Ultrasonic Thickness Measurement: Scope, Preparation and Reporting

Ultrasonic thickness measurement establishes the remaining thickness of ship structures without cutting into the steel. It is widely used during class renewal and intermediate surveys, condition assessments, repair planning and investigations of local corrosion. Useful results depend on survey planning, access, surface preparation, calibrated equipment and accurate location reporting—not simply taking a large number of readings.

Acceptance limits and measurement extent are vessel-specific. The attending classification surveyor, applicable class rules, approved plans and original scantlings determine what must be measured and whether diminution is acceptable.

How ultrasonic thickness measurement works

A probe transmits an ultrasonic pulse through a couplant into the material. The instrument calculates thickness from the time taken for the sound to reflect from the back wall. Coatings, scale, pitting, curvature, laminations, temperature and poor coupling can affect the response. Operators must recognise unstable signals and verify questionable readings rather than recording the first number displayed.

For heavily corroded structure, local pits may govern condition even where surrounding average readings appear acceptable. Grid density and pit measurement techniques should therefore match the survey objective.

When thickness measurements are required

Class rules prescribe measurement patterns based on ship type, age, survey type and structural condition. Areas can include deck plating, shell strakes, bottom structure, transverse sections, water ballast tanks, cargo holds, hatch coamings, bulkheads, internals and suspect areas identified during close-up survey. Additional readings may be required when substantial corrosion, wastage or damage is found.

Owners also commission gauging before drydock to estimate steel renewal, after grounding or contact damage, when monitoring known corrosion, or when evaluating tanks for coating repair. Define the decision the data must support before setting the scope.

Use an appropriately approved gauging company

Where measurements form part of a class survey, the thickness measurement company generally requires approval by the relevant classification society. Approval should cover the company, procedures, equipment and qualified operators. Confirm whether the attending operator and reporting format are accepted for the vessel’s class.

For non-class condition work, competence remains essential. Ask for personnel qualifications, written procedures, calibration arrangements, equipment range and examples of traceable reports. Ultrasonic training alone does not establish familiarity with ship structural nomenclature or class measurement patterns.

Information to provide before quotation

• Vessel name, IMO number, ship type, year built, flag and class.
• Survey type and due date.
• Class measurement requirements or survey-status printout.
• General arrangement, shell expansion, tank plan and midship section.
• Previous thickness report and steel-renewal history.
• Known suspect areas, leaks, deformation, coating breakdown or pitting.
• Tank-entry status, cleaning condition, staging and lighting arrangements.
• Port, berth or anchorage, ETA, ETD and surveyor attendance.

Surface preparation and safe access

The probe needs a sufficiently clean contact surface. Loose scale, marine growth, cargo residue and thick degraded coatings can prevent reliable readings. Agree whether the crew, riding squad, yard or gauging company will prepare spots. Unplanned preparation is a frequent cause of incomplete surveys.

Tank and hold work requires enclosed-space controls, atmosphere testing, ventilation, communications, rescue arrangements and safe lighting. Staging, rafting or approved alternative access must suit the measurement locations. The operator should never be pressured to reach structure from unsafe ladders or makeshift platforms.

Calibration and quality control

Instrument calibration should be checked against traceable reference blocks suitable for the expected material and thickness range. Calibration is verified at appropriate intervals and whenever the instrument, probe, temperature or measurement conditions change. The operator should record instrument and probe identification and calibration checks.

Quality control includes repeat readings, comparison with adjacent structure, confirmation from both sides where practical and immediate escalation of unexpectedly low values. A surveyor may request verification readings or expanded gauging around a low point.

From readings to a usable report

A class-ready report identifies the exact structural member and location of every measurement. It should contain vessel particulars, survey scope, plans or sketches, original or rule thickness where applicable, measured thickness, diminution, equipment details, operator identity and dates. Ambiguous labels such as “tank plate 6 mm” are not sufficient.

Low readings should be highlighted, mapped and discussed while access is still available. Waiting until after sailing to discover missing locations can force repeat attendance. The owner, master, surveyor and gauging supervisor should review progress daily on larger scopes.

How corrosion findings are evaluated

The measurement company reports objective readings; acceptance is determined by class or another authorised technical authority. Criteria may consider percentage diminution, minimum allowable thickness, buckling strength, pitting intensity and the extent of affected structure. Do not apply a generic percentage across every plate and stiffener.

Where renewal is required, measurements support crop boundaries and steel estimates, but the final repair must follow approved material, welding and inspection requirements. Post-repair verification may be requested.

Common causes of delay or rejected reports

• The measurement company lacks approval for the vessel’s class.
• Plans and original thicknesses are unavailable.
• Tanks are not gas-free, clean or safely accessible.
• Measurement locations do not follow the class-agreed pattern.
• Readings cannot be traced to specific structural members.
• Low values are not expanded or verified before access is removed.
• The report is issued without surveyor review or required signatures.

A practical measurement-campaign workflow

Begin with a kick-off meeting between the surveyor, owner’s representative, chief officer and gauging supervisor. Confirm terminology, drawings, required transverse sections, suspect areas and how additional measurements will be authorised. Divide larger scopes into daily zones and establish a reporting cut-off so findings can influence cleaning, access and repair work on the following shift.

The gauging team should mark completed locations on controlled drawings and back up readings each day. Significant values should be reported immediately rather than hidden in a final spreadsheet. The surveyor may expand the pattern around corrosion or ask for close-up visual examination, and the owner may need to mobilise steel or coating resources while the vessel remains accessible.

At the end of each zone, reconcile required and completed points, low-reading verification, inaccessible areas and photographs. This simple review prevents a vessel sailing with a technically polished report that is missing a class-required strake, frame or structural member.

Limitations that should appear in the report

Ultrasonic results describe the accessible material at the measured locations. They may not reveal hidden corrosion behind liners, inaccessible connections, internal defects that do not affect the back-wall echo, or the condition between widely spaced points. Coating and surface condition can also reduce precision. A responsible report states these limitations and identifies areas that were not measured.

Thickness data should be interpreted with visual findings, coating condition, leak history and structural calculations. An apparently acceptable average does not negate isolated deep pitting, cracking, grooving or deformation. Conversely, one unusual reading should be cleaned, recalibrated and repeated before major steel renewal is ordered.

Arrange ultrasonic gauging

Use the ultrasonic thickness measurement service guide to prepare a request. vslSafety also provides planning pages for Egypt, Singapore and major ports such as Rotterdam. Submit class, vessel, drawing and access details through the quotation form.

Technical references

Consult the vessel’s classification-society rules, IACS Unified Requirements applicable to the ship type, IACS procedural requirements for thickness measurements and the approved gauging company’s procedure. Current requirements should be obtained directly from class before finalising the scope.