An Automated Tool-Joint Inspection Device for the Drillstring
- Mark C. Moyer (Exxon Production Research Co.) | Bruce A. Dale (Exxon Production Research Co.) | Felix N. Kusenberger (Southwest Research Inst.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- June 1984
- Document Type
- Journal Paper
- 982 - 986
- 1984. Society of Petroleum Engineers
- 1.11 Drilling Fluids and Materials, 4.1.5 Processing Equipment, 1.10 Drilling Equipment, 4.1.2 Separation and Treating, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.6.1 Drilling Operation Management, 4.2.3 Materials and Corrosion, 1.6 Drilling Operations, 5.5.2 Core Analysis, 1.4 Drillstring Design
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This paper discusses the development of an automated tool joint inspectiondevice-i.e., the fatigue crack detector (FCD), which can detect defects in thethreaded region of drillpipe and drill collars. Inspection tests conducted at aresearch test facility and at drilling rig sites indicate that this device candetect both simulated defects (saw slots and drilled holes) and service-induceddefects, such as fatigue cracks, pin stretch (plastic deformation), mashedthreads, and corrosion pitting. The system operates on an electromagnetic-fluxleakage principle and has several advantages over the conventional method ofmagnetic particle inspection.
There has been concern in the petroleum industry over the increasing numberand cost of downhole drillstring failures that plague production operationsworldwide. Exxon Co. U.S.A. studies have shown that a high percentage of thesefailures occur in the connections of percentage of these failures occur in theconnections of the drillstring, thus calling to question the quality of currentend-area inspections. Typically, separations occur in the second or thirdthread root from the shoulder in the pin member and in the threads just outsidethe makeup pin member and in the threads just outside the makeup region in thebox member (Fig. l). Table l presents results of those studies, showing failureratios for the drillstring. Note that a high percentage of drillstring failuresoccur in the tool-joint connections.
Inland and offshore records for 1977 through 1980 indicate that drillstringseparations have an average cost of about $106,000 and occur on an estimated14% of all rigs. These figures do not include washouts-i.e., fatigue cracksthat allow drilling fluid to penetrate the drillpipe without actually partingthe string. Records from the Persian Gulf' dating from 1974 to 1977, whichinclude separations and washouts, showed an average cost of $47,000 perincident and one failure for every 6,500 ft [1980 m] drilled.
It is generally agreed that most drillstring failures are caused by someform of metal fatigue. Fatigue cracks start and grow as a result of cyclicstresses and corrosion, until failure occurs. The petroleum industry attemptsto guard against these failures by periodic inspections of the tool joints tocheck for cracks and excessive pin stretch.
Conventional inspections consist of using magnetic particles to locatecracks visually and a thread profile particles to locate cracks visually and athread profile gauge to check for excessive pin stretch (Fig. 2). Theseinspections are highly dependent on obtaining a very clean, dry surface as wellas the skill, patience, and visual acuity of the inspector. Calibration methodsand magnetization-field strength checks are almost nonexistent in conventionaltool-joint inspections. As a result of the questionable reliability ofconventional methods, drillstrings often have failed shortly after a rig-siteinspection.
Based on technology developed for the inspection of the threads onhigh-strength steel aircraft bolts, a development program was undertaken tobuild an automated device (the FCD) for the inspection of the threaded regionof tool joints. Fig. 3 shows a typical laboratory setup used to develop aprototype of the FCD for pin and inspection and to confirm applicability of themethod for box-end inspection.
New Inspection Method
The automated FCD system shown in Fig. 4 consists of a detector head thatthreads onto the tool joint, a control console that regulates the system'soperation and output, and a head suspension hoist. The detector head contains asolenoid, which produces a strong longitudinal magnetic field in the tool jointas the flux flows in a closed loop formed by the tool joint, a steel guide, andthe steel outer case of the detector head.
This method of crack detection is based on the theories of electromagneticflux leakage inspection. The method analyzes the perturbations (i.e., leakageflux) in an applied magnetic field, caused by the presence of a flaw ordiscontinuity, to detect and even estimate the severity of the flaw; the moreabrupt the discontinuity, the more pronounced the perturbation. The magneticfield is pronounced the perturbation. The magnetic field is produced in adirection roughly parallel with the produced in a direction roughly parallelwith the longitudinal axis of the pipe. The off-axis flux perturbations causedby the presence of the flaw are then perturbations caused by the presence ofthe flaw are then picked up by a magnetic sensor probe. This probe may be apicked up by a magnetic sensor probe. This probe may be a semiconductormaterial such as a Hall-effect element, magnetodiode, or magnetoresistor, allof which measure the magnetic flux statically. Or the probe may be a wire coilthat dynamically measures the magnetic flux.
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