WO1998038082A1

WO1998038082A1 - Preventing deposition of fouling larvae on a ship's hull

Info

Publication number: WO1998038082A1
Application number: PCT/GB1998/000535
Authority: WO
Inventors: Lars Steinar ØSTLIE
Original assignee: Oet Holdings Plc
Priority date: 1997-02-28
Filing date: 1998-02-20
Publication date: 1998-09-03
Also published as: GB9803499D0; NO970947D0; NO970947L; NO304506B1; AU6299798A; GB2327211A

Abstract

Larvae are prevented from depositing on a ship's hull by having low frequency transducers mounted in pairs on the inside of the hull create vibrations in the water adjacent to the hull. A method is used with two alternating signal forms: in one signal form the phase of one transducer in each pair is swept in relation to the constant phase signal of the other transducer, in order to create travelling pressure maximum positions in the water along the hull, and in the other signal form, both transducers in every pair are driven synchronously and equally with pulses having a low duty cycle.

Description

PREVENTING DEPOSITION OF FOULING LARVAE ON A SHIP'S HULL

The present invention concerns a method and a means for preventing deposition of larvae on a ship's hull using mechanical vibrations. In particular it is of interest to fight the so-called "barnacle larvae", but other types of marine biological fouling can also be fought using the method and the means in accordance with the invention.

The most usual combat method against biological fouling on a ship's hull underwater is to apply paint or some other type of coating with a repelling effect on marine biological vegetation and animals, so as to prevent these from getting stuck. It is also previously known to use mechanical vibration methods, most often this relates to using sound waves where the actual sound pressure is intended to inhibit fouling. Usually this is done by killing the organisms, often in the form of small larvae, by means of a strong sound pressure.

For instance in Norwegian patent no. 82.676 a system is used with high frequency sound, i.e. ultrasound, from transducers mounted inside and on the hull, in such a manner that the hull propagates ultrasonic frequencies (the wavelengths in the hull will then be shorter than about 25 cm, and in the water just outside, shorter than about 7cm).

It is also previously known to use separate transducers on the outside of the ship's hull, compare Norwegian patent no. 100.272. However, the problem with ultrasound-based systems is that they are only able to maintain fouling-free regions which are quite limited as to area. Covering larger surfaces with such ultrasound systems has turned out to be problematic.

However, it has been shown that vibration systems using low frequency oscillations are capable of providing an effect which inhibits fouling. Because one has previously not realized which physical processes are of importance in connection with prevention of fouling when using low frequency vibrations, these systems have resulted in a relatively poor effect. Is has often been supposed that it is the actual sound pressure which is of importance, in the same manner as in the ultrasound case, but lately more clarity has been achieved regarding the important physical processes. It has turned out that the marine organisms of most interest, namely certain larvae in the size range 0.15-0.4mm, feel uncomfortable with strong water particle movement in the infrasonic vibration range 20- 60 Hz, i.e. when water particles move with amplitudes of ± 0.1 -0.2mm and at a particle maximum velocity which lies above a certain lower limit. Under such conditions, these larvae will try to avoid settling down on the ship's side.

Norwegian patent no. 168.513, with the same inventor as in the present invention, discloses such a low frequency vibration system operating with vibration frequencies in the range 20-30 Hz. In this case transducers are mounted on the inside of the ship's hull, which is set in transverse oscillations. The transverse hull vibrations prevent fouling by the above mentioned types of larvae. The ship's hull is put in an oscillatory motion in a direction substantially perpendicular to the hull, and the effect which prevents the larvae in question from sticking to the hull, is that a water particle movement is achieved in toward and out from the hull due to the motion of the hull itself.

From Norwegian patent no. 170.320, also with the same inventor as in the present invention, a more advanced low frequency system is known, in which transducers are mounted in pairs on respective sides of clamped lines on the ship's hull, i.e. lines along the hull which are mechanically clamped, that is clamped from the inside of the hull by construction details, most often bulkhead walls, and which lines therefore cannot make any transverse oscillations. By driving two transducers in such a pair in opposite phases, there is achieved a longitudinal "washing movement" in the water on the outside of the clamped hull line, so that fouling in such a region is also prevented.

However, it has turned out that also this last mentioned anti-fouling system is encumbered with drawbacks or limitations. The methodology of providing a high particle velocity using constant opposite phase operation for the transducers in a pair, which gives a "fixed dipole velocity pattern" in the water between transducers, gives a standing node in the substrate (i.e. the hull) which in turn results in a deposition of larvae in these regions. When the larvae first have settled down, this will be a signal for other larvae, and the deposition will spread out rapidly. A very favourable effect will be obtained if the fixed node pattern is broken in such a manner that the nodes virtually "wander" or sweep over the outer surface of the hull. ^* Besides, recent research shows that a combination of, or rather an alternation between velocity and pressure patterns in the water (opposite phase dipole and in-phase deflection) will influence swimming larvae to a much higher degree than merely a velocity pattern.

