On This Page
Internal and external methods compared
Separate the aircraft's access route from the sensor's capability. A camera
records visible surfaces. A contact ultrasonic testing probe, usually shortened
to UT, measures thickness at the places where a valid reading is obtained. A
laser scan records surface geometry. These outputs answer different questions
even when one aircraft collects them together.
Scroll horizontally to compare all columns.
This is an editorial comparison of method capabilities and project choices,
drawing on
BINDT's external visual-inspection guidance summary,
published tank-robot field research, and
Flyability's contact UT workflow.
It is not an inspection checklist prescribed by a tank standard.
Where API 653 governs the inspection program, involve the responsible tank
inspector when setting the scope.
API's inspector certification description
covers tank inspection and repair knowledge; piloting an aircraft does not
establish that qualification. Agree which remote records the inspector will
accept and which examinations or engineering assessments remain necessary.
How an external inspection works
Start with the asset drawing and previous findings. Define a repeatable location
scheme, such as shell course plus circumferential position, and distinguish roof
areas, nozzles, stairs, and other scoped items. The inspection lead should
identify the surfaces and details that matter before the pilot chooses capture
positions.
A useful collection sequence is an overview to locate the feature, a closer
image to show its condition, and additional angles where the first view hides
detail. Have the reviewer accept representative images before extending the
survey around the tank. This is a proposed job sequence, not a promise that
every surface can be reached from the air.
BINDT's summary of HOIS guidance describes application-specific image
resolution, viewing direction, lighting, and camera settings. Those variables
explain why a high pixel count alone does not establish inspection quality. A
distant image may locate a coating change while still being unsuitable for
assessing a small indication.
An exterior survey may be possible while the tank remains in service, subject to
site and flight restrictions. That does not authorize a flight near vents or
other classified areas. In U.S. general industry,
29 CFR 1910.307
requires electrical equipment in hazardous classified locations to meet the
applicable suitability provisions. Confirm the location classification and the
actual aircraft and payload configuration before commissioning work there.
If insulation or cladding covers the shell, record that as the surface
inspected. Images of the covering cannot establish the condition of the
concealed steel. Likewise, seeing the roof exterior does not document the
underside of its supporting members. These gaps should drive additional
inspection tasks rather than disappear inside a general statement that the tank
was surveyed.
How an internal inspection works
An internal aerial mission requires a usable flight volume, a route through the
opening, and a way to illuminate and locate the surfaces. The owner and
inspection team must first establish the tank's condition, contents, isolation
requirements, and suitability for the proposed equipment. Do not assume that an
emptied tank is safe to fly in, or that a protective cage establishes explosion
protection.
Scope launch and retrieval as carefully as capture. Record the opening
dimensions, route to each target, places where the aircraft could become
trapped, and the response to a lost aircraft. A proposal that avoids entry
during normal capture can still create an entry task during recovery.
The U.S. general-industry
confined-space rule, 29 CFR 1910.146,
defines entry as occurring when any part of a person's body crosses the opening
into a permit space. Reaching through a manway to place or retrieve equipment
therefore matters when claiming a mission involves no human entry. Establish the
applicable entry controls for any such task; a remote camera does not remove
them.
Inside, localization can combine laser, camera, and inertial measurements to
estimate the aircraft's movement and build a map. The 2024 tank-robot field
paper demonstrates this approach in ship ballast tanks and describes narrow
passages, mapping difficulties, and lighting constraints. These are useful
engineering lessons for enclosed inspection, but its ship trials do not certify
performance in a particular land-based storage tank.
Plan separate views of the relevant faces of roof supports and other members.
Flying past a structure does not mean the camera has seen behind it. Preserve
inaccessible faces in the coverage register, and check the captured detail
before closing the tank or releasing the inspection crew.
If thickness is required, the workflow changes from image capture to controlled
contact measurement. Flyability's documented UT process includes placing the
probe, dispensing couplant, inspecting the returned ultrasonic signal, and
preparing an unsuitable surface where necessary. Couplant provides the acoustic
connection between probe and surface; the signal display, called an A-scan,
helps the UT inspector judge the reading. Probe access and valid contact must be
demonstrated at the required locations.
An internal flying drone should also be distinguished from an immersed tank
robot.
Square Robot documents on-stream robotic tank-bottom and shell UT services.
That is a different access and sensing approach. Where the question concerns the
floor beneath stored liquid, ask whether an appropriately qualified immersed
system or a separate bottom-inspection program fits; do not substitute roof
imagery for floor measurements.
What accuracy means for a tank inspection
There is no single useful accuracy figure for all of these outputs. Require the
provider to separate three questions.
