Environmental Control of Fouling

Field of the Invention

The invention relates to an apparatus and method for controlling fouling and in particular to an apparatus and method which controls marine fouling by modifying the environment around a surface.

Background of the Invention

Marine fouling is the undesirable accumulation of biological and/ or inorganic matter on the surfaces of structures submerged in the sea. It can be divided into two main groups: namely microfouling and macrofouling, which together form a fouling community. Examples of microfouling include bacterial adhesion and the formation of biofilms or slime, as well as the crystallisation of salts, oxides and hydroxides known as scaling. Macrofouling, on the other hand, is the attachment of larger organisms such as barnacles, mussels, worms, sea squirts, bryozoans and seaweed but also includes plant matter and manmade refuse. Biofouling or biological fouling is most problematic and affects both static and mobile structures. The growth of slime and larger organisms is a particular problem for ships where such growth can form an enormous mass on the hull and diminish the ship's speed, manoeuvrability and carrying capacity, as well as increasing fuel consumption and corrosion. This is more severe on naval vessels since they are usually moored for long periods of time, where the formation of slime alone can reduce fuel consumption by up to 2%. As a result, governments and industry currently spend billions of pounds a year trying to control biofouling.

In order to minimise the impact of biofouling, many underwater structures are protected by antifouling coatings containing biocides which are toxic to marine fouling organisms. In the early days of sailing ships, lime, copper sheets, and later arsenic were used to coat the ship's hull until the

modern chemical industry developed antifouling paints using biocidal compounds. Compounds commonly used in antifouling paints include cuprous oxide, metallic copper, and until recently tri- butyl-tin oxide (TBT) and other organic biocides such as zinc pyrithione. These compounds, however, are designed to dissolve in a controlled manner to release the biocide over a period of time. This reduces the effectiveness of the coating, and as a result, they have to be reapplied in regular intervals, in some cases as often as every 5 months. More importantly, studies have shown that the compounds can persist in the water for long periods of time, killing non-target sealife, harming the environment and possibly entering the food chain. Furthermore, these coatings are rarely completely effective against all forms of fouling, and even on modern biocidal coatings, slime films form. The US navy have since called for an antifouling agent which is both non-toxic and has a 12 year life span.

The International Convention on the Control of Harmful Antifouling Systems on Ships set up by the International Maritime Organisation (IMO) was adopted in October 2001 and comes into force in September 2008. This convention prohibits the use of harmful substances in antifouling paints used on ships. Under the terms of the convention, member states are required to prohibit and/or restrict the use of harmful antifouling systems on ships flying their flag, as well as ships which operate under their authority and those that enter their ports, shipyards and offshore terminals. The ban of these compounds presents a severe problem for the shipping industry. A major challenge, therefore, is to develop alternative technologies to prevent and treat biofouling, and to date, a number of different solutions have been proposed:

The use of electrochemical apparatus is described in EP0985639 and JP2000008338. This method involves the passage of a dc current through a conductive coating on the surface of the submerged structure to kill off any bacteria and overlying organisms. However, fine control of the applied potential is necessary to avoid the production of harmful chlorine gas when sea water is

electrolysed. With ships, care is also needed to prevent the current from passing through the underlying hull and dissolving it.

A different solution described in US4046094 uses a continuous flow of fresh water and an opaque curtain to discourage and inhibit the growth of biological material on a ship's hull. The curtain shields the fresh water from the mixing action of ocean currents and also acts to block out sunlight. This method would appear to be very impractical for a large ocean going vessel.

US6551536 describes a reverse osmosis membrane containing titanium dioxide particles. In this invention, titanium dioxide is used as a photo catalyst that decomposes organic contaminants and kills microorganisms when exposed to UV or solar radiation. When biofoulants attach to the membrane they are destroyed, without the production of any harmful bi-products. This technique has applications in water purification systems but is unsuitable for coating the hull of a ship.

Several non-toxic silicone-based coatings have been developed which make the surfaces of mobile structures sufficiently slippery that fouling organisms have difficulty attaching themselves and are washed off as the structure moves through the water. Although these coatings are implied to have no limited lifespan, they gradually foul, and have to be cleaned periodically. They are also unsuitable for use on static structures.

Summary of the Invention

When carbon dioxide mixes with sea water, carbonic acid is initially formed (equation 1). The two hydrogen atoms of carbonic acid then dissociate from the parent molecule in turn to form bicarbonate or hydrogen carbonate (equation 2) and carbonate (equation 3). It has been found that the low pH of carbonic acid creates an inhospitable environment for marine organisms inhibiting their growth. However, as carbonic acid is non-toxic in low concentrations, occurs naturally in seawater and is quickly buffered, there is no adverse environmental legacy.

