The end of radiographic film for pipe inspection?

Radiography is the most common way of inspecting pipes for corrosion and weld defects – usually using traditional X-ray film. So why is it taking so long for the industry to migrate to a faster and more cost-effective digital approach?

Digital X-ray film (DFX) takes a different approach to the challenge of inspecting pipe welds, with flexible digital X-ray technology designed to be used just like radiographic film – but with a digital output. This approach allows multiple pipeline welds to be prepared for X-ray inspection in advance, before the crawler is inserted – increasing X-ray inspection efficiency and reducing down time significantly compared with traditional film (Figure 1). So, will 2024 be the year that we see the first truly flexible digital re-placements for traditional radiographic film?

Radiographic film for pipeline inspection

The main advantage of radiographic film is the fact that it is thin and flexible, enabling it to be wrapped around a pipe while the image is taken. And the film is available on rolls that can be cut to length to match almost any pipe size.

Image intensifier screens – in the form of thin layers of lead – are usually required in front and/or behind the X-ray film. These have to be loaded into light-blocking envelopes in a dark room, to pre-vent light affecting the film and the subsequent image quality. But modern roll packs are provided with the image intensifier screens already attached to the film and pre-loaded into a light-blocking sleeve, so that loading doesn’t have to be done on site.

After exposure, the film is removed from the sleeve in a dark room and developed manually or automatically to produce the final X-ray film/image. For traceability reasons, these films have to be digitised and electronically stored – or physically stored for up to 60 years in environmental chambers.

Where access allows, pipeline inspection can be done using an X-ray or gamma-ray source mounted on a crawler that travels down the pipe to the weld under inspection. With the film wrapped around the pipe, it is then possible to do a panoramic exposure of the whole weld on a single film – a method called single wall single image (SWSI) (Figure 2).

This is a very efficient method for capturing the image – but has some drawbacks. Firstly, the pipeline has to be taken out of com-mission to allow access for the crawler. The second drawback is the size of the film to be developed. Depending on the diameter of the pipe, the film could be over 1 m long – making developing it more complicated.

When access to the pipe is not possible, a double wall single image (DWSI) approach is used (Figure 3), where the X-ray or gamma-ray source is on the outside of the pipe, with individual strips of radiographic film on the opposite side of the pipe. This method is much slower than the SWSI approach due to the number of images that have to be captured per weld joint. It also requires a lot more film and the associated chemicals to develop the final images.

So, although X-ray film itself is low cost, thin and flexible, it involves a lot of preparation, consumables (chemicals, image intensifiers and light-blocking sleeves) and storage costs – making its total cost of ownership relatively high. Which brings us back to the question of why the industry has not switched to digital detectors.

Current digital detectors for pipeline inspection

Digital flat-panel (DFP) detectors – also sometimes called digital radiography (DR) – have their origins in the medical industry. They are typically square, flat, rigid, heavy and expensive. More modern DFP detectors are more robust, with protective housings – but that adds more weight and cost to the panels.

The biggest issue with DFP detectors for the pipeline industry is the fact that they are not flexible – which means they can’t be wrapped around a pipe. For larger diameter pipes, this doesn’t lead to any significant distortion of the images – but it does mean the weld joint being inspected has to be inspected slowly, with many images captured to cover the full pipe circumference. These images can be captured with the source (gamma or X-ray) mounted on a crawler inside the pipe, as described above, using the SWSI approach (Figure 4). In this case, however, the DFP detector has to be moved several times around the pipe to capture the full image of the weld. These digital images are later stitched together digitally to form a complete weld image. The same issue arises if the DWSI method is used.

There are more automated alternatives for digital radiography of large-bore pipes. One option involves attaching welding bands or similar support rails on either or both sides of the weld under inspection. An X-ray source mounted on a motorised stage is attached to the welding band on one side of the pipe, with a DFP detector on a similar stage on the opposite side of the pipe. When the set-up is complete, the source and detector are rotated around the pipe to capture a continuous image of the weld (Figure 5). The same method can also be deployed with the source mounted on a crawler inside the pipe (Figure 6). The main disadvantage of this method is the set-up time to mount the rails, the X-ray source and the detector.

Another alternative is to use a DFP, with a motorised stage, held to the pipe with permanent magnets. The DFP on the motorised stage is then rotated around the pipe with the source on a crawler inside the pipe. This method has the advantage that it doesn’t require the installation of welding bands or rails on the outside of the pipe on which to mount the source and detector. The use of magnets to keep the motorised stage and DFP in contact with the pipe, however, limits…

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