Apparatus and Method for Prevention and Treatment of Marine Biofouling

Abstract

An apparatus and method for prevention and treatment of marine biofouling on a surface to be treated by the apparatus includes a body defining a cavity, the body terminating in a distal open end that is adapted, in use, to be in the vicinity of a surface to be treated such that the surface forms a first end wall of a chamber within the cavity; an acoustic transducer mounted within the cavity to form an opposite second end wall of the chamber, the acoustic transducer being adapted to cause acoustic pressure fluctuations; and a reservoir for a liquid, the reservoir including an inlet passage and a plurality of mutually spaced outlet passages for liquid flow from the reservoir into the chamber, the outlet passages at least partially surrounding the chamber in the vicinity of the distal open end.

Claims

1. An apparatus comprising: a body defining a cavity, the body terminating in a distal open end that is adapted, in use, to be in the vicinity of a surface to be treated such that the surface forms a first end wall of a chamber within the cavity; an acoustic transducer mounted to the body proximate an opposite second end wall of the chamber, the acoustic transducer being adapted to introduce acoustic energy into the chamber which is directed from the second end wall towards the first end wall thereby to allow acoustic pressure fluctuations to be generated at the surface to be treated, the acoustic pressure fluctuations being sufficient to cause cleaning of the surface to be treated; and a reservoir for a liquid, the reservoir being located laterally to the chamber and including an inlet passage for liquid flow into the reservoir and a plurality of mutually spaced outlet passages for liquid flow from the reservoir into the chamber, the outlet passages at least partially surrounding the chamber in the vicinity of the distal open end.

2. (canceled)

3. An apparatus according to claim 1, wherein the reservoirs laterally adjacent to the chamber and the reservoir and chamber are separated by a common wall.

4. An apparatus according to claim 3, wherein the reservoir at least partially surrounds the chamber.

5. An apparatus according to claim 1, wherein the chamber is surrounded by a wall that is acoustically rigid for sound impinging on it from liquid in the chamber, whereby acoustic pressure in the liquid, generated by the acoustic transducer, substantially reflected at the wall without a phase change in the pressure waveform in the liquid adjacent to the wall.

6. An apparatus comprising: a body defining a cavity, the body terminating in a distal open end that is adapted, in use, to be in the vicinity of a surface to be treated such that the surface forms a first end wall of a chamber within the cavity; an acoustic transducer mounted to the body proximate an opposite second end wall of the chamber, the acoustic transducer being adapted to introduce acoustic energy into the chamber which is directed from the second end wall towards the first end wall thereby to allow acoustic pressure fluctuations to be generated at the surface to be treated, the acoustic pressure fluctuations being sufficient to cause cleaning of the surface to be treated; a reservoir for a liquid including an inlet passage for liquid flow into the reservoir and an outlet passage for liquid flow from the reservoir into the chamber; and at least one first gas vent for venting gas from the chamber, the first gas vent being located from the distal open end.

7. An apparatus according to claim 6, further comprising at least one second gas vent for venting gas from the reservoir, the second gas vent being located remote from the outlet passages.

8. An apparatus according to claim 6, further comprising a one-way valve in each gas vent to prevent reverse gas flow into the chamber.

9. An apparatus according to claim 6, further comprising a suction device coupled to each gas vent to increase gas flow through the vent away from the chamber.

10. An apparatus according to claim 1, further comprising a cooling system for the acoustic transducer.

11. An apparatus according to claim 10, wherein the cooling system comprising a coiled pipe at least partly surrounding the acoustic transducer and a coolant supply system for supplying a flow of coolant through the coiled pipe.

12. An apparatus according to claim 10, wherein the cooling system comprises a fan for directing a flow of a fluid onto or into the vicinity of the acoustic transducer.

13. An apparatus according to claim 1, wherein the acoustic transducer is adapted to generate acoustic resonance within the chamber when the apparatus is positioned with the surface to be treated forming the first end wall of the chamber, and the acoustic transducer has an acoustic frequency and a spacing from the distal open end are adapted such that an acoustic pressure antinode in a plane wave mode is formed at or adjacent to the surface to be treated, whereby an oscillatory pressure field, on the surface to be treated and/or in the liquid adjacent to the surface to be treated, has substantially a same phase across the surface to be treated at any given instant, and wherein the acoustic frequency and the spacing from the distal open end of the acoustic transducer are adapted such that only one acoustic pressure antinode is formed in the chamber, which is located at or adjacent to the surface to be treated.

14. An apparatus according to claim 13, wherein the acoustic transducer has an acoustic frequency and a spacing from the distal open end such that, in use, only one acoustic pressure antinode is formed in the chamber, which is located at or adjacent to the surface to be treated.

