Inspectioneering Journal

Long-range Ultrasonic Testing (LRUT) of Pipelines and Piping

By Peter Mudge, Operations Director at Pi Ltd., and Julian Speck, Structural Integrity Department Manager at TWI Ltd. This article appears in the September/October 2004 issue of Inspectioneering Journal.

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.

Figure 1. Principle of long range UT
Figure 1. Principle of long range UT

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.

Figure 2. Inflatable collars for transducers
Figure 2. Inflatable collars for transducers

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.

Figure 3. Detection results for Teletest®
Figure 3. Detection results for Teletest®

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Comments and Discussion

Posted by Nace Susnik on September 24, 2015
Can a LRUT test be used to test pipes inside a... Log in or register to read the rest of this comment.

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