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Koninklijke Vereniging - Société Royale

DOSSIER

 

Keeping the hull clean


Ever since our predecessors first took to the water, it was realised that smooth surfaces glide through the water much more efficiently, and consume less energy, than rough surfaces.*


Unfortunately, even if a surface starts out smooth it will soon attract weed, slime, barnacles and other forms of growth, which increase resistance and make the vessel harder to push through the water.

Years ago, the solution was to keep the bottom of the ship clean. With wooden hulls this was not easy, ships either had to be scraped by divers or beached for a more thorough scrubbing. Then, it was found that if a coating of copper was applied, fouling was reduced, as well as the surface being easier to clean.

Anti-fouling paints were introduced around 65 years ago. The best anti-foulings contained tin compounds – commonly tributyl tin (TBT) – which was designed to act as a biocide; ie over time the tin compounds leached out of the paint to kill off any biological life forms that attached to the hull.

As is now known, their effectiveness was their downfall, causing undue harm to the marine ecosystem in places where there was a high concentration of biocides in the water. The highest concentrations tended to be where a lot of pleasure boats were moored, such as in marinas, as a high concentration of boats meant a high concentration of toxic tin in the water. This led to a ban on TBT paints for pleasure craft, closely followed by an IMO blanket ban across all vessels.

With TBT paints, the difficult task of hull cleaning largely disappeared. The nature of the coating meant that its effectiveness diminished gradually over time as the biocide leached out, thus needing drydocking and recoating every few years, rather than a frequent cleaning regime.

Industry efforts then concentrated largely on finding a less-toxic alternative to TBT. The obvious answer was a return to copper. Cuprous oxide was initially substituted for TBT in biocidal coatings. This did the job, but nowhere near as effectively, and still led to contamination. Copper may be less harmful than tin, but it is still toxic, and its widespread adoption has meant that in many ports, copper levels in water have risen well above the perceived safe limits.

The problem was exacerbated by the fact that because copper-based paints are less effective than TBT, recreational craft in particular have increased the frequency of their hull cleaning regimes. As well as removing fouling, the action of in-water cleaning releases additional biocide. Estimates suggest that up to 50% of the pollution is a direct result of in-water cleaning. The result is that application and in-water cleaning of copper-based coatings on pleasure boats is now becoming subject to restrictions and bans.

Washington State in the US was the first to issue a ban on copper anti-foulings for recreational craft under 19.8 m in length, with California likely to follow. Europe is travelling down the same path, with restrictions in Sweden, Denmark and the Netherlands. It seems inevitable that copper will go the same way as tin, with talk of a ban for leisure craft by 2020 and a universal ban sometime after.

Foul release coatings were another significant development. These are claimed to be non-toxic, relying on a smooth, slippery surface on which it is difficult for fouling organisms to gain a hold. When a ship travels at speed through the water, the fouling is displaced.

Most of these paints rely on silicone-based polymers for their effectiveness, but silicon itself can contaminate its surroundings. The paints can be difficult to apply, and tend to be less durable than the harder biocidal coatings. Because of their slippery nature, and the need for primers, tie coats and top coats, they are difficult to repair.

With shipping, keeping fuel consumption at an optimal level is crucial not only to economics, but also to emissions. But the environmental benefit of an effective means of minimising fouling extends beyond the atmosphere. Heavy fouling on ships hulls can lead to introduction of invasive species, in similar fashion to the transport of non-native species in ballast water.

A fouled hull is likely to offer around 20% more resistance than a new hull, and a 20% increase in fuel costs is an intolerable burden in today’s economic climate. Cleaning can easily remove 30% or more of an antifouling coating, which not only adds to local contamination but also significantly shortens the lifetime of the paint.

With conventional paints, in-water cleaning using divers with scrapers, pressure washers or self-propelled hull cleaning devices is therefore impractical. Estimates suggest that full blasting and repainting will be needed at least every three to five drydockings, that is to say every 10 years or so, probably more frequently with foul release type coatings.


Repairs needed

Any hull coating is likely to need local repairs in drydock to rust, scratches, chips and general wear and tear. Such repairs are likely to result in a less-than-ideal surface, so a repaired hull will never be as smooth as a newly-coated one.

