GB2260611A

GB2260611A - Acoustic scanning of underwater wall structures

Info

Publication number: GB2260611A
Application number: GB9221525A
Authority: GB
Inventors: Joseph Jackson; David Fitzherbert Jones
Original assignee: UMC International PLC
Current assignee: UMC International PLC
Priority date: 1991-10-16
Filing date: 1992-10-14
Publication date: 1993-04-21
Anticipated expiration: 2012-10-14
Also published as: GB9221525D0; GB9121992D0; GB2260611B

Abstract

Apparatus for scanning an underwater wall structure 8 having a metallic primary layer and optionally at least one secondary layer. e.g. of paint, comprises a wheeled vehicle 1 with means 5, 6 for holding the vehicle against the wall structure. Ultrasonic scanning means 9 and detection means are mounted on the vehicle and enable the electronic processing of ultrasonic echo signals reflected from the wall structure in order to determine e.g. its thickness. A method of scanning and underwater wall structure is also disclosed. <IMAGE>

Description

Scanning of underwater wall structures This invention relates to apparatus for scanning an underter wall structure having a metallic primary layer and optionally at least one secondary layer. In particular, but not exclusively, the underwater wall structure comprises the hull of a ship, in which case the metallic primary layer may have at least one protective layer, e.g. of paint or the like, applied thereto and constituting the secondary layer(s). The invention also relates to a method of, and a system for, scanning an underwater structure.

it is already known to employ acoustic measuring devices to measure wall thicknesses. For example, in eB-A- D2GO32, there is disclosed a measuring device in which a rotating acoustic transducer periodically emits a sequence of acoustical forces directionally into a borehole towards the borehole wall. Echo signals reflected back from the borehole wall are analysed and displayed on a cathode ray tube. Typically the display represents a mapping of the borehole wall thickness. However this known device is not intended to operate underwater and does not traverse across a surface to be scanned.

the present inventIon seeks to enable the scanning of an underwater, at least partly metallic, wall structure by arranging for ultrasonic scanning means to be mounted on an underwater vehicle which is movable over the wall structure to provide information regarding the wall structure.

According to one aspect of the present invention there is provided apparatus for scanning an underwater wall structure having a metallic primary layer and optionally at least one secondary layer, the apparatus comprising a wheeled vehicle having holding means for holding the vehicle against the underwater wall structure to be scanned, drive means for manoeuvring the vehicle over the underwater wall structure, ultrasonic scanning means mounted on the vehicle for emitting ultrasonic signals toward the wall structure and for generating sensing signals representative of ultrasonic echo signals detected after reflection from said wall structure and electronic processing means for processing said sensing signals to provide information about the scanned underwater wall structure.

The scanning of the wall structure enables the metallic primary layer thickness to be measured or mapped.

Alternatively, or additionally, the scanning of the wall structure enables defects in the primary layer to be detected and identified. Since the wheeled vehicle is able to manoeuvre under water and is able to be held against the underwater wall structure, the scanning of underwater wall structures can be achieved relatively easily and quickly.

In the case where the underwater wall structure is the hull of a ship, the primary layer comprises a metallic base layer of the hull having at least one coating (constituting at least one second layer) of paint, such as anti-fouling marine paint, applied thereon. in this case the ultrasonic scanning means may measure either the thickness of the metallic primary layer alone or the thicknesses of both the primary layer and the coating layer(s). Conveniently the primary layer thickness measurement is achieved by detecting, for each emitted ultrasonic signal, the time delay between successive ultrasonic echo signals reflected from the back surface of the metallic primary layer.

Several echo signals are generated for each emitted ultrasonic signal because part of any signal reflected from the back surface of the primary layer will be reflected back into the primary layer from its front surface for subsequent further reflection from the back surface of the primary layer. By measuring the delay between successive echo signals a measure of the thickness of the primary layer can be calculated. This method of measuring the time delay between successive echo signals is unsuited for the detection of the presence of pitting on the back surface of the primary layer. A more reliable method of detecting pitting is to compute the thickness of the primary and secondary layers.This is achieved by measuring and recording the time that elapses following a pulse emission for the first echo to be detected after reflection from the back surface (a "first time interval") and the time interval between the detection of first and second echo signals reflected from the back surface (a"second time interval ).

