Reduction in Fatigue Failures Through Crack Detection by Alternating Current Field Measurement
- T.M. Gaynor (Sperry-Sun Drilling Services) | D.L. Roberts (Sperry-Sun Drilling Services) | E. Homan (Sperry-Sun Drilling Services) | W. Dover (University College)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- March 1997
- Document Type
- Journal Paper
- 37 - 42
- 1997. Society of Petroleum Engineers
- 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 4.5.7 Controls and Umbilicals, 5.5.2 Core Analysis, 1.10 Drilling Equipment, 1.6.1 Drilling Operation Management, 1.6 Drilling Operations
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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.
Crack Detection Methods
Magnetic particle inspection.
The near-universal method of detecting cracks in the carbon steels used for downhole motor body components is commonly known as MPI or "black-light inspection." The component is magnetized and sprayed with a medium containing magnetic particles in suspension. These cling preferentially to surface defects. When the component is viewed under an ultraviolet light, the surface defects can be seen as bright lines.
There are several drawbacks inherent in this system; the principal one is that the process does not identify cracks, but makes surface defects more easily visible.
The inspector must have line of sight to the defect. Because, for example, a cracked box connection is most likely to crack in the root of the innermost thread, this may require a certain amount of physical contortion, and mirrors, to see.
Further, the process allows measurement of the length of a defect, while what is important is its depth. The only practical way to establish whether a defect is a crack, and how deep it is, is to grind or polish it out and make a judgment. If the "crack" disappears, it was a surface defect. If it does not, further grinding may be attempted or the component scrapped.
Essentially, the process is on-the-spot assessment. It relies on the quality of the complete operation from cleaning to defect identification. It is also subject to variations in reliability depending on the consistency of interpretation between different operators.1 No audit trail is possible because the evidence is not available as hard copy to be revisited.
ACFM is a crack-detection method which has been in use for offshore structure and aerospace inspection for several years. The technique works on magnetic and nonmagnetic, ferrous and nonferrous metals by detecting anomalies in a current flow along a metal surface. If the surface is defect-free, the current flow will be uniform. If the current flow is disturbed, the disturbance can be detected.2
ACFM creates a uniform "thin film" electrical field in a metal surface, and a corresponding magnetic field is created in the free space above it. A crack or defect on the metal surface may be thought of as causing the current to deflect around and beneath the crack. These current deflections cause anomalies in the magnetic field. By measuring perturbations in this magnetic field the cracks or surface defects which caused them can be detected.
The value and direction of the perturbations in the magnetic field allow a close estimate of the length and (more importantly) the depth of the crack or defect, to be obtained. Because the values of the components of the magnetic field vary in a consistent pattern (Fig. 1) when a crack is detected, operator interpretation can be eliminated and the process automated.
At a practical level, because it detects anomalies in the free space above the surface, ACFM detecting probes need not contact the parent metal. This simplifies the design, eliminates lift-off problems (losing electrical contact) and increases the life of probes. Further, cleaning to "bright metal" condition is not necessary, reducing the time required to complete an inspection.
The principles are explained in more detail in Appendix 1.
ACFM Inspection Station.
The system used in the test consisted of a hand-held probe connected by an umbilical to a crack microgauge. The crack microgauge (a 41´31´16 cm box weighing 14 kg) contains an oscillator and amplifier which outputs to the magnetic field generator contained within the hand-held probe. The probe returns sensor readings to the microgauge, which amplifies the signal, digitises it, and transmits it to a standard laptop computer. The complete system can be hand-carried.
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