Tat-Hean Gan, Graham Edwards, Malik Kayous and Bryan Bridge
Paper published in World Pipelines, May 2007
The occurrence of corrosion, erosion and mechanical damage to pipes and pipelines means that there is great interest in advances in methods of inspection. Of particular importance are improvements in the speed of inspection, tool accessibility, inspection range and costs. This paper presents an evolution of pipeline inspection techniques since 1970s and offers a comparison of techniques such as Pigs and Crawlers and the more recent guided wave systems.
1. Historical perspective and problems
The need for pipeline non-destructive testing (NDT) grew very rapidly with the increase of offshore gas and oil exploitation in the 1970s, which arose from a gross inflation in OPEC prices of oil from 'traditional' land sources. New pipelines for offshore supplies brought new and sometimes unexpected corrosion problems. The presence of sediment and chemicals in offshore risers often caused corrosion and erosion of the risers, leading to wall thinning at several times the expected rate. The consequence of this was that expensively laid pipelines would fall short of their design lives. Very substantial investments were made in the 70s in the development of both pipeline pigs for electromagnetic and ultrasonic detection of wall thinning, and X-ray crawlers for weld inspection in long runs of pipe. The perceived economic importance of these techniques at the time is evident from the publicity they received. The first smart pig was developed in 1964 using Magnetic Flux Leakage (MFL) technology to inspect the bottom portion of the pipeline ( Figure 1).
Fig.1. A 'pig' used to clean natural gas pipelines
The first X-Ray crawler was used for weld inspection during pipe laying. A crawler that could fit pipes as small as 8 inches was developed by OIS. Pigs and crawlers excelled at the time in being the only means of inspecting for erosion, corrosion and other types of defect in pipes buried inaccessibly below ground or on the sea bed and/or encased in concrete or other protective coatings to protect outer pipe walls from corrosion.
Both approaches have the capability to inspect long lengths of pipes and pipelines in a short period of time. These pipes and pipelines do not need to be emptied for the inspection. However, not all the pipelines can be inspected in this way. Entry and exit points may not be present for the 'pig'. Pipe bends or steep gradients may occur, which prevent the 'pig' passing through. There are therefore very large proportions (perhaps as high as 75%) of pipelines that cannot be inspected with the 'pig'.
Hence long runs of pipe have to be taken out of service and bypassed to allow inspection. The problem of inspecting these non-piggable pipelines has been the subject of significant research in recent times. One technique that has been demonstrated as a viable solution is long range ultrasonic (LRU) inspection, although it is recognised that other techniques offer alternative solutions.
In cases where the pipes are accessible (i.e. when running overland and not having protective coatings or not buried behind other objects) inner wall corrosion and other defects are inspected by ultrasonics and weld defects by double wall through transmission radiography. A variety of external pipe crawlers and magnetically adhering robot vehicles have been used in an attempt to deploy the inspection sensors more rapidly on long runs of pipe.
However, ultimately, the time taken to achieve total coverage is still of serious economic concern, even with the present state of the art of external robotic deployment. Most pipelines in process and manufacturing plant are inaccessible for external inspection by the above techniques, along most of their length, because of their proximity to other pipes and structures ( Figure 2).
Fig.2. Typical examples of pipelines in process and manufacturing plant that are inaccessible for inspection by internal pigs and crawlers or external crawlers
In summary, traditional methods of pipeline corrosion inspection have the drawbacks of being cumbersome and time consuming, and they can not inspect efficiently runs of pipe that possess one or more of the following features:
- Protective coatings, the removal of which is undesirable as part of the inspection process;
- An inner diameter too small for internal crawlers or pigs;
- Bends of small radius;
- Proximity to other pipes or other structures which do not allow access to place an external crawler and sensors.
2. A novel solution: long range ultrasonics for global inspection
The above problems have been overcome using the LRU technique. Over the past 10 years, TWI has been pioneering the technique of LRU for corrosion and erosion detection and monitoring, which, in principle, allows inspection of long runs of pipe from just one access point.
One of the main applications of this technology has been the inspection of non-piggable pipelines, although other applications of LRU inspection are equally valid, where conventional inspection methods are already accepted.
