Regardless of the anti-fouling or foul-release coating applied, most ships will suffer a degree of fouling between drydockings that can adversely affect their performance, contribute to non-performance claims under time charters and could be responsible for transferring species between different environments.
Operators of fouled ships have a number of choices open to them including having the hull cleaned by a specialist contractor. The time to the next drydocking and the extra fuel being consumed must be weighed against the cost of cleaning the hull and, for ships with a limited lifespan coating, the possibility of damaging what coating remains is also a factor to be considered.
Ships that have been hard-coated are far less likely to suffer damage from cleaning operations. Manufacturers such as Subsea Industries recommend that cleaning is undertaken regularly if the full benefit of such premium coatings is to be enjoyed. Cleaning can be done by almost any diving contractor with appropriate equipment.
Underwater maintenance of Subsea Industries’ Ecospeed is carried out with specially-designed underwater hull cleaning tools that simultaneously remove all fouling and optimise the smoothness of the paint surface. A complete set of complementary equipment was designed in-house to allow divers to clean the flat areas as well as the harder-to-reach parts of the hull without damaging the coating. Sea chests and other nooks and crannies are best cleaned out using underwater high pressure water jet equipment.
This combination makes it possible to have a 100% clean hull after each maintenance session, resulting in the best possible hydrodynamic condition of the underwater hull throughout the service life of the vessel and the removal of any potentially harmful invasive aquatic species which the ship may have picked up. The tools best suited to cleaning hulls are often developed by the specialist contractors and sometimes marketed to independent contractors. Most use manually-operated tools but there are a small number of alternatives.
CleanHull for example offers efficient hull cleaning, based on its CleanROV (remotely operated vehicle) technology. The company was originally a Norwegian operation but now operates from Singapore and Spain. The CleanROV uses only high-pressure seawater for cleaning and does not harm anti-fouling, which is susceptible to brush damage. It is designed to crawl around a ship’s hull, rotating around its own axis. Cleaning is carried out using water at intermediate pressure, carefully removing any fouling. The machine documents the whole cleaning operation with several cameras, enabling random quality controls of the cleaning process.
UMC is another contractor that has developed its own tools, one of which is the Mini-Pamper that was initially designed for cleaning acoustically-clad submarines where the absence of any anti-fouling paint meant that device had to deal with extreme marine growth without damaging the fragile surface. The Mini-Pamper allows the operator to bring a choice of cleaning heads gently into contact with the hull until they are just cleaning and no more. Having selected the correct cleaning pressure, the machine will automatically maintain this level.
In 2015 CleanHull and UMC began working together. A similar service is offered by GAC EnvironHull with its HullWiper ROV. As with the other systems, HullWiper is a brushless cleaning method and it also incorporates a control system that allows the operator to control water pressure and monitor the cleaning process through forward- and aft-facing CCTV cameras. GAC claims that the system is twice as fast as using divers for cleaning hulls and offers a cost savings calculator on its website. The company says the system can clean up to 2,000m²/hr, completing a full hull in just a few hours.
Although hull cleaning is recommended as a means of reducing fuel use and therefore exhaust emissions, it is not an operation that every port authority is prepared to permit. However, HullWiper does not discharge removed residues and harmful materials into the sea. Instead, it collects them with a unique onboard filter unit that is collected by a locally-approved environmental waste disposal company. This cleaning method reduces the risk of cross-pollination of waters with alien species and was a factor in the Australian port of Townsville accepting it for use in local waters in 2018.
Nations reluctant to allow hull cleaning because of risks may need to reconsider if the IMO biofouling guidelines ever become mandatory as without the ability to clean hulls anywhere shipowners would have a legitimate defence against any possible penalties.