WO2001058750A1 - Method and device for preventing fouling - Google Patents

Abstract

The invention relates to a method for preventing fouling by micro-organisms on a surface located underwater by periodically generating ultrasonic vibrations in the water in the vicinity of the surface. Such periodically generated vibrations disturb the cell division of the micro-organisms without this having any other effect on the surrounding water. The generated vibrations can advantageously be directed parallel to the surface. The vibrations can be generated in the form of pulse trains with a length in the order of several hundredths of a second and a period in the order of a few seconds to several tens of seconds. The frequency of the generated vibrations can be varied. The invention also relates to a device for performing this method. Such a device has periodically acting means for generating vibrations in the vicinity of the surface, which vibration generating means comprise one or more piezo-electric elements placed in series.

Description

METHOD AND DEVICE FOR PREVENTING FOULING

The invention relates to a method for preventing fouling by micro-organisms on a surface located underwater by periodically generating vibrations in the water. Such a method is known, for instance from DE-B- 1031166.

Fouling by such micro-organisms, in particular algae, on surfaces located underwater is a generally known problem. For stationary surfaces, such as the underwater parts of mooring posts, sheet pilings, lock gates and other engineering structures, this problem is not so serious, although particularly the legibility of for instance water-level gauges, which sometimes lie under water and sometimes not, can be greatly reduced by such fouling. The problem of fouling is however of particular importance for ships, part of the hull of which is always situated below the water surface. The friction drag of these parts of the hull increases considerably due to the fouling, whereby the performance of the ship is adversely affected. In order to prevent fouling by algae on ships use has usually been made heretofore of "anti-fouling" paint. A known type of anti-fouling paint is so-called chlorinated rubber anti-fouling. This paint contains a toxin which acts against the micro-organism, but has the drawback that the surface water is poisoned thereby and has therefore now been prohibited for the pleasure cruise sector.

Another type of anti-fouling paint is a varnish which dissolves slowly in water, whereby during sailing the top layer of the varnish, together with all algal growth thereon, is as it were washed off the hull. This anti-fouling therefore also has the drawback that the residues thereof end up in the surface water. When the hull of a vessel treated with this anti-fouling is spray- cleaned at its berth at the end of the season, the paint residues must therefore be collected and disposed of.

There are also methods known with which fouling can be prevented without the use of a special paint layer. A system is thus described in the above stated publication DE-B-1031166 for preventing fouling by generating ultrasonic vibrations in the water around a ship. In order to generate these vibrations use is herein made of transducers mounted on the hull of the ship. These transducers are herein oriented such that they will emit ultrasonic signals transversely of the direction of the hull. In respect of limiting the required power it is proposed in this older patent to feed the transducers alternately, whereby each transducer only generates an ultrasonic signal periodically.

A similar system is described in US-A-5, 532 , 980. Here also a number of transducers are mounted on the hull of a ship and set this hull into vibration transversely of its surface, whereby ultrasonic signals directed transversely of the hull are generated in the surrounding water.

Finally, US-A-5, 386 , 397 describes another system wherein a continuous ultrasonic signal is generated in the ship's hull. Although this does not become very clear from the document, it would appear that this signal is directed in the longitudinal direction of the hull.

It has been found that the growth of micro-organisms can be inhibited or even stopped completely by the vibrations generated in the water using the above described systems . Although the underlying mechanism here is not yet fully understood, it is thought that the vibrations disturb the cell division of the microorganisms, whereby growth is suppressed.

The above described known systems all have the drawback that the vibrations are generated in or against the ship's side, and cause the ship's side to vibrate, whereafter this vibration is transmitted into the water in the form of a transverse vibration perpendicularly of the ship's side. The construction and design of the ship's side hereby have an influence on the vibrations eventually generated in the water. Stationary vibrations can thus for instance occur in the ship's side in which 5 nodes and antinodes occur, wherein the water at the position of the nodes is not set into vibration. In addition, a ship's wall will always have discontinuities, such as frames, welds and the like, by which the generated vibrations can be reflected. Once again a o stationary wave can hereby occur, or the generated vibration can in any case be impeded, which results in beating .

The invention now has for its object to provide a method of the above described type, wherein these s drawbacks do not occur. According to a first aspect of the invention this is achieved in that the vibrations are generated in the vicinity of the surface. By thus generating the vibrations not in or against the surface but directly into the water in the vicinity of the o surface, the influence of the surface on the vibrations in the water is minimized. A uniform vibration pattern is thus generated in the water in the vicinity of the surface, irrespective of the form and construction thereof at that location. 5 According to another aspect of the invention an active and uniform pattern of vibrations is generated in the water in that the generated vibrations are directed substantially parallel to the surface. The shear forces generated by such vibrations are more effective than o compression forces generated when the vibrations are directed transversely of the surface, as in the discussed earlier proposals.