Thus, the present invention is provided to achieve just this enhanced effect, and to avoid completely standing nodes in any region on the hull.

The invention providing this effect is defined precisely in the appended patent claims.

For a further illumination of the invention, detailed embodiment examples will now be described, and this is done while referring to the appended drawings, wherein

Fig. 1 shows a simplified diagram curve forms for the pressure signals in the water for two transducers in a pair, as a function of time, and additionally a synchronous disclosure of the motions of a swimming larva,

Fig. 2 shows the same curve forms as in Fig. 1, however with broken time axes to render the curve forms and the shifts therebetween in a more realistic fashion,

Fig. 3 shows a diagram of instantaneous pressure conditions in the water just outside a ship's hull in the region adjacent to transducers in a pair, the transducers at the moment oscillating in phase, and Fig. 4 shows a corresponding diagram to Fig. 3, in which the transducers^" at the moment oscillate in opposite phases.

The prior art, in particular the art disclosed in NO 170.320 which is mentioned above, is based on a type of acoustics which has an effect on a larva approaching the hull, using one single negative stimulus. In the present invention two negative stimuli are used against the larva, first a stimulus with a combination of low frequency velocity and pressure progressions which influence the deposition pattern of the larvae, and in addition a pressure pulse is emitted in which all oscillators operate in phase, with another type of signal. This last signal affects the swimming pattern of the larvae in such a manner that during a short time lapse (about 0.5 seconds) they stop swimming due to a received pressure pulse with instantaneous frequency in the range 100 Hz-1 kHz. Since the larvae exhibit a small negative buoyancy, they will sink, but in a few seconds they will start swimming again, whereafter they are exposed to a new pressure pulse period of the same type. In practice it has been found favourable to run the combination signal for about 4 seconds, and then the pressure pulse signal for about 0.5 seconds, and this continues periodically.

In Fig. 1 is shown a diagram containing the curve forms achieved in the water outside two single transducers, SI and S2, a time axis running toward the right side. The signals which are sent to every second transducer sitting in one or two rows on the inside of the ship's hull, are actually signals consisting of synthesized forms, such as shown in the upper left circle in Fig. 1. One transducer, SI, in a pair receives the simplest type- of signal, which in the water results in pressure conditions substantially like a square pulse, in the example indicated in curve SI as a "27.5 Hz signal". The frequency is not necessarily 27.5 Hz, it is preferably within the range 10-50 Hz. The square signal is run for a period of about 5 seconds, and is then succeeded by a different type of signal, indicated as a "27.5 Hz burr". This is a signal in which there is still in essence used square pulses, however, each pulse is so short as to appear as a straight line or a spike in this diagram. The spacing between spikes in this diagram is somewhat erroneous, importance has here only been attached to showing that a different type of signal appears. It is referred to Fig. 2 regarding a more correct showing of the time relations. The "burr" signal is held for a period of about 0.5 seconds, and is then succeeded by the "main signal" which once more lasts for about 4 seconds, and so on. In the upper right circle in the figure the "burr" signal is shown in closer detail, however still with a somewhat incorrect time ratio, since the spacing (T) between the two double pulses is in reality considerably larger in the example shown. Thus, in reality the fundamental frequency, i.e. the frequency of occurring double pulses in the "burr" signal given by a frequency 27.5 Hz in this case, which corresponds to a period (T) of 33.4 milliseconds. The duty cycle of the "burr" signal is low, as indicated in the example, a full double pulse takes 1/32 of the complete period (T). This gives a duty cycle of about 3%. The instantaneous frequency attached to the actual double pulse is about 880 Hz in the case shown. The S2 curve is superficially quite similar to the SI curve, and similarity actually exists in the "burr" periods, in which both transducers SI an S2 are driven in full synchronism and quite similarly. However, in the "main signal" region the S2 signal deviates from SI in the following manner: In the beginning also S2 is a 27.5 Hz square signal output from the transducer, however in this case a phase sweep is applied in relation to the

51 signal. In the example shown, S2 starts, that is just after the first indicated "burr signal", with a 90° phase shift in relation to the SI signal (this does not appear clearly from Fig. 1, but appears more clearly in Fig. 2). During the approximately 4 seconds now at disposal, the phase of the

52 signal is changed in such a manner that half way it is in direct opposite phase with the SI signal, and in the end, i.e. just before the next "burr", it is 270° out of phase with the S 1 signal. In reality this means, if the phase shift occurs in a gliding manner, that the fundamental frequency here is a little different from 27.5 Hz, in the case shown the S2 square pulse may effectively be 27.6 Hz.

