Electromagnetic testing emt-mft chapter 9b

One such system for automated thread inspection on drill pipe and collars is
described: The device consists of an electromagnet and an array of sensors
mounted outside a nonmagnetic cone that threads onto the tool joint. The
assembly is driven in a helical path along the threads by a motor/clutch
assembly. To minimize the leakage flux signal variations caused by the
threads, signals from the sensor array are compared differentially. The
system is capable of operating in a high field strength mode for the detection
of cracks and corrosion pits and also in a residual field mode for the detection
of other forms of damage. At last report, the system was undergoing field
tests and was found to offer advantages, in terms of ease of application and
defect detection, over the magnetic particle technique normally used for
thread inspection.
Charlie Chong/ Fion Zhang