15. An apparatus according to claim 1, wherein an acoustic energy transmitting material is located in the chamber between the acoustic transducer and the outlet passages, wherein the acoustic energy transmitting material comprises a liquid or solid and has an acoustic impedance that is substantial the same as the acoustic impedance of water.

16. An apparatus comprising: a body defining a cavity, the body terminating in a distal open end that is adapted, in use, to be in the vicinity of a surface to be treated such that the surface forms a first end wall of a chamber within the cavity; an acoustic transducer mounted to the body proximate an opposite second end wall of the chamber, the acoustic transducer being adapted to introduce acoustic energy into the chamber which is directed from the second end wall towards the first end wall thereby to allow acoustic pressure fluctuations to be generated at the surface to be treated, the acoustic pressure fluctuations being sufficient to cause cleaning of the surface to be treated; a reservoir for a liquid including an inlet passage for liquid flow into the reservoir and an outlet passage for liquid flow from the reservoir into the chamber; and a liquid conditioning unit adapted to remove bubbles from the liquid supplied to the chamber, wherein the liquid conditioning unit has an input for liquid and an output for liquid, the output being connected to the inlet passage of the reservoir, wherein the liquid conditioning unit comprises a casing having a plurality of compartments serially located between the input and output, at least one of the compartments including a headspace at an upper part thereof for collecting gas released from liquid flowing through the liquid conditioning unit.

17. An apparatus according to claim 16, wherein the plurality of compartments define a serpentine path therebetween.

18. An apparatus according to claim 16, wherein the input is located at the centre of the plurality of compartments and the output is located at a periphery the plurality of compartments.

19. An apparatus according to claim 16, wherein the volume of the compartments of the plurality of compartments increases in a flow direction from the input to the output so that the flow rate decreases in the flow direction.

20. An apparatus according to claim 16, wherein the liquid conditioning unit is fitted to the body so that the headspace is oriented in a direction away from the distal open end.

21. An apparatus according to claim 16, wherein the liquid conditioning unit is fitted to the body by a gimbal mechanism to permit the liquid conditioning unit to pivot relative the body so that when the liquid conditioning unit is submerged in water the headspace is maintained in an upwardly oriented position by buoyancy of the headspace in the water.

22. An apparatus according to claim 16, wherein the liquid conditioning unit is rigidly fitted above the body, the input is at an upper end of the liquid conditioning unit and the output is at a lower end of the liquid conditioning unit and communicates directly with the inlet passage of the reservoir.

23. An apparatus according to claim 16, wherein each compartment comprises a filter element.

24. An apparatus according to claim 23 wherein the filter element substantially fills the respective compartment.

25. An apparatus according to claim 23, wherein the filter element comprises a porous open cell foam or sponge.

26. An apparatus according to claim 16, wherein the compartments are annular and are mutually separated by annular walls.

27. An apparatus according to claim 16, further comprising a vent for the headspace to permit gas flow away from the upper part of the casing.

28. An apparatus according to claim 16, wherein the liquid conditioning unit comprises a vortex chamber.

29. An apparatus according to claim 28, wherein the vortex chamber is fitted to the body by a gimbal mechanism to permit the vortex chamber to pivot relative the body so that when the vortex chamber is submerged in water a headspace in the vortex chamber is maintained in an upwardly oriented position by buoyancy of the headspace in the water.

30. An apparatus according to claim 16, wherein the liquid conditioning unit is adapted to remove from the liquid bubbles having a diameter of greater than 100 μm.

31. An apparatus according to claim 16, further comprising a liquid degasser upstream of the inlet of the liquid conditioning unit which is adapted to remove dissolved gas from the liquid.

32. (canceled)

33. A surface treating assembly for prevention and treatment of marine biofouling, the assembly including multiple apparatuses according to claim 1, wherein the multiple apparatuses are assembled together in a mutually adjacent or tessellated form to form a linear or two-dimensional array of a plurality of the bodies to form a linear or two-dimensional array of a plurality of a mutually adjacent or tessellated chambers, each chamber being associated with a respective acoustic transducer.

34. A surface treating assembly according to claim 33 further comprising a phase controller for providing that the plurality of acoustic transducer emit acoustic energy in phase with each other.

35. A surface treating assembly according to claim 33, wherein each chamber is acoustically isolated from any adjacent chamber.

36-61. (canceled)

62. An apparatus according to claim 1, wherein the apparatus is configured to be operated while submerged below a liquid surface of a submersive liquid, and wherein the acoustic transducer is configured for controlling an amplitude of the acoustic energy to vary with a depth of the transducer below the liquid surface.

