Development of Pipeline Measurement Sensors for Large Diameter Foam Pigs

Abstract

The Pathfinder in-line pipeline inspection tool, produced by Pipeline Innovations, is a maintenance tool for verifying the integrity of a pipeline. It uses a novel magnetic sensing system to measure the bore of pipeline it passes through, with its magnetic sensors encased within the watertight core of the tool, and a ring of permanent magnets embedded into the tool's body of compressible foam. The changes in measured magnetic field are proportional to the changes in magnet position, which is in turn proportional to pipeline bore.

This system suffers from a key drawback of a low overall sensitivity caused by a pair of conflicting factors: the magnetic sensitivity is decreased as the distance between the mounting position of the embedded magnets and sensor is increased, and in contrast the mechanical sensitivity is decreased under the inverted conditions. Therefore, this work investigates several novel designs of magnetic fluxguides - shapes of high permeability material - to increase the magnetic flux density and so sensitivity in the system without affecting the mechanical sensitivity.

An initial study of the magnetic behaviour of the existing Pathfinder sensing system is performed and compared to alternative arrangements of magnets. Whilst it has been noted that both increasing or decreasing the number of magnets could produce higher sensitivities, the work continues to consider 24 magnet systems as a direct comparison to the Pathfinder tool, therefore giving a baseline sensitivity of 0.4293 T/m. This is followed by the main investigation of fluxguide designs.

Initially, two designs of fluxguide have been compared: a cuboidal rod, and a tapered pyramidal frustum. The two designs are shown, when optimised, to be within 0.05 T/m of sensitivity, at around 1.2 T/m each, or a 190% increase from the baseline. The rod is of the two the more effective when considered with the greater simplicity of its construction, whilst the frustum opens options of further variants.

A further improvement of using a flat plate concentrating shield is also investigated, with an optimised design yielding a 78% improvement in measured flux density across the range of compression. It is also noted that this behaviour is independent of prior discussed fluxguides, and can when combined with the frustum produce a sensitivity of approximately 2.2 T/m, or a 412% increase.

Finally, novel variant frustum designs are investigated, with focus on increasing the compressibility and so potential mechanical sensitivity of the measuring system. A design incorporating a 'diamond' of magnetic material is noted to be able to produce significant sensitivity increase over a tunable range, a feature beneficial to the Pathfinder tool. With this, an optimised design is found to produce an increase in sensitivity of 347% over a 5 mm compression range, improving to a 636% increase with the inclusion of the flat plate concentrator, at the cost of 0.2 T/m sensitivity over the broader range.