- Email: [email protected]
Identification and sizing of defects in metallic pipes by remote field eddy current inspection
Pergamon
Trenchless Technol. Res., Vol. 13. Nos. 1-2, pp. 17-27, 1998 BG Technology. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain SO886-7798(9H)00090-X 0886-7798/99/$ - see front matter 0 1999
PII:
Identification and sizing of defects in metallic pipes by remote field eddy current inspection Declan Robinson BG Technology, Loughborough
LEl I 3GR, UK
The remotefield eddy current (RFEC) techniqueis a through-wallnon-destructive method of evaluating pipelinesfor defectswhich can be usedin metallic pipes, both magneticand non-magnetic.Although pipe replacementpolicies in the UK have beensuccessful,corrodedpipe can be missedand sometimespipe with significant remaininglife hasbeenreplacedunnecessarily.When effective inspection devices become available essentialpipeline replacementand maintenanceprogrammescan be prioritised. The cost of theseprogrammeswill then be significantly reduced.This paper outlines the requirementsof a system suitablefor inspecting100mm diametermetallic pipe and gives a brief descriptionof some of the reportedapplicationsof the technique.The developmentroute being pursuedby the current author andthe resultsof initial laboratory testsandfield trials undertakenwith the prototype vehicle are described.Defectshave beensuccessfully detected in schedule80 steel and ductile iron pipe. Finally, the proposed future developmentof the technologyand how the miniaturisedvehicle will open up new market opportunitiesare mentioned.The RFEC inspectionsystemis currently under developmentat the company and it is expected that preliminary evaluation trials, in a number of defective pipelines,will take place in 1999. 0 1999BG Technology. Publishedby Elsevier ScienceLtd. All rights reserved
has been replaced unnecessarily. The effective management of this infrastructure demands the availability of a range of high-resolution, in-pipe, nondestructive testing (NDT) tools to detect and determine the size of significant instances of metal loss and cracks in the pipes. These results can then be used to prioritise essential pipeline replacement and maintenance programmes. The cost of these programmes, when tools with live inspection capability are available, will be significantly reduced. There is currently available a range of in-pipe inspection vehicles, based on ultrasonic and magnetic sensing, to carry out this work. However, existing technology restricts the use of these vehicles to pipes greater than 150 mm in diameter. A key factor in the future development of these tools is a reduction in size of the electronics and sensor assemblies to ahow inspection of currently inaccessible pipelines. The technology described herein aims to achieve this and offers the potential of a non-contact m(ethod which is of particular importance for the assessment of lined pipe. The inspection system must be compatible with
1 INTRODUCTION The gas distribution mains network represents an investment covering a period of over 100 years. The reliability of this network is crucial to industrial, commercial and domestic consumers. Iron mains form the major part of the distribution network (almost 50% or 123 000 km at present), the remainder being either steel or polyethylene. Three varieties of iron have been used for distribution mains: l l l
pit cast iron (40 to 110 years old); spun cast iron (less than 30 years old); and ductile iron (up to 24 years old).
The problems caused by corrosion are well documented; small areas of pitting can lead to leakage and large areas of general corrosion-internal or extemal+an weaken a pipe making it less able to resist stress. Subsequent pipe failure may lead to expensive unplanned pipe maintenance or catastrophic failure. Although pipe replacement policies have been successful, corroded pipe can be missed and sometimes pipe with significant remaining life 17
D. Robinson
18
the configurational and environmental constraints inherent in gas distribution systems. Target goals for the development of a practical system include the following. It should be potentially adaptable to 50, 150, 200 and 250 mm lines, after successful application to 100 mm systems, which is the next size down from 150 mm. It should be compatible with live launch top entry and keyhole threading technology, both of which are likely to be in common usage in the gas industry in the future. Live launch top entry technology will allow the inspection vehicle to be deployed under live conditions while the keyhole technology’ will allow threading of the live main with the vehicle winch cable. It should have a continuous inspection range of up to 200 m (i.e., a total inspection length of 400 m by approaching from both ends) in straight runs of typical gas main piping, or runs containing one-diameter (1D) bends (see Fig. 1) and protrusions of less than 15 mm. This range capability of the device, plus judicious selection of entry points, should provide effective inspection coverage. The system should also be capable of bi-directional operation; i.e., the ability to carry out an inspection run in both directions. The inspection system should work in conjunction with a pipe entry system which will permit insertion and deployment over the range of system pressures normally seen in gas distribution systems. A maximum operating pressure of up to 689 kN/m* would be desirable, but 207 kN/m* is the initial target goal.
The system should allow free flow of gas with minimal obstruction or pressure loss. It should be compatible with, and operate in the presence of, various liquid and solid contaminants found in distribution piping. These include rust, scale, hydrates, water and distillate. The system should be operable at line temperatures in the range range 0” C to 15” C, 100% humidity, and in the presence of liquid pools in the piping. The limitations of the existing equipment to the inspection of pipelines with diameters greater than 150 mm means that market opportunities for the sale of inspection services are severely limited. The length of pipes available for inspection with smaller diameter is much greater, and the development of pipeline inspection equipment with a capability to inspect pipes down to 50 mm will open up new market opportunities, especially in the areas of petrochemical plants and in power-generation sites, where the Icost of replacement of pipeline is prohibitive or access is restricted. In addition, there is increasing acceptance of the need for inspection for both reliability and regulatory reasons. The key issue for smaller-diameter high-resolution inspection devices of this type is higher-density electronics, allowing the advanced signal capture and processing functions to be achieved on-board. The ultimate objective of the work is to produce a remote field eddy current pig, with a 1D bend passing capability, that is capable of detecting and sizing defects in 100 mm diameter metallic pipe under live conditions. This paper aims to provide details on the approach ta.ken in the development of the prototype vehicle and. how an initial performance assessment of this technology was carried out by evaluation of laboratory and field trials. The prototype vehicle is restricted tlo inspecting straight pipes of 100 mm diameter. FGtture developments of this technology will utilise Multi-Chip Modules (MCM) to reduce the size of the electronics assemblies and create a device capable of negotiating a 1D bend and inspecting 100 mm mains pipelines with protrusions of up to 15 mm.
2 REPORTED TECHNIQ!UE
Fig. 1. A one-diameter bend in a schedule 80 steel pipe
APPLICATIONS
OF THE
The remote field eddy current (RFEC) technique is a through-wall non-destructive method of evaluat-
Identijcation
and sizing of defects in metallic pipes by remote jeld
ing pipelines for defects which can be used in metallic pipes, both magnetic and non-magnetic. The principle has been known for some time, but effective devices have only recently become viable. The system employs a coil, fed with a low-frequency signal, which generates an alternating magnetic field, and an array of detector coils, axially displaced from it. The received signal travels by two separate paths: the direct path and the indirect path which couples via the pipe wall. Each passage through the pipe wall will affect the amplitude and phase of the received remote field eddy current signal. The progress of the tool through the pipe can be monitored by using the data-capture software which communicates with the vehicle and records both the distance travelled and the angular rotation of the vehicle. Signals from the detector coils are communicated via an umbilical cord to a PC for storage to disk and subsequent analysis. Analysis of changes to the amplitude and phase of the signal allows various features to be identified and sized. The range of defects identifiable include internal and external metal loss features, slots and hard spots. Using the RFEC technique on steel pipes, variations in the noise level have been observed in samples with large permeabilities and conductivities.* Noise levels are also directly proportional to frequency,’ which means we are restricted to low operating frequencies. In addition, the permeability values may vary locally due to the manufacturing process of the pipe itself. For non-magnetic pipes, the permeability of the material is approximately that of free space, resulting in a much larger skin depth (see Section 6). It is therefore possible to increase the frequency of the signal to produce a larger signal at the receiver. As the area of remote field eddy current testing expands, academic institutions are developing models and experimental systems for use in a wide range of applications. - Typical applications are described in the sections below.