The thickness of the metallic primary layer plus the thickness of the secondary layer(s) are computed from the first time interval and the metallic primary layer thickness is computed from the second time interval. By subtracting the two computations from each other, a coating or secondary layer thickness can be computed. if the second time interval measurement fails, due to pitting of the back surface of the primary layer, but the first time interval measurement is successful, then the last known computed secondary layer thickness is deducted from the successful second time interval measurement to obtain a close approximation to the actual metallic primary layer thickness. Inaccuracies could occur in the unlikely event cf the scanned wall structure having both a non-linear, e.g.

pitted, primary layer back surface and a coating or secondary layer thickness which suddenly changes. However any such inaccuracies are minimised by the rapidly updated time interval measurements and in practice such inaccuracies are inconsequentiai.

Triggering means may be provided for enabling the processing of signals only after the vehicle has travelled predetermined distances over the underwater wall structure.

For example, the triggering means may control the emission of the ultrasonic signals, or the sensing of the reflected ultrasonic echo signals, so that signals are only emitted or detected after the vehicle has travelled the predetermined distances. Alternatively the triggering means may form part of the electronic processing means so that sensing signals are only processed after the vehicle has travelled the predetermined distances. Typically the triggering means will enable signals to be emitted, received and or processed at regular intervals of travel of the wheeled vehicle over the wall structure. Conveniently pulses are emitted at every 1 mm of travel, but this distance is not too critical and greater distances of up to 2 or 3 mm, e.g. 2.5 mm of travel have given adequate results. Suitably the triggering means includes a distance measuring transducer.

Conveniently the holding means comprises at least one rotatable member which, in use, is rotated either in contact with, or at a distance from, the underwater wall structure so as to generate sufficient suction to hold the vehicle against the wall structure. Typically, for example, the at least one rotatable member is designed to generate, in use, a suction force of up to 100 kilogram weight to hold the vehicle against the underwater wall structure. Preferably the or each rotatable member comprises a rotatable brush intended in use to contact the underwater wall structure.

In this case the rotatable member(s) is (are) positioned in front of the scanning means so as to clean the wall structure before it is scanned. Suitably the vehicle has a neutral or slightly positive buoyancy in sea water.

Preferably the electronic processing means is remote from the vehicle. For example, the electronic processing means, in use of the apparatus, is conveniently positioned out of the water. In this case, connection means, e.g. a cable or other data transmission link, is provided between the electronic processing means and the vehicle. Preferably the electronic processing means comprises a computer.

According to another aspect of the present invention, there is provided a method of scanning an underwater wall structure having a metallic primary layer and optionally at least one secondary layer, the method comprising moving an underwater vehicle over the underwater wall structure, directing ultrasonic signals towards the underwater wall structure, and detecting reflected ultrasonic echo signals received from the underwater structure, as the vehicle moves over the underwater wall structure with the aid of ultrasonic scanning means mounted on the underwater vehicle, and processing the detected ultrasonic echo signals so as to provide information regarding the scanned wall structure.

In particular, but not exclusively, the method is intended to enable the measurement of the primary layer thickness of the underwater wall structure.

According to a further aspect of the present invention there is provided a system for ultrasonically scanning an underwater wall structure having a metallic primary layer and optionally at least one secondary layer to provide information regarding the underwater wall structure.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which, Figure 1 is a schematic sectional view through a wheeled vehicle of apparatus according to the invention, and Figure 2 is a map representing a layer thickness of an underwater wall structure obtained with apparatus according to the present invention.

Figure 1 shows a wheeled underwater vehicle 1 designed to have a substantially neutral or slightly positive buoyancy when submerged in sea water. The vehicle 1 comprises a chassis 2 carrying a single steerable front wheel 3, a pair of rear wheels 4 (only one of which can be seen in Figure 1) and two rotatable brushes 5 which in use are designed to rotate in opposite directions. The brushes 5 are driven by hydraulic motors 6 and the front wheel 3 is driven by a hydraulic propulsion unit 7. Hydraulic power is delivered to the motors 6 and unit 7 from an above-water power pack (not shown) via an umbilical line (not shown).

The vehicle shown is designed to be diver-controlled and in use is intended to move over an underwater wall structure 8, e.g. a hull of a ship, having a metallic primary layer and optionally at least one non-metallic secondary layer, such as a coating of anti-fouling marine paint. However, the vehicle can also be modified so as to be remote-controlled.

