Hydrex's Practical Guide to Industrial Underwater Ship Hull Cleaning
Underwater ship hull cleaning: cost-effective, non-toxic fouling control
Hydrex's White Paper No. 5 is a practical guide to industrial underwater ship hull cleaning, its value and limitations, and how to institute a workable program for optimizing ship hull performance and saving fuel, with due concern for the environment
Part I. Introduction
Historical note
Cleaning ship hulls of marine fouling has been a fact of maritime life since humankind first took to the sea in boats and ships. The Greek priest and author Plutarch (45–125 AD) discusses the cleaning of ship hulls in his Symposiacs.
...for the ship continuing dry, not yet made heavy by the moisture soaking into the wood, it is probable that it lightly glides, and as long as it is clean, easily cuts the waves; but when it is thoroughly soaked, when weeds, ooze, and filth stick upon its sides, the stroke of the ship is more obtuse and weak; and the water, coming upon this clammy matter, doth not so easily part from it; and this is the reason why they usually calk their ships.
The British Royal Navy was well aware of the importance of removing fouling from the hulls of their men-of-war in the 18th and 19th centuries when “Britannia ruled the waves.” Captain James Cook landed his ship Endeavour at a small harbor he found at the mouth of what he named the Endeavour River in Australia on his way around the world so that it could be careened (laid over on its side) and the hull repaired and scraped free of barnacles.
Captain Cook notes elsewhere in his journal that he was looking for a suitable location to careen the ship with the sole purpose of cleaning the bottom. It has long been known that fouling on a ship’s hull greatly increases hull friction and slows the vessel down, making it more sluggish and less maneuverable.
Technology advanced to the point where ship hulls could be cleaned with the ship still in the water by divers using a variety of hand and mechanical tools and scrapers. Cleaning the hull with the ship in the water avoided beaching and careening or the more modern equivalent, drydocking, thus saving time and expense while still getting the job done.
For a brief time towards the end of the 20th century, the introduction of the highly toxic TBT into hull paints gave the illusion that ship hull cleaning was an unnecessary thing of the past. The idea was that all the hard work required to keep a ship hull clean could be avoided, substituting chemicals for manpower. This was one of those unfortunate “if it seems too good to be true it probably is” scenarios. It was soon discovered that TBT was a two-edged sword and the damage the poisonous substance caused to the marine environment was extensive, severe and unsustainable.
Nevertheless, during the “TBT era” the subject and practice of underwater ship
hull cleaning went into decline in terms of repute, technology, skill and general availability.
The TBT deception also resulted in the current attitude towards underwater cleaning held by many shipowners/operators who consider it a hassle and a logistical nightmare. The tendency is to avoid underwater hull cleaning.
Vicious circle
However, the antifouling technology which replaced TBT-laden hull paint was relatively ineffective. All ship hulls develop a biofilm or slime layer at the very least, regardless of the bottom paint used, and this, combined with rough hull coatings, carries with it a fuel penalty of as much as 20% or more. And there is an additional liability to the antifouling technology which replaced the TBT paints: the coatings could not be cleaned without damage to the paint and to the marine environment.
Rising costs of bunker fuel mean that a 20% fuel penalty is intolerable.
The underwater hull paint industry has created a “damned if you do, damned if you don’t” situation for shipowners/operators which includes frequent drydocking and paint replacement, a built-in fuel penalty, and coatings which are ineffective in preventing fouling yet are not suitable for underwater cleaning (the only practical means of avoiding the fuel penalty incurred).
Today underwater ship hull cleaning thoroughly and efficiently done on an industrial basis and on a suitable hull coating is the answer to reducing fuel costs, cutting GHG emissions, preventing the spread of non-indigenous species and avoiding marine chemical pollution.
