Ultrasonic Thickness Measurement

Marine ultrasonic thickness measurement

Ultrasonic thickness measurement (UTM) establishes the remaining thickness of steel without cutting or damaging the structure. Ship owners use it to plan steel renewal, support class surveys, investigate corrosion, and verify the condition of hull plating, decks, bulkheads, frames, tanks, hatch covers, and selected piping. vslSafety coordinates an appropriately qualified measurement company, access arrangements, surveyor communication, and reporting at the vessel’s port or repair location.

When UTM is required

The extent of measurement is normally set by the attending classification society and depends on vessel type, age, previous readings, coating condition, and close-up survey findings. IACS Unified Requirements, including UR Z10 series survey requirements and UR Z17 service-supplier controls, commonly shape the work. Measurements expand at special surveys and may also be requested at intermediate surveys, after damage, before purchase, or during repair planning. Only the attending class society can confirm the final measurement pattern and acceptance criteria.

Typical measurement scope

• Review of class instructions, previous UTM reports, structural drawings, and suspect-area records
• Grid or spot readings on shell plating, decks, bulkheads, internals, web frames, and tank structures
• Calibration checks using traceable reference blocks and equipment suitable for the material and coating
• Immediate communication of substantial corrosion, anomalous readings, and areas requiring expanded gauging
• Tabulated readings, sketches, diminution calculations, and a signed electronic report in the required class format

Plan an efficient attendance

Provide the vessel name and IMO number, flag and class, survey type, port and berth, tank plan, measurement instructions, previous report, drawings, coating condition, and access status. Confirm whether spaces will be gas-free, ventilated, cleaned, illuminated, and safe for entry, and whether staging, rafting, rope access, or a class surveyor is needed. UTM is not a substitute for the surveyor’s examination; it supplies measurement evidence used by class and the owner to decide repairs and acceptance.

UTM reporting and common questions

Must the measurement company be class approved?

For measurements credited to a class survey, the supplier normally needs approval acceptable to the vessel’s classification society and must work within that approval’s vessel and measurement scope. Technician qualifications, equipment, calibration, reporting software, and quality procedures may be checked. An independent commercial survey can use a different appointment basis, but it should not be represented as class-credit UTM.

Can readings be taken through paint?

Some instruments can compensate for suitable coatings, but coating thickness, roughness, scale, curvature, and corrosion profile affect accuracy. Calibration and verification should match the surface condition. Local preparation may be required at doubtful points. The surveyor can request repeat readings or expanded measurements whenever results conflict with visual condition or expected structural patterns.

What is substantial corrosion?

Class rules use defined diminution criteria and renewal thicknesses rather than one percentage for every plate. The UTM company records actual thickness and may calculate diminution against original or rule thickness; the attending surveyor decides classification and repair action. Areas near limits usually require denser gauging to establish the true extent before steel quantities are planned.

How can owners reduce time onboard?

Obtain the class measurement programme early, mark frames and strakes, prepare plans and previous readings, clean and ventilate spaces, arrange safe access, and coordinate tank entry with the close-up survey. Daily transfer of preliminary readings lets class expand the pattern while the team is still onboard. That is faster than remobilising after a report review ashore.