CLEANING AND ETCHING METHODS UTILISING ACOUSTIC FIELDS

Abstract: There is disclosed methods of cleaning or etching the surfaces of objects immersed in liquid comprising focusing an acoustic field adjacent a surface of an object and producing cavitation in the liquid at the point of focus. The method may be utilized to loosen or remove fouling from offshore structures.

Description: CLEANING AND ETCHING METHODS UTILISING ACOUSTIC FIELDS This invention relates to uses of cavitation in liquids, produced by means of acoustic fields. In particular, but not exclusively, the invention relates to the use of focused acoustic fields in the cleaning and etching of surfaces. Further, the invention also relates to the use of cavitation in the treatment of marine life, and to apparatus for use in such methods. Offshore structures used in oil and gas production are subject to varying degrees of marine fouling. This marine growth impedes inspection of the structure as the majority of inspection techniques require access to a relatively clean surface. More particularly, inspection methods, such as Eddy current and magnetic particle inspection for location of surface breaking flaws, and ultrasonics for detection of internal flaws and detection of flooded members, require a good surface contact to produce reliable test results. Accordingly, to allow inspection the structure must be cleaned prior to the inspection being carried out. Marine fouling is generally classified as being either hard or soft. Hard marine fouling typically refers to growth such as barnacles, whereas soft marine fouling would be represented by seaweed and the like. Various methods for removing this growth, or discouraging settlement, have been considered and among the more popular methods used are manual scrapers and brushes, water jetting and rotary brushes. However, while these methods provide relatively efficient means for removal of soft fouling, they are generally unsatisfactory for removing hard fouling. Further, at present, most offshore inspection and cleaning operations require some degree of diver intervention and such work can be dangerous and is also relatively expensive. It is among the objects of the present invention to obviate or mitigate these disadvantages by utilising ultrasonics for cleaning offshore structures. Ultrasonic cleaning systems are currently used in many areas of industry, though have not previously been utilised in the cleaning of offshore structures. Conventional ultransonic cleaning methods typically utilise a water filled cleaning tank with ultrasonic transducers located in the base or walls of the tank. The transducers generate a continuous waveform of a sufficiently high signal amplitude to produce cavitation in the water. Objects to be cleaned are placed in the water. The temperature of the water may be controlled and chemical additives may be used to improve the cleaning rate. According to one aspect of the present invention there is provided a cleaning method comprising focusing an acoustic field adjacent the surface of an object to be cleaned which is immersed in a liquid, and producing cavitation in the liquid at the point of focus. The power, frequency and other parameters of the field may be varied according to the particular application of the method. If the method is used in a cleaning operation on an offshore structure the selected parameter will vary depending on the material to be removed: removal of hard fouling such as barnacles will tend to require less power than the removal of soft fouling, such as seaweed. Also, selected power levels will remove surface corrosion from metal structures. For use in cleaning offshore structures, the method is preferably implemented using a remotely operated vehicle (ROV) including means for producing the acoustic field and means for positioning the vehicle on the structure. As the method does not produce any significant reaction force it is not necessary to mount the vehicle as securely as a vehicle utilising, for example, an abrasive cleaning method. A vehicle for carrying out such a cleaning operation may also be provided with means for effecting physical removal of the fouling from the object being cleaned; the focussed acoustic field will loosen fouling, but may not actually remove the fouling from the object. The vehicle may further include appropriate sensors for use in determining the condition or integrity of the cleaned structure; Eddy current and magnetic particle inspection techniques may be utilised for location of surface breaking flaws, and ultrasonic techniques may be used for inspection of internal flaws and in flooded member detection. The location of the focus relative to the surface of the object to be cleaned may be moved during a cleaning operation to obviate any difficulty in accurately locating the focus at an optimum location for cleaning. In addition to cleaning the surface of an object, the method of the present invention also has application in the etching of surfaces, notably metals. By moving the focus of the acoustic field in a predetermined pattern over the surface of an object to be etched it is possible to produce a desired surface pattern or configuration. In a further aspect of the present invention there is provided apparatus for providing cavitation in a liquid, the apparatus comprising an array of transducer elements and control means for controlling operation of the transducer elements to provide a focussed acoustic field. The control means may produce a focussed acoustic field by time-delayed activation of selected transducer elements to provide constructive interference at a desired point of focus. In the preferred arrangement a plurality of transducer elements are located in a planar arrangement of concentric rings. Axial focussing of the acoustic outputs of the elements is achieved by use of time delay activation of the rings. Preferably, the control means can vary the time delay between the ring activation to allow axial movement of the focus. The transducer elements are preferably mass-spring mode piezoelectric transducer elements. The control means provides the excitation signal and, for the preferred arrangement, the necessary time delay focussing. Preferably, the acoustic field formed has a smooth, lobe free field profile with a sharply focussed central peak. Circular type arrays are