The hunt for hard spots


Matt Romney, Senior Product Manager Global Pipeline Integrity, T.D. Williamson, USA, discusses how low-field magnetic inspection technology enhances general pipeline integrity.

Considering the variety of threats to pipeline integrity – everything from manufacturing flaws to corrosion to backhoes that unintentionally strike the pipe – being able to detect, identify and size different types of defects and damage is non-negotiable. The key, though, is having the right technology.

For decades, inline inspection (ILI) has been a tried-and-true method for keeping pipelines operating safely throughout their lifecycle. After all, you can’t fix what you don’t know exists, and ILI literally provides insight into what’s happening inside your pipeline so you can make repairs and prevent incidents.
Traditional ILI systems typically incorporate technologies like geometry (GEO) and magnetic flux leakage (MFL) to gather pipeline integrity data. But when you’re dealing with more complex concerns such as hard spots where cracking might initiate, or material verification which is necessary for determining fitness for service, those tools alone may not provide all the information you need.

For example, while MFL is widely regarded as a state-of-the-art technology for assessing volumetric metal loss, early industry research concluded there’s no clear correlation between the induced saturation magnetic field for a material and its commonly measured mechanical properties, such as yield. In other words, MFL on its own can’t deliver enough information to assign pipe grade, one of the key elements in determining material properties.

Adding low-field magnetic flux leakage (LFM) capabilities to your ILI toolkit, however, gives you a more comprehensive picture of your pipeline’s health. At the lower magnetic energy state, minor changes in material permeability can be detected, and that can lead to a more strategic approach to mitigating risk.

Hard spots and hydrogen

While most integrity threats occur after the pipeline is installed, hard spots – areas where the material hardness is greater than the surrounding material – are often there almost from the start. This is because many arise early in the manufacturing process when quenching, the procedure that cools heat-treated plate, is not adequately controlled.

Hard spots are susceptible to cracking, especially in the presence of hydrogen. With decarbonisation initiatives expected to increase the volume of hydrogen travelling through existing pipelines in the years ahead, hard spots that have been stable for long periods may become more vulnerable to cracking and its serious consequences.

Although hard spots have generally been associated with pipe manufactured by a single mill before 1970, they can occur in other pipes too, which makes locating and repairing them more important than ever. Fortunately, LFM technology has a distinct advantage when it comes to detecting hard spots.

Like other forms of magnetic flux leakage, LFM magnetises the steel being exam-ined. The magnetic field ‘leakage’ will increase anywhere there is a defect. But unlike traditional high-field MFL technologies, which induce a magnetic field be-yond the material saturation point, LFM technology induces a lower magnetic energy state in the pipe wall. At this lower magnetic energy state, characteristics of the pipe material – some of the details whose signals are overpowered by high-field MFL – are visible in LFM data. While often both the MFL and LFM tech-nologies will detect a hard spot, the localised hardened area will have different material properties, so it will react differently to each applied magnetic field. The unique responses between MFL and LFM make it much easier to identify…

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