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

Abstract

Summary Once a crack is initiated by cyclic stress in a drillstring 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 defense against component fatigue failure (after design) is detection of the initial crack, conventionally by magnetic-particle or dye-penetrant inspection (respectively, MPI or DPI). A clean bill of health from MPI or DPI means only 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 from one another. No reviewable 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 defect is present. ACFM is able to determine the length and depth of a defect. It does not require a clear line of sight between operator and crack location. All data are recorded electronically and the evidence for the existence or nonexistence of a crack can be 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.