2.1 Heat exchanger tubes Y amaoka et al. have examined the responses produced from duplex and martensitic stainless steels.4 Estimation of the depth of thinning from the phase signal reportedly could be achieved with an error of 10% or less.
eddy current inspection
19
2.2 Nuclear reactor pressure tubes Sullivan and Atherton’ have examined the responses from zirconium-2.5% niobium (Zr-2.5% Nb). Results were produced from both circumferential and axial cracks up to a depth of 75% of the wall thickness. These results show the technique to offer the potential to detect cracks. 2.3 Theoretical
modelling systems
The finite element modelling systems identified in the literature include that of Atherton and his associates at Queen’s University, Colorados, who carry out all modelling using the MagNet 2 Infolyfica Corporation finite element program. A large proportion of the work produced by this research team is theoretical-based modelling only. 3 LIMITATIONS
OF CURRENT
SYSTEMS
Currently there are several commercial systems available, notably in Japan and the United States. The Japanese system is based on 50 mm steel pipe and has little or no on-board processing. This means that the signal-to-noise ratio is poor and the ultimate performance is declared to be less than for llux leakage. In order to achieve better noise immunity, the sensor signals in the vehicle being developed by the current author are amplified locally before being passed to the on-board processing modules. The modular system is designed to be straightforward to operate and can present logged data and analysis results on-site. The winching technology and processing expertise developed by BG Technology for inspection systems are additional key advantages to the system under development. 4 DEFECT
EVALUATION
0ne of the governing phenomena of the RFEC technique is the skin effect. This is the gradual attenuation of the alternating current signal due to induced currents in the material effectively limiting r.he area of unopposed passage. It has been shown that the equation* used to determine the magnitude of the skin effect is approximately true for a cylindrical ferromagnetic conductor, if the thickness of the conductor and its radius of curvature are several
D. Robinson
20
times greater than the skin depth. The skin depth, in this case, is a measure of how rapidly the wave is attenuated inside the pipe. The magnetic field in a semi-infinite conductor is given by the skin effect equation: B = B,exp(
- An&a}
(1)
sin{ 2n$ - ddnfpa}, where (Tesla) is the magnetic flux density at a depth d inside the conductor, BO is the magnetic flux density at the surface of the conductor, fis the test frequency (Hz), p (Henry/metre) is the magnetic permeability of the conductor, u (Siemens/metre) is the electrical conductivity of the conductor, and t is time. B
The skin depth S (metres) is defined as the depth into the conductor at which the phase of the magnetic field changes by 1 radian and is given by: 6 = lldnfpu.
(2)
The phase lag 4 (radians) in the skin effect equation is represented by: C$J= dhifjm
(3)
The eddy currents flow in the path of least impedance. If there is a crack present it will cause a concentration of eddy currents at the crack ends. The change in phase between the drive coil signal and the detector signal is examined and used to determine the size of the defect. 5 APPROACH
TO DEVELOPMENT
5.1 Prototype design Initial developments concentrated on proof of the principle, followed by the construction of a prototype pig system targeted at 100 mm schedule 80 (wall thickness 8.42 mm) steel distribution pipe. The combination of thick wall, high conductivity and magnetic permeability make this the most difficult material to inspect. The target inspection specification has been defined, for an inspection
rate of 6 m/min, as: 80% of metal loss defects should be measured with an accuracy of 20%; defects must be greater than 40 mm in length and 50 mm in width, deeper than 20.0% through the wall thickness and be greater than 0.15 m away from the weld or other pipeline features. Defects must be separated by more than 25.0 mm in any direction. The capacity to pass a one-diameter bend (see Fig. I ) will be incorporated later by utilising suitable packaging of the electronics and sensor assemblies. The RFlX pig system is towed by a winch with a speed control unit, which allows the speed to be varied in ,the range from 0.1 to 6 m/min. Fig. 2 shows a schematic representation of the doubleended prototype system which can be used to inspect straight sections of schedule 80 steel or ductile iron main. The pig comprises two electronics modules, two sensor rings, a power module and a drive coil. ‘The umbilical cable, for power and communications, is released over a distance wheel, connected to an encoder, which allows the distance the vehicle has travelled through the pipe to be determined accurately. In the demonstration rig, the pig was winched through a 3 m section test spool and successfully found a range of defects, which varied in size between 20% and 95% of the wall thickness. The defects in this test spool were separated by 400 mm and oriented at 120” from the centre line. All the defects were on the external surface of the pipe. A complete assessment of the resolution capability of the technology will be possible when the system has been used to inspect test spools with a range of defects at different orientation and spacing. This will include testing for axial and circumferential resolution and also a gun-barrel, stepped wall-thickness spool, to investigate whether wall thickness rnonitoring can be achieved. The most important design parameters are operating frequency, receiver/transmitter separation a.nd sensor coil number and size. The latter is defined by the accuracy and sensitivity required from the system. In this case, 24 detectors are used around the circumference of the pipe, to achieve the performance level required. A further enhancement was to deploy two rings of detectors equidistant from the central drive coil. By merging the amplitude and phase information from both rings it is possible to extract the signals generated when the drive coil passes a feature. This strategy builds in some redundancy, thus increasing reliability. Howe’ver, this means that the data processing rate
Identijkation
and sizing of defects in metallic pipes by remote fzeld eddy current
Pow& vnll
ShlUd
%d
inspection
21
Slave sensor ring
Fig. 2. Remote field eddy current prototype.
required is- higher than with a single ring system. This has been achieved by the appropriate design of the on-board electronics packages. The limiting factors are the bandwidth of the serial data link and the inspection speed. The preliminary laboratory data have been used to develop the data analysis (detection and sizing) software. The design of the miniaturised vehicle will be compatible with live launch top entry and keyhole threading technology, which is currently under development. The live launch technology allows access to gas mains for a range of technologies under live conditions while the keyhole technology allows a parachute system to thread the main. The design and construction of the RFEC vehicle allows it to be used in systems up to 200 kN/m2 gauge operating pressure.
