Pipeline corrosion control - the history and the long range future
TWI Bulletin, September - October 2008
The occurrence of corrosion, erosion and mechanical damage to pipeline invites great interest in advances in inspection methods.
Tat-Hean Gan joined TWI in 2006 and is now the LRU Section Manager. He has developed various NDT methods(ultrasonics, infrared and capacitance) for condition monitoring, structural health monitoring and defect detection. He previously managed several R&D and DTI projects at Warwick University.
Graham Edwards has over 30 years experience in non-destructive testing. He started his career in NDT with the Unit Inspection Company, where he received training in all aspects of NDT and weld inspection. In 1973, he was one of the first ultrasonic test technicians to gain CSWIP approval.
Malik Kayous is a senior project leader with TWI's subsidiary company Plant Integrity Ltd. He has been heavily involved in the development of the latest Teletest
® long-range ultrasonic test system for pipes and pipeline inspection.
Professor Bryan Bridge BSc, DSc, Finst NDT, CEng, FIET, Cphys, FInstP, FRSA, of London South Bank University (LSBU)is research Professor and Co-director of the Research Centre of Automated and Robotic Nondestructive Testing which he founded in 1992.
Of particular importance are improvements in the speed of inspection, tool accessibility, inspection range and costs. Tat-Hean Gan, Graham Edwards, Malik Kayous and Bryan Bridge present an evolution of pipeline inspection techniques since 1970s and offer a comparison of techniques such as pigs and crawlers and the more recent guided wave systems.
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 seventies, 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 seventies 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 (Fig.1).
Fig.1. A 'pig' used to clean natural gas pipelines
The first X-Ray crawler was used for weld inspection during pipelaying. A crawler that could fit pipes as small as 8" diameter 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 (ie 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 (Fig.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
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 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 around 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. Depending on many factors such as pipe diameter, wall thickness and bend radius, as well as the considerations 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 one metre in front of a weld.
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 an Alaskan oil field - obtained with the Teletest® MK2 system during a service inspection
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 (Pi Ltd).
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
Case histories drawn from the service records of Pi's Teletest® instrument
Pi Ltd has provided service inspection in the pipeline sector with the Teletest® system since the late 1990s and during this time has 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 oilfields.
A particularly interesting case was inspection of offshore risers in Lake Maracaibo (Fig.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. Figure 8 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 headers in gas compressor stations in Montana and North Dakota in the USA
Fig.8. Inspection of offshore risers in Lake Maracaibo
Another interesting result was obtained from the inspection of headers in gas compressor stations in Montana and North Dakota in the USA. 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
As Figure 7 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 13m (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.
Future developments
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
- often, for the variety of reasons already given, it will be the only means of defect detection.