The vehicle 1 carries at least one ultrasonic probe 9 mounted on the chassis 2. As shown, the or each probe 9 is pivotally mounted on the chassis 2 and is resiliently urged by spring means (not shown) into a scanning position (as shown) so as to be mounted a short distance away from the wall structure 8. If any surface obstructions are encountered as the vehicle moves over the wall structure 8, the probe(s) is (are) pivotally deflected out of the way against the resilient biasing of the spring means. However, other mounting arrangements (not shown) may be employed.

For example, the probe(s) may be mounted on a small subcarriage to give better control of the angle the or each probe sits relative to the surface being scanned. The main consideration is to keep the or each probe 9 at, or as near as possible to, 90" to the surface being scanned. The or each probe 9 is arranged behind the brushes 5 which, in use of the vehicle, rotate in front of the probe(s) 9 both to clean the underwater surface of the wall structure 7 before the probe(s) pass over the cleaned wall surface and in order to create a suction effect which causes the vehicle 1 to cling to the underwater wall structure 8. Typically up to i6 probes 9 are provided mounted a fixed distance, e.g.

between 50 to 100 mm, apart-.

The or each ultrasonic probe 9 suitably includes a crystal which vibrates at a known frequency for a known duration so as to emit an ultrasonic signal which is directed towards the underwater wall structure 8. This ultrasonic signal is reflected from the wall structure 8 and the crystal in the probe is caused to resonate on detecting the return echo signal or signals derived from each emitted ultrasonic signal. These detected or received echo signals are passed to a remote computer (not shown), or other electronic processing means, by means of a multi-core electric cable (not shown) connected to a junction box 10 mounted on the chassis 2. The computer is conveniently positioned above water and is programmed to process the signals received by the or each probe 9 and to control the emission of ultrasonic signals from the probe(s).

The vehicle 1 also carries a distance measuring transducer 11 mounted on the chassis 2. The transducer 11 comprises a wheeled device 12 which is rotated by one of the wheels 4 in order to sense the distance that the vehicle travels over the surface of the underwater structure 8.

Signals from the transducer 11 are supplied to the abovewater computer to enable data regarding the scanned underwater wall structure to be sensed or processed at regular distances. The above-water computer suitably comprises a proprietary desktop or portable computer. The above-water computer performs the ultrasonic signal generation and detection functions, relating the time between echoes received from the underwater wall structure 8 to the total thickness, or the metallic primary layer thickness, of the wall structure obviating the necessity to remove paint or other cladding from the wall structure to obtain the measurements. The computer is suitably selected to be capable of storing a single run of thickness measurements in internal battery-backed RAM before down loading the data to hard disk or other non-volatile storage media on completion of the run.However other computers, having larger or small memory capacities may be employed.

In use of the apparatus, the vehicle 1 is positioned against a ship's hull (or other underwater wall structure accessible only from one side) to be scanned. The brushes 5 are rotated so as to create a suction force, typically of up to 1GO kg weight, to hold or retain the vehicle against the ship's hull. The vehicle 1 is then propelled over the ship's hull, under the control of a diver, and typically in a number of passes, so as to cover a predetermined area of the hull. As the vehicle 1 advances over the hull, the rotating brushes 5 clean the water-contacting surface of the hull immediately in front of the vehicle mounted probe(s) so that a clean hull surface is scanned by the probe(s) 9.

Ultrasonic signals are emitted and detected many times per second, but the detected data is only stored to the abovewater computer on receipt of a trigger signal provided, or controlled, by the transducer 11.

The computer software for the above-water computer has three main functions, namely data acquisition, data processing and data storage. By way of example, it has been found convenient for the software to control the input to the ultrasonic probe(s) 9 and to measure the feedback from the probe(s). Signais are processed at regular intervals to calculate the desired wall or primary layer thickness of the wall structure being scanned. Each emitted ultrasonic signal is initially reflected from the back surface of the metallic primary layer as an echo signal. However, part of this reflected signal is subsequently reflected back into the primary layer from the front surface of the latter for further reflection from the back surface of the primary layer. Thus a number of echo signals, gradually decreasing in strength, are produced by each emitted ultrasonic signal.