5.2 Principle of operation The system employs a low-frequency coil, which generates an alternating magnetic field, and an array of detector coils displaced axially from it. The received signal attempts to travel by two separate paths: the direct path which is blocked by steel shielding and the indirect path which couples via the pipe wall. Each passage through the pipe wall will affect the amplitude and phase of the received eddy current signal. Fig. 3 shows a block diagram of the RFEC system. The data-capture software ensures that the received vehicle scan data are stored correctly. The analysis software indicates the depth of the metal loss and where it occurs in the pipe.
53 Module description The RFEC system can best be described by reference to its major components, each of which has a specific function. The components are described hlereafter. 53.1 Magnetic module A drive coil consisting of a.solid iron core and a copper coil generates a flux density of around 88 Gauss in air. The solid core intensifies magnetisation. The sensor ring for field detection contains 24 sensors mounted around the circumference of the sensor module. The sensor assembly is constructed so that the sensor coils are near the surface of the module, and permits compression of the sensor ring tlo allow the unit to accommodate variations in the inner diameter of the pipe. The sensors are calibrated on receipt to improve performance accuracy. All test runs were carried out using a 20 Hz signal to the drive coil. The vehicle length was the rninimum possible using the present design; i.e., 150 mm from the drive coil to receive coils. The stainless steel shield was positioned centrally between the drive and receive coils. The runs were carried out at a speed of approximately 1 m/min. The bend-passing device (still under development), with selectable sampling rates for different pipe materials, will allow operations at up to 6 rn/min. This is particularly important, as inspection ranges could be up to 400 m. The increased speed will alllow the inspection to be completed in a much shorter time and reduce the cost of the whole operaltion.
22
D. Robinson
sKlsof ring
Slave pfocessing
POWK
Unit
dCctrtiCS
Fig. 3. Bfock diagramof the remote field eddy current vehicle.
5.3.2 Electronic module The electronic module contains several circuit boards that provide and monitor the excitation to the sensors and process the returning signals. The drive coil current must be kept constant as it dictates the field at the pipe wall. Delrin, the coil former material, has a maximum working temperature of 90°C and this is sufficient to achieve the required performance. The signal processing requirements for the RFlZC equipment are illustrated in Fig. 4. The analogue signal, received from the sensor coil, is preampli-
fied, further amplified in two gain stages and used as an input to the subsequent amplitude and phase processing circuits. The sensor signal is averaged using a root mean square to direct current converter, the output ripple from this averaging function being removed by filtering prior to analogue-to-digital conversion. The phase difference between the sensor signal and the exc:itation reference signal is extracted via counter circuitry implemented in a field programmable gate array (FPGA). This information is combined with the amplitude information, and trans-
cl
Sensor Coil
Electronicsmodule
U
Coil
Fig. 4. Block diagramof electronicsproc’essing.
Identification
and sizing of defects in metallic pipes by remote field eddy current
ferred via an RS-485 data link from a central microprocessor located in the electronic pack module (see Fig. 4). The resolution of the amplitude and phase processing systems is 12 bits, giving sufficient detail to enable subsequent processing algorithms to detect variations in pipe wall thickness. 5.3.3 Power module Electrical power is provided by a generator at the launch site, and is transmitted by an umbilical cable which also connects the electronics module to the personal computer. The cables for incoming power and outgoing data are separated within the power module. A regulator supplies power for the electronics module. The umbilical cable is released through a standard shaft encoder; the encoder produces a pulse train which allows the distance the vehicle has travelled through the pipe to be determined. The data are assembled and formatted and an RS485 serial data link is employed to transmit the information along the tether. The base station (for logging and processing) uses a standard personal computer, for economy and reliability. The interface unit contains a loo/50 V power supply, a direction discriminator and circuit boards for processing the signal from the distance-measuring equipment. There are two serial interfaces, an input for the distance transducer and a line in for power. 53.4 Sensor module The vehicle is designed to inspect both schedule 80 and ductile iron (DI) distribution mains of 100 mm nominal bore. The inside diameters of schedule 80 and DI are 97 mm and 106 mm, respectively. The signal at the pipe wall, received by the sensor coil, is very small and a close sliding tolerance between the sensor module and the pipe wall is necessary for optimum performance. Each sensor coil pair is mounted on a polyurethane sledge (Fig. 5) which allows the unit to
connection
23
compress and negotiate protrusions that are up to 16% of the internal diameter. The unit then recovers to full height. 5.3.5 Umbilical cable The umbilical cable, which consists of a seven-core cable surrounded by armoury, delivers power to the vehicle and provides base station-vehicle communications. The cable has a working load rating of 10.3 kN. 53.6 Vehicle winches The laboratory-based experiments with the vehicle, to detect metal loss on spool samples of short length of pipe (3.5 m), were carried out with a 0.5 tonne electric winch operating from a 12 V battery. A variable-speed control unit allowed the speed to be varied in the range 0.1-6 m/min. A large, variable-speed, diesel-engine-driven winch with a capacity of 3.7 tonnes was used for field trials. The maximum line pull recorded was 100 kgf during a 60 m line pull. 53.7 Material selection Deli-in, the coil former material, was selected because it has good electrical insulation and excellent machinability properties. During operation, the drive coil assembly was found to raise the temperature of the Dehin encasement to 70°C. The recommended operating temperature for Delrin lies between - 50°C and + 90°C in air, with intermittent use up to 160°C. The mlaterial specification for the effects of heat under load, moisture, stress and creep will be taken into account for the bend-passing device. 563.8 Analysis equipment The serial data are passed to the base station through one of the serial ports at a rate of 72 kbaud. The progress of the tool is monitored using the data-capture software which communicates with the vehicle via the base station. The software is Windows 95 executable and written in Visual C f + (version 5.0).
self 8djusting nabber sledge -
inspection
to sensor
intcreonneet
board Fig. 5. Detection semor coil pair.
24
D. Robinson
The following information is displayed when run in the data-capture mode: distance travelled by the vehicle since launch in metres; number of scans received; rotation of the vehicle in degrees; drive coil current value in amps; channel status; and all phase and amplitude channels selected. The software has the ability to expand a single trace to fill the trace area. Traces are updated as data are received from the vehicle. The computer used for the analysis software is a Pentium 200 MHz machine which allows stability and speed when executing the data analysis software. 53.9 Analysis software Raw data are streamed back from the remote field eddy current vehicle and recorded with the data-capture software (see Fig. 6). These data are subsequently analysed in a “stand-alone” RFJX analysis software package. The first stage of analysis involves generating a formatted version of the data using calibration data from the sensors. Normalisation of the data is car-
ried out next, thus removing the transient background signal. At this stage defects are detectable by visual analysis, appearing as peaks against an approximately constant background signal. Algorithms are being developed to automatically detect these defect peaks and calculate their signal parameters, i.e., amplitude, width and length. These values will then be passed into a sizing model to predict defect dimensions. The sizing model will be developed when the automatic detection is completed, and a consistent method of extracting the peak parameters is available. Data collected in the laboratory from machined defects of known size will be used to derive the model, using regression techniques and knowledge of the fundamental physics of the technique. The model will be modified and fine-tuned when acceptable data from real defects in the field are available. The field data to date did not contain any defects and were noisy in some cases. 6 LABORATORY PROCEDURE
SET-UP AND TEST
Different winching speeds were assessed for their ease of monitoring and the clarity of the signals
Fig. 6. Typical view of data using analysis software. Note that defect sites are characterised by peaks in the amplitude and phase data.
