Abstract: A new method using alternating current field measurement(ACFM) has been proposed in order to nondestructively inspect the critical welds of the in-service offshore platform drilling rig substructure with coatings.The detection principle of ACFM method is first introduced and is compared and analyzed with other nondestructive testing methods for surface defects,including MT and ET.Based on the structural characteristics and the special working environment of the platform drilling rig substructure,the corresponding ACFM detection system is presented,which is applied successfully to test surface or sub-surface defects of the critical welds of the rig substructure,and the scanning methods and signal recognition for the longitudinal and transversal cracks are emphatically discussed.At last,the general process and detection procedure of engineering application using ACFM method are summarized systematically,providing a good reference for ACFM inspection of other engineering structures.