Can the image reveal the feature?
Judge image suitability against the smallest feature the inspection lead needs
to assess, at the proposed distance and angle. Check sharpness, lighting, and
obstruction on the delivered image. The tank-robot field study specifically
reports that lighting direction can affect whether fine cracks are visible. A
clear general view is not proof that the capture method can reveal every small
defect.
Ask for representative captures under comparable conditions and define when
closer examination is required. Do not translate a camera's resolution
specification into a claimed probability of detecting defects without supporting
method testing.
Can the finding be located again?
A three-dimensional model can organize observations, but repeat inspection needs
a stable relationship to the tank. Specify the drawing reference, coordinate
system where applicable, feature identifiers, and how model locations will be
checked. For a dimensional task, require a stated uncertainty and checks suited
to that task before accepting deformation or settlement measurements. Treat a
model used to navigate or locate photographs as a different deliverable from a
verified dimensional survey.
As an illustrative reporting example, a finding might identify a shell course,
position relative to a fixed nozzle, an overview image, and a close view. That
gives the next inspector a route back to the observation even if they do not use
the supplier's model viewer. It is a proposed record structure, not a
measurement result.
Is the thickness reading valid and representative?
BINDT explains ultrasonic thickness gauging
as using pulse travel time, with calibration or knowledge of sound velocity in
the material. Specify the material, probe, calibration records, surface
condition, measurement units, and the qualified review of the signal.
Even a valid reading describes its sampled location. A collection of spot
measurements cannot be presented as complete floor or shell coverage. Require a
map of attempted, accepted, and rejected readings, with reasons for rejection.
Label a failed measurement as missing data, not zero thickness or an acceptable
surface. The tank inspector determines whether the sampling and any further
examination answer the integrity question.
For the broader distinction between an observation, a measurement, and an
engineering conclusion, see
what drone-inspection evidence can prove.
Compare the whole job cost
Compare quotations against the same surfaces, measurement locations, reporting
requirements, and owner acceptance process. An external camera flight and an
internal visual-plus-UT job are different purchases. Their day rates alone
cannot establish which is better value.
Use this cost structure to request comparable proposals:
Total project cost = tank preparation and outage impact + site access and
mobilization + data collection + processing and qualified review + confirmatory
work and recovery allowance.
This is an editorial budgeting structure, not a market price or a supplier
quotation. Keep costs outside the provider's invoice, such as owner labor and
lost tank availability, visible in the comparison.
For external work, ask how weather delays, the number of viewpoints, surrounding
obstructions, and any contact measurements affect the quote. For internal work,
ask what preparation the owner must complete, whether surface cleaning and probe
changes are included, and what happens when the aircraft cannot reach a required
location. For both, settle who pays for recollection and what constitutes usable
delivery.
The most plausible access saving occurs when scaffolding, a lift, or rope access
would otherwise be required solely to collect information the drone can obtain.
If that same access is still needed for repair, cleaning, or additional testing,
allocate only the portion actually avoided to the drone option. Do not count an
entire tank outage as a saving when the tank must already be shut down for the
remaining work.
Ask for two schedules as well as two prices: the proposed drone-assisted job and
the alternative method over the same accepted scope. Compare the dates when
preparation starts and the reviewed inspection package is available. Short
flight time does not necessarily shorten the critical path back to service.
Choose the method and specify the handover
Use the external method when the immediate decision concerns exposed exterior
condition and those surfaces can be captured at the required detail. Use
internal aerial inspection when roof undersides or internal structures are the
priority and the tank can be prepared for suitable remote operation. Add contact
UT where specified thickness measurements are needed and achievable.
If the main question is bottom corrosion beneath stored product, evaluate an
appropriate floor-inspection method separately. If repairs, inaccessible weld
examinations, or manual preparation remain necessary, plan a combined job. The
right commercial mission is often a defined part of the inspection program
rather than a replacement for every task.
Before award, agree that the handover will contain:
- A scope register showing each required surface or measurement location as
completed, rejected, or inaccessible.
- Located findings with original overview and detail images, descriptions, and
follow-up requests.
- UT readings with units, location references, calibration information, and
retained signal records where specified.
- Any model or dimensional output with its reference system, checks, and stated
limitations.
- The responsible inspection reviewer's conclusions, outstanding work, and files
the owner can retain and reopen.
Make the purchasing decision from those deliverables. The strongest proposal
explains both what the aircraft will collect and how the remaining questions
will be resolved before the tank's condition is judged.
Source notes
Last checked: September 6, 2026.
