New approaches for the quantitative wall thickness measurement in gas pipelines
In this paper a new wall thickness measurement sensor for pipeline inspection using electro magnetic acoustic transducer (EMAT) technology in combination with pulsed eddy current (EC) and magnetic flux leakage (MFL) is presented. The EMAT is developed for excitation and detection of linear polarised shear waves at normal incidence with the use of a horizontal magnetisation of the specimen. The sound field is optimised for the measurement of the wall thickness of steel plates and pipe joints. Special focus is on determining the remaining wall thickness in case of metal loss (e.g. general corrosion, pitting corrosion). If metal loss is present at the transducer-near side, it is not possible to determine the remaining wall thickness because the lift-off of the EMAT sensor caused by the wall thickness reduction may lead to a complete loss of the ultrasonic signal. In order to ensure reliable measurement for this case, the EMAT technique is combined with the EC technique and MFL. The EC technique, that uses the EMAT excitation signal as a pulsed EC excitation, is able to detect metal loss at the transducer-near side by measuring the transducer lift off, simultaneously to the electromagnetic ultrasonic inspection. Additionally, a MFL signal is derived by making use of the horizontal magnetisation and the EMAT receiver coil as flux leakage sensor.