PR7373 July 2003

Development of Phased Array and Swept Frequency Guided Wave Long Range

Ultrasonic Techniques for Fitness-For-Service Assessment of Pipe Corrosion For: A Group of Sponsors

0 +50m-50m

A-scan

Transducer Ring

Road Crossing

Figure 1 Guided Wave Ultrasonic Technique

PipeAxis

PipeAxis

PipeAxis

Axi-symmetric modes

Flexural Modes

Torsional Mode Figure 3 Guided Wave Modes

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��������On Bends At Branches

Under Supports In Weld Grooves

Axi-symmetricAxi-symmetric

FlexuralFlexural

Transmission Reception

Transducer

Pipe wall

Pipe Vibration

Figure 4A Figure 4B

Figure 4D Figure 4C

Figure 2 Corrosion at Geometric Features Figure 4 – Phased Arrays for Transmission and Reception of Guided Waves.

The guided wave Long Range Ultrasonic Technique (LRUT) has been shown to be capable of inspecting pipe for several tens of metres from the point of access, even when the pipe is buried or under insulation. More sophisticated operation of the equipment will allow identified defects to be quantified, and the discrimination defects from signals generated by welds, bends, branches and other geometric features in the pipe. This project aims to develop guided wave LRUT as an assessment tool as well as a screening tool. It will validate the performance of the more sophisticated use of guided wave LRUT including using phased arrays and swept frequency, and provide guidelines for its successful operation in industry.

BACKGROUND Many pipes are inaccessible for inspection, because they are buried or insulated. The guided wave Long Range Ultrasonic Technique (LRUT) is a special ultrasonics NDT technique that is capable of inspecting pipe for corrosion several tens of metres from the point of access [1] even when the pipe is inaccessible ie buried or under insulation. The guided waves are propagated from a ring of ultrasound transducers wrapped around the pipe (Figure 1). A typical application is detection of corrosion in pipe under a road crossing. The technique was introduced commercially by Pi Ltd.in 1998, and has been under continuous improvement since then with support from TWI and Penn State University. This programme of research will make a step change in its capability. Currently guided wave LRUT can broadly discriminate between minor and major corrosion and, very importantly, can measure the distance to the corrosion from the transducer ring [2]. The pipe can therefore be exposed at the corroded position, where it can be assessed visually, if the corrosion is on the outside surface or with another NDT technique such as radiography or B-scan ultrasonics, if it is in the inside surface of the pipe. Currently, guided wave ultrasonics has been limited to detection only. Other techniques have to be applied to evaluate the corrosion. This programme of work aims to develop guided wave LRUT as an assessment tool as well as a screening tool by improving procedures with current techniques and introducing new techniques. Guided waves are complex. There are a great many wave modes travelling at different velocities, depending on the wall thickness - diameter combination of the test pipe. They are also dispersive, that is to say their velocity varies with frequency. The A-scans must therefore be calibrated from dispersion curves; such curves being available for each wave mode and pipe diameter and thickness. LRUT systems allow for this in the analysis software. However, the complex nature of guided waves could be turned to advantage. The bulk waves used in conventional ultrasonics provide information about discontinuities from signal amplitude and time-of-flight data only. Guided waves on the other hand, also provide wave mode data. There is an interaction between guided wave mode, wave frequency, pulse flight-time, the pipe wall thickness – pipe diameter combination and discontinuity that could be used to characterise the discontinuity. Laboratory trials have shown that a discontinuity could be investigated by using the transducer ring as a phased array and sweeping through a range of ultrasound frequencies [3]. Trials conducted in this project will quantify the results of this work and validate existing and new test procedures. BENEFITS • Greater confidence in the use of existing

guided wave LRUT after collecting performance data covering a wide range of test conditions.

• Enhanced guided wave long range ultrasonic techniques that:

o Provide better discrimination between different levels of severity of corrosion.

o Detect corrosion in presence of geometric features in the pipe such as branches, bends, supports and welds (Figure 2).

