Thickness Measurement (Ultrasonic Thickness Measurement, UTM)
The number that decides whether a plate stays in service or must be replaced: how a thickness measurement campaign is planned and why areas of substantial corrosion require an extension of the measurement.
Operational Explanation
Ultrasonic thickness measurement (UTM) verifies the ship's structural condition by comparing residual plate thickness with original construction thickness and the minimum permissible limits defined by the Classification Society. It is a central element of the Special/Renewal Survey, especially for ships approaching or exceeding 10-15 years of age.
When thickness measurement reveals an area of substantial corrosion (defined according to percentage reduction thresholds relative to original thickness), Class requirements mandate extending the measurement to adjacent areas, to determine the actual extent of the phenomenon before the survey can be considered complete.
Regulatory Reference
IACS UR Z10 requirements (series, according to ship type): define the minimum extent of measurements required for the Special/Renewal Survey, substantial corrosion thresholds, and the obligation to extend thickness measurement to adjacent areas when such thresholds are exceeded.
Scope of Application
Every ship subject to Special/Renewal Survey, with the measurement campaign's extent increasing with ship age according to applicable IACS requirements.
Procedure / How to Complete It
- Plan the thickness measurement campaign according to the minimum extent required for the ship's age and type, coordinating with the Classification Society.
- Carry out measurements with a qualified technician and calibrated instrumentation, with the surveyor present for areas relevant to the survey.
- Compare measured values with original thickness and minimum permissible limits, identifying any areas of substantial corrosion.
- Extend the measurement to adjacent areas when substantial corrosion is detected, according to applicable reference tables.
- Plan the renewal of plates found below minimum permissible limits before the survey is completed.
Practical Example
Example of extension: substantial corrosion detected on a ballast tank bulkhead during the standard thickness measurement campaign; the measurement is extended to adjacent bulkheads according to the IACS reference table, to determine whether the phenomenon is localized or more widespread before the survey is considered complete.
Real Cases
Common Mistakes Mistake Library
| Mistake | Consequence | How to avoid it |
|---|---|---|
| Thickness measurement campaign limited to the minimum extent without considering the ship's actual age | Advanced corrosion areas not detected in time | Plan the campaign's extent in line with the actual age and corrosion history of the specific ship |
| Measurement not extended to adjacent areas after substantial corrosion is detected | The survey is not accredited as complete by the Classification Society | Always extend the measurement according to reference tables when substantial corrosion thresholds are exceeded |
| Replacement of plates below minimum limits planned after, not during, the dock period | Need for a second dry dock entry to complete the work | Plan the replacement of critical plates before completing the current survey |
PSC Observations
Operational Tips
- Plan the thickness measurement campaign's extent based on the ship's actual age, not just the formal minimum required.
- Don't stop at the first substantial corrosion threshold detected: always extend the measurement according to reference tables.
- Plan the replacement of critical plates within the same dock period, to avoid a second entry.
Checklist
- Thickness measurement campaign planned according to the extent required for ship age/type
- Measurements carried out by a qualified technician with calibrated instrumentation
- Values compared with original thickness and minimum permissible limits
- Measurement extended to adjacent areas in the event of substantial corrosion
- Replacement of critical plates planned within the current dock period