Practical approach
Much of the literature reviewed on the subject of underwater ship hull cleaning takes a theoretical approach to the subject or is produced by people who are remote from the slime, weed, barnacles and other fouling on the typical ship hull, have not looked at fouled hulls underwater or cleaned ship hulls or talked to those who have. Often they try to compare underwater cleaning to apparently similar activities carried out on land, demonstrating an unfamiliarity with hydrodynamics and the differences between operations carried out on land and in the water. The real issues are not necessarily identified. Potentialities are missed. Restrictions are also missed. The result is that the view of underwater cleaning currently in circulation in the maritime industry and in academic writings tends to be rather impractical and divorced from reality, leaving shipowners and operators ill-informed on the subject.
This paper is a realistic survey of the subject of underwater ship hull cleaning, examining benefits as well as difficulties and limitations, with a view to providing an accurate and useful summary which can be put into use by shipowners and operators on an immediate basis using today’s technology to save 20% or more on fuel costs and GHG emissions, reduce the need for drydocking, prevent the spread of non-indigenous invasive species and all without polluting the marine environment. Knowledgeable experts on and practitioners of underwater ship hull cleaning have been consulted with a view to describing real situations, issues and solutions.
Part II. Vectors of change
In the second decade of the 21st century, a number of vectors are converging, driving the shipping industry towards a more efficient and environmentally safe approach to hull coating and fouling control. The vectors involved are:
1. rising cost of fuel,
2. pressure to safeguard the marine environment from the harmful effects of chemical biocides contained in conventional antifouling paints,
3. the problem of accumulating pollution and contamination of ports and harbors and their immediate surroundings, along with the great difficulty of dredging or trying to clean up those areas,
4. rising concern about harmful atmospheric emissions such as nitrous oxides (NOx), sulfur oxides (SOx), so-called green house gases (GHG),
5. efforts to limit the spread of invasive non-indigenous species (NIS) via ship hull fouling
6. the economic need to extend the interval between drydockings.
Fast, effective, widely available industrial grade underwater hull cleaning plays a key role in each of 1 - 6 above. Let’s see how.
1. Rising cost of fuel
This has been well documented in a previous White Paper in this series, Hydrex White Paper No. 3, Clean Ships Hulls and Ports–Without Compromise. The price of bunker fuel has been rising and all indications are that it will continue to do so. This is a major concern for all shipowners/operators who are looking for ways to reduce this cost so that they can maintain a profit margin without having to raise their prices excessively.
Therefore keeping a ship’s hull in smooth condition and free even of slime can add up to savings as high as 20% or more. Currently available antifouling (AF) coating systems are not particularly effective in preventing a build-up of slime. Fouling release (FR) coating systems also accumulate slime. Surface treated composite (STC) systems also build-up slime. In fact all available coatings tend to accumulate biofouling in the form of slime or biofilm quite rapidly, depending on the ship’s disposition and the waters in which it sails. Thus a fuel penalty of as much as 20% or more is usual with any ship, no matter what hull coating system is in use, unless the fouling is removed.
2. Avoiding chemical pollution of the oceans and waterways
This factor has been a major concern since TBT was found to be so damaging. Even though TBT has been banned and is no longer in use as an antifouling biocide, the biocides currently in use including copper and a number of so-called booster biocides or co-biocides are under scrutiny and are being increasingly regulated against (the latest development is the Washington State ban on the use of copper in antifouling paint for recreational craft, the first US state to restrict the use of copper for this purpose). Part of the Synopsis of the bill as Enacted, C 2248 L 11, is quoted here:
Background: Aquatic antifouling paints are used on water vessel hulls to prevent the growth of aquatic organisms such as barnacles and algae. Most of these antifouling paints use copper to reduce the growth.
According to a 2007 study, the Department of Ecology (DOE) has conducted research measuring copper concentrations in marinas and found the primary source of copper to be from the antifouling paints found on boat hulls. Research has shown copper to be highly toxic to aquatic life.