preferred as they are relatively simple to focus, though square and rectangular the dimensional array structures may also be utilised, but would require more complex timing circuitry. In accordance with another aspect of the present invention there is provided a method of increasing the rate of ingestion of substances by fish and the like by feeding the fish with foodstuffs including a medication, and producing cavitation in the water adjacent the fish. As used herein, the term fish and the like is intended to encompass shellfish and crustaceans. The method has particular application in the treatment of farmed fish and the like in the treatment of immunisation against disease. These and other aspects of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic view of an acoustic cleaning system in accordance with a preferred embodiment of the present invention; Figure 2 is a view of a transducer array of the cleaning system of Figure 1; Figure 3 is a schematic representation of the array controller of the cleaning system of Figure 1. Figure 4 is a schematic illustration of the counter section of the array controller of Figure 3; and Figure 5 is a schematic representation of the memory device of the array controller of Figure 3. Reference is first made to Figure 1 of the drawings which illustrates, in schematic form, an acoustic cleaning system in accordance with a preferred embodiment of the present invention. Where the cleaning system is utilised in the removal of marine fouling of offshore structures, part of the system will be mounted on a remotely operable vehicle (ROV) capable of location on portions of a structure. Although not described in any detail herein, such a vehicle would also preferably be provided with mechanical cleaning means for brushing off fouling loosened through use of the illustrated system and also inspection apparatus for checking the integrity of the cleaned portions of structure. The system includes a transducer array 10, and array controller 12 and a master controller in the form of a micro-computer 14. Signals from the array controller 12 are also amplified by means of respective amplifiers 16. The transducer array 10 is illustrated in Figure 2 of the drawings and includes a planar, circular mounting structure 20 which accommodates a plurality of transducer elements 22 arranged in nine concentric rings. In operation, the array control provides axial focusing of the acoustic fields create by the individual elements. The array controller 12 generates the excitation signal and performs the necessary time delay focusing, both of which are variable. A nine channel, software controlled system operating from the micro-computer 14 separately drives each ring of the array. As will be described, each channel contains a memory section for storage of the digitized signal waveform for each transducer and a counter section, used for transmission control, is common to all channels. Focussing is achieved by relative time displacement of the excitation signals for each ring. Time delays for a given focussing situation are calculated by the system software, and included in the stored signal for the respective channels. The individual transducer elements 22 are spaced apart by one wave length. For high cavitation performance it is desirable that the system produces a smooth, lobe free field profile with a sharply focused central peak. Other, non circular array structures could be controlled to provide a field of this form, though, for example, square and rectangular two dimensional array structures require more complex electronic timing circuitry than a circular type array. The transducer elements 22 are each 25 millimetres in diameter and are mounted within a PVC transducer housing 24 strengthened by a steel back plate. The assembly is bolted together using stainless steel headcap screws around the periphery of the housing 24 and at selected positions on the face of the array 10. The transducer elements 22 are contained within a circular area of 570 millimetres diameter, while the array housing 24 has a diameter of 700 millimetres and a thickness of 100 millimetres. For complete flexibility in steering and focusing the acoustic field produced by the transducers, each transducer element 22 would require independent timing circuitry and signal amplification. However, a considerable reduction in electronic circuit complexity is available by simply employing axial focusing, with electronically variable focal length. Mass-spring mode piezoelectric transducer elements are utilised and allow a single power amplifier to provide effective excitation for up to six individual transducer elements. The elements 22 are arranged on the array 10 in concentric rings comprising multiples of six elements to minimise design and manufacturing costs. The centre element is grouped with the first ring which contain five elements, the remaining annuli containing N*6 elements, N being the ring number. The preferred embodiment has a total of 270 transducer elements. To meet cavitation requirements, each power amplifier unit 16 is capable of delivering 60 Watts electrical power to the respective six transducer elements. Each amplifier 16 is rated for a maximum input signal amplitude of 5V p-p and each amplifier unit is individually matched to the respective set of transducers, though testing has shown that an input amplitude level of 4V p-p is sufficient for production of cavitation at the transducer front face when all six elements are excited simultaneously. The array controller 12 generates the excitation signal for the transducer elements 22 and performs the necessary time delay focussing. Variable control of the excitation signal and time delay focussing is achieved using a software controlled, digital system using digital analogue converters at the system output stage to supply the transducers with the required analogue excitation signal, via the power amplifiers. The preferred controller 12, as illustrated in Figure 3 of the drawings contains a memory section 30 for storage of the digitized signal waveform, and a counter section 32 (Figure 4) for transmission timing control. The hardware is designed to interface with the micro-computer 14, in this example an IBM PS2, using software to calculate the signal and focussing parameters, load the memory and timing devices, and control