• Cost savings through avoidance of exposing

pipe for inspection, eg. £100,000 for digging up a road crossing.

OBJECTIVES • To gather sufficient data on known

discontinuities in sample pipes to validate the performance of the existing guided wave LRUT techniques.

• To validate the performance of new enhanced techniques, which use phased arrays and swept frequencies.

• To provide guidelines for selecting wave modes, test frequencies and other procedure parameters.

• To provide guidelines on interpretation of test data e.g. discriminating corrosion signals from signals due to welds, bends, branches, supports and other geometric features in the pipe.

• To provide guidelines for setting thresholds in terms of corrosion that can be left for future inspections, corrosion that should be evaluated further and corrosion that should be remedied immediately.

PROJECT APPROACH Test procedures will be validated by studying influencing parameters on lengths of pipe in the laboratory and in the field, with support from numerical modelling. Provision of test samples Samples of test pipe will be collected to show a wide range of test conditions including: • Corrosion/erosion, general, localised,

corrosion pitting, laminations, isolated cracks, fretting.

• Corrosion of differing depth, radial extent and longitudinal extent relative to the pipe axis.

• Corrosion in the vicinity of pipe elbows, pipe branches, pipe supports and other attachments to the pipe.

• Corrosion in the weld toe. • Corrosion on internal and external surface of

the pipe. • Pipe coatings, including soil, concrete,

bitumastic, insulation (wet and dry). • Carbon steel pipe and stainless steel pipe. Where possible, use will made of existing pipe samples with well documented corrosion e.g. pipe samples used in the RACH/CRIS research programmes and the pipe test loop at TWI. Pipes may be welded up to give representative lengths. If pipe with existing flaws is not available, then artificial flaws will be manufactured on the pipe. For example, machined flats on the pipe surface could simulate erosion. Electrolysis through pools of liquid kept on the pipe surface for several hours could simulate localised corrosion. Cracks could be simulated by spark-eroded slots. Flaws will be simulated on internal as well as external pipe surfaces. Validation of Current Techniques Current equipment propagates guided waves as axi-symmetric (symmetric about the pipe axis) longitudinal or torsional waves in pulses along the pipe (Figure 3). Where they meet a discontinuity that presents a change in the thickness of the pipe wall, the pulses are reflected or mode converted. The thickness change may be an increase in thickness, such as a weld with cap and root, or it may be a decrease in thickness, such as corrosion. If the discontinuity is symmetrical around the complete pipe circumference, the pulse will be reflected as an axi-symmetric wave mode. This would be the case where the discontinuity is a pipe butt weld. Normally in the case of corrosion however, the discontinuity is localised to one section of the pipe circumference and the pulse is mode converted to a flexural wave. Detecting flexural waves in the pulse reflected back to the transducer ring is the principal method of detecting corrosion. The plane of a simple flexural wave that is causing the pipe to vibrate in one plane can be polarised in an infinite number of direction around the pipe circumference. Current test equipment discriminates only the vertical and horizontal vibrations. The current techniques will be validated using Teletest multimode (Plant Integrity’s new 24 channel LRUT system with phased array capability) and studying the influence of the following parameters:

• Test frequency. • Wave mode i.e. longitudinal, torsional, and

flexural. • Signal/noise ratio at test range. • Distance amplitude correction. • Pipe dimensions. • Pipe coatings. • Corrosion/erosion characteristics. • Dimensions of corrosion/erosion. • Location of erosion/corrosion.

