Detection of metal defects on gas distribution pipeline by remote field eddy current (RFEC) using finite-element analysis

It is necessary to find out whether there are metal defects on underground gas distribution pipelines 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 leak or partial damages to a gas pipeline. Therefor e, it is required to develop an effective method in the form of an in-line inspection concept that could be implemented internally into a gas pipeline . In this study, theoretical formulations of the magnetic vector potential an d magnetic flux density including axial and radial wave numbers based on Ma xwell equations are presented analytically to find out the changes of ampli tude and phase in the magnetic flux density and the consequent induced volt age which can be a criterion in the remote field eddy current principle for detecting the metal defects in gas pipelines. Three-dimensional finite-ele ment analysis is also presented to analyze the physical phenomenon in metal defects according to each defect size, excitation frequency and moving vel ocity, which can overcome inaccuracy of the two-dimensional approach, using axisymmetry condition, and simulate local pit conditions occurred severely in real gas pipelines; otherwise only metal loss such as whole circumferen tial decrease in wall thickness can be modeled. Some experimental works are performed to validate the analytical and finite elemental results regarding the magnetic flux density and induced voltage in the detector.