start up and interruption of the transmission mode. Based on a 50kHz transducer signal, a 2MHz clock rate produces acceptable signal resolution. The control system is split into nine channels, each serving a separate array ring. Each channel contains a memory device 30 and a digital to analogue converter (DAC) 34. The memory employed is 8 bit x 8k samples, static, random access memory (RAM) 30. This is sufficient for signal pulse lengths of up to 4ms. The signal data thus stored in consecutive memory addresses and 8 bit data are capable of producing 256 signal levels, or, in bipolar range, +127. The full range of signal levels is designed to correspond to a +2.5V signal at the system output stage. As illustrated in Figure 5 of the drawings, every memory device 30 is buffered to separate the data pass used during loading and transmission loads. The counter section of the controller is common to all channels, and is illustrated in Figure 4 of the drawings. It contains a direct memory access controller (DMAC) 36 and programmable counters 38 for controlling the signal transmission. Using sixteen bit (4x4bit) programmable counters 38 to control the overall signal repetition rate, a period of up to 32ms is obtainable. When initialised, the DMAC 36 controls the memory location addresses for a complete pulse length, clocking at a rate of 2MHz. The operation is then halted for the remainder of the signal period. A controlled signal generated by the counters restarts DMAC operation and both the DMAC 36 and the counters 38 are automatically reloaded with the required parameters allowing a signal with controllable period and duty cycle to be maintained indefinitely. The DMAC 36 has a number of internal registers determining its operations and these are loaded from the micro-computer 14. Similarly, the counters 38 are programmed from the micro-computer 14 with the overall signal period. The system is programmed to produce a gated signal of variable period and pulse length and the software may generate a sinousidol or triangular waveform. An exponential on/off ramp may also be superimposed. In operation, the focus of the acoustic field is located preferably around .3 metres from the array 10, at the front face of the portion of the structure to be cleaned. The timing delays as calculated (Exact) , set by the hardware (Dig) , and measured at the analogue means (Meas) for 0.3 and also 0.6m and 1.0m focussing range are shown on table 1 below. As the effectiveness of the cleaning depends to a significant extent on the correct location of the focus of the field and the subsequent cavitation produced, in the majority of applications it is desirable to move the focus axially to ensure that, at least at one point, the focus is at the desired location. Testing will indicate the most appropriate amplitude and time period necessary to loosen different forms of fouling, to allow efficient operation for specific applications. The power required to drive the system will also be determined by the cleaning effect desired; increased power levels may lead to the removal of corrosion or corrosion protecting coatings from the elements of the structure being cleaned, in addition to loosening the marine fouling. The apparatus may also be utilised in other applications, including the etching of metals where relatively high power levels may be used to etch a metal surface. Through relative movement of the transducer array 10 and the object being etched, a desired surface appearance may be obtained. It has also been found that the system can be used to increase rates of ingestion of medications by farmed fish, although the mechanism which produces this advantageous effect has not been identified. TAB f: 1

Claims: CLAIMS ' 1. A cleaning or etching method comprising focusing an acoustic field adjacent the surface of an object to be cleaned or etched which is immersed in a liquid, and producing cavitation in the liquid at the point of focus. 2. The method of claim 1 including moving the location of the point of focus relative to the object to be clean or etched. 3. The method of claim 2 wherein the location of the focus is moved axially relative to the object. 4. The method of claim 2 wherein the location of the focus is moved in a predetermined path over the surface of the object to produce a desired surface pattern or configuration. 5. Apparatus for providing cavitation in a liquid comprising an array of transducer elements and control means for controlling operation of the transducer elements to provide a focused acoustic field. 6. The apparatus of claim 5 wherein the control means provides time-delayed activation of selected transducer elements to provide constructive interference at a desired point of focus. 7. The apparatus of claim 5 or 6 wherein the transducer elements are located in a planar arrangement of concentric rings. 8. The apparatus of claim 7 wherein the acoustic outputs of the transducer elements are axially focussed by time-delay activation of the rings. 9. The apparatus of claim 8 wherein the time-delay between ring activations is variable, to allow axial movement of the point of focus. 10. The apparatus of any one of claims 5 to 9 wherein the transducer elements are mass-spring mode piezo-electric transducer elements. 11. The apparatus of claim 10 wherein the control means provides the excitation signals for the transducer elements. 12. The apparatus of any one of claims 5 to 11 wherein the apparatus is mounted on a remotely operable vehicle including means for positioning the vehicle on a structure 13. The apparatus in combination with the remotely operable vehicle of claim 12 and further including means for effecting physical removal of fouling from a structure. 14. The apparatus in combination with the vehicle of claim 12 or 13 and further including sensors for determining the condition or integrity of a structure. 15. A method of increasing the rate of ingestion of substances by fish and the like by feeding the fish with feedstuffs including a medication, and producing cavitation in the water adjacent the fish.

Inventors: ALMAAS SIV GJENTRUDE, HAYWARD GORDON, ALMAAS, SIV, GJENTRUDE, HAYWARD, GORDON
Applicants: UNIV STRATHCLYDE, UNIVERSITY OF STRATHCLYDE
Application year: 1992
Main classification: B08B3
IPC codes: A01K61, B06B3, B08B3, B08B17, B63B59, C23F1, C23G5, E02B1, E02B17, G10K11