Identi$cation
and sizing of defects in metallic pipes by remote field eddy current
produced. It was decided to carry out the data runs with the prototype at 1-2 m/min. This speed ensured that no loss of data occurred. The optimum frequency level was obtained by experimentally increasing the frequency from zero to 50 Hz and viewing the resultant phase change. It was found that a frequency of around 20 Hz produced the maximum phase change. The average pipe wall thickness of the ductile iron defect spools used was 7.75 mm and the nominal diameter was 100 mm. The defect dimensions for the test spool referred to in Figs 6 and 7 were as follows: Distance (mm> 600 1000 1400 1800 2200 2600
Rotation (degrees) 0 120 240 0 120 240
Length (mm) 100 45 75 60 90 30
Width (mm) 55 55 55 55 55 55
% Depth 45 45 45 45 45 40
This ductile iron test spool has an overall length of 3200 mm. The distances are measured from the start of the test spool in millimetres and the rotation given in degrees from the centre line (top dead centre). All the defects are on the external surface of the pipe and the % depth was based on the average overall wall thickness value for the whole spool. The Analysis Software (see Fig. 6) was used to display the amplitude and phase data from all channels. The peaks in the data can easily be located at this stage of the processing. Each peak located was then measured and these measured values were used to determine the size of the defects that the peaks represent. The percentage depth corresponds to the height of a peak, the length of a defect corresponds to the peak width and the defect width is represented by the number of channels the defect spans. Data obtained from laboratory runs on the machined defect spools have confirmed the theoretical linear relationship between phase and defect depth (see Fig. 7). The defects could be reliably detected at a through-wall depth down to 40%. The smallest test spool defect consistently identifiable had dimensions of 3 1% of metal loss, length 45 mm and width 50 mm.
7 FIELD
inspection
25
OPERATIONS
7.1 Site preparation The following is an outline description of how the prototype RFEC tool is used in the field. The only excavation required is a 4 m long by 1.5 m wide trench at the beginning and end of the inspection run. The section of main must be decommi:ssioned and full-bore entry/exit is required. A camera inspection is carried out to check that the section of pipeline is free from obstru&ions. The malin is threaded with a lightweight rod and a test pass is performed with a cleaner device before the actual inspection commences. This checks the pipe for protrusions. The launch tray, 3 m in length, is placed at the entrance to the main. The vehicle is placed in the tray and pulled through the pipe at a speed of approximately l-2 m/min (see Fig. 8). The progress of the tool can be monitored by using the data-capture software. 7.2 Field trials An initial analysis of the data collected in the field trial was performed. Of the six machined defects, four were consistently identifiable by visual analysis of the data, the smallest of these being a discshaped defect, with dimensions of 35% of the wall thickness and 25 mm radius. Substantial end effects were observed, due to wall thickening at the end of each pipe length. This is potentially very useful as it means the system could be used to monitor wall thickness variations. Further research is required to investigate the potential of this application.
8 FUTURE
BENEFITS
TO INDUSTRY
The UK gas distribution network consists of 255 000 km of mains pipelines of which 60% is between 50 mm and 150 mm in diameter. This characteristic of the UK distribution network is “mirrored” in the European and American market, and the ability to address the gas pipeline inspection requirement for the smaller pipe diameters is expected to increase. Gas companies with similar mature distribution systems will value an RFEC system that has the potential to inspect live gas mains. MCM is an electronic packaging solution on a
D. Robinson
26
4-3.7
43.8
3.9
Distance
4.0
1 4.1
1 4.2
(m)
3.7
3.8
1 3.9
Distance
4.0
8 4.1
, 4.2
(m)
Fig. 7. Typical view of a defect using Analysis Software. * indicates the location of the centre of defects. The defect spans seven channels and represents 40% metal loss.
Fig. 8. Equipment arrangement for field operation.
substrate utilising bare die which minimises the space required for electronic components. The MCM module consists of an analogue front end, a digital processing section and connections to the main electronics processing module. This technology will allow the implementation of a frequency-selectable option for the drive coil, self-test and in situ calibration of the sensor coils. The crucial factor in the selection of an appropriate component technology for the miniaturisation of the RFEK vehicle is the minimum space requirements for the processing of the sensor signals. This space limitation requires two sensor channels to be processed in a circuit board area of 8 mm X 20 mm. The detected remote field signal is amplified and processed by a microprocessor for phase and amplitude information. Remote field information may be present in signals ranging up to 1 kHz, depending on the pipe material being inspected. The MCM
module is one of 24 in a ring of sensors. The phase and amplitude information acquired from each module is transmitted to an off-module microprocessor.
9 CONCLUSIONS The RFEC technology is still under development and only when the MCM vehicle is operational and evaluations have been carried out, both in the laboratory and the field, can a complete performance assessment be made. The following conclusions are made from the results of the initial laboratory tests and field trials with the prototype vehicle. a Using the present design, defects have been successfully detected in schedule 80 steel and ductile iron pipe in the laboratory and field trials.
Identijcation
and sizing of defects in metallic pipes by remote ,field eddy current
The smallest defect, from the field trial data, consistently identifiable by visual analysis was a disc-shaped defect, with dimensions of 35% of wall thickness and 25 mm radius. Further testing is required to investigate the detection limits of the prototype system. Further field testing with the double-ended system, in pipelines with metal loss, is necessary before a complete performance assessment of the vehicle can be made. The prototype system meets the specification for inspection of straight sections of 100 mm schedule 80 steel and ductile iron pipe. By further miniaturisation of the technology, the system can be used to inspect 100 mm pipework with 1D bends and protrusions of up to 15 mm. The bend-passing device, with a frequency-selectable option and utilising MCM technology, will allow the inspection of different pipe materials and operation at speeds of up to 6 m/min. The development of the miniaturised device will open up market opportunities, especially in the
inspection
27
areas of petrochemical plants and power-generation sites.
REFERENCES 1. Arunakumar, G., Live gas main repair via keyhole excavation. Trenchless Technology Research, Tunnelling and Underground Space Technology, 1998, 13(Sl-2), 29-36. 2. Lord, W., A survey of electromagnetic methods of non-destructive testing. In Mechanics of Non-destructive Testing, ed. W. W. Stinchcomb. Plenum Press, New York, 1980, pp. 77-100. 3. Schmidt, T. R., The remote field eddy current inspection Materials Evaluation, technique. 1983, 42(March), 225-230. 4. Yamaoka, S., Inoue, N. and Kimoto, S., Investigations using remote field eddy current defect testing of ferromagnetic stainless steel heat exchanger tubes. Journal of Non-Destructive Inspection, 1992, 41(7), 406-413. 5. Sullivan, S. and Atherton, D. L., Analysis of the remote field eddy current effect in nonmagnetic tubes. Materials Evaluation, 1989, 47(l), 80-86.
