Large Stand-off Magnetometry for Non-invasive Pipeline Inspection

This research sought to answer the question what improvements can be made to the magnetometry survey tool developed at the University of Leeds in collaboration with Speir Hunter to increase the detection and location of features on steel natural gas pipelines. Natural gas is a major component of the UK's energy usage and the pipeline network which transports this gas is vital to the UK's energy infrastructure. The predominant risk to the supply of gas is from infrastructure failure, the largest primary cause of these failures on the aging pipeline network is corrosion of the pipe wall. This corrosion develops over time, early detection and repair of these features can prevent these pipeline incidents. Pipeline incidents have many consequences such as loss of life, interruption of gas supply, property damage, regulator fines and reputational damage to the pipeline operator. Current inspection techniques are not available for all pipelines therefore complimentary inspection techniques are required. One such technique is large stand-off magnetometry. To answer the primary research question this was broken down into three further specific questions. These were: how can the survey data best be presented to maximise anomaly identification, how does the magnetometer separation in the array effect the sensitivity of the array to anomalies in the magnetic gradient field, and, how does the location of the anomaly in the magnetic field relate to the location of the feature on the pipeline. Laboratory experiments were conducted using 1/2", 3/4", 1" and 1 1/2" diameter steel conduits, AlNiCo magnets and magnetised short sections of 1" diameter conduit to conduct small scale experiments on known simulants. Linear integral convolutions were found to be the best way to present the data from magnetometry scans for the easy identification of anomalies in the magnetic field by the user. A peak angular magnetometer separation at the conduit centreline of 32° at a height of 50 mm and 23$^{\circ}$ at 300 mm were found, the relationship between this peak and the height above the centreline of the conduit becomes steady at 23°. The anomaly location in the magnetic field was found to deflect from the pipe feature along the conduit, this showed an average deflection of 16° which was linear with increasing height above the conduit. These findings give recommendations for the development of the large stand-off magnetometry survey array to increase the number of magnetometers in the array to five, to increase the separation between the centre and outermost magnetometers to 0.75 m and to add an additional magnetometry array layer. These will allow the data to be presented in a way which makes anomaly identification easier, increase the amplitude of anomalies by approximately 20% in the magnetic field increasing the sensitivity of the array and allow the angular deflection along the pipe to be determined to increase the locational accuracy of the feature by approximately 1-2.5 m.