In-depth | COATINGS
Figure 2: Calculated annual fuel savings (left) and accumulated fuel costs (right) for two Fouling Release coatings vs. a 100% clean self-polishing copolymer paint (10% self-smoothening is assumed on the SPC: Weinell et al., 2003). Simulations run for a 7000TEU container vessel burning 170tonnes fuel/day and with an activity of 80% (US$470/tonne of 380-cst bunker fuel).
applied silicone paints have a significantly lower friction coefficient with seawater compared to conventional anti-fouling paints has been generated in the past few years (Table 1). While tin-free self polishing coatings (SPC) presented a spiky “closed” texture, the silicone topcoat featured a wavy “open” texture with a smaller proportion of short-wavelength roughness (Hellio and Yebra, 2009; Chapter 26). In other words, the smoother surface of silicone coatings results in measurable fuel savings. Te study by Westergaard (2008) for
Hempel A/S introduced a new parameter: it is of little practical use to measure drag on smooth panels (e.g. Rz50
30μm), when
no less than 100 microns of average hull roughness are typically measured on newly built hulls. The study by Westergaard actually showed that, while the absolute friction resistance increased with the hull roughness, the savings resulting from the use of silicone-based coatings instead of SPC paints were larger on rougher substrates (Rz50 smooth plates.
467μm) than it was on Company Product name
While such studies are of relevance to
assess fuel efficiency with clean hulls, the fact is that underwater hulls are rarely 100% clean for the entire service period. Table 1 (see Part I in the November issue) shows the estimated decrease in performance of a containership when its hull coating becomes fouled with different marine organisms. It is now possible to use such data, combined with the measurements by Westergaard (2008), to simulate the fuel consumption of a container vessel coated with different technologies and developing different degrees of fouling. As a hypothetical example, we
compare a self-polishing paint that manages to stay 100% fouling-free (best case scenario) for five years to a FR paint which develops progressive fouling (thin slime). As shown in Figure 1, the fuel savings obtained while the hull is largely free from fouling (first three years) exceed the losses experienced at the end of the drydocking period. In this worst case scenario for a FR coating, the
Special Features
final accumulated savings achieved still exceed US$1million compared to an ideal biocide-based paint. Figure 2 shows the results of simulating
a more realistic fouling scenario for both the second (e.g. incorporating amphiphilic fluoropolymers) and the third (i.e. hydrogel-based) generation of FR coatings (see section below for further details).When compared to the same ideal SP antifouling paint, both technologies yield major fuel savings, with the delayed slime formation on hydrogel-based topcoats translating to about US$1.3 million extra fuel savings compared to non-hydrogel based coatings. According to Thomason (2010), the fact is that currently available FR coatings perform at the level of self-polishing copolymer paints, so Figure 2 (which assumes a 100% clean SPC) is likely to show conservative savings estimates.
Commercial topcoat technologies Current commercial non-stick FR coatings, all of them based on silicone matrixes, are based on a dual mode of action (Hellio and Yebra, 2009; Chapter 26):
• They prevent the adhesion of fouling organisms by providing a low-friction,
Hempel A/S Hempasil X3
Above 8 knots. Hydrophobic silicone matrix modified with hydrogels. The documented non-fouling properties of hydrogels are sum- marized in e.g. the AMBIO-funded Ekblad (2010).
International Paint Chugoku Marine Paints PPG-Sigma Coatings
Sherwin-Williams Jotun A/S
Intersleek 900 Bioclean HB SigmaGlide 990
SeaGuard Surface Coat SeaLion Repulse
Above 10 knots. Silicone matrix with amphiphilic fluoropolymer modification. Not known “Pure silicone with improved slime resistance”
Above 10 knots. Extra mechanical toughness. Silicone matrix modified with “nano-springs”
• Once fouling has settled the high polymer chain flexibility characteristic of
ultra-smooth surface on which organisms have great difficulties in adhering
polydimethylsiloxane (PDMS) polymers result easy the release of attached organisms.
Table 2: Most recent commercially available FR technologies since 2007.
18 The Naval Architect January 2011
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