The LRU technique 
- dispenses with the need for pigs or internal and external crawlers and all the expense and running time that these entail
- allows instantaneous inspection of long pipe runs
- allows inspection of pipes inaccessible to other approaches through the reasons (i) to (iv) specified earlier.
In this technique a pulsed guided wave mode is propagated in a pipe wall from a family of equally spaced ultrasound probes supported by a collar wrapped round the pipe. The wave is reflected from the pipe end, circumferential welds and defects in the wall, and the reflected echoes (usually mode converted) are received by the transmitting probes. Therefore, all defects in the entire run of pipe are detected simultaneously, provided they are large enough to produce an echo amplitude above the random noise level.
The promise of the technique as a global monitoring tool stems from the fact that low frequency guided waves have a very long range in pipes because
- absorption in the pipe material is low at low frequencies.
- for pipes in air, leakage of waves out of the pipe is very low because of the high acoustic impedance mismatch at the solid-air boundaries. Therefore, all the energy propagates down the pipe with little attenuation of the energy density (wave amplitude).
- A wave mode with low dispersion (frequency dependence of phase velocity) can be selected so that the rate at which the wave pulse spreads out in time is small.
The pipe acts as a wave guide, an effect that can be demonstrated at audible frequencies by someone whispering from one end down a long length pipe to be heard at the other.
With this combination of conditions the wave amplitude incident on a defect decreases only slowly with wave propagation range and correspondingly the minimum detectable defect increases only slowly with propagation range.
The test range can be defined as the range at which a defect which requires detection gives a detectable echo. So the probe collar need only be repositioned along the pipe at intervals equal to the twice the test range, with a small allowance for overlap to achieve total inspection coverage of an indefinite length of pipe. Depending on many factors such as pipe diameter, wall thickness and bend radius, as well as considerations (i) to (iii) above, the test range can be as much as 100 metres for an uncoated pipe in air.
Figure 3 shows a typical A scan display showing an echo from a corrosion defect of 3% CSA (cross-sectional area) located 12 metres from the transducers and 1 metre in front of a weld.
TWI has exploited the long range ultrasonic technique through their subsidiary Plant Integrity Ltd. Figures 4, 5 and 6 illustrate the first, second and third generation of Teletest ® long range ultrasonic equipment [ - ] developed and marketed by Plant Integrity Ltd (P i Ltd).
Fig.3. Long range ultrasound defect echo from a corrosion defect of 3%CSA at a depth of 100m in a pipe of 10inch diameter and 6mm wall thickness in a Alaskan oil field - obtained with the Teletest ® MK2 system during a service inspection
Fig.4. First generation MK1 Teletest ® system
Fig.5. Second generation MK2 Teletest ® system
Fig.6a) Third generation MK3 Teletest ® system
Fig.6b) MK3 Teletest ® system in operation
3. Some case histories drawn from the service records of P i's Teletest ® instrument
P i Ltd have provided service inspection in the pipeline sector with the Teletest ® system since the late 1990s and during this time have gained vast experience of the growing potential of the long range ultrasound technique, particular in the Alaskan Kazakhstan, Saudi Arabia, East and South East Asia and Middle Eastern Oilfield. 
A particularly interesting case was inspection of offshore risers in Lake Maracaibo ( Figure 7). Plant Integrity has been collaborating with a local company TechCorr to bring Teletest ® technology to Venezuela. A number of demonstrations have been carried out for Venezuelan oil and petrochemical companies. One of particular interest concerned the inspection of offshore risers. PDVSA own a number of small, unmanned gas platforms in Lake Maracaibo. They were concerned about the possibility of corrosion affecting the risers in the splash zone. The purpose of this exercise was to demonstrate in principle that Teletest ® was capable of inspecting this zone. The photograph shows the transducer ring clamped around a 6in riser. The 'Splashtron' coating, whilst causing some slight attenuation did not significantly affect the ability to inspect the critical region. 
Fig.7. Inspection of offshore risers in Lake Maracaibo
Another interesting result was obtained from the inspection of inspection of headers in gas compressor stations in Montana and North Dakota in the USA ( Figure 8). The final client, the stations' owner, was Northern Borders Pipeline (NBPL). AITEC were sub-contractors to Mears Engineering LLC, NBPL's principal inspection company. The challenges presented by these inspections were:-
- The presence of some twenty 12in branches.