Description

BRIEF DESCRIPTION OF FIGURES

[0073] Embodiments of the present invention will now be described by way of example only, with reference to the accompanying drawings, in which:

[0074] FIG. 1 illustrates an apparatus for prevention and treatment of marine biofouling according to a first embodiment of the present invention;

[0075] FIG. 2 illustrates an alternative apparatus for treating a surface according to a second embodiment of the present invention; and

[0076] FIG. 3 is a graph showing the relationship between bifoulant thickness and treatment of various materials (comparing treatment of an area of hull once a week (2), twice a week (3) versus control) in accordance with an Example of the invention and a Comparative Example.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0077] FIG. 1 shows a treating apparatus 1, for prevention and treatment of marine biofouling on a surface to be treated by the apparatus, in accordance with an embodiment of the present invention. The apparatus 1 comprises a metal, for example steel or aluminium, or polymeric, for example acrylic, body 10 defining a cavity 10a in the form of a circular, square or rectangular cylinder. The body 10 terminates in a planar distal open end 12 that is, in use, held in the vicinity of a planar surface 2, which is the surface to be treated by the apparatus), as shown in FIG. 1.

[0078] When the body 10 is held with the distal end 12 of the body 10 in the vicinity of the surface 2, the surface 2 forms a first end wall 7 of a chamber 11 within the cavity 10a. The body 10 is provided with casters 13 for positioning the apparatus 1 relative to the surface 2.

[0079] The apparatus 1 further comprises a supply of a treating liquid such as water which can be supplied to the chamber 11 from a treating liquid reservoir 3. The reservoir 3 is located laterally to the chamber 11 and includes an inlet passage 14 for liquid flow into the reservoir 3 and a plurality of mutually spaced outlet passages 15 for liquid flow from the reservoir 3 into the chamber 11. The outlet passages 15 at least partially surround the chamber 11 in the vicinity of the distal open end 12. The outlet passages 15 have an outlet end 14 oriented towards the distal open end 12. The reservoir 3 is laterally adjacent to the chamber 11. The reservoir 3 preferably at least partially surrounds the chamber 11, and most preferably entirely surrounds the chamber 11. The chamber 11 is surrounded by a wall 16, typically a common wall 16 between, and separating, the chamber 11 and the reservoir 3, which is acoustically rigid as seen by the sound field within the liquid in the chamber, whereby, in use, acoustic pressure waveforms in the water in the chamber 11 are substantially reflected at the wall without a phase change. Typically, for acoustic waves in the liquid in the chamber that are approaching the wall to then be reflected off it, the normal incidence pressure amplitude reflection coefficient R of the wall 16 has a value of from +0.89 to +1, i.e. from 80 to 100% of the acoustic energy is reflected, and the phase change in the pressure waveform adjacent to the wall has a value of approximately 0°, i.e. the incident and reflected acoustic pressure waves are approximately 100% in phase in the chamber liquid adjacent to the wall (with a tolerance in the phase change of up to +/−20% or lower as described above).

[0080] At least one first gas vent 6 is provided for venting gas from the chamber 11. The first gas vent 11 is located remote from the distal open end 11. At least one second gas vent 8 is provided for venting gas from the reservoir 3, the second gas vent 8 being located remote from the outlet passages 15. A one-way valve 30, 32 in each gas vent 6, 8 prevents reverse gas flow into the chamber 11 and/or reservoir 3. A suction device 34 may be coupled to each gas vent 6, 8 to increase gas flow through the vent 6, 8 away from the chamber 11 and/or reservoir 3.

[0081] An acoustic transducer 18 is mounted within the cavity 10a on the top wall 10b of the body 10. The acoustic transducer 18 forms an opposite second end wall 9 of the chamber 11, and is adapted to introduce acoustic energy into the chamber 11 which is directed from the second end wall 9 towards the first end wall 7 thereby to allow acoustic pressure fluctuations to be generated at the surface 2 to be treated. The acoustic transducer 18 is controlled by a controller 19 and can be driven at a frequency of 60 kHz to 140 kHz, preferably from 60 kHz to 80 kHz, for example about 70 kHz. A modulator allows amplitude or frequency modulation of pulses of acoustic energy. Acoustic isolation devices (not shown) in the treating liquid inlet 14 and the treating liquid outlet 15 prevent sound propagation out from the chamber 11.

[0082] FIG. 1 shows an assembly including multiple apparatuses 1 according to the present invention, wherein the multiple apparatuses 1 are assembled together in a mutually adjacent or tessellated form to form a linear or two-dimensional array 111 of a plurality of the bodies 10 to form a linear or two-dimensional array 112 of a plurality of a mutually adjacent or tessellated chambers 11, each chamber 11 being associated with a respective acoustic transducer 18.

[0083] A cooling system 36 for the acoustic transducer 18 comprises a coiled pipe 38 at least partly surrounding the acoustic transducer 18. A coolant supply system 40 supplies a flow of coolant through the coiled pipe 38. Typically, the coolant is water, such as seawater, sourced from the underwater environment. Alternatively, the cooling system comprises a fan for directing a flow of a fluid, such as air or a liquid, onto or into the vicinity of the acoustic transducer 18. When the fan is arranged for driving a liquid flow, there is a need to ensure electrical safety between the liquid and the electrical power supply to the transducer, through insulation or use of a thermally conducting but electrically insulating liquid such as a transformer oil.

