There are currently three different coating types in wide usage, and they offer different resistance to fouling, have a different impact on hull roughness, and have different requirements for cleaning frequency. These are [ABS 2013]
Controlled Depletion Polymer (CDP) – A traditional antifouling type based on water-soluble natural or synthetic pine rosin mixed with a biocide. An insoluble reinforcing polymer resin is added to create a skeleton to give the rosin better mechanical properties. The controlled dissolution of the rosin matrix releases the biocides.
Fig:Types of underwater fouling: green-weed
Over time, a build-up of insoluble materials at the surface in a leached layer slows the release of biocide and makes recoating difficult. Moving water (or cleaning) is required to wear off this resin skeleton and release the next layer of coating and biocide. Ordinary life before recoating is three years, but because of the build-up of this leached layer and reduced biocide release micro fouling (green slime or weeds) can become a problem in less than two years. The average hull roughness increase is estimated at 40 µm per year in surface profile, but this can vary greatly.
Self-Polishing Copolymer (SPC) – An insoluble metallic or organic synthetic polymer (e.g. copper-acrylate or silyl-acrylate) that contains a biocide. Through a chemical reaction – hydrolysis – the polymer becomes soluble. Its subsequent dissolution releases the biocide. The chemical reaction provides good control of the dissolution rate and results in a much thinner leached layer and smoother surface profile than possible with CDPs. No ship movement is required as there is no residual 'skeleton,' and the surface is self-smoothing. Five years of service for high-quality systems can be achieved. The average hull roughness increase is estimated at 20 µm per year.
Foul-release Coating – A biocide-free coating that uses non-stick properties to control fouling. It is usually silicone or fluoro-silicone based and designed to shed micro or macro growth when the vessel is underway. For slower vessels (less than 15 knots), this is a challenge for even the best coatings, so some 'soft' cleaning is usually required to remove the micro fouling. If the vessel is stationary for some time, barnacles and other macro-size biotas can become attached. Achieving a full release of all fouling through ship speed impact has proven to be a challenge in some cases. The coating gains some of its effectiveness from its extremely smooth surface, which must be maintained for best performance.
Roughness
in a foul-release coating will reduce its ability to discourage adhesion and slime/ micro fouling can take hold. Mechanical damage from example from tugs is especially critical for these types of coatings requiring special care in operations as the damaged parts has no fouling protection. Average hull roughness increase is estimated at 5 µm per year, but this is based on a very limited service experience.
Fig:Types of under water fouling: burnacles & mussels
Figure - Example of fouling [International Paint]
The factors that govern the type of underwater hull coating that may be applied to a ship are;
Cost: The more effective anti-fouling coatings tend to be more expensive.
Speed of vessel: Fast vessels such as HSC tend to have harder coatings.
Fresh-water or salt-water: The coating quality should be chosen accordingly.
Compatibility: Some coatings cannot be used on top of others due to adverse physical and chemical interactions
National regulations: Some types of anti-fouling paints are banned in certain countries.
Area of operation: Severity of fouling in area vessel is trading, some sea areas are much worse than others.
When searching for the best hull coating, it is important to consider a coating that provides a smooth surface that can be reasonably maintained in its smooth state, and that prevents adhesion of fouling organisms. The coating must also be applied properly, monitored, and managed to maintain its best qualities.
If done correctly, the right coating upgrade can offer significant fuel saving improvement. In general, the application of a good high-quality coating can yield an average reduction of up to 4% in propulsion fuel consumption. Reducing an already rough hull to smoother one (via getting rid of fouling, surface blasting, etc.) and applying the advanced coating even can provide a 10-12% decrease in fuel cost. A full blast to remove surface roughness and application of prime, anti-corrosive, and high-quality antifouling coating can cost about the US $10/m2. This converts for a VLCC to about US $300,000 [ABS 2013]
Since the banning of TBT (Tributyltin) based on "The International Convention on the Control of Harmful Antifouling Substances on Ships, 2001", most antifouling coatings are self-polishing copper and tin-based paints but it should be borne in mind that some countries are either banning or considering banning the use of copper-based paints in certain areas particularly in inland waters.
Biocide-free silicon-based coatings are also available, but their market share is minimal due to their high cost. These coatings are commonly referred to as "foul-release coatings" as they have a soft surface onto which it is difficult for most organic growth to hold. Research has shown that these new coatings are equally as effective as TBT-based systems, but there is still some debate and not everyone is convinced.
In general, the more advanced products yield better results; however, if a particular ship operating in a particular area is getting fouled up faster with one product, it is worth considering changing to a different one. It is also worth talking to other ship operators in the same area and asking which product works best for them, as they can give unbiased advice which one may not get from an original vendor.
Hull Cleaning
Regular in-service cleaning to remove fouling organisms is beneficial unless carried out in a way that results in a damaged coating or a 'roughed' up the surface. From a fuel efficiency point of view, the emphasis should be on hull and propeller "roughness" management and not just on the control of "fouling."