Trenchless Technol. Res., Vol. 13. Nos. 1-2, pp. 17-27, 1998 BG Technology. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain SO886-7798(9H)00090-X 0886-7798/99/$ - see front matter 0 1999
PII:
Identification and sizing of defects in metallic pipes by remote field eddy current inspection Declan Robinson BG Technology, Loughborough
LEl I 3GR, UK
The remotefield eddy current (RFEC) techniqueis a through-wallnon-destructive method of evaluating pipelinesfor defectswhich can be usedin metallic pipes, both magneticand non-magnetic.Although pipe replacementpolicies in the UK have beensuccessful,corrodedpipe can be missedand sometimespipe with significant remaininglife hasbeenreplacedunnecessarily.When effective inspection devices become available essentialpipeline replacementand maintenanceprogrammescan be prioritised. The cost of theseprogrammeswill then be significantly reduced.This paper outlines the requirementsof a system suitablefor inspecting100mm diametermetallic pipe and gives a brief descriptionof some of the reportedapplicationsof the technique.The developmentroute being pursuedby the current author andthe resultsof initial laboratory testsandfield trials undertakenwith the prototype vehicle are described.Defectshave beensuccessfully detected in schedule80 steel and ductile iron pipe. Finally, the proposed future developmentof the technologyand how the miniaturisedvehicle will open up new market opportunitiesare mentioned.The RFEC inspectionsystemis currently under developmentat the company and it is expected that preliminary evaluation trials, in a number of defective pipelines,will take place in 1999. 0 1999BG Technology. Publishedby Elsevier ScienceLtd. All rights reserved
has been replaced unnecessarily. The effective management of this infrastructure demands the availability of a range of high-resolution, in-pipe, nondestructive testing (NDT) tools to detect and determine the size of significant instances of metal loss and cracks in the pipes. These results can then be used to prioritise essential pipeline replacement and maintenance programmes. The cost of these programmes, when tools with live inspection capability are available, will be significantly reduced. There is currently available a range of in-pipe inspection vehicles, based on ultrasonic and magnetic sensing, to carry out this work. However, existing technology restricts the use of these vehicles to pipes greater than 150 mm in diameter. A key factor in the future development of these tools is a reduction in size of the electronics and sensor assemblies to ahow inspection of currently inaccessible pipelines. The technology described herein aims to achieve this and offers the potential of a non-contact m(ethod which is of particular importance for the assessment of lined pipe. The inspection system must be compatible with
1 INTRODUCTION The gas distribution mains network represents an investment covering a period of over 100 years. The reliability of this network is crucial to industrial, commercial and domestic consumers. Iron mains form the major part of the distribution network (almost 50% or 123 000 km at present), the remainder being either steel or polyethylene. Three varieties of iron have been used for distribution mains: l l l
pit cast iron (40 to 110 years old); spun cast iron (less than 30 years old); and ductile iron (up to 24 years old).
The problems caused by corrosion are well documented; small areas of pitting can lead to leakage and large areas of general corrosion-internal or extemal+an weaken a pipe making it less able to resist stress. Subsequent pipe failure may lead to expensive unplanned pipe maintenance or catastrophic failure. Although pipe replacement policies have been successful, corroded pipe can be missed and sometimes pipe with significant remaining life 17
D. Robinson
18
the configurational and environmental constraints inherent in gas distribution systems. Target goals for the development of a practical system include the following. It should be potentially adaptable to 50, 150, 200 and 250 mm lines, after successful application to 100 mm systems, which is the next size down from 150 mm. It should be compatible with live launch top entry and keyhole threading technology, both of which are likely to be in common usage in the gas industry in the future. Live launch top entry technology will allow the inspection vehicle to be deployed under live conditions while the keyhole technology’ will allow threading of the live main with the vehicle winch cable. It should have a continuous inspection range of up to 200 m (i.e., a total inspection length of 400 m by approaching from both ends) in straight runs of typical gas main piping, or runs containing one-diameter (1D) bends (see Fig. 1) and protrusions of less than 15 mm. This range capability of the device, plus judicious selection of entry points, should provide effective inspection coverage. The system should also be capable of bi-directional operation; i.e., the ability to carry out an inspection run in both directions. The inspection system should work in conjunction with a pipe entry system which will permit insertion and deployment over the range of system pressures normally seen in gas distribution systems. A maximum operating pressure of up to 689 kN/m* would be desirable, but 207 kN/m* is the initial target goal.
The system should allow free flow of gas with minimal obstruction or pressure loss. It should be compatible with, and operate in the presence of, various liquid and solid contaminants found in distribution piping. These include rust, scale, hydrates, water and distillate. The system should be operable at line temperatures in the range range 0” C to 15” C, 100% humidity, and in the presence of liquid pools in the piping. The limitations of the existing equipment to the inspection of pipelines with diameters greater than 150 mm means that market opportunities for the sale of inspection services are severely limited. The length of pipes available for inspection with smaller diameter is much greater, and the development of pipeline inspection equipment with a capability to inspect pipes down to 50 mm will open up new market opportunities, especially in the areas of petrochemical plants and in power-generation sites, where the Icost of replacement of pipeline is prohibitive or access is restricted. In addition, there is increasing acceptance of the need for inspection for both reliability and regulatory reasons. The key issue for smaller-diameter high-resolution inspection devices of this type is higher-density electronics, allowing the advanced signal capture and processing functions to be achieved on-board. The ultimate objective of the work is to produce a remote field eddy current pig, with a 1D bend passing capability, that is capable of detecting and sizing defects in 100 mm diameter metallic pipe under live conditions. This paper aims to provide details on the approach ta.ken in the development of the prototype vehicle and. how an initial performance assessment of this technology was carried out by evaluation of laboratory and field trials. The prototype vehicle is restricted tlo inspecting straight pipes of 100 mm diameter. FGtture developments of this technology will utilise Multi-Chip Modules (MCM) to reduce the size of the electronics assemblies and create a device capable of negotiating a 1D bend and inspecting 100 mm mains pipelines with protrusions of up to 15 mm.
2 REPORTED TECHNIQ!UE
Fig. 1. A one-diameter bend in a schedule 80 steel pipe
APPLICATIONS
OF THE
The remote field eddy current (RFEC) technique is a through-wall non-destructive method of evaluat-
Identijcation
and sizing of defects in metallic pipes by remote jeld
ing pipelines for defects which can be used in metallic pipes, both magnetic and non-magnetic. The principle has been known for some time, but effective devices have only recently become viable. The system employs a coil, fed with a low-frequency signal, which generates an alternating magnetic field, and an array of detector coils, axially displaced from it. The received signal travels by two separate paths: the direct path and the indirect path which couples via the pipe wall. Each passage through the pipe wall will affect the amplitude and phase of the received remote field eddy current signal. The progress of the tool through the pipe can be monitored by using the data-capture software which communicates with the vehicle and records both the distance travelled and the angular rotation of the vehicle. Signals from the detector coils are communicated via an umbilical cord to a PC for storage to disk and subsequent analysis. Analysis of changes to the amplitude and phase of the signal allows various features to be identified and sized. The range of defects identifiable include internal and external metal loss features, slots and hard spots. Using the RFEC technique on steel pipes, variations in the noise level have been observed in samples with large permeabilities and conductivities.* Noise levels are also directly proportional to frequency,’ which means we are restricted to low operating frequencies. In addition, the permeability values may vary locally due to the manufacturing process of the pipe itself. For non-magnetic pipes, the permeability of the material is approximately that of free space, resulting in a much larger skin depth (see Section 6). It is therefore possible to increase the frequency of the signal to produce a larger signal at the receiver. As the area of remote field eddy current testing expands, academic institutions are developing models and experimental systems for use in a wide range of applications. - Typical applications are described in the sections below.
