Reduction in Fatigue Failures through Crack Detection by Alternating Current Field Measurement

Once a crack is initiated by cyclic stress in a drill string component it will grow under further service loading and will fail when insufficient uncracked material remains to carry the applied load. Because the levels of cyclic and subsequent stress in drilling may be unknown - or unknowable - the drilling equipment supplier's main defence against component fatigue failure (after design) is detection of the initial crack, conventionally by Magnetic Particle or Dye Penetrant Inspection. A clean bill of health from conventional inspection means no more than that no crack indications were found. The likely locations of cracks (e.g. thread roots in box connections) are often difficult to examine. Detection and interpretation are subjective, and depend on the skill of the inspector. A crack and a surface defect may be indistinguishable. No re-viewable evidence of component inspection is left to allow an audit of inspection previously performed. Alternating Current Field Measurement (ACFM) induces a current in the surface of a component. If ACFM detects a perturbation in the magnetic field created in the free space above the surface, a surface break is present. ACFM is able to determine length and depth, and hence the severity of any defect. It does not require a clear line of sight between operator and crack location. Because all data is recorded electronically, operator interpretation of results may be eliminated completely and any inspection revisited. The paper describes the theory of the technique, the equipment used and practical results from the first application of ACFM to downhole motor components. The tests, conducted in the Middle East, allowed complete confidence that an ACFM clean bill of health meant no cracks and not no cracks discovered. Components laid down on hours or MPI crack indications were returned to service when ACFM reinspection found no cracks. During the period of the pilot test, no downhole motor fatigue failures occurred.