The vibrations are preferably generated in the form of one or more pulse trains. A large number of vibrations 5 is thus generated in concentrated form in a very short time, which is found to work better in practice than the same number of vibrations spread over a longer time. The length of the or each pulse train can herein be in the order of several hundredths of a second, while the total period for which the vibrations are generated is preferably in the order of one to several tens of seconds. A large number of concentrated vibrations is thus generated in each case for a very short time, whereafter nothing further happens for a longer period.

The generated vibrations advantageously have a frequency lying outside the audible range . Such ultrasonic vibrations are found to be the most effective in suppressing the growth of the micro-organisms. The generated vibrations herein preferably have a frequency in the order of 20,000 to 100,000 Hz.

The frequency of the vibrations is preferably varied. Different vibration frequencies can thus be used which can each be directed specifically at an organism to be combatted. It is herein possible to give successive pulse trains different frequencies, although it is also conceivable to make use of different vibration sources, each having their own fixed frequency. The frequencies can herein be mutually adjusted such that they have no common divisor, so that even in the unlikely case that stationary waves were to occur in the water, the nodes and antinodes would in any case not coincide, so that the water is therefore set into vibration everywhere. The invention further relates to a device for performing the above described method. Such a device is provided according to the invention with periodically acting means for generating vibrations in the vicinity of the surface. A very compact device is obtained when the vibration generating means comprise at least one piezo-electric element. In order to achieve a sufficiently large amplitude of the vibration, a number of piezo-electric elements can herein be placed in series, for instance in the form of a multilayer piezo-electric member.

In order to amplify the vibrations generated by the piezo-electric elements, the or each piezo-electric element is preferably received in a housing closed by at least one membrane. This housing and membrane function in fact as a kind of "drum" .

When the device comprises a number of piezo-electric elements, each received in a separate housing with 5 membrane, vibrations of varying frequencies can be generated herewith. Each piezo-electric element can herein then have its own vibration frequency.

The device can be further provided with means for underwater suspension of the or each housing, for o instance in the form of a suspension cable or frame which is lowered along the surface for cleaning.

When the device further has means arranged outside the or each housing for energizing the or each piezoelectric element, the part of the device situated s underwater can be given a very small form. Furthermore, the suspension means can herein also function as power supply cable. The energizing means can then advantageously be adapted to successively energize the or each piezo-electric element at different frequencies. o Different vibrations can thus once again be generated in the water, whereby diverse organisms can be combatted.

The device is preferably further provided with means for orienting the or each housing, thereby ensuring that in all conditions the emitted vibrations run 5 substantially parallel to the surface for cleaning.

The invention will now be elucidated on the basis of an embodiment, wherein reference is made to the annexed drawing, in which:

Figure 1 shows a schematic perspective view of a o device for periodically generating vibrations under the water surface according to the invention;

Figure 2 is a schematic cross-sectional view showing the operation of the device of Figure 1;

Figure 3 shows schematically the placing of a number 5 of devices according to the invention along the hull of a moored ship; and

Figure 4 shows schematically the progression in time of the generated vibrations. A device 1 for preventing fouling by micro-organisms on a surface 11 lying underwater comprises a transducer 2 which is to be placed underwater in the vicinity of surface 11 and which is connected over a multi -core cable 5 3 to means (not shown here) for activating or energizing transducer 2.

Transducer 2 comprises a cylindrical housing 4 which is closed on both sides by a membrane 5. Accommodated in housing 4 is a number of piezo-electric elements 6 which o are mutually connected in series so as to amplify the action thereof. The piezo-electric elements 6 are herein integrated to form one multilayer piezo-electric member 7. When a voltage is applied by the energizing means over this piezo-electric member 7, it will deform, wherein the s total deformation is the sum of the deformations of the individual piezo-electric elements 6. As a result of the deformation thereof, the piezo-electric member 7 will strike against the membranes 5 arranged on either side of housing 4, whereby these are deformed (shown in dotted o lines in fig. 2) and a longitudinal vibration 8 is generated in the water 9 surrounding transducer 2.

Transducer 2 is herein also provided with caps 10 protruding on either side, whereby the vibration 8 is directed into the water 9. It has been found that the 5 vibrations 8 are most effective in inhibiting the growth of micro-organisms when they are oriented roughly parallel to the surface 11 on which the micro-organisms would settle and multiply. The shear stress generated by such vibrations along the surface evidently has more 0 influence than a compression as would be obtained if the vibrations were to be directed transversely of the surface .

