TELETEST® Guided wave technology - case histories

P J Mudge and J D Harrison
Plant Integrity Limited
www.plantintegrity.co.uk

Mr Mudge and Dr Harrison are Directors of Plant Integrity Limited, a Member of the TWI Group.

Authors qualifications and professional affiliations:

Peter Mudge: BSc (Hons)
Chartered Engineer
Fellow, British Institute of Non-Destructive Testing
Member of the Institute of Materials;
Senior Member of The Welding Institute

John Harrison: MA, PhD
Fellow, Royal Academy of Engineering
Member Institution of Civil Engineers
Fellow, Institution of Mechanical Engineers
Fellow, The Welding Institute

Paper presented at Nondestructive Testing. 1st Middle East Conference and Exhibition, Bahrain, 24-26 Sept.2001

Synopsis

The Teletest ^® long-range ultrasonic testing (LRUT) system for monitoring pipes and pipelines in-service was introduced commercially by Plant Integrity Ltd (P i) in early 1998. It is becoming widely accepted by the oil and gas industry for detecting corrosion and other metal loss flaws, and as a valid means of assessing pipe condition, particularly where access for inspection isdifficult or expensive. This paper describes the equipment and its performance and outlines a number of case histories to illustrate its application in the field.

1. Introduction

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, 30mor more in each direction, may be examined from a single test point. The benefits are:

• Reduction in the costs of gaining access 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 the technique to be able to detect flaws down to 3% of the cross section of the pipe or less. In order to limit the number of 'calls' P i normally set a reporting level of 9%.

The impetus to use LRUT is that other methods of inspecting pipe for corrosion or erosion (ultrasonic thickness gauging, pulsed eddy current, digital radiography, etc.) are highly localised to an area under the 'footprint' of the particular search device. To survey large pipe lengths requires many measurements and access to much of the surface of the component being examined. (Some of these devices can be used without removing insulation. However, this does not help for buried or sleeved pipe or for pipe elevated on racks. Furthermore, the whole of the pipe area must be scanned). Where access is difficult or costly, a detailed survey becomes unattractive economically, with the result that often only limited sampling is carried out. Partial inspection of this type is unlikely to be effective in reducing the numbers of significant flaws which may cause leaks or failure, as the probability of detecting flaws in uninspected areas is zero. The benefit of using long range testing to examine 100% of the pipe wall is therefore considerable. Evidence for this is provided by a study carried out by the UK Health and Safety Executive, ^[1] 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 downtime and leaks of potentially hazardous materials. The use of LRUT to ensure that the whole pipe wall volume is tested provides a commercially attractive means of improving coverage.

In this paper we describe the equipment and outline the principal features of Teletest ^® and then illustrate its application in a variety of industrial situations by describing some recent case histories.

2. Equipment

The equipment consists of a bracelet transducer array, which is placed around the pipe. The transducer is energised by a special low frequency flaw detector ( Fig.1a), which is driven by a specifically produced package on a personal computer.

Fig.1a. Teletest ^® low frequency flaw detector and ruggedised personal computer

The computer, which controls the test, communicates with the Teletest ^® unit via up to 100m of umbilical cable. For small diameters the transducer bracelet is a solid clamp. For diameters of 6 inches and above a flexible transducer array is used ( Fig.1b).

Fig.1b. The Teletest ^® system in use on a 24-inch foam insulated pipe, testing a sleeved road crossing

3. Data presentation

A typical result from a site test is shown in Fig.2. 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 range may be 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.

Fig.2. Result from a site test on an uncorroded pipe. The large signals are from butt welds. Test range 36m.

4. Features of Teletest ^®

4.1. General

Teletest ^® has the following characteristics.

• Range - typically 30m in each direction from a single point. As much as 180m has been achieved.
• Productivity - typically 300 to 500m per day.
• Able to distinguish between flaws and pipe features, particularly welds.
• Accuracy of longitudinal positioning of flaws - better than ±100mm.
• Detects internal and external metal loss flaws.
• Diameter range from 2 to 48-inch.
• Temperature range from -40 to +125°C.
  

