Teletest® operating envelope
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, the technology is equally suited for application to pipelines including road crossings, bridge piers and poorly accessed pipework generally.
Areas highlighted by Teletest® used as a screening tool, can be further examined using Teletest FocusTM. Alternatively they may be identified for more detailed assessment using conventional NDT test methods. Teletest® is particularly suited to fingerprinting exercises, allowing the pipe condition to be checked on a periodic basis without the need to remove the entire insulation.
The field reporting threshold is area metal loss equivalent to 9% of the pipe wall cross-section. Metal loss features have been detected far smaller than this level. However, a lower reporting level can result in an increase in false calls.
Teletest® will provide information on the metal loss feature in terms of range from the transducer (or agreed datum) and severity (minor, moderate or severe).
Up until the present, long-range ultrasonic testing, has been unable to distinguish between a wide shallow flaw and a deep narrow one of similar cross sectional area. Furthermore, it could not identify the precise location of the flaw around the pipe. However, with the advent of Teletest FocusTM, these two limitations have, to a significant extent, been overcome.
Pipe diameters
Teletest® tooling currently held by Pi is suitable for testing all pipe diameters (ANSI/ASME nominal bore) from 1.5 to 48 inches. Other sizes both smaller and larger (based upon standard pipe diameters) are available to order.
Access
Access is required to 0.5m of bare pipe in order to mount the transducer ring. The ring also needs to be at least 1m from the nearest girth weld.
Pipe configurations
Teletest® really scores on straight sections of pipework, where inspection of tens of metres in either direction can be achieved. Testing around swept or pulled bends generally causes no problems. Testing around elbows can result in mode conversion of the guided ultrasound wave and thus reduced testing capabilities. However, the introduction of Teletest FocusTM has alleviated this problem. Testing from a main line will not cover branch lines. These must be tested separately.
Temperatures
Pipe surface temperatures can be in the range of +5°C to +125°C.
External coatings
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.
External environment
The Teletest® signal can be transmitted along pipe that is immersed in water, with good results. However, neither the unit itself nor the transducer tool is suitable for sub-sea operation.
Internal environment
As the viscosity of the pipe contents increases, the inspection range decreases due to loss of ultrasound strength. Heavy deposits on the inside of the pipe can also be highly attenuative.
Pipe condition
Teletest® works by detecting echoes from corroded regions of the pipe. Each region acts as a reflector, 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. It must be remembered that this in itself is a result and the corrosion would be reported accordingly.
Heavy corrosion at the place where the Teletest® tool is placed is particularly attenuative because it prevents the formation of a symmetrical wave. Test areas should be examined with a scan from a conventional 0° ultrasonic probe beforehand.
Test range
The pipe is interrogated first in one direction and then in the other from the one transducer location. Typically ranges of ±30m are achieved. Under ideal conditions, this has gone up to ±180m. However, it can be less than 30m, if conditions are unfavourable.
The table 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 2 Factors affecting performance
| Degree of difficulty | Surface condition | Geometry | Contents | |
|---|---|---|---|---|
  |
Easy |
Bare metal | ||
| Smooth well bonded paint | Straight lengths | Gas | ||
| Mineral wool insulation | ||||
| Fusion bonded epoxy | Infrequent swept/bends |
Low viscosity liquid | ||
| Light pitting | Attachments/brackets | |||
| Heavy pitting | High viscosity liquid | |||
| Plastic coating | ||||
| Bitumastic coating | ||||
| Concrete coating | Branches | Waxy or sludgey deposits | ||
| Many bends | ||||
Productivity
Test rates of up to ½ km per day have been achieved. As with conventional NDT, the rate of inspection depends largely on the condition on the pipework being inspected.
Proven applications
Teletest® has been used commercially over eight years. During this time its benefits have been proven on:
|
|
Potential applications
Teletest® is in a continual state of development. Among the applications currently under consideration are:
- Wind turbine towers
- Offshore platform jacket structures
- Railway lines
- Cables
Requirements on site
Teletest® is foremost a rugged site inspection tool, simple to operate and with few parts that can develop faults. It is self sufficient, with integral battery and air pump for collar inflation.
- Teletest® operation requires a Hot Work Permit
- Insulation removal (0.5m) at the tool location