The new age of pipeline inspection

Paul Jackson and Tat-Hean Gan

Paper published in Inspectioneering Journal, July/August 2007.

The integrity of pipelines is a natural concern for pipeline operators, and so the ability to detect corrosion, erosion and mechanical damage in pipes is therefore of significant interest. Traditional methods of detection, such as pigging and crawlers, have been used for many years to inspect pipelines with great success.

Particular benefits of these methods include their ability to inspect long lengths of pipe in a short period of time and pipes that are inaccessible (buried, placed on the sea bed or encased in protective coatings). The fact that the pipes don't actually have to be emptied for inspection is a further advantage.

However, these techniques do have their limitations. For example, the use of pigs requires entry and exit points, and pipe bends and steep gradients can prevent the pig from passing through the pipe. As a result, a significant proportion (suggested to be as high as 75%) of all pipelines cannot be inspected by pigs. In situations where pipelines are in close proximity to one another, the use of external crawlers along the entire length of the pipeline becomes impractical (Figure 1). Therefore, alternative solutions need to be considered for the inspecting pipelines that otherwise prove difficult to inspect.

One such alternative is long-range ultrasonic testing (LRUT) - arguably the most significant development in the field of non-destructive testing to have taken place over the last two decades.

This method was first introduced as a commercial technique when Plant Integrity Ltd (Pi Ltd - a wholly owned subsidiary of TWI Ltd) launched Teletest in 1997 (Figure 2). Since its introduction, Teletest has been through a number of development programmes to become the highly versatile inspection tool it is today.

What does Teletest detect?

Teletest is a global monitoring tool for the detection and analysis of metal loss features, such as corrosion and erosion, in pipes. It is capable of measuring significant lengths of pipe (typically 60m, but up to 350m) from a single point, detecting and locating areas of corrosion rapidly. The technique is capable of detecting 9% metal loss flaws. Once these flaws have been located, conventional inspection techniques are deployed to characterise the defects.

How does Teletest work?

In LRUT, a pulsed guided ultrasonic wave mode is propagated in a pipe wall from a ring of equally spaced ultrasound probes supported by a collar wrapped round the pipe. The ultrasonic waves are reflected by features such as flanges, circumferential welds, branches and defects in the wall, and the reflected echoes are received by the transmitting probes. Therefore, all features in the entire run of pipe are detected during the same scan, provided the response is large enough to produce an echo amplitude above the random noise level.

The success of the technique stems from the fact that low frequency guided waves can be transmitted a long range in pipes. This is because:

1. absorption of the waves in the pipe material is low due to the low frequencies.
2. 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).
3. 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.

Since these waves can propagate for some considerable distances within pipes, the technique can be used to inspect significant lengths of pipe from a single point. This is advantageous in a number of specific situations:

1. Inspection of coated pipe. By exposing a small area around the circumference of the pipe on which to fit the transducers, it is possible to inspect coated or insulated pipe without stripping the coating from the entire length of the pipe.
   
   
2. Inspecting elevated pipe. Placing a ring of transducers around an elevated pipe and using LRUT to inspect lengths of pipe removes the need to construct extensive scaffolding along the pipe to perform a full inspection. It also reduces the risk to personnel of extended periods of working at height.
   
   
3. Inspecting buried pipe. In some instances, over ground pipelines intersect roads. Here, the roads are built up over the top of the pipe, and the pipe is therefore buried. LRUT inspection techniques such as Teletest can inspect the entire length of the pipe situated under the road without the need for excavation.
   

The benefit of the long range nature of the Teletest signal is illustrated in Figure 3.

a) Conventional techniques inspect a few cm² under the device
b) Teletest^® inspects 100% of the pipe wall for tens of metres in each direction

Teletest has been designed as a 'modular' system (Figure 4). Tools are built up using the appropriate number of modules (for example 36 for testing 12" pipe). Each module has five individual transducer elements as shown in Figure 4. Three are orientated parallel to the pipe's axis to generate longitudinal waves and two are orientated circumferentially to generate torsional waves. This gives Teletest 'Multi Mode' capability. This is an important feature of Teletest because for some pipes and flaw types longitudinal testing is most sensitive, whereas, in other situations, torsional testing is more satisfactory.

The modules are forced into contact with the pipe by means of a lightweight inflatable collar. Figure 4 also shows a collar that has been populated with multi-mode modules. The collar is assembled around the pipe by a rapid clamping mechanism. Modules can be quickly removed from the collars and re-fixed in another collar for use on a different pipe diameter. Unlike conventional UT, a liquid couplant between transducer and pipe surface is not applied. There merely needs to be sufficient, evenly distributed pressure of the transducer on the test surface.

A typical display from a Teletest A-scan (signal amplitude versus distance from the transducer ring) is shown in Figure 5. Distance Amplitude Correction (DAC) curves are plotted on the display. These are based on the decaying signals from subsequent girth welds. From experience, it is known that the reflection from a girth weld with normal cap and root profile is 14dB (a factor of 5) less intense than the reflection from the pipe end (i.e. total reflection). This is the blue line plotted on the display shown in Figure 5. Furthermore, experience also shows that an area of thinning which has resulted in a loss of cross-sectional area of 9% in the pipe wall will produce a signal that is a further 12dB less intense than the signal from the girth weld. This -26dB level is used as a threshold for evaluating signals and is plotted as the green line in the A scan.

