1. Define what the thickness data must decide
| Question | Measurement plan | Boundary |
|---|---|---|
| How much steel remains? | Location-tagged readings tied to a controlled drawing and a supplied original, nominal or previous thickness. | UTM measures present thickness; it does not create the renewal criterion. |
| Is wastage general or local? | Screening grid followed by close readings or scanning around confirmed minima and suspect areas. | A broad grid can miss isolated pitting. |
| Is a pipe or tank changing over time? | Repeatable coordinates, material, service, surface condition and comparable instrument settings. | A trend is defensible only when the location and method remain comparable. |
| Is the work for class credit? | Society-agreed extent, qualified supplier route, surveyor coordination and required report format. | Independent owner screening is not automatically valid for a class survey. |
2. Establish the document hierarchy
Collect the general arrangement, shell expansion, tank or piping drawings, plate and member identities, material grades, original or nominal scantlings, previous thickness report, repair history and the applicable survey instruction. Mark which source controls each comparison. A copied nominal value, undocumented corrosion margin or assumed renewal threshold can turn an accurate echo into a wrong repair decision.
For class-related work, use the attending society's current rules and approved measurement programme. IACS Recommendation 77 provides guidance for surveyor control of the thickness-measurement process, while the applicable member society remains responsible for its own rules, scope and acceptance.
3. Make every reading traceable to structure
- Identify plate, strake, frame, deck, bulkhead, tank, compartment or piping segment.
- Distinguish required points, screening points, close-up grids, confirmed minima and inaccessible points.
- Use drawings, coordinates and photographs so repair teams can find the same location.
- Record nearby insert plates, welds, doublers, coating breakdown, erosion paths and previous repairs.
4. Prepare access, safety and the test surface
Confined-space entry, isolation, gas testing, ventilation, lighting, staging, rafting or rope access remain separate safety controls and must be ready before measurement. Remove loose scale and marine growth only to the extent needed for stable coupling, without hiding the original surface condition. Record coating presence, roughness, curvature and temperature where they can influence the result.
At berth, anchorage, shipyard or dry dock, confirm which spaces can be opened, cleaned and accessed during the available window. A marine UTM provider cannot recover a defensible grid when planned locations are unsafe or physically inaccessible.
5. Control the instrument, probe and reference checks
ISO 16809:2025 specifies principles for ultrasonic thickness determination using pulse time of flight. For the actual job, record the gauge and probe, material sound velocity, calibration or verification status, reference blocks, zeroing, couplant, measurement mode and through-coating basis. Check the system on suitable references before use and after conditions that could affect stability.
| Condition | Technical response |
|---|---|
| Coated steel | Validate the permitted measurement mode and compare representative prepared locations; do not assume every coating can be ignored. |
| Heavy pitting or rough back wall | Clean sufficiently, change probe or method where appropriate, inspect the echo response and confirm the minimum with adjacent readings. |
| Small-diameter or hot piping | Account for curvature, temperature, probe footprint, coupling and material velocity. |
| Unexpected low value | Repeat, reposition, examine the surface and echo, then map the surrounding area before reporting a minimum. |
6. Validate readings before calculating diminution
Separate instrument output from engineering interpretation. Confirm questionable readings, retain the lowest repeatable value with its location, and state when geometry, coating or surface condition limits confidence. Compare only against an identified reference thickness. Where corrosion rate or remaining life is requested, state the dates, comparable locations, calculation basis and uncertainty; two unmatched surveys do not create a reliable rate.
7. Build a repair-ready thickness report
- Vessel identity, survey purpose, date, location and applicable instruction.
- Drawings and structural identifiers used to control the scope.
- Instrument, probe, reference checks, calibration evidence and method.
- Surface, coating, access and environmental limitations.
- Measured values, confirmed minima, inaccessible points and photographs.
- Supplied original/nominal/required thickness and clearly labelled diminution calculations.
- Suspect areas requiring closer mapping, engineering review, repair or remeasurement.
- Revision control and data files suitable for owner, yard, class or flag review.
8. Class, flag and supplier-qualification context
For ABS-, BV-, CCS-, CRS-, DNV-, IRS-, KR-, LR-, ClassNK-, PRS-, RINA- or Türk Loydu-classed ships, confirm the exact service-supplier approval, vessel type, survey scope, validity, geography and attending-surveyor requirements before booking work for class credit. IACS develops common requirements and recommendations but does not approve service companies. Flag, contract and owner requirements remain separate, and this guide makes no approval or acceptance claim.
Authoritative references
- ISO 16809:2025 — principles for ultrasonic thickness determination.
- IACS Recommendation 77 — surveyor control of the thickness-measurement process.
- IACS UR Z archive — hull classification survey requirements; apply the current relevant instrument through the class society.
9. What to send before attendance
Send the vessel/IMO number, vessel type, purpose of measurement, class and flag where relevant, controlled drawings, original scantlings, previous report, known repairs, coating and access condition, tank-entry readiness, required locations, port or shipyard, attendance window and decision deadline. Attendance can be assessed in Singapore, Malaysia, India, Sri Lanka, Indonesia, Thailand, the Philippines, the Middle East, Asia-Pacific and worldwide, subject to safe access and availability.