Paper published in Inspectioneering Journal, September 2004.
LRUT for the in-service surveying of metal loss in pipelines and piping was introduced by Plant Integrity (Pi) Ltd in the form of the Teletest ® in 1998. Pi is a subsidiary of TWI (formerly The Welding Institute) and provides a commercial outlet for TWI's technologies.
The impetus for the development of LRUT is that ultrasonic thickness checks for corrosion, erosion, etc. are localised, in that they only measure the thickness of the area under the UT transducer. The operation of the LRUT technique is shown schematically in Fig.1. Teletest ® uses low frequency guided ultrasound travelling along the pipe providing 100% coverage of the pipe wall.
Up to 350 metres of pipework can be inspected from a single test point (using a battery- or mains-power supply). LRUT reduces the costs of gaining access and avoiding extensive removal of insulation (where present). The whole pipe wall is tested, achieving a 100% examination (including areas such as at clamps, and sleeved or buried pipes).
Continuous development
Over the past decade, the development of LRUT has been funded through a variety of sources including most recently, the Pipeline Research Council International, for the application of LRUT to large gas transmission pipelines.
The equipment can now generate any of the three main long-range wave types (longitudinal, torsional and flexural). It uses phased-array technology making it possible to focus ultrasound at any point both along and around the pipe). The inflatable collars and multi-mode transducer modules increase its high temperature capability to 160°C.
Inspection sensitivity
The earliest work showed that the smallest area of metal loss which LRUT can detect is approximately 3% of the pipe wall cross-section. The reporting level which is normally used is a signal amplitude equivalent to 9% area, to ensure that false call rates are kept to an acceptable level. These thresholds were verified through 'blind trials' without knowledge of any defects and the results were independently evaluated.
Figure 3 shows the results from the Teletest ® technique on 36 individual defects. The plot is in terms of depth and circumferential extent of the defects and indicates whether each was detected or not. The lines representing 3% and 9% defect area are also included. The data show the classic probability of detection characteristics, with an increasing likelihood of detection above the 3% level. All flaws at or greater than the 9% reporting level were detected.
The main factors which affect the sensitivity of LRUT are:
- The size of the corroded area, as 'seen' by the wave propagating along the pipe. Detectability is related to the proportion of the pipe wall cross-section which is lost (the combination of the depth and the circumferentialextent).
- The axial extent of the corroded area. The technique is less sensitive to this dimension, although long defects produce a stronger signal than shorter ones.
- Pipe features. All discontinuities affect the ultrasound signals and therefore give rise to responses, such as butt welds, bends, attachments, etc.
- Coatings. Some types of coating affect the rate of attenuation of the ultrasound and therefore reduce the test range achievable.
- Test sensitivity. To perform an adequate test, a certain level of ultrasound has to be generated (a minimum signal-to- noise ratio), in order to maintain the expected sensitivity to defects.
14" Ammonia line case study
This line was the feed to a reactor vessel in a chemical complex and was insulated, so external corrosion under insulation (CUI) was suspected. It emerged from the reactor around 2m above ground level, ran vertically for around 7m then horizontally for a further 10m. The Teletest ® A-scan output is shown in Fig.4.
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 at around 5m from the transducer is a weld at the elbow where the pipe turned to the horizontal. A number of defects 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, Fig.5.
10" Buried line case study
This Teletest ® survey was carried out on a network of water injection lines at an oil processing facility. The concerns were external corrosion around the soil to air interface and where the coating had been damaged in the underground sections. Several locations were identified on the A-scan output as being moderate-to-severe defects and were excavated. At one location, the line was found to be so heavily corroded that repairs were immediately required, Fig.6. It's unlikely that this defect would have been detected by other means before it had caused a failure of the line.
Summary
The growing body of evidence for the performance of LRUT in general and Teletest ® in particular, is supporting the wider application of this novel technology. LRUT has already crossed the technology transfer threshold from a curiosity to a usable and highly effective tool. It is particularly useful for locating corroded areas that may then by conventionally sized for fitness for service assessments. In many situations, it is a vital tool for inspection engineers to hold in their arsenal of NDT techniques.