No two tankers are the same and there is no one fouling control coating that covers all the required criteria. Safinah Group’s general manager of marine consulting Carl Barnes steers a course through the maze of anti-fouling and foul release coating options
Fouling control coatings can make a significant contribution to the operational efficiency of tankers, helping maintain a smooth and foul-free underwater hull. These coatings also deliver benefits such as extending dry dock intervals and improve the reputation of the shipping industry via a reduction in emissions (via reduced fuel consumption). They also play an important role in preventing the translocation of invasive species, a major concern of IMO.
Fouling control coatings work by deterring the settlement of fouling species. The choice of technology can have a significant effect on the roughness of the underwater hull; increasing hull roughness increases the frictional resistance of the hull. This means that to maintain a specific vessel speed there will be a requirement for an increase in power (at constant speed) and hence an increase in fuel consumption and emissions.
Frictional resistance is essentially caused by the hull ‘rubbing’ against the water it is moving through and there are two main types of roughness on hulls: physical roughness caused by mechanical damage, coatings cracking, detachment and corrosion; and biological roughness caused by fouling.
There are approximately 5,000 different fouling species found in the world’s oceans and these can be classified as: micro fouling (slime fouling); and macro fouling (weed fouling; animal fouling).
Slime fouling is always present to some extent on immersed surfaces, and whilst it is not a major source of translocation of invasive species, it still has an impact on the performance of a ship’s hull. The actual effect is not commonly agreed within the industry and values from a few percent up to 19% are quoted. Weed and animal fouling has the greatest effect on vessel performance (in extreme cases of shell fouling this can be in excess of 60%) and also presents the major source of invasive species.
The fouling to which a ship’s hull is subjected is influenced by many factors, which include:
Thus, selection of a fouling coating should be specific to an individual vessel.
Fouling – a potted history
Controlling fouling on ships has been a requirement since humans started using water as a means of transport. Historically, mercury, arsenic, lead, pitch/tar and copper were typical ingredients to prevent the build-up of fouling. One famous use of fouling control involved the battle of Trafalgar, where, in October 1805, the British fleet used copper cladding which gave them much more manoeuvrability than their French counterparts.
In the 1970s tributyltin (TBT) was introduced, tributyl tin oxide (TBTO) was a well-known biocide, but expensive and the target was to bind TBTO to an acrylic polymer to make paints last longer and cost less. The result was a hydrolysing polymer which released TBTO and dissolved away itself – Self Polishing Copolymers (SPC) were born. However, negative effects on non-target organisms ultimately led to TBT being banned under the IMO AFS Convention in 2008.
Since TBT was banned, the choice of fouling control coatings has become very confusing.
As a provider of services on coatings, but not a coating manufacturer itself, Safinah Group is able to provide advice to shipowners and operators on the type of fouling control coatings available. Safinah has a database of the performance of past underwater hull coating applications and can, without prejudice, provide a third-party opinion. Safinah also works with coating and paint manufacturers on the testing and development of new applications.
Today, there are two main groups of TBT-free fouling control technologies:
Biocidal anti-fouling technology accounts for around 90-95% of the fouling control coating market, but there is pressure to reduce the use of biocides. The EU currently has the toughest requirements when it comes to the application of biocidal coatings via the EU Biocidal Products Regulation (BPR). Signatories to the AFS Convention agreed to banning the use of cybutryne (Irgarol) from 2021. Today there are only a handful of main biocides that can be used in the formulation of an antifouling coating. The only products that can be applied in the shipyard of a particular country are those that the paint manufacturer has sought and gained regulatory approval for. This may mean that an owner is unable to have the coating they have chosen applied in a yard because it has not been registered in that country.
It is not only in the EU where the environmental impact of coatings is being re-assessed. China has enacted a series of tough environmental protection laws that will require a similar regulatory process, as has South Korea. As the number one and two shipbuilders, this will be a challenge for owners and coating supplier.
The other 5-10% of the coating market falls into the newer foul release category. Overall, foul release coatings are a small part of the market at the moment. One of the reasons for this is that these coating systems are more difficult to apply, there needs to be dedicated spray equipment in the shipyards, masking of other parts of the vessel and protection of any other vessels in the dry dock to prevent contamination. It causes the shipyards a number of issues and generally impacts shipyards productivity; hence the cost to apply these products can be expensive on what is already a premium product.
Choosing your coating
The fouling control solution chosen should be selected based on individual vessel parameters, operating location and an understanding of product performance. Typically: