Taking the long view...

TWI Bulletin, March - April 2003

The oil and gas industries benefit from a revolutionary means of detecting in-service metal loss defects

Peter Mudge joined TWI's NDT Research department in 1976 and was manager of that department from 1985-97. His major interest has been the development of ultrasonic testing techniques, latterly concentrating on the use of guided waves for long-range inspection. Peter is a past President of the British Institute of Non-Destructive Testing and a Member of the Institute of Materials. In 1990 he was awarded the Leslie Lidstone/ESAB Gold Medal for his contribution to welding technology. As Operations director of P i Peter is responsible for managing both manufacture of Teletest ^® equipment and the delivery of services and demonstrations.

Long range ultrasonic testing (LRUT) was introduced commercially by Plant Integrity Ltd in the form of the Teletest ^® technique in early 1998 for the in-service monitoring of pipes and pipelines. As Peter Mudge reports in part one of this two part piece it is largely used by the oil and gas industry for detection of corrosion and other metal loss defects, and is becoming widely accepted as a valid means of assessing the condition of pipes and pipelines, particularly where they are difficult or expensive to access for inspection.

The Teletest ^® technique has now been extensively used in the field for evaluating the condition of pipes in the diameter range two inches to 48 inches and has performed well in identifying corrosion in pipes in a variety of situations.

As with any new technology, a crucial stage in gaining acceptance by industry as a 'front line' inspection or monitoring tool is the generation of adequate evidence of the performance achieved. This paper describes the evidence available for the performance of the technique from laboratory studies and from field work, including both applications trials and contract testing.

The Teletest ^® low frequency ultrasonic guided wave technique has been developed for the rapid survey of pipes, for the detection of both internal and external corrosion. The principal advantage is that long lengths, 30m (~100ft) or more in each direction, may be examined from a single test point. The benefits are:

• Reduction in the costs of gaining access to the pipes for inspection, avoidance of removal and reinstatement of insulation (where present), except for the area on which the transducers are mounted,
• The ability to inspect inaccessible areas, such as under clamps and sleeved or buried pipes,
• The whole pipe wall is tested, thereby achieving a 100% examination.

Site trials have demonstrated that this method is capable of detecting corrosion <30% wall thickness deep and <25% circumference wide. The technique is now commercially available as an inspection tool for use in hydrocarbons transmission and processing facilities.

The impetus for the use of long range ultrasonics is that ultrasonic thickness checks for metal loss due to corrosion or erosion are highly localised, in that they only measure the thickness of the area under the transducer itself. To survey a large area requires many measurements and access to much of the surface of the component being examined. Where access is difficult or costly a detailed survey becomes unattractive economically, with the result that often limited sampling only is carried out. Similar restrictions also apply to other methods of measuring wall thickness, such as radiography, eddy currents etc. Partial inspection of this type is not likely to be effective in reducing the numbers of significant defects which may cause leaks or failure being present in pipes as the probability of detection of defects in uninspected areas is zero. The benefit of using long range testing to examine 100% of the pipe wall along the length tested is therefore considerable. Evidence for this is provided by a study carried out by the UK Health and Safety Executive, which reported that over 60% of the reportable hydrocarbon release incidents from offshore platforms in the UK North Sea sector were related to pipework. The adoption of adequate inspection and maintenance practices for pipework therefore has a considerable effect on the incidence of both unscheduled plant down-time and leaks of potentially hazardous materials. The use of long range ultrasonics to ensure that the whole pipe wall volume is tested provides a commercially attractive means of improving coverage.

Development of long range ultrasonic testing

To meet these inspection needs TWI led two major projects aimed at developing a method of inspection of insulated pipework for detection of external corrosion under insulation. The method is also sensitive to internal corrosion in pipes. The initial objectives were to demonstrate the viability of using low frequency ultrasonic tests for detection of corrosion under insulation in long lengths of pipe, and to carry out limited trials using pre-prototype procedures and equipment. Detailed experimental studies and site trials, backed up by numerical modelling using finite elements, showed that guided waves propagating in the pipe wall can be used to detect corrosion damage in insulated pipework systems. The principle of the technique is that guided waves may propagate over long distances in metals with minimal attenuation and are reflected from discontinuities. Thus it is feasible to examine many tens of metres of pipe from a single test point by detecting the reflections from features and defects in the pipe wall. The operation of the technique is shown schematically in Fig.1.

This initial development was followed by two studies to provide the data on which the transition from a laboratory technique to a usable inspection method for operational plant could be based. The approach taken was to perform extensive trials using the long range-low frequency technique in order to gather data on performance on real plant and in the presence of pipework features. This information was used to establish the requirements for refinements which needed to be made to the technique, and to determine the specifications for both transducers and equipment. The initial projects, funded by both UK and European agencies plus a group of industrial companies, examined pipes in the diameter range 2-12 inches (50-300mm). Further work was funded by PRC International, through their NDT Supervisory committee, with the aim of determining the applicability to large diameter gas transmission pipelines. For both, the minimum sensitivity required was the detection of corrosion 50%t deep, where t is the pipe wall thickness, and D/2 x D/2 in area (approximately 9% pipe wall area), where D is the pipe diameter, over a usable test range of at least 15m. As corrosion may occur as deep pits which are small in area, the desired target for a practical test sensitivity was detection of corrosion 50%t deep and 3t x 3t in area (approximately 3% pipe wall area).

