Operational Guide · Registers & Logbooks

# 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.

thickness measurement UTM corrosion special survey

## 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

1. Plan the thickness measurement campaign according to the minimum extent required for the ship's age and type, coordinating with the Classification Society.
2. Carry out measurements with a qualified technician and calibrated instrumentation, with the surveyor present for areas relevant to the survey.
3. Compare measured values with original thickness and minimum permissible limits, identifying any areas of substantial corrosion.
4. Extend the measurement to adjacent areas when substantial corrosion is detected, according to applicable reference tables.
5. 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

Underestimating the required extent of thickness measurement, with measurements limited to only the originally planned areas despite the detection of substantial corrosion, has in several cases led to a survey not accredited by the Classification Society until the required extended measurements were completed, resulting in an unforeseen prolongation of docking time.

## 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

Thickness measurement is not typically subject to direct PSC verification, but the validity of the Class Certificate (which presupposes accredited completion of the survey, including any required measurement extensions) is verifiable at every inspection.

## 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

## FAQ

What is 'substantial corrosion' for thickness measurement purposes?
A reduction in thickness relative to the original construction value that exceeds a percentage threshold defined by applicable IACS requirements, requiring the measurement campaign to be extended to adjacent areas.
Does the extent of the thickness measurement campaign always increase with ship age?
Yes: IACS requirements typically provide for an increasing minimum extent of required measurement as the ship ages, reflecting the growing risk of structural deterioration.
What happens if a plate is found below the minimum permissible limit?
It must be renewed before the survey can be considered complete and accredited by the Classification Society.
🎬 Additional photos, videos and interactive diagrams for this topic will be available in a future version of the platform.

## Related Topics

Coating and PSPC Survey in Dry Dock

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