A recent development in the Teletest technology now allows the ultrasonic waves to be focused around the circumference of the pipe.

At the core of the new system is the new 24-channel Teletest Focus^TM Multi-Mode flaw detector. A major advance on the existing, well-proven 12-channel unit, it operates in the same way but with 24 channels and 50% more power, and offers the potential for greater inspection distance and resolution.

The equipment can generate any of the three main wave types used in guided wave technology, longitudinal, torsional and flexural. It has been designed to be used as a phased array. This makes it possible to focus ultrasound at any point both along and around the pipe, thus improving flaw detection capability. It also means that it is possible to determine the approximate circumferential extent of any flaw that has been identified using the equipment in its original screening mode.

The modules in a Teletest Focus tool are grouped in eight octants around the tool's perimeter. The unit triggers the octants separately so that the tool acts as a phased array. Furthermore, the power to each octant can be adjusted to compensate for any variation in coupling. The phasing of the firing of the transducer modules enables ultrasound to be focused at a predetermined position both along and around the pipe. Thus, when a normal screening test has identified the longitudinal position of a flaw which might normally be deemed marginal, ultrasound can be focused at the position and the focal point can then be swung around the pipe in eight steps. This means that it is possible to determine both the circumferential position and the circumferential extent of a flaw.

The display from a Teletest Focus scan is shown in Figure 6. The display shows an A-scan (left hand window) showing the condition of the pipe, with the highlighted area indicates where the ultrasound is focused in terms of distance along the pipe. In this case, the focal point is at a longitudinal position of -15.42m and at an angle of 225° from top dead centre. The polygon plotted inside the polar plot, on the bottom right hand side of the screen, shows the amplitude of each octant relative to all of the others. The maximum amplitude is displayed as contact between the gray polygon and the black circle. The red dot on the outside of the black circle indicates the circumferential position of the A-scan being displayed in the main window.

Capabilities of Teletest

General

The Teletest technology was developed to screen pipework for metal loss features such as corrosion and erosion. Originally developed for the inspection of corrosion under insulation in petrochemical plant pipework but, as we have already seen, the technology is equally suited for application to pipelines including road crossings, bridge piers and poorly accessed pipework generally.

Teletest is particularly suited to pipeline monitoring operations, allowing the pipe condition to be checked on a periodic basis without the need to access the entire length of pipe (remove significant lengths of insulation, excavate roads, etc.).

As mentioned previously, the field reporting threshold is area metal loss equivalent to 9% of the pipe wall cross-section. However, features of between 3-9% cross-section are commonly found.

Teletest Focus will provide information on the metal loss feature in terms of the distance from the transducer (or agreed datum), location around the circumference of the pipe (by octant) and severity in terms of both signal amplitude and circumferential extent.

Conventional LRUT cannot distinguish between a wide shallow flaw and a deep axial narrow flaw of similar cross sectional area, but the introduction of focusing capabilities, as mentioned earlier, makes this distinction possible.

Pipe diameters

The modular nature of Teletest means that the tooling can be configured to inspect pipe diameters (ANSI/ASME nominal bore) from 1.5 to 48 inches.

Access and set up

When inspecting pipes using Teletest it is necessary to expose 0.5m of bare pipe in order to mount the transducer ring. The location of this ring would ideally be at least 1m from the nearest girth weld. The transducer ring has been designed so that it only needs 65mm (or less in some instances) of clearance between adjacent pipes. Due to its rapid clamping mechanism, an assembled Teletest collar can be mounted on a pipe in under a minute.

The inspection is carried out under control of the ruggedised lap-top PC loaded with the Teletest FastTrack software. Since the umbilical between the Teletest unit and laptop can be up to 100m long, the data can be analysed in the comfort of a portable office - a truck for example.

Pipe configurations

Teletest is ideally suited for inspection of straight sections of pipework, where inspection of tens of metres in either direction can be achieved. Testing around large radius swept or pulled bends generally causes no problems, but testing around elbows can result in mode conversion of the guided ultrasound wave and thus reduced testing capabilities.

Pipe situations

Teletest has been successfully used to inspect pipes with surface temperatures up to 125°C (257°F). It is possible to transmit the Teletest signal along pipes that are immersed in water, with good results. Figure 7 shows a few of the many situations that Teletest is used in.

a) Road crossing on the North Slope in Alaska
b) Bayou crossing
c) Ahia manifold

As for this pipe contents, it has been found that as the viscosity of the pipe contents increases, the range of the inspection decreases due to loss of ultrasound energy. The presence of heavy deposits in the pipe can also reduce the range of inspection.

The condition of the pipe itself can affect the efficacy of the inspection. Areas of corrosion on the pipe wall act as reflectors to the Teletest signal, each in turn reducing the intensity of the ultrasound travelling beyond it. On pipework exhibiting general heavy corrosion, ultrasound will be reflected from all the corrosion, effectively reducing the inspection range. However, it must be remembered that this in itself is a result and the corrosion would be reported accordingly.

