Fixed-Roof Tank Inspection: Where Plate UT Stops

What does a fixed-roof plate reading tell you?

A qualified UT point gives thickness at the located plate position. It cannot clear a seam, vent curb, roof-to-shell junction or supporting member. Nor can one point identify which face lost metal, prove a weld sound, show leak tightness or establish the roof's load capacity. Record the point as a point, not as coverage of the surrounding roof.

API's 2026 API 653 inspector outline lists roof type, roof condition and roof-to-shell junctions as separate topics. Give those features separate entries in the work pack. The tank owner's adopted standard and approved procedure govern examination and disposition.

Which roof zones and fittings belong on the map?

Begin with the controlled roof drawing and check it against what is installed. Give each accessible plate sector an ID. Add seams, vents, hatches, manways, attached fittings and the full roof-to-shell edge as separate entries. Show supports where the drawings locate them; confirm whether this roof is supported or self-supporting instead of assuming a typical arrangement.

Mark permanent obstructions and any difference between the drawing and the roof you find. A radial distance and clock position can locate points on a cone roof if the team agrees on one datum and viewing direction. Link the roof and shell maps, but keep their achieved coverage separate.

Where should visual inspection start?

Photograph the roof before approved cleaning or surface preparation changes it. Start with an orientation view. Then mark coating loss, corrosion products, standing water, deposits, stains, deformation and visible openings on the zone map. Photograph each fitting from several directions and mark the surfaces its curb hides.

A stained seam shows visible change; it does not tell you which face lost metal or how much. Record its extent and send the inspection question to the qualified inspector. For UK flammable-liquid tanks, HSE guidance calls for routine visual checks, formal examinations and a record of deterioration.

How should a roof UT location be recorded?

For every attempted point or scan segment, record the tank ID, drawing revision, roof-zone and plate ID, radial distance or grid coordinate, clock datum and direction. Tie the photograph and raw response to the same location. Keep the equipment setup, operator, date and prepared-surface extent with the reading so a later crew can return to the actual patch.

A location named only ‘near vent’ will not support a repeat inspection. The position remains in the log even if the UT attempt is rejected; deleting it makes a method gap look like an unplanned blank. Use the same datum in the route sketch, image index and final table.

When is a thickness reading acceptable?

Accept a value only when the approved procedure covers this material, geometry and setup. Check surface preparation, material-velocity basis, equipment verification, probe seating, couplant and a stable, repeatable backwall response. Save enough setup and response evidence to show why the point passed. If a displayed number fails the procedure's criteria, log a rejected attempt and its reason.

ASTM E797/E797M-21 describes manual contact pulse-echo thickness measurement, primarily on flat parallel surfaces, with limited applicability to nonparallel or concentric surfaces. It does not qualify a robotic route. ISO 16809:2025 addresses time-of-flight thickness determination; ISO 16831:2025 addresses thickness-equipment verification. Apply the project's qualified procedure at a curb or seam rather than extending a good plate response across changed geometry.

Where can MagRover help on a fixed roof?

PetroBot lists magnetic adhesion, a camera option and UT spot or scan options for MagRover on suitable ferrous surfaces. Its tank service includes external roof work. Assign it to accessible plate routes only after the selected setup shows repeatable location control and procedure-valid UT; keep fittings on a separate schedule.

Trial the selected configuration on representative material, coating, slope, plate condition and roof geometry. Confirm adhesion, probe contact, response, tether path and recovery before assigning coverage. A roof-route trial does not authorize travel across a raised seam, vent, hatch or edge; stop and hand off where the route or measurement fails.

What stops the roof route?

Before anyone or anything goes onto the roof, the owner confirms its construction, loading and access plan. The site team sets the product and atmosphere controls, permits, isolation, ignition controls, weather limits, fall protection, tether route and recovery arrangement. A robot does not remove the need for structural review or controls for people working near exposed edges.

Stop at an unverified support zone, suspect plate, ponding, poor adhesion, lost coupling, a raised fitting or an edge that the approved route cannot clear. Give the stop a coordinate and a reason. If support, access or recovery cannot be demonstrated, select another qualified access or examination plan rather than treating the route as complete.

How do you report seams and penetration gaps?

Use four states in the evidence table. Accepted means a located visual observation or thickness result met its procedure. Rejected means an attempted measurement failed a validity check. Unexamined means there was no safe or workable examination at the mapped location. Deferred means the examination has a recorded trigger, next method and owner. Neither a rejected value nor an unexamined area counts as accepted thickness coverage.

One row should answer one feature question. Keep the drawing reference, attempted method, result state, reason, evidence link and named follow-up owner together. A plate value beside a vent curb stays in the plate row; the curb keeps its own row even when its next method is pending.

When does another method or access plan take over?

Use the gap register to ask what remains unanswered. A seam with coating loss may need a closer visual examination after approved preparation. A curb with invalid UT needs a different qualified setup or access. An underside or support question may require internal access at a planned opportunity. The qualified inspector selects the method for the feature and suspected condition; ordinary plate thickness UT does not examine a weld or hidden face.

The owner and qualified inspector choose the next method under the tank program. Before arranging access, confirm the adopted standard, permits and isolation requirements for this tank.

What should the owner receive after the survey?

Deliver one roof-zone map reconciled to the controlled drawing and fitting schedule. Attach accepted visual and UT evidence, the setup and quality log, rejected traces, bounded unexamined areas and deferred examinations. Give each unresolved item a next method, trigger and named owner. This lets the inspector see the achieved examination beside the original scope, rather than infer coverage from a route line.

The owner and qualified inspector then combine this evidence with design, service history and prior inspection results to decide whether further examination, repair or engineering assessment is needed. The final register should show the vent, seam and edge decisions beside the accepted plate values.

Frequently asked questions

Technical references

• API 653 — 2026 inspector body of knowledge
• ASTM E797/E797M-21 — manual contact UT scope
• PetroBot — MagRover
• ISO 16809:2025 — ultrasonic thickness determination
• ISO 16831:2025 — thickness equipment verification
• HSE HSG176 — storage of flammable liquids in tanks
• HSE — work at height
• OSHA 29 CFR 1910.28 — walking-working surfaces
• PetroBot — tank inspection services