The invention provides a simple apparatus and method for controlling marine fouling without contamination of the environment. When used in conjunction with carbon dioxide, a micro- environment in which the pH is lowered is created at the surface around which the carbon dioxide is released. This is able to keep the surfaces of structures submerged in static, fully marine water clear of visible marine fouling.

Based on the results of field trials, the invention is particularly suited to preventing the formation of slime, algae and macro invertebrates on the surfaces of small and large mobile and static structures in marine environments.

The invention may extend beyond carbon dioxide gas, to other gases capable of modifying other conditions in the micro -environment and which may be selected such that said micro- environment is rendered unfavourable to the accumulation of a specific foulant. One such example is the release of a noble gas or nitrogen gas to create oxygen-depleted conditions, inhibiting the growth of macro fauna and flora, or aerobic microbes. Also, the invention may use a combination of gases with different properties simultaneously to modify more than one condition in the micro- environment.

Further, whilst the release of carbon dioxide into the marine environment in the amounts required to control fouling is easily buffered, thereby preventing environmental damage, the invention is not limited to the use of gases which do not have an environmental impact. The invention may be deployed in a situation where an environmental impact is acceptable and the fouling requires a gas which may result in an environmental impact.

The gases used in the invention may be supplied from gas canisters or could come from recovered exhaust gases. Large vessels, such as tankers, store scrubbed exhaust gases from engines for use as fire suppressants. Such gases are rich in carbon dioxide and would be suitable for use without any prior purification.

According to the invention, there is provided an anti-fouling apparatus comprising a gas permeable element attachable to a surface and means to introduce gas to the gas permeable element, wherein the gas is released into a micro-environment immediately adjacent the free surface of the gas permeable element and wherein the gas is selected to modify at least one condition in said micro- environment such that the said micro -environment is unfavourable to the accumulation of foulants therein.

According to another aspect of the invention, there is provided a method of modifying at least one condition in a micro-environment such that the micro-environment is rendered unfavourable to the accumulation of foulants therein, the method comprising the steps of:

i) releasing a gas into said micro-environment;

ii) reacting the gas with at least one constituent of the micro -environment to modify said at least one condition.

This technology is particularly suited to the prevention of fouling on static structures such as oil rigs, cooling water pipe intakes for coastal power stations and desalination plants, windows /sensor housings on remote intelligent ocean monitoring buoys and land based marine aquaculture systems, whilst likely mobile beneficiaries include ships, submarines and barges. The invention may also be used to prevent calcium carbonate accumulation at the point of entry in domestic and industrial water supplies.

A tie-coat may be used to bond the gas permeable element to the surface of the submerged structure. In general, tie-coats are paints formulated to provide a transition from an undercoat to a topcoat under specific conditions. In this case, the tie-coat could be applied directly to the surface of interest with the gas permeable element fixed on top. The gas permeable element itself might be a gas permeable membrane, a micro-porous layer, a gas permeable solid, a gas chamber, a perforated pipe or an air stone. An air stone is a block of porous material that when fed a source of pressurised gas, diffuses the gas into the water in the form of bubbles.

Whilst most antifouling paints are capable of controlling specific types of fouling material, few are able to prevent all forms of fouling. Also, most of these paints have a limited lifespan as a result of their solubility in water and must be reapplied on a regular basis. Even silicone-based nontoxic coatings which are implied to have no limited lifespan gradually foul and need periodic cleaning. On the other hand, this invention is suitable for controlling all types of marine fouling and has no foreseeable restriction on lifespan.

Brief Description of the Drawings

In the drawings, which illustrate preferred embodiments of the invention and are by way of example:

Figure 1 is a cross-sectional schematic of the coating according to a first aspect of the invention;

Figure 2 is a cross-sectional schematic of the coating according to a second aspect of the invention;

Figure 3a is a cross-sectional schematic of a roughened PVC test panel;

Figure 3b is a cross-sectional schematic of a PVC test panel coated with a smooth silicone membrane;

Figure 3c is a cross-sectional schematic of a PVC test panel with a smooth silicone membrane stretched over PVC support ribs and enclosing a gas space containing carbon dioxide;

Figure 4a is a plan view photograph of the test panel in Figure 3a taken immediately after the 5 month static water field trial;

Figure 4b is a plan view photograph of the test panel in Figure 3b taken immediately after the 5 month static water field trial;

Figure 4c is a plan view photograph of the test panel in Figure 3c taken immediately after the 5 month static water field trial;

Figure 5 is a bar chart of the experimental results showing the summed percentage cover of slime formed on each panel type as determined immediately after the 5 month static water field trial;

Figure 6 is a bar chart of the experimental results showing the total number of macro fouling species formed on each panel type as determined immediately after the 5 month static water field trial; and

Figure 7 is a schematic cross-section of the coating indicating pH as measured at the surface of the membrane and the surrounding water.