4.2. Sensitivity

Teletest ^®'s sensitivity has been assessed in two independent blind trials. The first was the European RACH project ^[2] in which various methods were assessed against their ability to detect controlled corrosion flaws in 6-inch pipe. Teletest ^® was the only long-range method considered. There were 36 flaws and Teletest ^® detected all those exceeding its nominal reporting level of 9% of cross sectional area. A number of flaws between 3 and 9% of cross section were also detected. The second trial was for PRCI ^[3] on 24-inch pipes with 86 machined flaws of various areas. Results from this trial suggested that the technique is substantially more sensitive for larger pipe sizes. Whilst the number of flaws assessed in both trials is strictly limited, the results may be used to determine an approximate probability of detection (POD) as a function of flaw area as shown in Fig.3.

Fig.3. Probability of detection from RACH and PRCI trials as a function of flaw area

The PRCI results suggest that, for larger diameter pipes, flaws as small as 3% of the cross sectional area should be detected with a probability approaching 100%. The combined data provide valuable evidence of the performance expected in the field.

5. Case studies

5.1. Case study 1 - 14" ammonia line

This insulated line was welded to a vessel in a chemical plant at a point 2m above ground level, ran vertically for 7m then horizontally for a further 10m. Due to the insulation, visual inspection was difficult, and scaffolding would be necessary to access the elevated section. Corrosion under insulation (CUI) was suspected. The Teletest ^® transducer was attached at the base of the vertical section. An A-Scan output is shown in Fig.4a. The two lines are distance amplitude correction (DAC) curves, the upper representing the amplitude from butt welds in the pipe and the lower being the reporting level. The large peak about 5m from the transducer is a weld at the elbow where the pipe turned to the horizontal. A number of flaws were reported in the region 13 to 19m from the transducer (marked '+' on the plot). On removal of the insulation for cleaning and visual inspection, these were confirmed as areas of CUI attack. See, for example, Fig.4b.

Fig.4a Teletest ^® result from a 14" Ammonia line with CUI

Fig.4b View of the region marked '+' in Fig.4a after removal of insulation and cleaning

5.2 Case study 2 - 24" slurry line

This uninsulated line carried water-based slurry and was at ground level, thus access for inspection was not difficult. However, high levels of local internal erosion, where turbulence caused particles in the slurry to impact on the pipe wall, had led to leaks. Service history showed the positions of these leaks to be unpredictable. Thus spot thickness checks were ineffective in detecting thinned areas before leakage. Teletest ^® overcame this problem, as 100% of the pipe wall was examined. An initial trial was conducted on a section including a small leak. The result is shown in Fig.5. The signal approximately 12m from the datum is from a butt weld. The large signal which follows it (marked '+') coincided with the location of the leak. This suggested extensive metal loss. Subsequent examination showed there to be a band of erosion almost through the wall for the majority of the pipe circumference. The pipe was therefore at the end of its service life.

Fig.5. Result from test on a 24" slurry line

5.3. Case study 3 - 10" buried line

This work was on water injection lines, partially above ground but mainly buried, at an oil processing facility. The concerns were external corrosion at the soil to air interface and in the buried sections, where the coating had been damaged. Tests were carried out from above ground or from 'bell hole' excavations. Figure 6 shows the result from a buried section, which was totally inaccessible at the time of inspection. The scale is enlarged to show the feature of interest, about 26m from the transducer. There are welds at 11.5 and25.5m. The steepness of the DAC curves indicates the higher levels of attenuation, which are generally observed on buried lines with protective coatings.

Fig.6. Result showing flaw marked '+' on the plot

Immediately beyond the second weld is a signal with associated mode converted components, which are plotted in colour (marked '+' on the plot). This region was reported as a severe flaw and the area was excavated. The pipe was found to be heavily corroded at that point and a repair was made immediately. The section removed is shown in Fig.7. Note the weld with the heavy corrosion adjacent to it. This flaw would not have been detected by any other means before it had caused a failure of the line.

Fig.7. Corroded section of 10" line removed following detection by Teletest ^®

5.4. Case study 4 - 48" road and bayou crossings

On sub-contract to Cooperheat MQS, P i carried out a Teletest ^® inspection of a 48" diameter brine line, the largest pipe tested to date. Figure 8 shows the transducer ring assembled on the pipe just before a bend leading into a bayou crossing.

Fig.8. Transducer ring on 48" diameter pipe before support, clamp and 90° elbow

Despite the presence of two bends, Teletest ^® was able to cover the full extent of the crossing. This meant 'shooting' more than 26m on pipe submerged in water.

A problem of particular concern to the oil industry is corrosion at supports. This line is clamped to saddles with a neoprene interlayer. Teletest ^® was able to 'shoot' past the supports, the signal being unaffected by them. This meant that corrosion in the support area, either over the saddle or under the clamp could be detected.

