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ANALYSIS


TEST AREAS The procedure for selecting test areas and the number of readings per unit area varies considerably according to the standard used.


• Under SSPC, reference areas of about 10 square metres are identified. Within each ten square metres, five separate spot measurements are taken. For structures not exceeding 30 square metres, each 10 square metres must be measured. For structures greater than 100 square metres, three areas are measured, followed by one area for each additional 100 square metres. The average of the spot measurements taken in each reference area must be greater than the specified minimum thickness, with no individual measurement less than 80% of the minimum specification.


• ISO 19840 has more complex requirements regarding reference areas and the number of required readings, requiring about twice as many readings per unit area as SSPC, with special requirements regarding the number of readings required for pipes and beams. Similar to the SSPC, it also requires that no readings fall below 80% of the specified minimum, with the added requirement that less than 20% of individual readings fall below (the ‘80/20’ rule).


• PSPC requires one gauge reading per five square metres for flat areas, regardless of total area, but as this Standard is designed for ballast tanks and double-skinned spaces of bulk carriers, it is unlikely that this requirement will be widely employed. It requires that no spot measurement be below 90% of the specified DFT, and no more than 10% of all readings be less than the specified DFT (the ‘90/10 rule’).


The number of readings required for a


300,000 square metre ballast tank would thus vary greatly according to the Standard employed. Under SSPC, 15,000 spot measurements are required – but each spot measurement would require three gauge readings resulting in 45,000 readings. At a rate of one reading per second, that represents 12 man-hours. Under ISO, 30,000 measurements would be required, but only one gauge reading per measurement, representing 8 man-hours. Under PSPC however, at one reading per square metre, 300,000 readings would be necessary, representing 83 man-hours.


LESS MAY BE MORE Francis illustrates the differences of the standards in practice: “Some idea of the difficulty in determining the required number of readings and their acceptability can be shown by looking at a case study: a thick, multi-coat system over underwater hulls. DFT readings were taken after recoating the underwater hulls of two US Navy aircraft carriers. Both hulls were


28 PCE APRIL-JUNE 2013


coated with two anti-corrosion coatings and three anti-fouling coatings, to a total specified DFT of 0.675mm. Over two thousand readings were taken on each hull.” “Analysis of the spread of readings showed


the spread of DFT results that should be expected in a “real world” coating situations. “Manual painters, whatever surface is being painted, must inevitably produce an uneven coating with a significant spread of results. Painting the hull of a ship should be relatively simple as it is a large flat area, and should result in relatively small spread of results. Against this, a five coat system would have greater spread than a typical two or three coat anti-corrosion system. “It is probable that the spread of DFT results


obtained in practice is far greater than the specifiers imagine. Engineers used to very tight tolerances of modern manufactured goods are unlikely to be aware of the significant variation in thickness achieved by manual spray painting. Even the most skilled painter coating a large flat surface with an easily applied paint will have trouble achieving a uniform thickness. With less skilled applicators, awkward shapes and difficult-to-apply paints, variations across the surface become even greater. “ (See illustrations for USS Nimitz and USS Lincoln)


UNNECESSARYWORK? Francis’ analysis of real world examples suggests that the high volume of readings required by all three Standards is probably overkill. Plotting relatively small numbers of readings predicts quite accurately the variation and spread of high-volume readings. More importantly, while DFT on flat areas


may meet specifications, a more common place for coating failure is at edges and welds, and even modern gauges are only capable of measuring further than 5mm or so from an edge. Francis concludes: “A programme of requiring thousands of DFT readings during coating work is not going to guarantee that the coating system will provide the desired durability. In fact, ship owners may be lulled into a false sense of security by believing that such thorough inspection using the latest computerised thickness gauges is covering the entire painted area, when in fact the critical areas are not being measured and cannot be inspected, at least by film thickness measurements alone. “It must be recognised that PSPC does not


rely on DFT measurements alone to ensure durability, and factors such as selection of quality coating systems, edge grinding, stripe coats, high standards of surface preparation are all required as part of the new IMO requirements. Coating durability is general should noticeably improve as a result of these initiatives. The concern is that such a large number of readings with its associated costs and delays are not providing durability improvement commensurate with its cost.”


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