Summary: Recreational water vessels are defined as a vessel that is less than 65 feet in length, and used primarily for pleasure or leased, rented, or chartered to a person for the pleasure of that person. It does not include a vessel that is subject to United States Coast Guard inspection and is engaged in commercial use or carries paying passengers.
After January 1, 2018, new recreational water vessels with antifouling paint containing copper may not be sold in the state. Beginning January 1, 2020, the sale of copper antifouling paint intended for use on recreational water vessels is prohibited.
At time of writing, a similar bill has been introduced in California, SB 623, which is in final stages of approval in the California Senate.
This is a valid concern. The trend here is towards a complete ban on biocides in hull coatings where there is any doubt about the environmental safety of these biocides. So far no biocide has been developed which is completely safe environmentally, which targets only the fouling which actually settles or attempts to settle on the hull, and which loses its toxicity within a very short time or distance from the hull so that no non-target organisms are affected, the surrounding water is not polluted and the poisons do not settle in sediment in port areas.
Until such a biocide is developed and tested, the regulation against all biocides which do not meet these specifications and which continue to pollute the oceans, ports and waterways will increase. In Chapter 25 of the 2009 book Advances in marine antifouling coatings and technologies, A. J. Scarding includes the following:
25.1.1 The need for non-toxic alternatives The control of biofouling has finally reached an important crossroad. No longer is it acceptable to use TBT or indeed any other toxins which will harm non-target marine organisms. It is extremely difficult to predict the impact biocides and heavy metals will have on marine life in the future. The precautionary principle suggests that non-toxic strategies are the safest approach to adopt.
Already many ports in the world understandably ban the underwater cleaning of hulls coated with biocidal AF paint in order to protect their waters and environment.
3. Problem of accumulation of pollution and contamination in ports and
harbors
This is the same general situation as covered in point number 2 above but with local consequences of grave concern to ports and harbors. It extends to inland waterways. Due to the activity in ports, the fact that ships can remain there for some time and the work carried out in drydocks and ship repair yards, the pollution and contamination of these waters and the local seabed is intensified many times compared to the effects of biocides in deep water. Port authorities are
rightfully protective of their immediate environment, concerned for the health and safety of those using the port and the general cleanliness of the water and seabed.
4. GHG, CO2, NOx, SOx
Harmful emissions go hand in hand with fuel consumption. Responsible governments, environmental agencies and a number of NGOs are working hard to reduce the worldwide emission of greenhouse gases, carbon dioxide (CO2) nitrous oxides (NOx), sulfur oxides (SOx), particulate matter (PM) and other atmospheric pollutants. International shipping is not the main culprit when it comes to these emissions but nevertheless plays a significant role. Thus part of the worldwide endeavor to reduce these emissions from all sources are the efforts by the IMO and others to reduce that component of the overall emissions which can be attributed to shipping.
These emissions tend to be in direct proportion to the amount of fuel burned by ships. Propulsive fuel consumption can be reduced in a number of ways. A major factor is avoiding the extra fuel required to overcome the hull friction increase caused by fouling. This brings us back to the points covered above under “Rising cost of fuel.” Reduce fuel consumption by removing slime and other biofouling in a timely manner and this will automatically reduce the emissions of atmospheric pollutants.
5. Invasive, non-indigenous species (NIS)
Greater and greater pressure is being exerted to prevent or limit the spread of invasive nonindigenous species (NIS) via shipping, both from ship hull fouling and from ballast tanks.
Trying to prevent this spread by applying highly toxic AF coatings is a self-defeating activity from an environmental point of view. Like robbing Peter to pay Paul, the gains that might be made in limiting the NIS spread are outweighed by the damage done by the chemicals. The AF coatings currently in general use are not very effective in preventing the spread of NIS. In fact there is evidence that they contribute to the creation of a sort of super-NIS which are resistant to biocides and better armed to take over a new marine environment than the local species they displace. Neither are the FR coatings effective since some NIS are translocated even if others do not adhere or are washed off. If some NIS are transported, the damage is done. In order to really prevent the spread of NIS, ships must sail with a completely clean hull. They will not usually pick up fouling while en route and therefore if they sail with a clean hull from Port A they will arrive at Port B with a clean hull: no NIS. But this would require thorough cleaning before the ship sails, not a 20%, 40%, 75% job.