[0084] The acoustic transducer 18 is arranged, in use, to generate acoustic resonance within the chamber 11 when the apparatus 1 is positioned on or adjacent to a surface 2 (as shown in FIG. 1) with the surface 2 to be treated forming the first end wall 7 of the chamber 11 and the chamber 11 is filled with treating liquid. The acoustic transducer 18 has an acoustic frequency and a spacing from the distal open end 12 such that, in use, an acoustic pressure antinode in a plane wave mode is formed at or adjacent to the surface 2 to be treated. Preferably, the acoustic transducer 18 has an acoustic frequency and a spacing from the distal open end 12 such that, in use, only one acoustic pressure antinode is formed in the chamber 11, which is located at or adjacent to the surface 2 to be treated.

[0085] The acoustic transducers 18 are driven by a transducer drive system. A phase controller may be connected to the transducer drive system for providing that the plurality of acoustic transducer emit acoustic energy in phase with each other. Additionally or alternatively, each chamber may be acoustically isolated from any adjacent chamber, for example by providing each chamber with acoustically rigid walls.

[0086] In use, liquid flowed into the chamber 11 from the outlet passages 15 may entirely fill the chamber 11. Alternatively, an acoustic energy transmitting material 42 may be located in the chamber 18 between the acoustic transducer 18 and the outlet passages 15. The acoustic energy transmitting material 42 may comprise a liquid or solid and has an acoustic impedance that is substantially the same as the acoustic impedance of the liquid flowed into the chamber 11 from the outlet passages 15, most typically water. Water is preferably used as the liquid to be flowed into the chamber 11 towards the surface 2 to be treated.

[0087] A liquid conditioning unit 20 is located upstream of the treating liquid inlet 14, and is adapted to remove bubbles from the treating liquid supplied to the chamber 11, which would otherwise attenuate the acoustic field within the chamber 11. It can also filter particulates and other contaminants from the liquid. The liquid conditioning unit 20 has an input 46 for liquid and an output 48 for liquid, the output 48 being connected to the inlet passage 14 of the reservoir 3. The liquid conditioning unit 20 is typically adapted to remove from the liquid bubbles having a diameter of greater than 100 μm.

[0088] Typically, the liquid conditioning unit 20 comprises a casing 50 having a plurality of compartments 52 serially located between the input 46 and output 48 which define a serpentine path 54 therebetween. At least one of the compartments 52 includes a headspace 56 at an upper part 58 thereof for collecting gas released from liquid flowing along the serpentine path 54. The input 46 is located at the centre of the plurality of compartments 52 and the output 48 is located at a periphery of the plurality of compartments 52. The volume of the compartments 52 of the plurality of compartments 52 increases in a flow direction from the input 46 to the output 48 so that the flow rate decreases in the flow direction.

[0089] The liquid conditioning unit 20 is fitted to the body 10 so that the headspace 56 is oriented in a direction away from the distal open end 12. The liquid conditioning unit 20 may further comprise a vent 60 for the headspace 56 to permit gas flow away from the upper part 58 of the casing 50.

[0090] In some embodiments of the present invention, as shown in FIG. 1, the liquid conditioning unit 20 is fitted to the body 10 by a gimbal mechanism 62 to permit the liquid conditioning unit 20 to pivot relative the body 10 so that when the liquid conditioning unit 20 is submerged in water the headspace 58 is maintained in an upwardly oriented position by buoyancy of the headspace 58 in the water.

[0091] In other embodiments of the present invention, as shown in FIG. 2, the liquid conditioning unit 200 is rigidly fitted above the body 100. The input 202 is at an upper end 204 of the liquid conditioning unit 200 and the output 206 is at a lower end 208 of the liquid conditioning unit 200 and communicates directly with the inlet passage 210 of the reservoir 212. The serpentine path 214 extends downwardly from the input 202 to the output 206 between successive compartments 216 of a stack 218 of compartments 216. Each compartment 216 comprises a filter element 220. The filter element 220 may substantially fill the respective compartment 216. Typically, the filter element 220 comprises a porous open cell foam or sponge, optionally composed of a synthetic polymer, metallic mesh or mineral, or porous ceramic or stone. Preferably, the compartments 216 are annular and are mutually separated by annular walls 222.

[0092] FIG. 2 also illustrates a fan 230 for directing a flow of a fluid, which is an electrically insulating liquid such as a transformer oil, onto or into the vicinity of the acoustic transducer 18.