5.5. Case study 5 - offshore risers in lake Maracaibo

PDVSA own a number of small unmanned gas platforms in Lake Maracaibo. They are concerned about the possibility of corrosion affecting the risers in the splash zone. This inspection, performed on sub-contract to TechCorr of Venezuela, showed that Teletest ^® was capable of inspecting this zone. Figure 9 shows the transducer ring clamped around a 6" riser. The range achieved was 13m, but was only limited by a change in pipe section. The 'Splashtron' coating used to protect the splash zone, whilst causing some slight attenuation did not significantly affect the ability to inspect the critical region.

Fig.9. Teletest ^® set up on 6" riser

6. Factors influencing performance

The main factors affecting sensitivity to flaws are:

• The size of the corrosion as 'seen' by the wave propagating along the pipe. Detectability is related to the proportion of the pipe wall cross-section that is missing, i.e. it is a combination of the depth and the circumferential extent. P i's experience is that the limit of detection is an area of 3% of the original pipe wall cross-section. The technique is equally sensitive to internal and external flaws. Similarly, the sensitivity is uniform around the pipe, unless it is affected by pipe features (see below).
  
  
• Depth of corroded area. Although the primary parameter affecting detection is area, The technique is more sensitive to deep short flaws than to shallow wide ones of the same area. This is satisfactory since deep short flaws are more detrimental.
  
  
• The axial extent of the corroded area. The technique is less sensitive to this dimension, as the waves approach the flaw end-on. However, we do have some evidence that a long flaw produces a stronger signal than a short one, provided that the circumferential extent is large enough for it to be detected.
  
  
• Pipe features. All discontinuities and changes of geometry affect the ultrasound signals and therefore give rise to responses. There are a number of pipe features, which may affect the ability to detect flaws. Some of these are listed below:
  
  

  - Butt welds. These are obviously widespread in any pipe system and it is essential that there is a means of distinguishing between these and corroded or other defective regions. Teletest ^® achieves this by examining the different characteristics of the weld and flaw signals during the data treatment and display process.

  - Bends. Bends give a response owing to their geometry. When testing a pipe system containing bends it is necessary to determine that there is sufficient sensitivity in the region following the bend to allowtesting.

  - Attachments. Items such as welded supports again produce signals. It is necessary to determine an interpretation procedure for such cases, depending on the geometry of the support brackets.

  - Ts and branches. The system copes adequately with small diameter branches, such as instrumentation lines. However, if the branch diameter approaches that of the main pipe, the signals may become very complicatedand this may prevent satisfactory inspection of and beyond that region.

  - Coatings. Some types of coating increase the rate of attenuation of the ultrasound and therefore reduce the test range achievable. It is known that the polyurethane foam insulation does affect test range, but notto the extent that testing will not be feasible. Internal deposits may absorb some of the ultrasound and may reduce the test range.
  

  
  
• Test sensitivity. To perform an adequate test, a certain level of ultrasound has to be generated. Also, there needs to be a minimum signal to noise ratio in order to maintain the expected sensitivity to flaws. These aspects determine the effective range and are assessed at the time of testing.
  

7. Concluding remarks

The growing body of evidence for the performance of long range ultrasonic testing in general and Teletest ^® in particular, as illustrated here, supports the wider application of this novel technology. LRUT has already crossed the technology transfer threshold from a curiosity to a usable and highly effectivetool, and the range of applications continues to grow. There are other instances of corroborated performance, for example reference. ^[4]

Proven applications are:

8. References

1. Patel R and Rudlin J, 'Analysis of corrosion/erosion incidents in offshore process plant, and implications for non-destructive testing' Insight - Journal of the British Institute of NDT, Vol. 42 No.1, January 2000.
   
   
2. Reliability Assessment for Containment of Hazardous Materials RACH), European Commission Project OG 112/FR/UK, Final Report, 1999.
   
   
3. Mudge, P J and Lank, A M, 'Detection of corrosion in pipes and pipelines', ASNT International Chemical and Petroleum Industry Inspection Technology Topical Conference V, Houston, Texas, 16-19 June 1997.
   
   
4. Mudge P.J. and Hipkiss A. 'Rehabilitation of a 3km 12 inch pipeline at a UK refinery'. Insight - The Journal of the British Institute of Non-Destructive Testing, Vol. 42, No. 2, February 2000.