In-Service Tank Floor UT: Map Weld and Sump Gaps

What does a tank-floor UT point actually answer?

A valid UT point gives bottom-plate thickness at one located position. It cannot identify which face lost metal, clear a nearby weld, prove leak tightness or describe the rest of the plate. Put those questions on separate feature rows.

API's inspector outline treats bottom thickness, annular rings, records and repairs as separate topics. The owner and qualified inspector compare these located results with the tank history and their adopted inspection basis before deciding what they mean.

Which floor features belong on the map?

Start with the controlled floor drawing and check it against the installed tank. Assign IDs to plates, seams, any annular plates, the shell-to-bottom junction, sumps, drains, support bases, internal pipes and repairs. Outline each feature instead of dropping a pin at its center. Mark known deposits, lining and leak-history areas as questions for inspection, not diagnoses.

Use that feature schedule to plan the route: where can the crew collect plate thickness, where must it test seam-adjacent geometry, and how will it examine a sump? Reconcile missing or unexpected features with the tank drawing before anyone treats the route as complete.

How do you locate a reading for the next inspection?

Agree on a fixed datum, coordinate direction and plate IDs before acquisition. For each attempt, record the tank ID, drawing revision, feature ID, coordinates, orientation, date, operator and setup. Link the raw response and any visual record to that position. When the route stops, mark its last confirmed position and outline the area it did not examine.

‘Near the sump’ will not help the next crew find a low point. Give them the drawing, datum and feature reference they need to revisit it or understand a rejected attempt. Record the location even when there is no accepted thickness.

When is a bottom-plate UT result acceptable?

Check the selected setup against the floor material, geometry and surface or lining. Verify the equipment and material-velocity basis under the approved procedure. Save a stable, repeatable response with its position and setup. If the response, location or surface check fails, keep the attempt and its reason in the log. Leave the displayed number off the accepted-thickness map.

ASTM E797/E797M-21 describes manual contact pulse-echo measurement, primarily on flat parallel parts. It does not qualify a robotic tank-floor route. ISO 16809:2025 describes ultrasonic time-of-flight thickness determination by contact or immersion technique; ISO 16831:2025 addresses verification of thickness equipment. The project procedure still has to establish what counts as valid on this floor.

What changes at a lap weld or annular edge?

At a lap seam, one plate rises over another. The bead, edge and any deposit can upset probe seating or the UT response. Demonstrate the chosen setup's seating, location control and response on representative geometry before accepting seam-adjacent points. An open-plate result cannot clear the weld or an annular band.

Near the shell, map the annular arc and shell-to-bottom junction separately. Record the last valid plate point, rejected attempts and the remaining unexamined arc. That boundary lets the inspector select a suitable edge or weld examination under the tank program.

How should a sump or internal obstruction be reported?

Outline the sump lip, wall and low point. Mark the planned approach, the last confirmed position and why the route stopped. Do the same for a support base, repair plate or fixed internal fitting. A blank cell hides the obstacle; a bounded unexamined row makes it actionable.

Name the next access arrangement or method, its owner and a trigger such as the next approved outage. A planned examination is still a plan. Check whether the actual tank has the illustrated feature before putting it in the field scope.

Where can ITIS Rover help?

ITIS Rover supports UT acquisition on bottom plates in suitable operating storage tanks. PetroBot's tank-inspection service lists visual records, UT readings where applicable and a condition report. Plan an accessible plate route and an exception list; confirm feature-specific access and measurement on the actual tank.

Before deployment, collect the product and operating conditions, access opening, recovery path, repairs, lining, deposits, obstacles and prior findings. The site team checks atmosphere, area classification, isolation and operating controls, then trials the selected configuration on representative safe geometry. Stop when the route, response or recovery plan fails its approved check.

What if deposits or lining block a useful response?

Describe what the team can actually see: deposit, lining, staining or an obscured edge. If UT was attempted but failed the response or location check, retain it as rejected. If no attempt could be made, outline the unexamined feature. Neither record supplies a thickness value.

The owner decides whether to prepare the surface later, use another qualified NDT setup or wait for outage access. Keep that assignment beside the route record. For UK flammable-liquid tanks, HSE HSG176 provides inspection and maintenance guidance; the owner still sets the method and scope for this tank.

How do you hand off a low value and an unexamined band?

Give each feature a clear evidence state. Accepted means a located UT point passed the procedure. Rejected means an attempted point failed a named check. Unexamined means no usable examination covered the bounded feature. Deferred records the owner's assigned follow-up method, person and due trigger; it never means measured coverage. Keep the route and setup with each attempted point.

Send a low accepted point to the qualified inspector for confirmation and engineering review under the owner's adopted basis. Send an unexamined band for method or access planning. A low value and a gap need different actions; neither establishes a repair threshold or proves the surrounding steel sound.

What should the tank owner receive?

Hand over the floor and exception map against the controlled drawing. Include the planned and actual routes, accepted points, rejected attempts and reasons, visual records, setup and quality log. Outline every unexamined area. For each deferred feature, name the next method, owner and trigger so the inspector can see exactly what the survey achieved.

The owner and qualified inspector set those records beside tank design, service history and prior findings. They decide which low points need confirmation and which weld, sump or annular areas need another method or outage access. Put their assignments in the final register beside the plate results.

Frequently asked questions

Technical references

• API 653 — 2026 inspector body of knowledge
• API 653 — public fifth-edition contents preview
• ASTM E797/E797M-21 — manual contact UT
• ISO 16809:2025 — ultrasonic thickness determination
• ISO 16831:2025 — thickness equipment verification
• HSE HSG176 — storage of flammable liquids in tanks
• PetroBot — ITIS Rover
• PetroBot — tank inspection services