2.1 Heat exchanger tubes Y amaoka et al. have examined the responses produced from duplex and martensitic stainless steels.4 Estimation of the depth of thinning from the phase signal reportedly could be achieved with an error of 10% or less.
eddy current inspection
19
2.2 Nuclear reactor pressure tubes Sullivan and Atherton’ have examined the responses from zirconium-2.5% niobium (Zr-2.5% Nb). Results were produced from both circumferential and axial cracks up to a depth of 75% of the wall thickness. These results show the technique to offer the potential to detect cracks. 2.3 Theoretical
modelling systems
The finite element modelling systems identified in the literature include that of Atherton and his associates at Queen’s University, Colorados, who carry out all modelling using the MagNet 2 Infolyfica Corporation finite element program. A large proportion of the work produced by this research team is theoretical-based modelling only. 3 LIMITATIONS
OF CURRENT
SYSTEMS
Currently there are several commercial systems available, notably in Japan and the United States. The Japanese system is based on 50 mm steel pipe and has little or no on-board processing. This means that the signal-to-noise ratio is poor and the ultimate performance is declared to be less than for llux leakage. In order to achieve better noise immunity, the sensor signals in the vehicle being developed by the current author are amplified locally before being passed to the on-board processing modules. The modular system is designed to be straightforward to operate and can present logged data and analysis results on-site. The winching technology and processing expertise developed by BG Technology for inspection systems are additional key advantages to the system under development. 4 DEFECT
EVALUATION
0ne of the governing phenomena of the RFEC technique is the skin effect. This is the gradual attenuation of the alternating current signal due to induced currents in the material effectively limiting r.he area of unopposed passage. It has been shown that the equation* used to determine the magnitude of the skin effect is approximately true for a cylindrical ferromagnetic conductor, if the thickness of the conductor and its radius of curvature are several
D. Robinson
20
times greater than the skin depth. The skin depth, in this case, is a measure of how rapidly the wave is attenuated inside the pipe. The magnetic field in a semi-infinite conductor is given by the skin effect equation: B = B,exp(
- An&a}
(1)
sin{ 2n$ - ddnfpa}, where (Tesla) is the magnetic flux density at a depth d inside the conductor, BO is the magnetic flux density at the surface of the conductor, fis the test frequency (Hz), p (Henry/metre) is the magnetic permeability of the conductor, u (Siemens/metre) is the electrical conductivity of the conductor, and t is time. B
The skin depth S (metres) is defined as the depth into the conductor at which the phase of the magnetic field changes by 1 radian and is given by: 6 = lldnfpu.
(2)
The phase lag 4 (radians) in the skin effect equation is represented by: C$J= dhifjm
(3)
The eddy currents flow in the path of least impedance. If there is a crack present it will cause a concentration of eddy currents at the crack ends. The change in phase between the drive coil signal and the detector signal is examined and used to determine the size of the defect. 5 APPROACH
TO DEVELOPMENT
5.1 Prototype design Initial developments concentrated on proof of the principle, followed by the construction of a prototype pig system targeted at 100 mm schedule 80 (wall thickness 8.42 mm) steel distribution pipe. The combination of thick wall, high conductivity and magnetic permeability make this the most difficult material to inspect. The target inspection specification has been defined, for an inspection
rate of 6 m/min, as: 80% of metal loss defects should be measured with an accuracy of 20%; defects must be greater than 40 mm in length and 50 mm in width, deeper than 20.0% through the wall thickness and be greater than 0.15 m away from the weld or other pipeline features. Defects must be separated by more than 25.0 mm in any direction. The capacity to pass a one-diameter bend (see Fig. I ) will be incorporated later by utilising suitable packaging of the electronics and sensor assemblies. The RFlX pig system is towed by a winch with a speed control unit, which allows the speed to be varied in ,the range from 0.1 to 6 m/min. Fig. 2 shows a schematic representation of the doubleended prototype system which can be used to inspect straight sections of schedule 80 steel or ductile iron main. The pig comprises two electronics modules, two sensor rings, a power module and a drive coil. ‘The umbilical cable, for power and communications, is released over a distance wheel, connected to an encoder, which allows the distance the vehicle has travelled through the pipe to be determined accurately. In the demonstration rig, the pig was winched through a 3 m section test spool and successfully found a range of defects, which varied in size between 20% and 95% of the wall thickness. The defects in this test spool were separated by 400 mm and oriented at 120” from the centre line. All the defects were on the external surface of the pipe. A complete assessment of the resolution capability of the technology will be possible when the system has been used to inspect test spools with a range of defects at different orientation and spacing. This will include testing for axial and circumferential resolution and also a gun-barrel, stepped wall-thickness spool, to investigate whether wall thickness rnonitoring can be achieved. The most important design parameters are operating frequency, receiver/transmitter separation a.nd sensor coil number and size. The latter is defined by the accuracy and sensitivity required from the system. In this case, 24 detectors are used around the circumference of the pipe, to achieve the performance level required. A further enhancement was to deploy two rings of detectors equidistant from the central drive coil. By merging the amplitude and phase information from both rings it is possible to extract the signals generated when the drive coil passes a feature. This strategy builds in some redundancy, thus increasing reliability. Howe’ver, this means that the data processing rate
Identijkation
and sizing of defects in metallic pipes by remote fzeld eddy current
Pow& vnll
ShlUd
%d
inspection
21
Slave sensor ring
Fig. 2. Remote field eddy current prototype.
required is- higher than with a single ring system. This has been achieved by the appropriate design of the on-board electronics packages. The limiting factors are the bandwidth of the serial data link and the inspection speed. The preliminary laboratory data have been used to develop the data analysis (detection and sizing) software. The design of the miniaturised vehicle will be compatible with live launch top entry and keyhole threading technology, which is currently under development. The live launch technology allows access to gas mains for a range of technologies under live conditions while the keyhole technology allows a parachute system to thread the main. The design and construction of the RFEC vehicle allows it to be used in systems up to 200 kN/m2 gauge operating pressure.
