Thickness Measurement (Ultrasonic Thickness Measurement, UTM)
A UTM reading is useful when it measures the material with a suitable method and remains linked to location, reference and conditions. The report supports hull assessment; the displayed number does not itself decide acceptance.
Operational Explanation
Ultrasonic thickness measurement (UTM) estimates material thickness from the time taken by the ultrasonic signal to reach a reflecting surface and return to the transducer. Converting that time into thickness requires the material’s sound velocity and settings consistent with instrument, transducer and method. A displayed number alone does not demonstrate a valid measurement.
For hull surveys, a reading becomes useful when linked to an identified member and location, the surface condition and the applicable thickness reference. Coatings, irregular surfaces or inadequate signals can affect a reading; the selected method must account for them. Evident: calibration; Evident: measurements through coatings.
| Stage | Question to clarify |
|---|---|
| Measurement | Which material, transducer, mode and calibration verification were used? |
| Traceability | Which plate or member and which location does the reading represent? |
| Assessment | Which thickness reference and applicable criteria does the decision-maker use? |
Reliable measurements and a traceable report support structural assessment. They do not constitute hull acceptance or replace visual examination and other survey evidence.
Regulatory Reference
2011 ESP Code, as amended, and applicable class Rules. Bulk carriers and oil tankers within SOLAS XI-1/2 follow the relevant ESP regime and category provisions; UR Z10 is not one table for every ship. Other ships require identification of the survey regime and pertinent Rules.
PR19 Rev.1 addresses control of measurements in the referenced hull classification surveys and the meeting for the pertinent intermediate/special surveys. Recommendation 77 Rev.4 guides control of the process; it is not a corrosion limit for all ships. IACS PR19; IACS Recommendation 77.
PR19 provides for a planning meeting before measurements involving the surveyor, ship’s representative and thickness-measurement firm. Agree access, preparation, criteria and communication of results; final measurement extent and locations also depend on the overall survey of representative spaces. IACS PR19.
Make each reading traceable to its location and structural member, with reference to applicable thicknesses and limits. Promptly report results that may require further measurements or repairs; do not merely deliver an aggregated report at the end of docking. Measurements support the surveyor’s decision rather than replace it. IACS PR19: monitoring and reporting.
Within the 2011 ESP Code scope, MSC.553(108), in force from 1 January 2026, updates Administration approval and auditing of thickness-measurement firms. It does not introduce a universal number of points or one allowable diminution. IMO MSC.553(108).
Scope of Application
This card concerns hull measurements prepared and used as part of a survey and structural assessment. Extent, locations and criteria depend on ship type/construction, age and survey, areas, observed condition and applicable Rules.
Identify the correct reference before comparing numbers: original thickness or another specified reference, pertinent diminution/renewal criteria and their version. “Substantial corrosion” is a term of the applicable regime; do not assign it a universal percentage. The reader does not assume the qualified operator’s role or that of the competent authority assessing the outcome.
Procedure / How to Complete It
- Define the work and references. Gather the survey plan, drawings identifying members/thicknesses, previous reports and applicable limits. Agree initial extent, preparation and reporting with the competent parties.
- Prepare access and identification. Make areas available safely under the prescribed procedures. Link points to compartment, member, frame or another drawing reference; describe surface and coating.
- Verify the measurement method. The appointed firm identifies instrument, transducer, mode and calibration/checks appropriate to material and conditions. A known-thickness check must be consistent with the method and applicable instructions.
- Take readings and clarify abnormalities. Record readings and locations. Unstable readings, doubtful signals or unmeasurable areas require clarification; repeating a number alone does not prove the method correct. Promptly report findings that may require further measurement or repair.
- Agree survey follow-up. The surveyor finalises locations and extent according to findings. Make additional measurements, incomplete areas and information needed for assessment explicit.
- Check report and outcome. The report must allow reconstruction of areas, members, references and results, including limitations and remaining work. Compare preliminary and final reports under the prescribed process and retain the competent decision, repairs and their closure.
Practical Example
At an identified shell location, a reading is unusually low compared with nearby evidence. Before interpreting it as material loss, the operator checks position, surface/coating, signal and settings according to the method. The reading and checks performed remain traceable.
If the finding is confirmed or significant doubt remains, it is reported to the surveyor, who decides the extent of further examination and follow-up. A general cargo ship uses criteria pertinent to its survey: it does not automatically receive a Z10.2 bulk-carrier table.
What Typically Goes Wrong
Apparently orderly readings may not be comparable if position, structural reference, material or method changes without documentation. A value may include coating effects, while an average report may conceal local deterioration.
Other errors include measuring only readily accessible areas, treating a missing reading as zero and submitting only a final total. The process must make results, doubts, incomplete areas and communication to the surveyor identifiable.
Common Mistakes Mistake Library
| Mistake | Consequence | How to Avoid It |
|---|---|---|
| Reading without member/location | It cannot be repeated or compared with drawings or a later survey. | Use consistent structural references and point mapping. |
| Method unsuitable for material or coating | The number may not represent the thickness sought. | Verify pertinent mode, transducer and calibration; document limitations. |
| Average report conceals local abnormalities | Assessment loses necessary detail. | Make abnormalities and the required distribution of readings traceable. |
| One limit or table for every ship | Diminution and acceptance are assessed under the wrong scope. | Identify applicable Rules, category, survey and reference. |
| Report signature mistaken for survey closure | Measurements or repairs remain incomplete. | Follow competent assessment, remaining work and recorded outcome. |
What the PSCO Checks
Operational Tips
- Prepare a point-identification convention that another operator can locate, including after repairs or renewals.
- Distinguish measured value, thickness reference, applicable criterion and assessment: they are connected information with different roles.
- Communicate findings during the work so necessary extent can be defined before access to the areas is lost.
Preparation checklist
- Survey and applicable criteria identified.
- Drawings, structural references and previous reports available.
- Firm/personnel suitable under the required regime.
- Access, surfaces and coatings considered.
- Instrument, transducer, mode and checks recorded.
- Readings, abnormalities and remaining areas identifiable.
- Report and competent outcome, including repairs, followed to closure.
FAQ
Related Topics
Certified substantive revision: 12 October 2026 · page fingerprint 585fe02c53ab