Numerical modelling TWI have extensive expertise in the use of Finite Element models to provide support to experimental studies of guided wave propagation through pipes and, more recently rails. They will provide additional evidence to validate the procedures and extrapolate between measurements. Validation of New Techniques The new techniques will use equipment that extends the mode of operation of Teletest to include transducer ring operation as a phased array and the sweeping of test frequencies during a test. Investigating the same influential parameters as were investigated with existing procedures will validate the new procedures. . Phased Array There are many complex flexural wave modes, where there are multiple nodes, or ‘pinch points’ around the pipe circumference. Figure 3 illustrates two, three and four nodes. The new test procedures will be sensitive to these more complex wave modes. This will be accomplished by using the transducer ring as a phased array. To produce a wave that is axi-symmetric, the transducers in the ring are fired in unison (Figure 4A). When stationary and in reception mode, the transducers will recognise returning pulses containing axi-symmetric waves because the transducers all receive the same wave simultaneously (Figure 4B). If they do not receive the wave simultaneously, the waves will be recognised as flexural (Figure 4C). Using the transducers in reception as a multiple array, for example top and bottom, left-hand side and right-hand side, and diagonally across it will be possible to determine the width of corrosion and its position around the pipe. A more difficult proposition is to use the transducers in transmission as a multiple array (Figure 4D), therefore propagating pulses with flexural waves. In this way it may be possible to differentiate internal from external corrosion and corrosion in the presence of geometric discontinuities. Figure 5 below shows that the transducers can be fired using a phased array technique to focus the guided waves. This will increase the defect detection capability and also

differentiate small amounts of corrosion and their location in the pipe.

Swept Frequency Marked changes in the amplitude of reflected pulses from a discontinuity have been observed when the frequency of the guided wave is increased or decreased. This is partly a function of the pipe diameter/wall thickness ratio and partly a result of the equipment’s operating characteristics. There is also evidence however, that these changes may also be due to interactions between the pipe wall thickness – diameter combinations and the discontinuity. The technique will sweep through a range of test frequencies and identify peaks in the signal amplitude. Field Trails The techniques will undergo field trails. These are necessary in order to measure performance under realistic conditions, particularly as parametric studies will be restricted to relatively short lengths of pipe. Guidelines Based on the validation exercises, guidelines will be written for writing procedures using existing and new techniques. The guidelines will cover: • Wave mode selection. • Frequency selection. • Interpretation of results. • Test sensitivity levels. Summary of evidence on capability and limitations. Based on a literature survey, experience with other work and the results of this project, a summary of evidence will be prepared of the

capability and limitations of both the existing and new guided wave LRUT techniques. This will provide sponsors with a reference document for writing test procedures and planning inspections. INTELLECTUAL PROPERTY RIGHTS Any IPR arising from this project will be held by TWI and will be confidential to the sponsors of the project. Sponsors will be consulted on the handling of the IPR generated in this project. DELIVERABLES The deliverables for the project are. • Enhanced guided wave long range

ultrasonic techniques. • Data on the reliability and sensitivity of

existing and new techniques. • A guidance document for writing

procedures and interpreting results. • Information for setting thresholds in

inspection strategies. SUGGESTED PRICE AND DURATION The total estimated cost of the project is £210,000. It is planned to form a Sponsor Group of seven companies, each contributing £15,000 per year for two years Contact: Dr Aamir Khalid or Dr. John Harrison NDT Technology Group TWI Limited Granta Park Gt Abington Cambridge CB1 6AL Tel: +44 (0) 1223 891162 Fax: +44 (0) 1223 890952 E-mail: aamir.khalid@twi.co.uk E-mail: john.harrison@twi.co.uk REFERENCES 1. Mudge P.J., ‘Field application of Teletest

long range ultrasonic testing technique’, Insight Vol.43, No. 2, Feb. 2001, pp74-76

2. Cawley P., Lowe M.J.S., et al ‘Practical Long Range Guided Wave Testing, Application to Pipe and Rail’, Materials Evaluation, Vol.61, No.1, pp.66-74.

3. Hay T.R., Rose J.L., ‘Guided Wave Testing Optimisation’, Materials Evaluation, Vol. 60, No. 10, pages 1239-1252.

Figure 5. This picture shows a simulation of focusing beyond an elbow.