[0093] In other embodiments of the present invention, the liquid conditioning unit comprises a vortex chamber. The vortex chamber may be fitted to the body by a gimbal mechanism to permit the vortex chamber to pivot relative the body so that when the vortex chamber is submerged in water a headspace in the vortex chamber is maintained in an upwardly oriented position by buoyancy of the headspace in the water.

[0094] A liquid degasser may be provided upstream of the inlet of the liquid conditioning unit which is adapted to remove dissolved gas from the liquid.

[0095] An aggressive or chaotropic agent introduction system can be used to introduce one or more aggressive or chaotropic agents into the chamber 11, for example ozone, chlorine and/or hydrogen peroxide. A chemically active agent introduction system can be used to introducing one or more chemically active agents into the treating liquid, for example a detergent, a surfactant and/or a biocide.

[0096] Operation of the apparatus will now be described.

[0097] In use, the apparatus is used in a method of preventing or treating marine biofouling of a surface submerged in an underwater environment. In preferred embodiments of the present invention, the surface submerged in an underwater environment is a ship hull. It can also treat areas of the hull that are close to the waterline, and so can suffer from biofouling as a result of intermittent submersion, which is particularly facilitated if the outer walls of the device are made to be acoustically rigid whether submerged or not (as discussed above, metal walls more than 2 mm thick are acoustically rigid). In some embodiments, the method is a method of preventing marine biofouling of the surface submerged, permanently or intermittently, in an underwater environment, wherein the surface to be treated is coated with a biofilm in the absence of any macroscopic biofouling. The biofilm may be continuous or discontinuous.

[0098] In other embodiments, the method is a method of preventing marine biofouling of the surface submerged, permanently or intermittently, in an underwater environment, wherein the surface to be treated is coated with macroscopic biofouling.

[0099] Typically, the surface is composed of a material selected from a metal, such as steel or aluminium, a polymer, a rubber or a fibre-reinforced polymeric resin matrix composite material. In some embodiments, the surface is a bare surface of the material which has not been pre-coated with an anti-fouling paint or a biocide. However, in other embodiments the surface has been pre-coated with an anti-fouling paint or a biocide.

[0100] In the method, the apparatus described above is provided and the distal open end 12 is positioned in the vicinity of the surface 2 to be treated such that the surface 2 forms an end wall 9 of the chamber 11. The distal open end 12 of the body 10 may be slightly spaced from the surface to be treated in use, or may directly engage the surface 2. Such a spacing is typically up to 2 mm, for example from 0.5 to 2 mm, more typically from 0.5 to 1.5 mm, such as about 1 mm.

[0101] Liquid is supplied to the chamber 11 through the outlet passages 15 such that the liquid engages the surface 2. Preferably, the liquid has been subjected to an outgassing step which removes gas bubbles that are larger than the bubble size which is in pulsation resonance with the ultrasonic field (i.e. typically having a diameter of greater than 100 μm). The outgas sing step may optionally be preceded by a de-gassing step to remove dissolved gas from the liquid, as described above.

[0102] In preferred embodiments of the present invention, water is used as the liquid supplied to the chamber 11. Preferably, the water supplied to the chamber 11 is simultaneously sourced from the underwater environment. Typically, the water supplied to the chamber 11 is seawater.

[0103] The acoustic transducer controller 19 is used to control the acoustic transducer 18, and may be configured to generate pulses of acoustic energy. In this way it is possible to operate the transducer more efficiently by only generating acoustic energy in pulses and by reducing attenuation caused by bubbles. Typically, the acoustic transducer 18 is driven at a frequency in the range of from 60 kHz to 140 kHz, preferably from 60 kHz to 80 kHz, for example about 70 kHz. A frequency of about 70 kHz produces a bubble radius of typically about 40 to 45 μm in water close to the sea surface.

[0104] The zero-to-peak pressure amplitude can be controlled by the controller 19 to provide non-inertial bubble motion and non-inertial bubble cavitation. If desired, and unwanted surface damage is not a significant factor, the power emitted may be sufficiently high to stimulate inertial cavitation. The ultrasound frequency and the amplitude can be modified to modify the range of bubble sizes and the number of bubbles, and thereby also the acoustic pressure on the surface and the strength of the treatment. The acoustic transducer 18 may be operated to control the acoustic energy in the chamber 11 to generate surface waves in the bubbles and/or microstreaming. Surface waves may be controlled by varying the zero-to-peak pressure amplitude and/or the ultrasound frequency and/or bubble size. In general, the closer a bubble is to its pulsation resonance size, the lower the threshold acoustic pressure required to excite the Faraday wave (and other related waves). The sound field may be continuous or alternatively amplitude or frequency modulated, and the treating operation may comprise employing modulated acoustic energy to cause the non-inertial bubble motion and/or inertial cavitation and/or to generate the surface waves and/or microstreaming.