In one embodiment, the software utilises the time interval between successive back wall echoes coupled with a velocity constant to calculate the "primary layer" thickness, whilst negating the effect of surface coatings. The velocity constant can be altered to provide accurate thickness measurements on different materials. The data may conveniently be processed with the aid of an algorithm which compares a specific thickness measurement with the two preceding measurements. If the variation in thickness measurement falls outside a set limit the reading is labelled "suspect". If no reading is obtained (excessive thickness or zero thickness) the reading is labelled "failed". The processed data is only stored to the RAM storage area after a trigger pulse is received indicating a set distance has been travelled by the vehicle. However the use of such an algorithm is not essential and all readings may be recorded and used.

A typical printout showing the metallic primary layer wall thickness of an underwater wall structure having a metallic base layer and a protective paint layer thereover is shown in Figure 2. This printout shows the scanned or sensed metallic wall thickness at intervals of 2.5 centimetres for a run length of the vehicle of 2.3 metres.

One hundred readings, numbered from 1 to 100, are shown, with the metallic wall thickness for each reading being given both numerically (in mm) and diagramtically. At the start of a survey the software allows the input and storage of title data such as Job Number, Vessel Name, Survey Location, Date, etc. This data is shown appended to the start of the run data shown in Figure 2. In addition each individual run has further data appertaining to the run location on the vessel, starting point and direction. A run can be stopped and re-started, or aborted and started again.

Alternative methods of storing data such as internal battery-backed RAM are also envisaged. In this case the PAM memory, when full, may be downloaded to a second computer, e.g. a PC or laptop computer.

Once the data for each run has been received and processed, the output may be printed or plotted to give a visual representation of the plate thickness along the length of the run. The data can also be analysed to give the percentage of failed readings, percentage of spurious readings, mean plate thickness per metre run, minimum thickness readings, etc..

The invention is primarily intended for measuring the thickness of a metallic base or primary layer of a ship's hull (which is normally provided with one or more coatings or secondary layers of, for example, paint, such as antifouling marine paint). Although this primary layer thickness can be measured as described above by measuring time intervals between successive echo signals, a more effective solution, especially when the back surface of the primary layer is pitted, is to measure both the time interval (the "first time interval") between the emission and receipt, after an initial back wall reflection, of an ultrasonic signal (i.e. the time delay between emission of the ultrasonic signal and the receipt of the first echo signal) and the time interval (the "second time interval") between receipt of the first and second echo signals.Thus in accordance with this further embodiment, two time intervals are measured for each emitted ultrasonic pulse.

In the case of a backing metallic primary layer and a covering secondary layer of paint, an emitted ultrasonic pulse passes through the secondary layer and into the primary layer where it is reflected from the back surface of the primary layer. At the front surface of the primary layer this reflected signal splits, with a part of the reflected signal passing on through the secondary layer to be detected, at a time tl (the first time interval) after the initial pulse emission, as a first echo signal and with the remaining part of the reflected signal being reflected from the front surface of the primary layer back into the primary layer.This re-reflected signal is reflected from the back surface of the primary layer where it is against partly split at the front surface of the primary layer, with a part of the signal passing on through the secondary layer to be detected as a second echo signal at a time t2 (the second time interval) after the detection of the first echo signal. From the first time interval tl the combined thicknesses of the primary and secondary layers can be computed. From the second time interval t2, the primary layer thickness can be computed. By deducting the computed primary layer thickness from the computed combined layer thickness, an "equivalent" secondary layer thickness can be computed.It should be realised that it is difficult to accurately measure the actual secondary layer thickness from these time measurements since the speed of sound through the metallic primary layer is used to calculate the secondary layer thickness and is different from the speed of sound through the secondary layer. Depending on the type of paint used for the secondary layer, the speed of sound travel through the secondary layer could vary between being slightly faster than, to about one quarter, the speed of sound travel through the primary layer.

If the second echo signal cannot be detected (due, for example, to back surface pitting of the metallic primary layer) but the first echo signal is detected, then the last calculated equivalent secondary layer thickness can be deducted from the time t1 of the first echo signal detection to obtain a close approximation of the actual primary layer thickness. Inaccuracies would then only occur if there was a sudden change in the secondary layer thickness in the area of the non-linear, e.g. pitted, back surface of the primary layer. However it is unlikely that a non-linear primary layer back surface and a sudden change in secondary layer u kness, e.g. a break in the secondary layer, would occur together.However, even if these events do occur together, the resulting inaccuracies are inconsequential in practice due to the rapid updating of the various layer calculations, e.g. after every 1 mm of travel of the scanning apparatus.

