Abstract
In recent years, the rotating electromagnetic field eddy current (RoFEC) testing technique has attracted widespread attention due to its various advantages for inspecting tubular structures. However, most of the related work was focused on detecting inner wall defects on metal pipes using feed-through probes, which are often not applicable for outer wall defect detection. This work pushes forward the encircling RoFEC technique and demonstrates its feasibility for detecting outer wall defects. First, the basic principle of the encircling RoFEC technique is introduced. A three-dimensional finite element (FE) model was built in COMSOL to analyse the distribution of the rotating electromagnetic field and study the interaction between the defects and eddy currents. The axial component of the resultant magnetic field due to defects was selected as a characteristic signal and obtained from the FE models to study the factors, including pipe tilt, defect circumferential location, defect orientation and defect size, that influence the detection performance. An encircling RoFEC system using a probe with six excitation windings and a single bobbin pickup coil was constructed and used to inspect a steel pipe with one axial and one circumferential defect. The obtained voltage signal due to defects can form a Lissajous pattern in the impedance plane and be used for defect evaluation. The results showed that the encircling RoFEC technique can detect outer wall defects of both orientations and determine the circumferential location of the defect.
Highlights
Steel pipes are widely used in the petroleum industry
This paper proposed an encircling rotating electromagnetic field eddy current (RoFEC) technique to detect outer wall defects on steel pipes
The working principle of the RoFEC technique was introduced, and the effects of pipe tilt, defect circumferential location, defect orientation and defect size on the inspection performance were analysed in detail using the finite element (FE) method
Summary
Steel pipes are widely used in the petroleum industry. In the process of long-term service, steel pipes are prone to defects, such as deformation, corrosion, and cracks [1], leading to the possible leakage of the conveying medium and/or failure of the whole structure [2]. Extensive efforts have been presented into the nondestructive evaluation (NDE) techniques capable of detecting defects in steel pipes, such as eddy current inspection [3] to [5], magnetic flux leakage inspection [6] and [7], ultrasonic guided wave inspection [8] and [9], etc. The above-mentioned techniques have been widely used in steel pipe inspection, but some of them have inherent limitations, e.g., traditional eddy current can only detect defects in the area under the probe; circumferential and axial scanning are required to achieve full coverage; Some detection techniques, such as magnetic flux leakage and alternating current field measurement (ACFM), are more sensitive to cracks in certain directions. The rotating electromagnetic field eddy current (RoFEC) method [12] has emerged as a useful NDE technique for plate-like structures [13] to [15], tubular structures [16] to [18], and welding seams [19] and [20]
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