[0105] The acoustic transducer 18 therefore introduces acoustic energy into the chamber 11 and acoustic energy from the acoustic transducer is directed towards the first end wall 7 thereby generating acoustic pressure fluctuations at the surface 2 which are sufficient to cause cleaning of the surface 2. The acoustic transducer 18 generates acoustic resonance within the chamber 11 and, as shown in FIG. 1, forms an acoustic pressure antinode 300 in a plane wave mode (i.e. with constant phase, at any given time, across the surface to be treated) at or adjacent to the surface 2 to be treated. The acoustic pressure causes bubbles to form in the liquid at or in the vicinity of the surface 2. The acoustic pressure generates non-inertial cavitation of the bubbles within the liquid which treat the surface 2 to prevent or treat marine biofouling. Preferably, only one acoustic pressure antinode 300 is formed in the chamber 11, which is located at or adjacent to the surface 2 to be treated. This reduces attenuation of the acoustic energy by bubbles that would otherwise collect in the vicinity of any antinode remote from the surface 2.

[0106] The bubbles may rise within the chamber 11 and the reservoir 3 and be vented by the first and second vents 6, 8, optionally under suction to increase gas flow through the vents 6, 8 away from the chamber, as described above. The gas venting may be controlled to permit the apparatus to be oriented in any direction or attitude while in use, since the surface to be treated in the underwater environment, or just above the underwater environment if treating fouling on an intermittently fouled surface, may include a variety of regions of different orientation.

[0107] The chamber 11 and reservoir 2 are constructed of robust, stress resistant materials so as to be submergible in the underwater environment to depths of up to 30 m and to be able to resist hydrostatic pressure at such depths. Furthermore, the acoustic transducer 18 is configured to be capable of providing sufficient acoustic pressure under such hydrostatic pressures to be able to generate sufficient non-inertial cavitation of bubbles, and surface waves on bubbles, to achieve the desired surface treatment, and inertial cavitation if that is desired.

[0108] When treating marine biofouling, any microscopic or macroscopic biofouling is removed from the surface by a cleaning effect resulting from non-inertial cavitation of bubbles at or in the vicinity of the surface 2. If desired, and unwanted surface damage is not a significant factor, the power emitted may be sufficiently high to stimulate inertial cavitation. In both cases, the treatment is carried out for sufficient time to remove the microscopic or macroscopic biofouling in a treatment cycle.

[0109] Alternatively, the method is carried out in a prevention protocol to prevent marine biofouling in which the steps of the method forming a treatment cycle are repeated in plural successive treatment cycles separated by a period of from 72 to 240 hours, optionally from 120 to 240 hours. These successive treatments could, for example, be carried out whilst a ship is stationary in harbour, by mounting the device on a vehicle or arm that circles the vessel, possibly autonomously, covering (‘painting out’) successive rows one above the other (or side by side), touching or overlapping each other, until it has covered the whole hull, at which point it then returns to start painting the hull again, repeating its original path. This would allow a regular short-duration treatment of every region of the ship whilst it was in harbour, for example by a robot vehicle, which prevents the particular problem of biofilm and foulant build-up whilst the vessel is stationary in harbour (as merchant, naval surface ships and submarines are for extended periods).

[0110] It has been surprisingly found by the present inventors that the method of the present invention can prevent marine biofouling of the surface submerged in an underwater environment, wherein the surface to be treated is coated with a biofilm in the absence of any macroscopic biofouling. The biofilm may be continuous or discontinuous. In its early stages, the biofilm is also typically invisible to the naked eye, particularly during underwater inspections by divers or cameras. What has also been surprisingly found by the present inventors that the marine biofouling can be prevented when the surface is composed of a material selected from a metal, such as steel or aluminium, a polymer, a rubber or a fibre-reinforced polymeric resin matrix composite material and typically also when the surface is a bare surface of the material which has not been pre-coated with an anti-fouling paint or a biocide.

[0111] In other words, it has been surprisingly found that a prevention protocol of repeated treatment cycles at a desired frequency can prevent biofouling even when any continuous or discontinuous biofilm present is invisible to the naked eye and even when the surface is composed of a material typically used to produce ship hulls may be a bare surface of the material which has not been pre-coated with an anti-fouling paint or a biocide. Whilst the device also works with surfaces treated with antifoul, the ability to prevent fouling by attacking the biofilm on surfaces that do not have antifoul brings rewards in terms of reduction of environmental pollution through the use of antifouls and biocides.

[0112] The chamber 11 is surrounded by a wall 16 that is acoustically rigid to acoustic pressure waveforms in the liquid that impinge upon it. That is to say, acoustic pressure waveforms in the liquid, as generated by the acoustic transducer 18, are substantially reflected at the wall 18 without a phase change in pressure in the liquid at the wall. This can provide that the acoustic pressure antinode can be in a plane wave mode at or adjacent to the surface 2 to be treated, which provides that the acoustic pressure antinode has uniform phase across the area of the distal open end 12 of the chamber 11, and consequently across the area of the particular area of the surface 2 that is covered by the distal open end 12 of the chamber 11 at any given moment in time.