So, accepting that no coating has yet been invented that is completely resistant to fouling, and some maintenance will always be needed, Subsea Industries said it can offer a solution that goes back to basic principles – a hard, smooth coating that will last the lifetime of a ship, and, using modern technology, can be kept clean with a minimum of time and effort. This solution is centred around the use of non­toxic surface treated coating (STC) combined with routine in-water cleaning.

Subsea Industries’ Ecospeed is a durable, one-coat system based on a glass flake vinylester formulation. It can be easily applied using conventional equipment, in two coats of 500µm thickness, with no primer or tie coat required.

Its durability has been well proven by its adoption for the hulls of ice-going ships. It has been employed by the British Antarctic Survey (BAS). It was applied to BAS 80 m-long ‘Ernest Shackleton’ in 2009. When this ship was drydocked in Frederikshavn, Denmark, in 2017, the coating was found to be in very good condition.

BAS superintendent, Andrew Webb, said; “’Shackleton’s’ hull condition is the best I have seen after typical ice year operations. We tend to account for touch up coats every other year to areas impacted by the ice, but this year we needed to repair even less surface area than expected, despite the vessel encountering heavy Antarctic ice.”

This experience led to the choice of Ecospeed for the hull of the 15,000 gt ‘RRS Sir David Attenborough’, a new Polar research ship under construction at the Cammell Laird shipyard at Birkenhead, UK.

Webb added: “The shipyard initially wanted to apply its preferred supplier’s coating system, but based on our experience of this coating on the ‘James Clark Ross’ and ‘Ernest Shackleton’, we wanted Ecospeed. We already had this system on the entire hulls below the water line of both research vessels and found it much easier to repair. It doesn’t need to be applied under strict environmental conditions or require the hire of any specialist application equipment.”


Cold and warm seas

This experience is echoed by Hamburg-based Interscan Schiffahrt, which applied Ecospeed to one of its Arctic vessels, the 3,000 dwt Patriot, during a routine drydocking in 2005. The coating has now lasted well over its 10- year warranty period. Interscan has since applied Ecospeed to the hulls of six more of its ships.

Such a coating is highly suitable for Polar conditions, where the sea temperature is too cold for fouling to be a problem. But one of the Interscan ships, the 6,288 dwt Karin, operates in warmer waters.

Interscan head of chartering, Michael Tensing, said: “’Karin’ sails in warm waters and in those conditions it might need an underwater cleaning every six months but it is an easy procedure because the coating is very tough. Cleaning takes only six to eight hours.”

In-water cleaning may have gone out of fashion but Subsea Industries has developed its own equipment, that is offered on a worldwide basis through a network of strategically-located stations.

The systems are designed to remove fouling without causing damage to the coating – in the case of a hard coating like Ecospeed this is no problem, and the smooth nature of the Ecospeed surface means that fouling is removed easily, often in a matter of a few hours to thoroughly clean the whole underside of the ship.

Hydrex said it is company policy not to carry out underwater cleaning activities where these might result in an increase of pollution, but as Ecospeed is non-toxic, such problems are limited to other manufacturers’ coatings. The equipment can additionally be made available to shipowners wishing to arrange their own underwater cleaning.

An Ecospeed application and the cost of regular hull cleaning is easily offset by the reduction in drydocking time and costs along with the lower fuel costs an optimally smooth, foul free hull provides. Further savings arise because the coating is intended to last the lifetime of the ship, so there is no need to blast and re-coat at drydocking, with any necessary repairs able to be carried out simply, cheaply, and durably.

There is an additional bonus – the absence of toxins, lower emissions and reduced risk of non-native species benefits the environment, adding to an operator’s ‘green’ credentials.

Antwerp-headquartered Subsea Industries was established in 1983 and claims to be a pioneer in the development of hard hull coating systems and hull and propeller cleaning systems.

In 2002, after three years’ extensive research and development, the company introduced Ecospeed as an environmentally safe underwater hull coating system, capable of improving ship performance, providing long­term fouling protection and reducing the impact of ship operations on the environment. Ecospeed now has more than 700 marine references.

The coating is type approved by Lloyd’s Register as an abrasion resistance coating for Ice Class ships and has DNV GL approval for use as a coating in ballast water tanks.


*This article was taken from a paper written by Subsea Industries

 

 

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