Overcoming limitations of current in-line inspection technology by applying a new approach using spiral magnetic flux leakage (SMFL)
Proceedings Publication Date
Dr. Mike Kirkwood
Dr. Mike Kirkwood
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The use of conventional Magnetic Flux Leakage (MFL) In-line Inspection (ILI) tools are commonplace in the inspection and determination of the severity of metal loss in gas/liquid transmission pipelines. Magnetic inspection technologies are well suited to inspection of steel pipelines as they can flow at around the same rate as the product, they can run in gas and liquid mediums and are tolerant to debris that may reside in the lines even after cleaning. However, there is one draw back and this is the direction of the magnetic field has an impact on what the tool will “see”.

Conventional or axial MFL ILI tools induce a magnetic field that is aligned to the axis of the pipe being inspected. Due to the physics of flux leakage, traditional MFL will see defects that are perpendicular to the field (like a rock in a flowing stream) whereas defects that are long and slim (like a log in a flowing stream) will be difficult to see and even more difficult to size accurately.

The industry response to this phenomenon was simple, in theory, turn the magnetic flux through ninety degrees such that the flux is now radial to the pipe and thus the so named Transverse Field Inspection (TFI) tool will see long narrow defects. TFI can now see the “logs” but then loses accuracy for the “rocks”. Furthermore, the transposition of axial to circumferential field means that the magnets now get in the way of the sensors and thus the tool requires overlap of the magnets and sensors to work practically.

T.D. Williamson (TDW) has been developing a new approach which turns the magnetic field through an angle thus moving the flux into a spiral around the pipe wall. The Spiral MFL (SMFL) technology combines the best of both worlds and can see not only the “logs” but also the “rocks” which means operators can now use one tool to detect (see) and accurately dimension (size) defects that would require both an MFL and TFI to be used.

This paper will present the development of the tool from its inception to application and give the audience a view of our experience of what the tool is capable of through laboratory and field experiences and what this means to operators worldwide.

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