[0113] The acoustic transducer 18 is cooled using the cooling system 36 by supplying a flow of coolant, from the coolant supply system 40, through the coiled pipe 38 which at least partly surrounds the acoustic transducer 18. Alternatively, seawater can fill or be pumped directly around the transducer to cool it, provided acoustic emissions into the seawater do not generate significant power losses.

[0114] Typically, the apparatus is translationally slid over the surface to provide a continuous treatment action over an area of the surface 2 which is larger than an area of the distal open end 12. This may be achieved manually, for example when the apparatus is held by a diver, or by using a robot or autonomous vehicle, for example with the apparatus being attached to the surface to be cleaned by suction or magnetic attraction.

[0115] The present invention is now described further with reference to the following non-limiting Examples.

EXAMPLE 1

[0116] In this Example, mature biofouling was removed using an apparatus and method according to the invention. This was compared to the cleaning achieved by a mechanical rotating brush system which will be termed ‘Comparative Example 1’.

[0117] The thickness of mature biofouling before and after treatment was measured at multiple sites on each tested plate using an Episcopic Differential Interference Contrast (EDIC) microscope. This microscope works by having the light source above the sample, meaning it is possible to measure growth on solid surfaces. The depth of field on an EDIC microscope is very narrow and as a result it is possible to focus on the top of the biofouling, the sample or anywhere in between. What this allows is for a technique where by focusing the microscope on the base of the sample and setting an origin, it is then possible to move the sample down until the top of the sample is then in focus. The difference between the 2 points, which is measured by the microscope is the thickness of the foulant at this point. As the foulant will not be uniformly spread over the surface of the sample it will be necessary to measure the thickness at several points. Doing this for 10 points on every plate was sufficient to give statistically significant results.

[0118] Samples of steel, aluminium and rubber plates (50 samples of each material) of surface area 10 cm×10 cm were submerged in a seawater dock for a period of 36, 41 and 50 days respectively. During this period, mature biofoulant was established on the surfaces of the plates.

[0119] The samples were the removed from the seawater. The thickness of the biofoulant was measured in 10 places on each plate, and an average thickness calculated.

[0120] The plates were then treated to remove the mature biofoulant for a treatment period of 1 minute using the apparatus of FIG. 1. A single chamber was used for the testing, powered by a single transducer. The transducer was driven in continuous wave mode at 70 kHz, with a power amplifier supplying 100 W to the single transducer. The area of the target surface that was covered by the cleaning chamber at any given time was 36 cm.sup.2, whilst the area of the sample plates was 100 cm.sup.2 (the plates measured 10 cm by 10 cm). That means that, to cover the entire plate, the device was continuously moved over the plate containing established mature biofoulant placed for 1 minute, during which time any particular section might statistically be covered for 25% of the time.

[0121] The rms acoustic pressure amplitude that could be generated across the treated surface was measured prior to testing, using a plate of the material through which a hydrophone just protruded into the water, such that its active element was aligned with the surface to be cleaned (usually the upper surface of a plate laid flat). The rms acoustic pressure over steel was 16.8 kPa, over aluminium was 17.3 kPa, but over rubber was much less, at 11.2 kPa. These values show that the cleaning process here was by non-inertial cavitation (since inertial cavitation would have required in excess of 100 kPa). The rubber significantly reduced the rms pressure that could be generated across it, as expected because it is more absorbent than the metals (both in terms of not reflecting the sound back into the water to the same extent that the metals do, and absorbing a portion of the sound that enters it by converting it to heat). Nevertheless, good cleaning was still possible because the rigid walls of the chamber assisted in the formation of 11.2 kPa rms acoustic pressure even on the surface of the rubber.

[0122] Although the samples were out of the water for cleaning by the device, the device could have cleaned them when submerged. Removal from the water was to ensure electrical safety in this prototype, as the safety protocol for this experimental prototype included keeping the power amplifier and high voltage away from the water.

[0123] The thickness of the remaining biofoulant after the treatment was measured in 10 places on each plate, and an average thickness calculated. The difference between the initial and final thickness measurements was calculated and expressed as a percentage value for the biofoulant that was removed from the plate surfaces.

[0124] The same treatment was then completed using a rotating brush device, as described below for Comparative Example 1.

[0125] The results are shown in Table 1.

