It is necessary to find out whether there are metal defects on underground gas distribution pipeline without excavation in order to establish safety strategies for replacement or maintenance. The metal defects are classified into general corrosion, stress corrosion cracking, lamination, pits, and metal loss which cause the leak or partial damage to a gas pipeline.
Therefore it is required to develop an effective method in the form of in-line inspection concept that can be implemented internally to a gas pipeline. Magnetic flux leakage or ultrasonic inspection method can be a candidate for this purpose, but these are improper to gas distribution pipeline which is in environment of small diameter and low pressure, due to their weight problem of using permanent magnet and couplant problem, respectively.
In this study, theoretical formulations of magnetic vector potential and magnetic flux density including axial and radial wave number based on Maxwell`s equations are presented analytically to find out the changes of amplitude and phase in magnetic flux density and the consequent induced voltage which can be a criterion in the remote field eddy current principle for detecting the metal defects in the gas pipeline. Three dimensional finite element analysis is also presented to analyze the physical phenomenon in metal defects according to each defect size, excitation frequency and moving velocity, which can overcome inaccuracy in two dimensional approach using axisymmetry condition and simulate local pits condition occurred severely in real gas pipeline, otherwise only metal loss such as whole circumferential decrease in wall thickness can be modeled. Some experimental works are performed to validate the analytical and finite elemental results regarding magnetic flux density and induced voltage in detector.