In the case when only partial cleaning is possible due to operational circumstances, the hull areas should be cleaned in the following order to provide the best performance enhancement:
Forward third of hull
Remainder of hull working from forward to aft with emphasis on areas which have more exposure to light
A proactive approach that pre-empts any extensive macro fouling is always recommended because the cost of having such a fouling present outweighs the cost of cleaning by a considerable margin. Regular cleaning of micro-fouling is also often cost-effective if the proper cleaning technique is used so that the surface roughness is not degraded and the coating material is not removed.
For best results, the scheduling of cleaning should be based either on monitoring of performance indicators (like power versus speed) or on regular pre-cleaning inspections. In both cases, a threshold is established that identifies when cleaning is economically justified. For visual inspections, the threshold includes the percentage of the hull surface fouled and the fouling type. Regular inspection, photographs, and roughness measurements would be a prudent way to monitor the impact of cleaning and the condition of the coating.
The use of underwater cleaning techniques and equipment should be done with care and with due consideration to the original coating as well as the amount of fouling. For example, removal of macro size fouling is difficult to be done without removing a significant amount of paint. If the antifouling is applied in different color layers, then these colors can be used to monitor paint removal during cleaning. In general, SPCs have a thinner leached layer than CDPs, so the cleaning should use a less aggressive technique.
Cleaning of foul-release coatings should only be done with a light touch and soft pads. In all cases, the advice of the paint manufacturer should be followed. Cleaning aspects should be reviewed with the cleaning company and condition before and after documented with good underwater photography of the cleaned surface.
Cleaning a light slime can yield up to a 7-9% reduction in propulsion fuel consumption. Cleaning a heavy slime could give 15-18% and cleaning of a macro heavy fouling up to 20-30%. Hull cleaning by divers can cost about the US $1.5 to 2.5 in the Far East. It could convert to the US $50,000 for a full hull cleaning for a VLCC based on ABS's guide written in 2013. Of course, there is always no need to do a full hull cleaning, thus reduce the time and cost of hull cleaning but everything must be carried out with due care for the preservation of the coating system; otherwise, things may give negative results.
Other aspects
Regulations: When planning for hull cleaning, it is also important to be aware of other regulatory instruments that govern the coating system. The IMO at its MEPC meeting in July 2011 adopted as a voluntary instrument, the MEPC.207(62) resolution on "Guidelines for the Control and Management of Ships' Biofouling to Minimize the Transfer of Invasive Aquatic Species." It asks for a Bio-fouling Management Plan and a Bio-fouling Record Book to be on-board. It is clear that while the proposed regulations undoubtedly will lead to added cost for ship maintenance and operation, it can also reduce the hull drag and fuel consumption. However, it is to be well managed with consideration for the selection and maintenance of the best coatings in reduced total cost. Also, as indicated, there are local regulations that ban a certain type of antifouling coatings that also need to be taken into account.
Area of operation: Antifoul hull coatings used on seagoing vessels are generally designed for saltwater and ships that spend extended periods in freshwater or brackish water will tend to foul quicker.
If a vessel is going to operate in freshwater on a particular charter for some time, it will be worth considering reducing the hull cleaning period or changing the type of antifoul paint used. In winter, when the temperature of the water drops below a certain level, the fouling of the hull is reduced
but will increases in the summer months when the temperature of the water rises. In temperate zones, the maximum fouling occurs in late spring through to early autumn. An in-water inspection of the hull in the summer month may be beneficial for ships operating in such conditions such as short-sea passenger ships.
In water inspections: Regular in water inspections of the hull or dry-docking is the only certain way of assessing what the condition of the hull coating is. The vessel's speed and power output should be continually monitored to establish if fouling is reducing the vessel's performance. It can help the ship operator decide what the in-water cleaning or dry-docking interval should be.
The condition on the hull should be assessed regularly in line with the period detailed by the company, and this period my need may need to be reduced if substantial fouling is evident. If a ship is laid up for any period an in-water survey of the hull is advisable and if the fouling is significant the ships hull should be cleaned, and the antifoul reapplied.
Hull cathodic protection system: The installation of a hull cathodic protection system should also be considered, reducing the corrosion of the hull. Corrosion will increase the friction and resistance of the hull to the water flow past the hull and increase fuel consumption. It has the added benefit of reducing pitting of the hull plating which reduces the strength of the hull.
Lay-up: If a ship has been in a high fouling area for a long time, it may need to be taken to dry dock to be cleaned before it can be put into service. When laying a ship for any period, it is worth considering where the ship is going to be moored and, if possible, avoid an area that is subject to high fouling.
References and further reading
The following list provides references and additional publications that may be used for more in-depth study :
1. "IMO train the trainer course material," developed by WMU, 2013.
2. ABS 2013 “Ship Energy Efficiency Measures, Status and Guidance“, http://ww2.eagle.org/
4. Munk, T and Kane D, "Technical Fuel Conservation Policy and Hull And Propeller Performance," Royal Institute of Naval Architects Design and Operation of Tankers Conference, June 2011.
5. International Paint "Coatings Technology: What Is Fouling?" http://www.international-marine.com/PaintGuides/WhatIsFouling.pdf
6. MEPC.207(62) resolution on "Guidelines for the Control and Management of Ships' Biofouling to Minimize the Transfer of Invasive Aquatic Species," 2011.
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