The first-mentioned technique of measuring the time between back surface echo signals for calculating the primary layer thickness cannot detect the presence of pitting on the back surface of the primary layer.

Accordingly the second-mentioned technique, which computes thicknesses of both the primary and secondary layers, provides a more reliable method of detecting the presence or pitting on the back surface of the metallic primary layer.

It will also be appreciated that instead of providing a "measure" of specific metallic layer thicknesses, the invention may have application in merely scanning an underwater wall structure to identify parts or areas of the wall structure where the metallic primary layer has a thickness below an acceptable minimum value.

Although the invention is primarily involved with scanning coated metallic hulls of ships, the invention also has application in scanning other underwater, at least partly metallic, wall structures, such as fixed marine instaliations.

Claims

1. Apparatus for scanning an underwater wall structure having a metallic primary layer and optionally at least one secondary layer, the apparatus comprising a wheeled vehicle having holding means for holding the vehicle against the underwater wall structure to be scanned, drive means for manoeuvring the vehicle over the underwater wall structure, ultrasonic scanning means mounted on the vehicle for emitting ultrasonic signals towards the wall structure and for generating sensing signals representative of ultrasonic echo signals detected after reflection from said wall structure and electronic processing means for processing said sensing signals to provide information about the scanned underwater wall structure.

2. Apparatus according to claims 1, including triggering means for enabling the processing of signals at predetermined distances of travel of the vehicle over the underwater wall structure.

3. Apparatus according to claim 2, in which the triggering means controls the emission of the ultrasonic signals, or the sensing of the reflected ultrasonic echo signals, so that signals are only emitted or detected after the vehicle has travelled the predetermined distances.

4. Apparatus according to claim 2, in which the triggering means forms part of the electronic processing means so that sensing signals are only processed after the vehicle has travelled the predetermined distances.

5. Apparatus according to any one of claims 2 to 4, in which the triggering means includes a distance measuring transducer.

6. Apparatus according to any one of the preceding claims, in which the holding means comprises at least one rotatable member which, in use, is rotated either in contact with, or at a distance from, the underwater wall structure so as to generate sufficient suction to hold the vehicle against the wall structure.

7. Apparatus according to claim 6, in which the or each rotatable member comprises a rotatable brush intended in use to contact the underwater wall structure.

8. Apparatus according to claim 7, in which the or each rotatable brush is positioned in front of the scanning means so as to clean the wall structure before the latter is scanned.

Apparatus according to any one of the preceding claims, in which the vehicle has a neutral or slightly positive buoyancy in sea water.

iG. Apparatus according to any one of the preceding claims, in which the electronic processing means is remote from the vehicle.

11. Apparatus according to claim 10, in which a cable or other data transmission link is provided between the electronic processing means and the vehicle.

i2. me method of scanning an underwater wall structure having a metallic primary layer and optionally at least one secondary layer, the method comprising moving an underwater vehicle over the underwater wall structure, directing ultrasonic signals towards the underwater wall structure, and detecting reflected ultrasonic echo signals received from the underwater structure, as the vehicle moves over the underwater wall structure, with the aid of ultrasonic scanning means mounted on the underwater vehicle, and processing the detected ultrasonic echo signals so as to provide information regarding the scanned wall structure.

13. A method according to claim 13, comprising detecting first and second echo signals derived from each ultrasonic signal directed towards the underwater wall structure.

14. A method according to claim 13, in which the detection of the ultrasonic first and second echo signals enables the calculation of the time interval between the first and second echo signals and the time interval between emission of the ultrasonic signal and the detection of the first echo signal.

15. A method according to any one of claims 12 to i, in which said signal processing provides a measure of the primary layer thickness of the underwater wall structure.

16. A system for ultrasonically scanning an underwater wall structure having a metallic primary layer and optionally at least one secondary layer to provide information regarding the underwater wall structure.

17. A system for ultrasonically scanning an underwater wall structure, substantially as hereinbefore described, and as illustrated in, Figures 1 and 2 of the accompanying drawings.

18. Apparatus for scanning an underwater wall structure substantially as herein described with reference to, and as illustrated in, Figure 1 of the accompanying drawings.