone UT Shell Survey Is Run
A survey follows a defined sequence so the data is traceable and the readings are valid. The instrument and probe are calibrated on a reference block of known thickness before and after the survey, and the procedure is written to ASME Section V and ISO 16809, the codes governing ultrasonic thickness measurement, so that every reading is defensible.
- Grid planning — the shell is divided into a measurement grid, with rows set on each shell course and columns around the circumference, so wall loss can be tracked course by course and trended against previous surveys.
- Calibration — the ultrasonic gauge is calibrated to the sound velocity of the tank's steel using a certified step block, and rechecked on completion to confirm no drift.
- Contact acquisition — the drone flies to each grid point, applies couplant, presses the probe to the plate, and captures a stable reading; multi-echo mode confirms it is reading metal, not coating.
- Data logging — every reading is stored against its grid reference, building the thickness map as the survey progresses.
- Assessment — the map feeds the corrosion-rate and remaining-life calculation, and the results are compiled into an integrity report for the tank engineer's API 653 evaluation.
Because the whole survey is flown from the ground, there is minimal disruption to the surrounding tank farm, no scaffold footprint in the bund, and no permit-heavy work-at-height operation to coordinate. On congested sites this is often the difference between fitting a survey into a live-plant window and having to wait for a full shutdown.
What We Measure
A shell survey targets the locations where thinning matters most and where the standard demands data.
Shell-Course Thickness
Gridded readings up every course, from the thick base ring to the upper courses, compared against the API 653 minimum required thickness for each course's head of liquid.
Corrosion & Erosion Mapping
Dense point clouds reveal localised wall loss — pitting bands, corrosion cells and erosion patches — that a sparse manual survey can walk straight past.
Remaining-Life Data
Current thickness set against original or prior readings gives the corrosion rate, and against the minimum thickness gives remaining life — the figure that sets the next inspection interval.
Nozzle & Appurtenance Areas
Readings around nozzles, manways and shell penetrations, where geometry concentrates stress and accelerated local corrosion is common.
Honest Scope: Shell, Floor and Internal
Drone ultrasonic thickness testing is a powerful tool for the external shell, and it is important to be clear about what it does and does not replace. Shell UT covers external wall-thickness screening and monitoring — the courses of steel you can reach from outside. It is not a floor inspection. Tank floor plates, which corrode from underside soil-side attack and from product-side water bottoms, are assessed with magnetic flux leakage floor scanning during an out-of-service internal inspection.
Likewise, the internal face of the shell and the roof are captured during a shutdown, when a caged remote visual inspection drone can fly the interior of the empty tank. A confined-space inspection drone removes the need for a technician to enter the vessel to gather that internal imagery. We scope the external shell UT alongside these complementary methods so the tank engineer receives coordinated data on the floor, shell and roof rather than a single-technique snapshot. This is part of a wider industrial drone inspection capability across plant assets.
Why Choose SG Drone Inspections
We are a specialist industrial drone inspection provider in Singapore, delivering the quantitative NDT data that asset-integrity teams and tank engineers rely on for API 653 decisions.
Contact-UAV Capability
We operate wall-sticking contact drones that carry an ultrasonic probe to the shell, capturing true wall-thickness readings — not just imagery — across the full height of the tank.
Coded to ASME & ISO
Thickness measurement follows calibrated procedures to ASME Section V and ISO 16809, with reference-block calibration before and after, so every reading is traceable and defensible.
Integrity-Ready Deliverables
You receive a gridded thickness map per shell course, a corrosion-rate assessment and a remaining-life indication in a report format your tank engineer can drop into the API 653 evaluation.
CAAS-Permitted Operations
Our flights run under the required CAAS operator and activity permits, and we handle airspace coordination on Jurong Island and other industrial sites so your team does not have to.
Frequently Asked Questions
Get a Drone Tank UT Survey Quote
Send us your tank details — diameter, height, number of shell courses, product and last inspection date. We reply with a survey scope and quote within 24 hours, no obligation.