Subsequent to the interruption with the next "burr" signal, a new phase sweep is made in the S2 signal, in the case shown, in exactly the opposite manner of what took place the first time, namely from 270° and back to

90° phase difference in relation to the SI signal. Effectively, the S2 square wave may then have a fundamental frequency 27.4 Hz.

What is achieved through this phase sweeping, is that the positions of the pressure and velocity maxima in the area between transducers, and also in the area outside the transducer pair, are shifted during a sweep. Fig. 2 shows the same two signals output from transducers SI and S2, however with gaps several places in the time axis in order to present genuine curve forms as a function of time along the axis. Besides, there is an enlargement in the lower part of the figure, corresponding to the upper right circle in Fig. 1, however with a more correct indication of the spacing between two double pulses. In Fig. 2 it is easier to see that the "main signals" S 1 and S2 indicated here in the same example embodiment as in Fig. 1 , start with a phase difference of 90° in the left end of the diagram (see also an indication between the two curves). Units are indicated along the time axis in order to indicate the correct time, thus the scale is indicated in the interval 0-0.1 second at the far left in the figure. A gap has been drawn in the time axis in the first "main signal", and the signal is then found as it approaches its end after the first sweep, i.e. approaching a time indication of 4.0 seconds. It here appears that a phase difference of 270° has been achieved between the two "main signals", see an indication between the two signal curves S I and S2 at 4.0 seconds. The "burr" signal, which in this example lasts for 0.5 seconds, consists also in this scale of quite short spikes, and in order to show more clearly how these spikes actually appear, an enlargement has been entered below the two curve diagrams. It appears from the enlargement that the "burr" signal, which as previously stated is synchronous and equal for the two transducers of the pair, is a square pulse signal also, however with a low duty cycle. During the half second at disposal, 16 such double pulses are accommodated, see the indication between the S 1 and the S2 curves. At a time of about 4.5 seconds, the "main signal" is started again, and a sweep

(regarding S2) is made from 270° phase difference to 90° phase difference, also in this case indicated by writing between the SI and S2 curves. A gap in the time axis has been marked also in this case. At 8.5 seconds a new "burr" period starts, and in this "burr" period there is also a gap in the time axis in the same manner as in the previous "burr" period. Thereafter the curve shapes are repeated further for a desired duration.

Referring once more to Fig. 1, it appears in the lower part of the figure how the "burr" signal affects a swimming larva. When this signal starts, the larva is stunned, and it stops swimming. Thus the swimming pattern of the larva is interrupted repeatedly, and this is an effect in addition to the effect of the "main signal" which is adapted to provide a strong particle movement in the water just outside the hull, and which by means of the sweep arrangement constantly shifts the areas where pressure, respectively particle velocity, are at a maximum.

In Fig. 3 is shown a diagram of the pressure conditions in the water outside the ship's hull near two transducers A and B in a pair, in a case where the two transducers are driven in phase, i.e. like in a "burr" period. Thus, the ship's hull is represented in the figure as the elongated line through A and B, and the water is situated above this line. Midway between A and B there is a perpendicular line, along which in the case shown there is no phase difference between signals from A and B, and along which maximum pressure is found. Corresponding maximum pressures are found along the other curved, emphasised lines on both sides of the perpendicular mid-line. In the particular case shown here, with a spacing of 213cm between transducers, and at a frequency of 27.5 Hz, a phase different of 1.4° is found between every such pressure maximum line, and this- means that a gliding phase shift introduced between signals from A and B will shift the pattern successively laterally in one or the other direction in such a manner that when a phase difference of 1.4° is achieved, that pattern will overlap the original pattern. During such a phase shifting, the maximum pressure lines will so to speak sweep the whole water area outside the hull, and adjacent to the hull.

An "opposite" case is shown in Fig. 4. Here the starting point is an inverse phase operation of the two transducers A and B, and the emphasized lines then indicate surfaces with minimum pressure. But in the same manner as explained regarding Fig. 3, it will be possible to shift these lines in a sweeping motion between and around transducers A and B by sweeping the phase of one of the transducers. This means of course then that both maximum and minimum pressures, as well as maximum and minimum particle velocities in the water can be swept and moved in the area outside and adjacent to the ship's hull, by means of an operation as discussed above. The arrows indicated in the water in Fig. 4 state particle motion in the case of opposing phases. Dimensions and physical parameters have, as mentioned above, been selected so that a phase difference of 1.4° provides a repetition of the pattern, and thus such a phase difference is the minimum usable phase difference for making a desired movement of the pattern in the water. However, a phase sweep may be much larger than this, compare the example of Figs. 1 and 2, where a sweep is made through a range of 180°. It should also be noted that in Fig. 1 it is stated in the upper left circle, just that 1.4° is added or subtracted for each cycle of the S2 signal, in order to change from 90° phase difference to 270° phase difference. This means then that the 180° sweep is achieved in the course of 128-129 cycles, and with a fundamental frequency of 27.5 Hz this means that the "main signal" for SI and S2 lasts for about 4.7 seconds, which is a little longer than the 4 seconds suggested in Fig. 2.