6. Fewer, shorter drydockings
As a further means of maintaining or increasing profit margins by keeping ships in service as much of the time as possible, trends are towards longer intervals between drydocking and less time spent in drydock. Many operations to the underwater hull and other parts of the ship below the water line can be accomplished more quickly and economically in the water, without pulling the vessel out of the water in drydock. This includes underwater hull cleaning and propeller polishing, both designed to reduce fuel consumption, as well as minor and major repairs which, if not done, would prevent a ship from operating at all.
Many of these cleaning, maintenance and repair activities can also be done in drydock just as well or in some cases better, but the expense of frequent drydocking is prohibitive in terms of the drydocking fees and labor costs themselves and the financial loss resulting from having a vessel out of service frequently and/or for extended periods of time. Others of these operations, such as hull cleaning on a surface treated composite (STC) coated hull, are better performed in the water due to the ease of access and the lubricant effect of the water when using mechanical tools. In fact an STC coated hull improves in smoothness and hydrodynamic properties when cleaned in the water, a combined effect of the tools and the water.
The pressure to drydock vessels less frequently and for shorter periods of time so as to keep costs down is increasing, as seen in the push towards a 7.5 or even 10 year drydocking interval.
The main obstacles to this extended interval are hull corrosion and fouling. The main incentive, if these factors are handled, is a great reduction in costs.
The following article published by the Baltic and International Maritime Council
(BIMCO) states the situation and trend very clearly:
Drydocking a ship periodically has been regarded as important, and an adequate network ofdrydocks and graving docks has been regarded as essential for both efficiency and safety. At the regular docking, the fouling over the ship’s submerged body can be scrubbed off, new coatings applied while the important underwater elements, such as the stern tube and its seals which prevent water getting into the ship (and any oil getting out) can be inspected. The propeller can be polished, and the various intakes such as the sea water suction, or the bow thrusters and its tunnel, can be inspected and overhauled.
It is fair to suggest that shipowners regard drydocking as a necessary evil, with the ship out of service for this periodic inspection. Drydocks are expensive to hire, and with so many very large ships at sea, it is sometimes difficult to find one available, especially for an emergency. It is usually necessary to book many months, or even years, ahead of the drydocking date, while big owners of big ships often contract their dockings on a block booking system that they hope will give them preferential treatment for their vessels.
Owners have tried to convince classification societies that the interval between dockings could be extended, on the grounds of more reliable and high performance hull coatings that keep growth at bay for longer, along with maintenance programmes that help to demonstrate the vessel’s ongoing quality. But what is a real breakthrough is underwater maintenance and repair equipment and technology that will virtually do everything a drydock can do, but without needing that drydock.
There are now underwater repair specialists that will undertake quite extensive repairs using “cofferdams” – chambers that can fit tightly to the underwater body of a ship and provide a dry refuge to the repair team. It is possible to scrub growth off the underwater shape, even to recoat with certain coatings, while propeller polishing is a job that can be done by divers as well as can be undertaken by people working in drydocks. It then becomes possible to extend the intervals between dockings, with savings in time and money, and without having to take the ship out of service.
Much of this technology has emerged from the offshore industry, where maintenance regimes and specialist coatings have been developed that will enable floating platforms and storage vessels to stay on station for 20 years or more. Ships will still need drydocking , but not so often, and the huge expense of building docks capable of taking today’s huge vessels might be mitigated.
These six vectors of change are all driving the industry in the same direction: use of a hard, inert, non-toxic coating and routine in-water cleaning. There are many advantages to this approach.