In-water surface cleaning
In-water cleaning or in-watter surface cleaning is methods of removing unwanted material in-situ from the underwater surface of a structure. This often refers to removing marine fouling growth from ship hulls, but also has applications on civil engineering structures, pipeline intakes and similar components which are impossible or inconvenient to remove from the water for maintenance. It does not generally refer to cleaning the inside of underwater or other pipelines, a process known as pigging.
- Removal of marine fouling for ship performance improvement.
- Removal of marine organisms to prevent infestation by alien species. Fouling of ships' bottoms by marine organisms is recognised as a major vector for the introduction of invasive alien species, which can have significant economic and environmental impacts. The risk can be mitigated by maintenance of the immersed surfaces to kill or remove fouling organisms before entry into the protected areas. The usual primary method is the use of anti-fouling coatings, which are themselves an environmental hazard due to toxicity. In-water cleaning is an adjunct to anti-fouling in cases where the coating has not been completely effective, and also carries a biosecurity risk due to the potential release of the removed organisms and toxins from the coatings.
- Removal of fouling and contaminants in preparation for inspection, maintenance or repair work.
Ship bottom cleaning
- Brushing – A tool with bristles, used for cleaning, grooming, or applying liquid coatings
- Brush cart. Brush carts do not usually include a system for removal or treatment of waste, but it can be done if there is sufficient demand. Brushes that do not directly contact the surface coating can remove fouling without damaging the coating itself. The standoff can be controlled when the cart rolls over the surface on wheels. Some removal of the surface of some anti-fouling coatings may actually improve the performance by exposing a fresh layer with more concentrated active materials.
- Rotary brush
- Water jetting
- High pressure water jetting. When correctly applied, high pressure water jetting can provide acceptable levels of cleaning without damaging anti-fouling coatings or releasing toxins to the environment, but this requires fairly accurate control of jet angle and distance from the surface, and duration of impingement. In other circumstances water jetting can be used to remove paint. Water jetting can be applied manually by divers, by diver-operated carts, or by ROVs.
- Abrasive water jetting. This system is intended to remove contaminants, coatings, and corrosion products down to the substrate. Abrasive grit is entrained in the jet of water and the impact of the grit has an aggressive cleaning action.
- Cavitation water jet. This system uses jets of water containing cavitation voids of water vapour, generated ultrasonically in the nozzle, which develop high localised impact pressures on hard surfaces when the bubbles implode at the surface to be cleaned. This is claimed to do less damage to surface coatings than high pressure jetting, and reduce the hazard to the operator. The cavitation jet can remove fouling, loose paint and rust, without damage to sound paint when used correctly, but can erode ablative and self-polishing paint coatings if applied too closely or for too long. Tools include hand-held pistols, diver and self-propelled carts and potentially, also robotic systems. Suction systems to recover waste are available, and the waste can be treated or filtered.
Capture and treatment of waste products
Depending on the reason for bottom cleaning, it may be desirable to capture and treat the waste dislodged from the surface. If the purpose is to remove potentially invasive alien species, then they must be removed from the water or killed. If the organisms are not a problem, it may be necessary to contain released toxins from the anti-fouling coating.
Effects on substrate
Some cleaning technologies can cause significant damage or degradation of the substrate, particularly removal or excessive abrasion of protective or biologically active surface coatings. In some cases removal of the upper layer of an anti-fouling paint can expose paint which has a stronger concentration of active biocides, which can reactivate the paint.
- Morrisey, Donald; Woods, Chris (November 2015). In-water cleaning technologies: Review of information. MPI Technical Paper No: 2015/38 Prepared for Ministry for Primary Industries (Report). Wellington, New Zealand: Ministry for Primary Industries. ISBN 978-1-77665-128-3. ISSN 2253-3923 – via www.researchgate.net.