5.2 Principle of operation The system employs a low-frequency coil, which generates an alternating magnetic field, and an array of detector coils displaced axially from it. The received signal attempts to travel by two separate paths: the direct path which is blocked by steel shielding and the indirect path which couples via the pipe wall. Each passage through the pipe wall will affect the amplitude and phase of the received eddy current signal. Fig. 3 shows a block diagram of the RFEC system. The data-capture software ensures that the received vehicle scan data are stored correctly. The analysis software indicates the depth of the metal loss and where it occurs in the pipe.
53 Module description The RFEC system can best be described by reference to its major components, each of which has a specific function. The components are described hlereafter. 53.1 Magnetic module A drive coil consisting of a.solid iron core and a copper coil generates a flux density of around 88 Gauss in air. The solid core intensifies magnetisation. The sensor ring for field detection contains 24 sensors mounted around the circumference of the sensor module. The sensor assembly is constructed so that the sensor coils are near the surface of the module, and permits compression of the sensor ring tlo allow the unit to accommodate variations in the inner diameter of the pipe. The sensors are calibrated on receipt to improve performance accuracy. All test runs were carried out using a 20 Hz signal to the drive coil. The vehicle length was the rninimum possible using the present design; i.e., 150 mm from the drive coil to receive coils. The stainless steel shield was positioned centrally between the drive and receive coils. The runs were carried out at a speed of approximately 1 m/min. The bend-passing device (still under development), with selectable sampling rates for different pipe materials, will allow operations at up to 6 rn/min. This is particularly important, as inspection ranges could be up to 400 m. The increased speed will alllow the inspection to be completed in a much shorter time and reduce the cost of the whole operaltion.
22
D. Robinson
sKlsof ring
Slave pfocessing
POWK
Unit
dCctrtiCS
Fig. 3. Bfock diagramof the remote field eddy current vehicle.
5.3.2 Electronic module The electronic module contains several circuit boards that provide and monitor the excitation to the sensors and process the returning signals. The drive coil current must be kept constant as it dictates the field at the pipe wall. Delrin, the coil former material, has a maximum working temperature of 90°C and this is sufficient to achieve the required performance. The signal processing requirements for the RFlZC equipment are illustrated in Fig. 4. The analogue signal, received from the sensor coil, is preampli-
fied, further amplified in two gain stages and used as an input to the subsequent amplitude and phase processing circuits. The sensor signal is averaged using a root mean square to direct current converter, the output ripple from this averaging function being removed by filtering prior to analogue-to-digital conversion. The phase difference between the sensor signal and the exc:itation reference signal is extracted via counter circuitry implemented in a field programmable gate array (FPGA). This information is combined with the amplitude information, and trans-
cl
Sensor Coil
Electronicsmodule
U
Coil
Fig. 4. Block diagramof electronicsproc’essing.
Identification
and sizing of defects in metallic pipes by remote field eddy current
ferred via an RS-485 data link from a central microprocessor located in the electronic pack module (see Fig. 4). The resolution of the amplitude and phase processing systems is 12 bits, giving sufficient detail to enable subsequent processing algorithms to detect variations in pipe wall thickness. 5.3.3 Power module Electrical power is provided by a generator at the launch site, and is transmitted by an umbilical cable which also connects the electronics module to the personal computer. The cables for incoming power and outgoing data are separated within the power module. A regulator supplies power for the electronics module. The umbilical cable is released through a standard shaft encoder; the encoder produces a pulse train which allows the distance the vehicle has travelled through the pipe to be determined. The data are assembled and formatted and an RS485 serial data link is employed to transmit the information along the tether. The base station (for logging and processing) uses a standard personal computer, for economy and reliability. The interface unit contains a loo/50 V power supply, a direction discriminator and circuit boards for processing the signal from the distance-measuring equipment. There are two serial interfaces, an input for the distance transducer and a line in for power. 53.4 Sensor module The vehicle is designed to inspect both schedule 80 and ductile iron (DI) distribution mains of 100 mm nominal bore. The inside diameters of schedule 80 and DI are 97 mm and 106 mm, respectively. The signal at the pipe wall, received by the sensor coil, is very small and a close sliding tolerance between the sensor module and the pipe wall is necessary for optimum performance. Each sensor coil pair is mounted on a polyurethane sledge (Fig. 5) which allows the unit to
connection
23
compress and negotiate protrusions that are up to 16% of the internal diameter. The unit then recovers to full height. 5.3.5 Umbilical cable The umbilical cable, which consists of a seven-core cable surrounded by armoury, delivers power to the vehicle and provides base station-vehicle communications. The cable has a working load rating of 10.3 kN. 53.6 Vehicle winches The laboratory-based experiments with the vehicle, to detect metal loss on spool samples of short length of pipe (3.5 m), were carried out with a 0.5 tonne electric winch operating from a 12 V battery. A variable-speed control unit allowed the speed to be varied in the range 0.1-6 m/min. A large, variable-speed, diesel-engine-driven winch with a capacity of 3.7 tonnes was used for field trials. The maximum line pull recorded was 100 kgf during a 60 m line pull. 53.7 Material selection Deli-in, the coil former material, was selected because it has good electrical insulation and excellent machinability properties. During operation, the drive coil assembly was found to raise the temperature of the Dehin encasement to 70°C. The recommended operating temperature for Delrin lies between - 50°C and + 90°C in air, with intermittent use up to 160°C. The mlaterial specification for the effects of heat under load, moisture, stress and creep will be taken into account for the bend-passing device. 563.8 Analysis equipment The serial data are passed to the base station through one of the serial ports at a rate of 72 kbaud. The progress of the tool is monitored using the data-capture software which communicates with the vehicle via the base station. The software is Windows 95 executable and written in Visual C f + (version 5.0).
self 8djusting nabber sledge -
inspection
to sensor
intcreonneet
board Fig. 5. Detection semor coil pair.
24
D. Robinson
The following information is displayed when run in the data-capture mode: distance travelled by the vehicle since launch in metres; number of scans received; rotation of the vehicle in degrees; drive coil current value in amps; channel status; and all phase and amplitude channels selected. The software has the ability to expand a single trace to fill the trace area. Traces are updated as data are received from the vehicle. The computer used for the analysis software is a Pentium 200 MHz machine which allows stability and speed when executing the data analysis software. 53.9 Analysis software Raw data are streamed back from the remote field eddy current vehicle and recorded with the data-capture software (see Fig. 6). These data are subsequently analysed in a “stand-alone” RFJX analysis software package. The first stage of analysis involves generating a formatted version of the data using calibration data from the sensors. Normalisation of the data is car-
ried out next, thus removing the transient background signal. At this stage defects are detectable by visual analysis, appearing as peaks against an approximately constant background signal. Algorithms are being developed to automatically detect these defect peaks and calculate their signal parameters, i.e., amplitude, width and length. These values will then be passed into a sizing model to predict defect dimensions. The sizing model will be developed when the automatic detection is completed, and a consistent method of extracting the peak parameters is available. Data collected in the laboratory from machined defects of known size will be used to derive the model, using regression techniques and knowledge of the fundamental physics of the technique. The model will be modified and fine-tuned when acceptable data from real defects in the field are available. The field data to date did not contain any defects and were noisy in some cases. 6 LABORATORY PROCEDURE
SET-UP AND TEST
Different winching speeds were assessed for their ease of monitoring and the clarity of the signals
Fig. 6. Typical view of data using analysis software. Note that defect sites are characterised by peaks in the amplitude and phase data.