- The large diameters - 36, 37 and 42in.
- The significant thickness - 44mm.
Fig.8. Inspection of headers in gas compressor stations in Montana and North Dakota in the USA
As the photograph shows, the headers were supported on concrete blocks. The aim of the inspections was to detect possible atmospheric corrosion at the 6 o'clock position at the interface between the headers and the concrete supports. Because of the thicknesses involved it was decided to inspect using torsional wave excitation. The Teletest ® collar was mounted at the quarter length positions of the headers, which were up to 60m (180 ft) long.
Despite the intervening branches, it was possible to 'see' to the dome ends. A small indication was seen at a range of about 42 feet corresponding to the position opposite the pressure take-off branch that can be seen in the photograph near the header end NBPL were completely satisfied by these inspections. A plan is now in place to use Teletest ® to inspect the headers on a regular three yearly basis.
Further examples of Long Range Ultrasonics inspection can be found in  .
4. Current state of the art
Ordinarily with ultrasonic techniques, defects need to have dimensions greater than a wavelength to be detectable so the low frequencies and correspondingly long wavelengths used in guided wave ultrasonics might be perceived as restricting the sensitivity of the technique, limiting it to the detection of gross defects. However, the advantages described, such as its global monitoring capability from a single position outweigh that sensitivity drawback.
All things considered, a couple of points should be noted:
- that the technique can detect defects from one position in a long pipe run well before the defects reach the size that would lead quickly to pipe failure - it is a case of 'better late than never';
- often, for the variety of reasons already given, it will be the only means of defect detection.
Work is well in hand to move this technique in the direction of a high sensitivity technique as well as global screening tool by using focussed waves. The Teletest ® Focus system includes focussing facility i.e. a long range phased array ( Figure 9). In addition, research is also being carried out with this instrument into a novel focusing method called time reversal focusing. [4,5] From initial experimental results it is expected that corrosion defects as small as 1% CSA with a 95% POD will be detectable with this approach.
Fig.9. Modelling of time delay focusing technique
5. Future applications
Long-Range Ultrasonic Testing is a rapidly evolving technology with advances of Teletest platform expected in the areas of Permanently Mounted Tools for remote monitoring, Internal tools for inspection of small diameter tubing and marinisation of the technology for sub-sea inspection of pipes such as risers.
TWI have been pioneering the long range ultrasonics technique for defect (i.e. corrosion, erosion and metal loss) detection and monitoring which in principle allows inspection of long runs of pipe from just one access point.
This provides the ability to inspect complete pipelines, rather than just those regions accessed by pigs and crawlers. What's more, the inspections are cost effective and reliable, and are done with the minimum of disruption.
The benefits of long-range ultrasonic testing are:
- Rapid screening for in-service degradation.
- Cost reduction in gaining access to the pipes for inspection.
- Avoidance of removal and reinstatement of insulation or coatings (where present), except for the area on which the transducer tool is mounted.
- The ability to inspect inaccessible areas, such as at clamps or sleeved or buried pipes.
- The whole pipe wall is tested, thereby achieving a 100% examination.
Teletest ® (an LRU system marketed by Plant Integrity Ltd) is such a system capable of 100% direct assessment for pipe lengths of typically 30m in either direction from the test position, although in ideal conditions significantly greater lengths of pipeline can be inspected from a single test point.
Whilst LRU technologies are thought to be inherently insensitive, recent developments, such as the ability to focus the ultrasonic waves, have added a further dimension to the technique's potential.
The authors would like to thank Plant Integrity Ltd for supporting this project. Plant Integrity designs, manufactures and provides services using the Teletest ® system.
- LRUT brochure
- P J Mudge
- C Ennaceur, P. Mudge, B. Bridge, M. Kayous. Application of Time Reversal Technique to the Focussing of Long Range Ultrasound in pipelines, Insight, Vol 49, No 4, 2007.
- C. Ennaceur T. H Gan, R. Sanderson, P Mudge and B. Bridge, Modelling of Time Reversal Focussing in Straight Pipes.