Remote field eddy current probes for the detection of stress corrosion cracks in transmission pipelines

Abstract

<p>Magnetic flux leakage (MFL) is a technique used widely in non-destructive testing (NDT) of natural gas and petroleum transmission pipelines. This inspection method relies on magnetizing the pipe-wall in axial direction. The MFL inspection tool is equipped with an array of Hall sensors located around the circumference of the pipe, which registers the flux leakage caused by any defects present in the pipe-wall. In general, the tool is constructed such that the magnetization occurs in axial direction as a result of which this method is not sensitive to axially oriented defects.;One type of defect, which is of a growing concern for the gas and petroleum industry is the stress corrosion crack (SCC). The SCCs are a result of aging, corrosion, fatigue and thermal stresses. SCCs are predominantly axially oriented and are extremely tight, which makes them impossible to detect using the current inspection technology.;One possible solution to this problem is to utilize the remote field eddy current (RFEC) effect to detect axially oriented defects. The RFEC method has been widely used in the industry in the inspection of tubular products. The method uses a pair of excitation and pick-up coils. The pick-up coil is located in the remote field region, usually two, three pipe-diameters away from the excitation coil. With RFEC the presence of defects is detected by the disturbance in the phase of the signal measured by the pick-up coil. Unlike conventional eddy current testing the RFEC method is sensitive to defects on the exterior of the inspected product, which makes it a good candidate for the development of in-line inspection technology.;This work focuses on the development of non-destructive testing technique, which uses remote field eddy currents induced by rotating magnetic field (RMF). A major advantage of the RMF is that it makes possible to not only detect a defect but also localize its position in circumferential direction. Also, it could potentially allow detection of defects, regardless of their shape and orientation. In this work the RFEC-RMF technique is investigated and is shown to be useful tool in the detection of axially oriented, circumferentially oriented and skewed notches, SCCs and round defects. A data acquisition system capable of performing the RFEC-RMF measurements automatically was designed.</p>