Pipeline Inspection with Crawlers

Pipeline Inspection with Crawlers is an advanced non-destructive testing technique in which a remotely controlled, self-propelled radiography unit (crawler) travels inside the pipeline, positions itself in line with the weld seam and captures a panoramic radiographic image of the entire circumferential weld in a single exposure. The physical basis of the method is the same as in conventional radiography: ionising rays emitted from an X-ray tube or a gamma isotope source (typically Ir-192 or Se-75) are attenuated as they pass through the material and are absorbed to differing degrees at discontinuities in the weld metal — such as porosity, slag, lack of penetration or cracks — creating density (darkening) differences on the film or digital detector. When the radiation source is placed exactly at the centre of the pipe, in the plane of the circumferential weld, the rays are emitted with equal geometry through 360 degrees and the entire seam is imaged in a single exposure; this geometry is known as the "single wall single image, panoramic" technique.

In practice the crawler is driven into the pipe interior from an open pipe end and, powered by batteries, travels tens or even hundreds of metres, positioning itself at each weld seam in turn. Stopping in line with the weld and initiating the exposure is generally commanded by a low-activity gamma marker placed on the outside of the pipe; on detecting the marker the crawler stops, projects the source, exposes for the set time and moves on to the next seam. Meanwhile the film or CR/DR detector strip is wrapped around the weld circumference from the outside of the pipe. The method is particularly efficient on large-diameter pipelines such as 16"–60": the many separate exposures required for one seam with the double-wall technique are reduced to a single exposure with the panoramic technique, which markedly reduces both shooting time and film/detector consumption and provides productivity suited to fast-moving pipeline construction sites.

On the equipment side, a crawler system is an integrated arrangement consisting of a drive and positioning module, a radiation source (X-ray tube or isotope projector), a battery unit and a command/detection system. X-ray tube crawlers eliminate the burden of isotope transport and licensing, while gamma source systems offer the advantage of independence from cables and mains power under site conditions. Image quality is controlled through the source-to-film distance, the focal spot size, the exposure time and the selected film class/detector resolution; the required sensitivity is verified on every shot using wire-type image quality indicators (IQI/penetrameters).

The specific value of the method in application is that it reveals volumetric discontinuities occurring through the pipe wall thickness (porosity, slag inclusions, lack of weld penetration, undercut) and planar flaws at a single glance, for the whole seam. Thanks to the panoramic geometry, magnification and distortion across the image are minimised, which increases the consistency of evaluation. Radiation safety is an inseparable part of this method: restriction of the controlled area, dosimeter monitoring and working in compliance with the legal regulations are mandatory; for this reason the application is carried out only by authorised personnel and with licensed equipment.

After the films have been processed or the digital images handled, the discontinuity dimensions are compared with the limit values of the relevant acceptance standard (for example API 1104 or those defined by reference to the applicable construction code); each seam is reported as "accept" or "reject" and, where necessary, a repair and re-inspection cycle is defined. TÜV AUSTRIA SILA KALİTE performs pipeline inspection with crawlers as a TÜRKAK-accredited inspection body under TS EN ISO/IEC 17020, with certified personnel and equipment under traceable calibration; the findings obtained therefore form an independent and reliable basis against national and international acceptance criteria.