Ultrasonic wave excitation and reception

Guided waves propagating in the pipe wall, similar in nature to Lamb waves in plates, can propagate many hundreds of metres in plain pipe. However, unlike bulk waves used for conventional ultrasonic testing, where generally only a single mode such as either compression or shear exists, a large number of guided wave modes are possible. In order for the responses from discontinuities in pipes to be interpretable it is necessary to use selected wave modes which allow the signals to be simplified. The most attractive is the fast-moving longitudinal L(0,2) mode. This also has the property that its velocity is independent of frequency over a range of frequencies, Fig.2.

It is vital to minimise the amplitude of all other, unwanted, modes in order that response signals from discontinuities may be identified and that the signal-to-noise ratio may be increased. To achieve this it is necessary to operate in a frequency range in which L(0,2) is non-dispersive, ie its velocity is independent of frequency, and which is below the theoretical cut-off for higher order waves modes to exist. For material thicknesses encountered in pipes in oil and gas systems this requires frequencies in the tens of kHz range to be used, Fig.2. It is also possible to exploit the fact that, in the frequency range of interest, only two modes, L(0,1) and L(0,2), are axisymmetric. All other flexural modes are non-axisymmetric and therefore may be eliminated if the excitation and reception conditions are axially symmetric. If piezoelectric transducers are used, a good approximation to axial symmetry can be obtained by placing a number of nominally identical transducers at equally spaced intervals around the pipe circumference, see Fig.3.

Sensitivity

A major objective of this work was to determine the sensitivity of the technique to corrosion in pipes. An important point to note is that the long range techniques currently available are screening tools and do not provide the same kind of resolution as local thickness measurements. The aim is to provide a rapid method of screening so that more appropriate test methods may be directed at areas requiring further attention in an efficient manner. Most importantly, long range UT does not provide a direct measurement of wall thickness, but is sensitive to a combination of the depth and circumferential extent of any metal loss, plus the axial length to some degree. This is due to the transmission of a circular wave along the pipe wall which interacts with the annular cross-section at each point. It is the reduction in this cross-section to which the long range technique is sensitive. This is shown schematically in Fig.4.

Theoretical and practical studies were carried out to determine the responses expected from loss of pipe cross-section. Figure 5 shows the results of the theoretical work compared to experimental results.

In Fig.6. it may be seen that not all points lie on the mean line. This is due to the aspect ratio of the flaws. Deep flaws which have a small circumferential extent give a higher response than long shallow flaws of the same projected area. This theoretical relationship was further tested experimentally on lengths of three inch and eight inch diameter pipe containing machined notches of known cross-sectional area. Figure 7 shows the results, compared with the finite element (FE) predictions.

From Fig.7 it may be seen that in most cases the practical result agreed closely with the finite element prediction, so that the relationship given in Fig.6 may be used as a basis for describing the sensitivity of the Teletest ^® technique. The one case where the practical result did not agree with the finite element study was for a 12% area notch which gave a much higher response than expected. It is not clear why such a high response was recorded, as there appeared to be no features of the notch which would be likely to account for this. However, this is a fail-safe result as, (a) this was a large flaw and (b) the higher than expected amplitude ensures detection. This result is shown in Fig.8.

Figure 8 also illustrates the format of the Teletest ^® display. The main features are:

The form of the display is similar to that of conventional ultrasonic testing; The horizontal scale is distance (either from the transducer, or from an offset datum). The range plotted in Fig.8 is 12.72m. The vertical scale is signal amplitude. Distance Amplitude Correction (DAC) curves are superimposed on the display to provide lines of equal sensitivity with distance from the transducer. Four curves are normally displayed:

• The uppermost represents the amplitude from a pipe end, which is designated as 0dB (this being a 100% area reflector).
• The second line, which intercepts the vertical axis at 45mV in Fig.8, represents the amplitude of responses likely to be obtained from welds. This is 14dB below the 0dB line.
• The third line, which intercepts at 12mV, is the reporting level. This is equivalent to a reflection from a 9% area flaw and is set at 26dB below the pipe end (0dB) level.
• The fourth, dashed, line is a target level for backscattered noise ('grass' signals). Where these consistently exceed this level the limit of test range has been reached.

Three traces are plotted, superimposed on each other. The main trace is the directly reflected longitudinal (L(0,2) mode) response from features and flaws. The others are vertical and horizontal components of mode converted signals, which are generated when the out-going longitudinal mode is reflected from rough and/or asymmetric reflectors. The presence of high levels of mode converted signals is indicative of flaws and is essential in interpretation.

Practical Implementation

Equipment

The equipment consists of a bracelet transducer array which is placed around the pipe, as shown in Fig.3. The transducer is energised by a special low frequency flaw detector, which is driven by a specifically produced package on a personal computer. Figure 9 shows the equipment.

The computer is connected to the Teletest ^® unit via an umbilical cable, which also provides low voltage power. The umbilical may be up to 100m long. The test is controlled for the computer. For pipe diameters of six inches and above a flexible transducer array is used. So far tools have been produced to work with diameters of up to 48 inches, Fig.10.

Results

A typical result from a site test is shown in Fig.11. This pipe is free from any corrosion and the trace shows a clean baseline with sharp peaks from the butt welds in the line. The test may range over 100 metres. It should be noted that by applying a phased input to the different parts of the transducer, it is possible to send the ultrasound in one direction only along the pipe, so that any confusion of interpretation may be avoided. By switching the input, the test may be performed in the opposite direction.

Part 2 some practical applications of Teletest.