Detailed Description of the Preferred Embodiments

Referring now to Figure 1, in one embodiment of the invention there is shown a surface coating 1 submerged in seawater 2, said surface coating 1 comprising of a smooth gas permeable membrane 3 stretched over support ribs 4 and enclosing a gas space 5 into which carbon dioxide 6 is

introduced. Over time, carbon dioxide gas 6 diffuses through the membrane 3 and reacts with the seawater 2 to produce carbonic acid 7. This in turn lowers the pH at the surface of the membrane which creates inhospitable environmental conditions for the organisms and inhibits their growth.

Referring now to Figure 2, in another embodiment of the invention there is shown a second surface coating 8 applied to a ship's hull 9, said surface coating 8 comprising of a smooth gas permeable membrane 3 adhered to a micro-porous tie -coat 10. The tie-coat 10 is in direct contact with the hull 9 and carbon dioxide gas 6 is introduced into the micro-porous tie-coat 10 and diffuses therefrom through the membrane 3 and reacts with the seawater 2 to produce carbonic acid 7.

Referring now to Figures 3a, 3b and 3c, there are illustrated cross-sections of PVC test panels with different surfaces designed to study the effects of surface structure and coating on marine fouling. All panels are made from grey PVC of size 10 X 10 X 0.3cm . The first type of panel 11 has been evenly roughened with coarse sandpaper, the second type 12 has been coated with a 0.15cm thick gas permeable silicone membrane 3 fixed to the PVC with a silicone-based adhesive, whilst the third type 13 has a 0.15cm thick gas permeable silicone membrane 3 stretched over PVC support ribs 4 and industrial grade carbon dioxide gas 6 introduced in the space behind the membrane 3 at a rate of 0.015 l/cm ² /h and a pressure of approximately O.lbar controlled by a pressure controller such as a pressure regulator. The carbon dioxide source may be contained in an engine exhaust gaseous mixture, which may have been subjected to a scrubbing process, or a more pure form of the gas.

Referring now to Figures 4a, 4b and 4c, there are shown plan view photographs of the test panels in Figures 3a, 3b and 3c respectively, taken immediately after the 5 month static water field trial. Slime 14 and multiple species of macro fouling material (tunicates, worms and bryozoans) 15 are clearly visible on the roughened PVC 11 and silicone coated 12 panels, whilst the panel 13 treated

with carbon dioxide appears clean, except for the area above the support ribs where slime and tunicates formed in small quantities.

Figures 5 and 6 show the experimental results of the static water field trial determined at the end of the five month test period. The summed percentage cover of slime formed on each panel type is given in Figure 5, whilst Figure 6 reveals the total number of tunicates, worms and bryozoans as indicated.

Referring now to Figure 7, there is shown a schematic cross-section of the coating 8, indicating pH as measured at the surface of the membrane 3 and the surrounding water 2. At the surface, carbonic acid production reduces the water by three pH units from pH ~8 (substantially neutral) to ~5 (acidic) creating a micro -environment that is unfavourable to the accumulation of marine foulants.

Experiment

A five month static water field trial was conducted to study the effects of surface structure and coating on marine fouling. Three test panels of each type were randomly arranged on a framework made from PVC tubing, and suspended in a temperate marina at a depth of one meter for five months. The panels were periodically photographed and any marine growth coarsely identified and quantified in terms of percentage cover and the total number of individual organisms.

Results

After five months, multi-species marine communities were found to be growing on the roughened PVC and silicone coated panels, with least growth present on the panels exposed to carbon dioxide. The small amount of fouling present on the latter type of panel was concentrated around the PVC support ribs. Panels covered in the silicone membrane without being exposed to

carbon dioxide had a limited antifouling capacity, whilst the test panels made from roughened PVC were worst affected.

Discussion and conclusions

The results of this experiment indicate that carbon dioxide gas permeating through a silicone membrane is capable of preventing the formation of slime, algae and invertebrates on the surfaces of structures submerged in static marine waters for at least five months. That small growth was present on the membrane above the PVC support ribs may be attributed to the fact that the PVC used is not permeable to carbon dioxide, and so this area of the membrane is unlikely to be exposed to the same acidic conditions as the surrounding regions. The difference between the levels of growth on the rough PVC and the smooth silicone coating also suggests that microorganisms favour an irregular surface over a smoother surface. This may be related to an improved adhesion allowing fouling materials to bind to the panel, or could be down to an increase in surface area providing a greater number of inhabitable sites.

Based on the results of these field trials, the invention provides a simple solution for keeping the surfaces of structures submerged in the sea clear of all forms of marine fouling. Such technology is likely to benefit large static and mobile structures including ships, barges, submarines, oil rigs and pipes amongst others.