NDTScan measures remaining wall thickness with calibrated ultrasonic gauges, on a grid the engineer can repeat, and reports the readings against the original section so loss, and the rate of loss, can be calculated.
What is measured
- Sheet and tubular piles, dolphins and jetty steelwork. Splash zone, tidal zone and mud line, from a survey boat at the right state of tide, the deck or the foreshore.
- Storage tanks. Shell courses, roof plates and nozzles, supporting API 653 style integrity programmes and insurance inspections.
- Pipework and vessels. Erosion at bends and tees, corrosion under insulation at entry points, and thinning on pressure systems.
- Structural steel. Beams, columns and connections in car parks, plants, bridges and gantries, through coatings where the coating is sound.
- Hulls, pontoons and lock gates. Plate thickness surveys for condition assessment and class support.
The instrument and the method
The gauge is the Olympus 38DL Plus, calibrated against a step wedge at the start and end of every session with the calibration recorded in the report. On coated steel it runs in multiple-echo mode so the coating is excluded and only the steel is measured; where the coating has failed or the steel carries marine growth, each reading point is cleaned to bright metal first. Thickness measurement follows BS EN ISO 16809. Instruments are calibrated by UKAS-accredited laboratories where the work requires it.
Spot readings answer whether a point is thin. Corrosion mapping takes readings on a grid across the area so the extent and depth of thinning can be drawn, the minimum found rather than assumed, and a repeat survey on the same grid turned into a corrosion rate. Where access is the constraint, the probe goes to the steel by boat, rope access or ROV rather than by scaffold. Phased array and automated crawler mapping, where a project specifies them, are arranged through specialist partners and coordinated by NDTScan.
What the engineer receives
- Every reading tabulated by element, level and grid position, with the nominal or original thickness beside it and the loss calculated.
- Minimum readings flagged against nominal thickness and corrosion allowance.
- Thinned zones dimensioned and located on the structure drawing, with photographs of the surface at each one.
- The gauge, probe, couplant, mode, calibration and surface preparation for each session.
- On repeat surveys, the change since the last survey and the corrosion rate it implies.
That is what an integrity engineer needs to defer a repair with confidence, or to stop deferring one. Findings are presented with the confidence of each reading and the limits of the method stated; the interpreted report follows within an agreed number of working days.
What thickness testing cannot do
A gauge reads the steel under the probe and nothing else: pitting between grid points is found only if the grid is tight enough, which is why the grid spacing is agreed with the engineer for the corrosion mechanism expected. It cannot read through loose scale, thick marine growth or a failed coating, and it cannot read a surface it cannot reach. Each of those limits is recorded where it applied.
Related work
Thickness testing is usually part of a wider marine survey or underwater inspection, and sits alongside the concrete scanning of the deck and soffit above. The same instruments are described on the equipment page, and the service is also written up under corrosion mapping services.
Can thickness be measured through paint or coatings?
Yes, on sound coatings. The gauge is set to multiple-echo mode so the coating is excluded from the reading and only the steel is measured. Heavy marine growth, loose rust and failed coatings must be cleaned to bright metal at each reading point, which is specified in the survey plan.
What standard is the work done to?
Ultrasonic thickness measurement follows BS EN ISO 16809, with the gauge calibrated against a step wedge at the start and end of each session and the calibration recorded in the report. Instruments are calibrated by UKAS-accredited laboratories where the work requires it.
Can readings be taken underwater?
Yes. In the tidal zone they are taken at low water from a survey boat, the deck or the foreshore. Below that, probes are deployed by ROV or by commercial diving partners where a project needs them.
Questions we are asked
What is corrosion mapping?
Thickness readings taken on a grid across an area of steel, rather than at a few spots, so that the extent and depth of thinning can be drawn as a map and the minimum found rather than guessed. Spot readings answer whether a point is thin; a map answers where the thinning is, how far it runs and how fast it is moving between surveys.