Detection of stress concentration zones using stand-off magnetometry based on the magnetomechanical effect has recently been applied to pipeline inspection.
This study addresses the lack of scientific evidence encountered in the development of the technology by using a combination of laboratory experiments, field observations, and computer simulations to study the magnetic indication of local stress concentrations in the bulk material.
It then offers new techniques to determine features of underground pipelines, and evaluates their performance using field survey data and standard inspection reports.
The models proposed in this study are capable of simulating the effects of stress cycles on magnetisation of steel bars and pipes placed in the earth's magnetic field.
The experimental and simulation results have shown that the magnetic indication of a local stress concentration zone (SCZ) is due to the distribution of magnetisation between the local SCZ and the surrounding area or the bulk material, which has explained the reverse in the polarity of the magnetic indication when varying the applied stress. It also implies the possibility to monitor stress conditions in ferrous material.
It is proposed that the gradient magnetic field should be used in order to extract the magnetic indication from the measured magnetic field, together with a parameter K as the criteria for detection and characterisation of local SCZ using the remote magnetic field.
It has found that K is linear with the initial magnetic condition induced in low field at a given stress.
Inversely, at a given initial condition, variation of K with stress follows the stress-magnetisation relationship of the material. K is more sensitive with stress at stronger initial conditions.
The study has also established the quantitative exponential relationship between K and the measurement distance, which implies a technique to solve for stress condition from the remote magnetic field.
A study on circumferential welded joints of pipelines has found magnetic features of the welds, which implies the possibility to locate them using above-ground surveys. A technique is proposed and is capable of locating 70% of the actual welds with an offset of 3 meters and the probability of false call of 20% for the pipelines of 17 meters constructed length buried with more than 2 meters depth of cover.
A new practical technique to estimate the depth of cover of underground pipelines from the remote passive magnetic field, which has the tolerance of 8% of the measurement depth, is also proposed.
Importantly, this study proposes a preliminary technique to detect SCZs in underground pipelines using the remote magnetic field, which can detect side bends and sag bends, and shows promises for detection of SCZs caused by mechanical defects.
The development of these techniques are important milestones in non-invasive remote pipeline condition monitoring.
