An Automated Tool-Joint Inspection Device for the Drillstring
Authors: Moyer Mark C. , Dale Bruce A. , Kusenberger Felix N. 
Published: December 2007

Summary
This paper discusses the development of an automated tool joint inspection device-i.e., the fatigue crack detector (FCD), which can detect defects in the threaded region of drillpipe and drill collars. Inspection tests conducted at are search test facility and at drilling rig sites indicate that this device can detect both simulated defects (saw slots and drilled holes) and service-induced defects, such as fatigue cracks, pin stretch (plastic deformation), mashed threads, and corrosion pitting. The system operates on an electromagnetic-flux leakage principle and has several advantages over the conventional method of magnetic particle inspection.

Introduction
There has been concern in the petroleum industry over the increasing number and cost of downhole drillstring failures that plague production operations worldwide. Exxon Co. U.S.A. studies have shown that a high percentage of these failures occur in the connections of percentage of these failures occur in the connections of the drillstring, thus calling to question the quality of currentend-area inspections. Typically, separations occur in the second or third thread root from the shoulder in the pin member and in the threads just outside the makeup pin member and in the threads just outside the makeup region in the box member (Fig. l). Table l presents results of those studies, showing failureratios for the drillstring. Note that a high percentage of drillstring failures occur in the tool-joint connections.
Inland and offshore records for 1977 through 1980 indicate that drillstring separations 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 cracks that allow drilling fluid to penetrate the drillpipe without actually parting the string. Records from the Persian Gulf' dating from 1974 to 1977, which include separations and washouts, showed an average cost of $47,000 per incident and one failure for every 6,500 ft [1980 m] drilled.
It is generally agreed that most drillstring failures are caused by some form of metal fatigue. Fatigue cracks start and grow as a result of cyclic stresses and corrosion, until failure occurs. The petroleum industry attempts to guard against these failures by periodic inspections of the tool joints to check for cracks and excessive pin stretch.
Conventional inspections consist of using magnetic particles to locate cracks visually and a thread profile particles to locate cracks visually and a thread profile gauge to check for excessive pin stretch (Fig. 2). These inspections are highly dependent on obtaining a very clean, dry surface as well as the skill, patience, and visual acuity of the inspector. Calibration method sand magnetization-field strength checks are almost nonexistent in conventional tool-joint inspections. As a result of the questionable reliability of conventional methods, drillstrings often have failed shortly after a rig-site inspection.
Based on technology developed for the inspection of the threads on high-strength steel aircraft bolts, a development program was undertaken to build an automated device (the FCD) for the inspection of the threaded region of tool joints. Fig. 3 shows a typical laboratory setup used to develop a prototype of the FCD for pin and inspection and to confirm applicability of the method for box-end inspection.

New Inspection Method
The automated FCD system shown in Fig. 4 consists of a detector head that threads onto the tool joint, a control console that regulates the system'soperation and output, and a head suspension hoist. The detector head contains a solenoid, which produces a strong longitudinal magnetic field in the tool joint as the flux flows in a closed loop formed by the tool joint, a steel guide, and the 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 magnetic field is pronounced the perturbation. The magnetic field is produced in a direction roughly parallel with the produced in a direction roughly parallel with the longitudinal axis of the pipe. The off-axis flux perturbations caused by the presence of the flaw are then perturbations caused by the presence of the flaw are then picked up by a magnetic sensor probe. This probe may be a picked up by a magnetic sensor probe. This probe may be a semiconductor material such as a Hall-effect element, magnetodiode, or magnetoresistor, all of which measure the magnetic flux statically. Or the probe may be a wire coil that dynamically measures the magnetic flux.
JPT
P. 982