The energizing means are adapted for periodic energizing of piezo-electric member 7. Energizing herein 5 takes place such that a pulse train 13 of vibrations is formed. A large number of vibrations is thus generated for only a very short time t, whereafter transducer 2 is then inactive for a longer period. Although the ratio between the length t of the pulse train 13 of vibrations and the total period P can be established by the skilled person on the basis of experiments, it is assumed for the time being that a pulse train 13 with a length in the order of several hundredths of a second followed by an inactive period of a few seconds to even several tens of seconds will produce very good results. The power taken up by the transducer can remain limited by only emitting vibrations periodically. The energizing means are further adapted to cause vibration of piezo-electric elements 6 at relatively high frequency. A vibration frequency is preferably even applied which lies outside the audible range, and can be in the order of 20,000 to 100,000 Hz. Such a vibration appears to disturb the cell division of the microorganisms such that growth can not only be inhibited but even stopped completely. The frequency of the vibrations within a pulse train 13 or between two successive pulse trains 13 can otherwise be varied so as to best combat different (parts of) organisms. Series of vibrations can thus be emitted at different frequencies, for instance four vibrations first at 30 kHz followed by ten vibrations of 50 kHz, and so on. The skilled person can establish on the basis of experiments which "mix" of frequencies is the most effective in a particular situation.

As stated, the amplitude of the vibrations 8 generated in the water 9 is determined by the deformation of piezo-electric elements 6 and can be increased or decreased by adapting the form and dimensions of piezoelectric elements 6 and the number of elements 6 that are combined to form a single piezo-electric member 7. Vibrations of a considerable activity can however still be generated with a relatively compact transducer 2 by making use of piezo-electric elements 6.

The number of transducers 2 necessary to keep a determined surface 11 free of fouling will of course depend on the power of transducers 2 and the nature of the water 9 in which they are used. In order to keep a yacht 12 free of fouling during its stay in a marina, it is possible by way of example to assume for instance four transducers 2 at regular mutual spacing oriented roughly 5 parallel to the hull of yacht 12. Transducers 2 could be further provided for this purpose with a guide element or foot which supports on the hull. Instead of the shown cable 3 it is also possible to opt for a rigid member from which transducer 2 is suspended and of which the o part protruding above the water can be mounted in any desired position on the hull of yacht 12. In this way the transducer 2 can be oriented in simple manner.

Although the invention is elucidated above with reference to an embodiment, it will be apparent to the s skilled person that this can be modified in many ways. The vibrations could thus be generated in a manner other than by means of the piezo-electric effect, while more or fewer than two membranes could also be applied in the transducer. Instead of varying the frequency of a single o transducer, a plurality of transducers could also be used, each having its own frequency. The direction of the vibrations relative to the surface for cleaning could also be varied within determined limits, while the repetition pattern of the vibrations can in principle be 5 freely chosen.

The scope of the invention is therefore defined solely by the appended claims.

Claims

1. Method for preventing fouling by micro-organisms on a surface located underwater by periodically generating vibrations in the water, characterized in that the vibrations are generated in the vicinity of the surface.

2. Method as claimed in claim 1 or the preamble thereof, characterized in that the generated vibrations are directed substantially parallel to the surface.

3. Method as claimed in claim 1 or 2 , characterized in that the vibrations are generated in the form of one or more pulse trains.

4. Method as claimed in claim 3, characterized in that the length of the or each pulse train is in the order of several hundredths of a second.

5. Method as claimed in any of the foregoing claims, characterized in that the period for which the vibrations are generated is in the order of one to several tens of seconds .

6. Method as claimed in any of the foregoing claims, characterized in that the generated vibrations have a frequency lying outside the audible range.

7. Method as claimed in claim 6, characterized in that the generated vibrations have a frequency in the order of 20,000 to 100,000 Hz.

8. Method as claimed in claim 6 or 7, characterized in that the frequency of the vibrations is varied.

9. Device for performing the method as claimed in any of the claims 1-8, characterized by periodically acting means for generating vibrations in the vicinity of the surface.

10. Device as claimed in claim 9, characterized in that the vibration generating means comprise at least one piezo-electric element.

11. Device as claimed in claim 10, characterized by a number of piezo-electric elements placed together in series .

12. Device as claimed in claim 10 or 11, characterized in that the or each piezo-electric element is received in a housing closed by at least one membrane.

13. Device as claimed in claim 12, characterized by a number of piezo-electric elements, each received in a separate housing with membrane, which are adapted to generate vibrations of varying frequencies.

14. Device as claimed in claim 12 or 13, characterized by means for underwater suspension of the or each housing.

15. Device as claimed in any of the claims 12-14, characterized by means arranged outside the or each housing for energizing the or each piezo-electric element .

16. Device as claimed in claim 15, characterized in that the energizing means are adapted to successively energize the or each piezo-electric element at different frequencies .

17. Device as claimed in any of the claims 12-16, characterized by means for orienting the or each housing.