Identi$cation
and sizing of defects in metallic pipes by remote field eddy current
produced. It was decided to carry out the data runs with the prototype at 1-2 m/min. This speed ensured that no loss of data occurred. The optimum frequency level was obtained by experimentally increasing the frequency from zero to 50 Hz and viewing the resultant phase change. It was found that a frequency of around 20 Hz produced the maximum phase change. The average pipe wall thickness of the ductile iron defect spools used was 7.75 mm and the nominal diameter was 100 mm. The defect dimensions for the test spool referred to in Figs 6 and 7 were as follows: Distance (mm> 600 1000 1400 1800 2200 2600
Rotation (degrees) 0 120 240 0 120 240
Length (mm) 100 45 75 60 90 30
Width (mm) 55 55 55 55 55 55
% Depth 45 45 45 45 45 40
This ductile iron test spool has an overall length of 3200 mm. The distances are measured from the start of the test spool in millimetres and the rotation given in degrees from the centre line (top dead centre). All the defects are on the external surface of the pipe and the % depth was based on the average overall wall thickness value for the whole spool. The Analysis Software (see Fig. 6) was used to display the amplitude and phase data from all channels. The peaks in the data can easily be located at this stage of the processing. Each peak located was then measured and these measured values were used to determine the size of the defects that the peaks represent. The percentage depth corresponds to the height of a peak, the length of a defect corresponds to the peak width and the defect width is represented by the number of channels the defect spans. Data obtained from laboratory runs on the machined defect spools have confirmed the theoretical linear relationship between phase and defect depth (see Fig. 7). The defects could be reliably detected at a through-wall depth down to 40%. The smallest test spool defect consistently identifiable had dimensions of 3 1% of metal loss, length 45 mm and width 50 mm.
7 FIELD
inspection
25
OPERATIONS
7.1 Site preparation The following is an outline description of how the prototype RFEC tool is used in the field. The only excavation required is a 4 m long by 1.5 m wide trench at the beginning and end of the inspection run. The section of main must be decommi:ssioned and full-bore entry/exit is required. A camera inspection is carried out to check that the section of pipeline is free from obstru&ions. The malin is threaded with a lightweight rod and a test pass is performed with a cleaner device before the actual inspection commences. This checks the pipe for protrusions. The launch tray, 3 m in length, is placed at the entrance to the main. The vehicle is placed in the tray and pulled through the pipe at a speed of approximately l-2 m/min (see Fig. 8). The progress of the tool can be monitored by using the data-capture software. 7.2 Field trials An initial analysis of the data collected in the field trial was performed. Of the six machined defects, four were consistently identifiable by visual analysis of the data, the smallest of these being a discshaped defect, with dimensions of 35% of the wall thickness and 25 mm radius. Substantial end effects were observed, due to wall thickening at the end of each pipe length. This is potentially very useful as it means the system could be used to monitor wall thickness variations. Further research is required to investigate the potential of this application.
8 FUTURE
BENEFITS
TO INDUSTRY
The UK gas distribution network consists of 255 000 km of mains pipelines of which 60% is between 50 mm and 150 mm in diameter. This characteristic of the UK distribution network is “mirrored” in the European and American market, and the ability to address the gas pipeline inspection requirement for the smaller pipe diameters is expected to increase. Gas companies with similar mature distribution systems will value an RFEC system that has the potential to inspect live gas mains. MCM is an electronic packaging solution on a
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26
4-3.7
43.8
3.9
Distance
4.0
1 4.1
1 4.2
(m)
3.7
3.8
1 3.9
Distance
4.0
8 4.1
, 4.2
(m)
Fig. 7. Typical view of a defect using Analysis Software. * indicates the location of the centre of defects. The defect spans seven channels and represents 40% metal loss.
Fig. 8. Equipment arrangement for field operation.
substrate utilising bare die which minimises the space required for electronic components. The MCM module consists of an analogue front end, a digital processing section and connections to the main electronics processing module. This technology will allow the implementation of a frequency-selectable option for the drive coil, self-test and in situ calibration of the sensor coils. The crucial factor in the selection of an appropriate component technology for the miniaturisation of the RFEK vehicle is the minimum space requirements for the processing of the sensor signals. This space limitation requires two sensor channels to be processed in a circuit board area of 8 mm X 20 mm. The detected remote field signal is amplified and processed by a microprocessor for phase and amplitude information. Remote field information may be present in signals ranging up to 1 kHz, depending on the pipe material being inspected. The MCM
module is one of 24 in a ring of sensors. The phase and amplitude information acquired from each module is transmitted to an off-module microprocessor.
9 CONCLUSIONS The RFEC technology is still under development and only when the MCM vehicle is operational and evaluations have been carried out, both in the laboratory and the field, can a complete performance assessment be made. The following conclusions are made from the results of the initial laboratory tests and field trials with the prototype vehicle. a Using the present design, defects have been successfully detected in schedule 80 steel and ductile iron pipe in the laboratory and field trials.
Identijcation
and sizing of defects in metallic pipes by remote ,field eddy current
The smallest defect, from the field trial data, consistently identifiable by visual analysis was a disc-shaped defect, with dimensions of 35% of wall thickness and 25 mm radius. Further testing is required to investigate the detection limits of the prototype system. Further field testing with the double-ended system, in pipelines with metal loss, is necessary before a complete performance assessment of the vehicle can be made. The prototype system meets the specification for inspection of straight sections of 100 mm schedule 80 steel and ductile iron pipe. By further miniaturisation of the technology, the system can be used to inspect 100 mm pipework with 1D bends and protrusions of up to 15 mm. The bend-passing device, with a frequency-selectable option and utilising MCM technology, will allow the inspection of different pipe materials and operation at speeds of up to 6 m/min. The development of the miniaturised device will open up market opportunities, especially in the
inspection
27
areas of petrochemical plants and power-generation sites.
REFERENCES 1. Arunakumar, G., Live gas main repair via keyhole excavation. Trenchless Technology Research, Tunnelling and Underground Space Technology, 1998, 13(Sl-2), 29-36. 2. Lord, W., A survey of electromagnetic methods of non-destructive testing. In Mechanics of Non-destructive Testing, ed. W. W. Stinchcomb. Plenum Press, New York, 1980, pp. 77-100. 3. Schmidt, T. R., The remote field eddy current inspection Materials Evaluation, technique. 1983, 42(March), 225-230. 4. Yamaoka, S., Inoue, N. and Kimoto, S., Investigations using remote field eddy current defect testing of ferromagnetic stainless steel heat exchanger tubes. Journal of Non-Destructive Inspection, 1992, 41(7), 406-413. 5. Sullivan, S. and Atherton, D. L., Analysis of the remote field eddy current effect in nonmagnetic tubes. Materials Evaluation, 1989, 47(l), 80-86.