But large variations are acceptable regarding frequencies, durations and widths of the sweep, the important thing is, as previously stated, that the "burr" periods are introduced synchronously and equally for both transducers in every pair, i.e. for all transducers, and that a sufficient sweep is made for S2 in relation to SI, that the water area outside the hull is swept through by pressure maximum surfaces and maximum velocity surfaces.

Finally it should be noted that the sweep during the "main signal" period must not necessarily be made in a gliding and regular manner, the sweep can also be introduced in a stepwise fashion, so that a constant phase difference is held through a lower number of cycles for S2, but so that a desired sweep from the beginning to the end of the "main signal" period is achieved anyway.

Claims

PATENT CLAIMS-

1. A method for preventing deposition of fouling larvae on ship's hulls, in particular "barnacle" larvae, using several electromechanical low frequency vibration transducers on the inside of the hull, and with the transducers grouped in pairs, characterised in that in each pair the first transducer is actuated in intervals with a periodic drive voltage having a fixed, predetermined fundamental frequency, e.g. 27.5 Hz, in repeated first time periods of e.g. 4 s, and with shorter intermission time periods, e.g. 0.5 s, while the other transducer is actuated according to the same interval scheme and with a fundamental oscillation based on an opposite phase swing in relation to the first transducer, however with a superimposed phase sweep through every first time period, from e.g. 90┬░ to e.g 270┬░ phase difference or vice versa in relation to the first transducer, and that both transducers in every pair are actuated synchronously in every intermission time period with pulses having a low duty cycle, e.g. 3%, but with substantially the same fundamental frequency as in said first time periods.

2. The method of claim 1, characterised in that the transducers are actuated by drive voltages which, apart from the fundamental frequency, contain superimposed harmonics in order to cause approximate square-shaped time progress curves for the dynamic pressure in the water outside the hull, both regarding transducers having a fixed time progression in said first time periods and regarding transducers having phase swept time evolvements, and regarding all transducers in the intermission time periods.

3. The method of claim 2, characterised in that each transducer is actuated by a drive voltage which comprises a superposition of a sine form at the fundamental frequency, and a number of odd harmonic forms in relation to said fundamental frequency, said harmonics being scaled in amplitude on the basis of knowledge regarding the characteristic physical parameters of the transducers.

4. The method of any of claims 1, 2 or 3, characterised in that the phase sweeps are executed regularly through each first time period, i.e. that in every other such first time period, every other transducer is actuated by respectively a somewhat lower and a somewhat higher fundamental frequency than the first transducers, e.g. 27.4 and 27.6 Hz as compared to a fundamental frequency of 27.5 Hz.

5. The method of any of claims 1, 2 or 3, characterised in that the phase sweeps are executed incrementally through every first time period, i.e. that each such first time period is subdivided into a number of time sub-periods in which every other transducer is all the time actuated by the same fundamental frequency as every first transducer, e.g. 27.5 Hz, but where the phase difference is advanced stepwise for each time sub-period from a start value of e.g. 90┬░ to an end value of 270┬░, or vice versa, using e.g. an increment of 18┬░ in eleven steps with ten oscillatory periods in each step, to fill a first time period of 4s.

6. The method of any one of the previous claims, characterised in that every phase sweep is run through from one phase extreme value to another, where the phase interval between the two extreme values is wider than a minimum value corresponding to a complete transfer of a pressure maximum in the water from one transducer in a pair to the other.

7. A system for preventing deposition of fouling larvae on ship's hulls, in particular "barnacle" larvae, said system comprising

(a) several electromechanical low frequency vibration transducers on the inside of the hull, said transducers being grouped in pairs with a given spacing between the two transducers in a pair, and

(b) a means for generating electrical drive energy to said transducers, said means being adapted for generating, amplifying and distributing electrical drive voltages to all of the transducers, characterised in that said generating means is adapted to drive the first transducer in every pair in intervals with a periodic drive voltage having a fixed, predetermined fundamental frequency, e.g. 27.5 Hz, in repeated first time periods of e.g. 4 s, and with shorter intermission time periods, e.g. 0.5 s, and to drive the other transducer of the pair in accordance with the same interval scheme and with a fundamental oscillation based on an opposite phase swing in relation to the first transducer of the pair, however with a superimposed phase sweep through every first time period, from e.g. 90┬░ to e.g. 270┬░ phase difference or vice versa in relation to the first transducer, and further to drive both transducers in each par synchronously in every intermission time period with pulses having a low duty cycle, e.g. 3%, but with substantially the same fundamental frequency as in the first time periods.