Atlantic Richfield Company

David L Foreman

Steven G Petermann

Pedro F Lara

Stanley T Low

A weld detection system for a pipeline survey pig includes a plurality of weld sensor units mounted spaced apart circumferentially on fore and aft pig support members and arranged in close proximity to the pipe wall. The sensor units comprise permanent magnet members for generating a magnetic flux field around a wire coil which is modified upon passing a girth weld or other magnetic anomaly in the pipe to produce a voltage signal which is conditioned by an amplifier and filter circuit to a signal suitable for input to a digital activation circuit. The digital activation circuit receives conditioned signals from opposed pairs of sensor units on the support members to distinguish transverse girth welds from spiral or longitudinal welds. The digital activation circuit includes circuit elements for timing the sensor output signals of the fore and aft detector arrays for measuring the velocity of the survey pig through the pipe. Each sensor unit comprises a high intensity permanent magnet disposed on a compact wire coil and support spool and arranged so that the flux field extends through the pipe wall during normal operation of the system. The output signal from the sensor units is capable of being conditioned for use in determining changes in pipe wall thickness and changes in axial strain on a pipe section of known length.

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Tucson, AZ, United States of America

David L Foreman