TABLE-US-00001 TABLE 1 Example 1 Comparative (the ultrasonic Example 1 device) Thickness (rotating brush device) Reduction % Thickness Reduction % (+/−standard deviation) (+/−standard deviation) Steel 91 (+/−4)  40 (+/−12) Aluminium 94 (+/−3) 97 (+/−1) Rubber 80 (+/−1) 64 (+/−1)

[0126] It may be seen from Table 1 that for each of the three materials tested, steel, aluminium and rubber, the treatment time of only 1 minute achieved a high removal percentage of the mature macroscopic biofouling.

COMPARATIVE EXAMPLE 1

[0127] Some of the plates used in Example 1 were subjected to a conventional cleaning treatment using rotary brushing for a period of 1 minute rather than using acoustic energy in accordance with the present invention. The % thickness reduction was again calculated and the results are shown in Table 1.

[0128] Table 1 shows that for the steel and rubber plates the use of acoustic energy provided significantly improved cleaning as compared to brushing. For aluminium, the thickness reduction were similar for Example 1 and Comparative Example 1, however the aluminium surface was significantly damaged by scratching by the bristles of the brush. The brush was also mechanically damaged and required replacement. The rubber plate was also damaged by scratching by the bristles of the brush.

[0129] Summary for Example 1: This data shows that the method and apparatus of the present invention are highly effective at removal of mature macroscopic biofouling. The removal by the ultrasonic device was significantly greater than removal by the rotating brush for the same treatment time, except for aluminium, where statically there was no difference in their cleaning performance.

[0130] After use, the brush was so badly damaged that it could no longer be used. Furthermore, the samples that were brushed showed scratches and damage to all three surfaces, particularly the rubber. No damage could be detected, either through visual or microscopic examination, of the samples that were cleaned using the ultrasonic device, and no damage was sustained to the ultrasonic device during these tests.

EXAMPLE 2 AND COMPARATIVE EXAMPLE 2

[0131] In this Example, biofouling was prevented using an apparatus and method according to the invention.

[0132] Samples of steel, aluminium and rubber plates (3 samples of each material) of surface area 10 cm×10 cm were submerged in a seawater dock for a period of 21 days. During this period, mature biofoulant was established on the surfaces of the plates.

[0133] For each material, one set of samples was used as a control in Comparative Example 2 and during the 21 day test period the plate was not removed from the seawater and mature biofoulant was established on the surface of the plate.

[0134] A second set of samples was removed from the seawater once per week and upon each removal the plate was then treated for a treatment period of 1 minute using the apparatus of FIG. 1 in Example 2. The transducers were driven in continuous wave mode at 70 kHz, with a power amplifier supplying 100 W to each transducer. The area of the target surface that was covered by the cleaning chamber at any given time was 36 cm.sup.2, whilst the area of the sample plates was 100 cm.sup.2 (the plates measured 10 cm by 10 cm). That means that, to cover the entire plate, the device was placed stationary for 15 s over each of the four quadrants of a plate. The cleaning time was set to 1 minute for each plate (15 s for each quadrant), which would allow for the entire sample area to be under the cleaning chamber for sufficient time for the surface cleaning action to take place.

[0135] The rms acoustic pressure amplitude that could be generated across each treated surface was as described in Example 1.

[0136] Although the samples were out of the water for cleaning by the device, the device could have cleaned them when submerged. Removal from the water was to ensure electrical safety in this prototype, as the safety protocol for this experimental prototype included keeping the power amplifier and high voltage away from the water.

[0137] The treated plate was then returned to the seawater to continue the submersion within the 21 day test period.

[0138] A third set of samples was removed from the seawater twice per week and upon each removal the plate was then similarly treated for a treatment period of 1 minute using the apparatus of FIG. 1 in Example 2. The exposures were identical to those given for the second set of samples.

[0139] The treated plate was then returned to the seawater to continue the submersion within the 21 day test period.

[0140] Thus in Comparative Example 2 each plate within the first set of samples was not treated in the 21 day test period, in Example 2 each plate within the second set of samples was treated a total of three times in the 21 day test period and each plate within the third set of samples sample was treated a total of 6 times in the 21 day test period.

[0141] Again, thickness measurements of any biofouling on the plate surfaces were measured and calculated as described for Example 1.

[0142] The results are shown in FIG. 3. FIG. 3 shows that for the second and third sets of samples of each material, according to Example 2, the repeated and regular acoustic energy treatment protocol prevented any biofouling from becoming established. For the weekly and twice weekly acoustic energy treatments, no significant biofilm, which was readily visible to the naked eye, was formed.

[0143] Therefore in Example 2 the plates were treated even when a skilled person would not have readily recognised that any treatment against biofouling was required. Such a treatment protocol unexpectedly provided a reliable and repeatable method for prevention of marine biofilm formation, and prevention of marine biofouling, on surfaces subjected to an underwater environment.

[0144] Various other modifications of the invention will be readily apparent to those skilled in the art, and are included within the scope of the invention as defined by the appended claims.