Heavy corrosion at the location where the Teletest transducer ring is placed can be particularly detrimental to successful inspection since it prevents the formation of a symmetrical wave. It is therefore important for test areas to be examined with a scan from a conventional ultrasonic probe beforehand.

External coatings

As reported earlier, one of the benefits of using Teletest is that it is only necessary to remove a short lengths of pipe coating to achieve successful inspection of a pipeline. However, there are some situations where the coatings themselves can affect the inspections.

Mineral wool insulation presents no difficulties. Bonded foam polyurethane insulation leads to a loss of ultrasound. However, this merely results in a reduced inspection range.

Some limited success has been achieved in testing pipe passing through concrete walls and pipe encased in lightweight fireproofing cement. However, concrete attenuates ultrasound rapidly and may prevent the effective operation of Teletest.

Bitumastic coatings currently inhibit the effective operation of Teletest, except where they have dried to a hard finish.

Some types of heavy adherent wrapping (Denzo wrap) can result in excessive loss of ultrasound. Newly applied material causes most problems. Testing has been successful on pipe where the tape has dried out and is no longer well adhered to the pipe surface. Testing of this type can be on a trial basis only.

Test range

During standard Teletest inspections, pipes are interrogated first in one direction and then in the other from the one transducer location. Typically ranges of 30m in each direction are achieved. Under ideal conditions, a range of 180m in each direction has been achieved. However, it can be less, if conditions are unfavourable.

We have seen from the commentary above that there are a range of factors that can affect the range of the inspection. Table 1 summarises the factors affecting performance, principally the test range over which adequate signal to noise separation is achieved. As the degree of difficulty of guided wave propagation increases, so the test range decreases and noise increases.

Table 1 Factors affecting performance

	Degree of difficulty	Surface condition	Geometry	Contents
	Easy	Bare metal
		Smooth well bonded paint	Straight lengths	Gas
		Mineral wool insulation	Infrequent swept/ pulled bends
		Fusion bonded epoxy		Low viscosity liquid
	Difficult		Attachments/brackets
		Light pitting		High viscosity liquid
		Heavy pitting
		Plastic coating	Branches
		Buried lines		Waxy or sludgey deposits
		Bitumastic coating	Many bends
		Concrete coating

Productivity

Test rates of up to 1km per day have been achieved, although as with conventional NDT, the rate of inspection depends largely on the condition of the pipework being inspected.

Cost comparison

The fact that Teletest can inspect a significant length of pipe from a single location, in conjunction with the technique's ability to be deployed in situations that prove difficult for alternative inspection methods, represents a significant opportunity for cost saving over these alternatives.

Pi Ltd has come up with estimates on the relative costs of using different testing methods to inspect comparative pipes in different situations (Table 2). A number of factors were taken into account when arriving at these estimates, including excavation and replacement of roadways, removal and reinstatement of coatings, and use of scaffolding to access elevated pipes. For the purpose of this estimation, it was assumed that the inspections provided 100% coverage in all cases except for manual UT, for which spot checking is assumed.

Table 2 Estimated costs as a ratio to the costs of a Teletest inspection for 1m of 12inch diameter pipe

It should be noted that the cost estimates were based on activities in the UK, but these should apply to any economically developed country.

Demonstrating the capabilities of Teletest

Pi Ltd is regularly asked by End-Users to demonstrate Teletest in order to prove its capability. Two of the more critical demonstrations are described here.

Jay oil field, Alabama/Florida

Teletest was being used to inspect a high-pressure water injection line buried in and around the main Jay facility. The pipe was buried in a light sandy soil and wrapped in plastic. The test results were acted upon immediately. The position of any anomaly on the Teletest A-scan was measured and paced out from the Teletest tool placed on exposed pipe in a bell-hole. One such indication is shown in Figure 8.

The anomaly, indicated by the (+) in the figure just beyond the weld, approaches the moderate threshold level - the blue DAC curve.

A bell hole was dug down to the pipe at the indicated position, where corrosion was revealed. It was decided to cut out the corroded section of pipe (Figure 9). This confirmed the presence of severe corrosion at the position indicated, just beyond the weld.

North Slope oil field, Alaska

Before this major survey of road crossings in the oil fields at Kuparuk and Prudhoe Bay, the Teletest system had to pass field trials to detect examples of 'weld-pack' corrosion. Figure 10 shows an example of this corrosion.

The corresponding Teletest A-scan is shown in Figure 11. It has been magnified, a useful feature of the software, to view the signal just in front of the weld. The corrosion gives rise to a moderate anomaly, about 750mm in front of the signal from the weld. The signal in the anomaly indicated by a peak in the red line indicating a horizontal flexural response due to the corrosion being concentrated near the three or nine o'clock positions.

Case histories

Pi Ltd have provided support to service inspection companies 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 12). Pi Ltd 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.

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 13). 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 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 on the Pi Ltd website (www.plantintegrity.com).