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Summary This paper describes a program for evaluating flaw-detection capabilities of electromagnetic inspection (EMI) and ultra-sonic inspection (UT) units used in oil-country tubular goods (OCTG) inspection. The test program involves use of two 7-in. [17.78-cm] joints of casing; each contains various OD and ID defects. The test procedures are similar to those used by other major operators, but the scope, design, and quality-control measures of Chevron's program make it unique. Results are presented for the 73 evaluations conducted to date, which indicate a two-to-one superiority of UT units in flaw-detection capabilities. The relatively poor performance of EMI inspection prompted re-evaluation of the use of this type of service for OCTG body wall inspection. Introduction In mid- 1984, the Drilling and Production Technology Sec. of Gulf Oil E and P Co. was concerned about the quality and limitations of body wall inspection services for OCTG, particularly the wall- thickness limitation of EMI units and the relationship between flaw orientation and detection capabilities for EMI and UT units. EMI was generally accepted by the industry as a valid body wall inspection technique on wall thicknesses up to 0.500 in. [1.27 cm]. In thicker members, the capability of generating a workable flux leakage from internal defects was suspect. Because internal defects are potentially the most harmful in heavy wall casing design, the accepted wall-thickness limitation needed to be tested and possibly revised. Another consideration on EMI units was whether angle-oriented defects could be effectively detected with existing longitudinal and transverse flux-leakage equipment. Flaw orientation was also of concern on the newer UT units that were increasing in availability at the time. A practical evaluation of the various body wall inspection units with test standards that simulated manufacturing defects was proposed to answer these questions. Two test joints were constructed from 7-in., 32-lbm/ft [17.78-cm, 47.62-kg/m] and 7-in., 45.3-lbm/ft [17.78-cm, 67.41-kg/m] casing. Each standard contained various OD and ID notches, wall reductions, and full penetration holes that could be used to determine the ultimate detection capabilities of EMI and UT units. This evaluation program was also designed to determine the inspection quality. Future inspection requirements would be awarded only to companies with units that demonstrated an acceptable level of performance on the test standards. EMI and UT Methodology A brief review of the theory of how each system functions will help to illustrate the significance of how the evaluation program was designed and how the results should be applied. EMI units generally include four inspection services combined into one package: longitudinal flaw detection, transverse flaw detection, gamma ray wall-variation measurement, and grade verification. This evaluation program focused on the first three services; therefore, grade verification is not discussed. For the first two flaw-detection methods, tubulars are magnetized with either a transverse or a longitudinal field. Ideally, magnetic flux passing through the tube will be deflected outside the pipe's surface by any existing flaws. The tube is passed through a series of coils designed to detect the diverted flux and to convert it into a voltage signal. This signal is modulated and then recorded on a strip chart. Depending on the magnitude of the deflection, the tube is marked as suspect and moved to a "prove-up" area where it is examined further. The magnitude of any flux-leakage indication depends on a number of factors. The most significant are magnetic field direction and strength and flaw orientation and location. If a flaw is located on the inside of the tube, the flux leakage must penetrate the entire wall thickness and still have enough strength to be detected by the coils. When thicker wall members are involved, the magnetic field strength required to generate a detectable flux leakage from an internal defect results in excessive baseline indications from insignificant OD surface imperfections. This causes considerable time to be spent in prove-up looking for nonexistent defects. Industry has generally considered this to be a problem at wall thicknesses of greater than or equal to 0.500 in. [greater than or equal to 1.27 cm]. Magnetic field direction and flaw orientation coincide to influence the magnitude of flux-leakage indications. Assuming constant field strength, maximum flux leakage is generated by flaws perpendicular to the field direction. Current EMI units used in the industry are limited to transverse and longitudinal field directions because of the geometrical configuration of OCTG. This implies that flaws oriented at 0 or 90 deg. with respect to the pipe axis will generate the maximum flux leakage. Flaws oriented at other angles will generate less flux leakage and thus have a more limited chance of being detected. Manufacturing flaws can and do occur at angles other than 0 or 90 deg., particularly in seamless-pipe mills where rollers are used to size and shape the tubes. The third EMI service, gamma ray wall-variation measurement, directs gamma radiation beams into the pipe wall, resulting in attenuation and/or measurement of the reflection. This system is used to detect eccentricity, wall reduction, and other related wall defects. UT body wail inspection uses pulsed-beam/sound-wave technology in a process similar to that of acoustic logging in wellbores. High-frequency sound waves are introduced to the test piece through transducers that are acoustically coupled to the tube with a water cushion. Physical discontinuities (e.g., cracks, laminations, and cavities) act as a metal/air interface that reflects sound waves. The reflected sound is picked up by receiving transducers and converted to voltage indications by a signal processor. The voltage is modulated and recorded on an oscilloscope and strip chart. Like the EMI process, if the indication demonstrates sufficient magnitude, the tube is marked as suspect and sent to a prove-up area, where the final determination on whether or not the flaw constitutes an API reject (87 1/2% remaining wall) is made. UT body wall inspection generally incorporates a compression sound-wave device to check wall thickness and eccentricity. The acoustic signal is directed through the tube wall perpendicular to the pipe axis. In this case, the internal surface represents a metal/air interface that reflects the sound wave to a receiving transducer. The travel time of the sound wave is determined and correlated directly to wall thickness. UT body wall inspection does not have the same thickness limitations as EMI, but it is very sensitive to flaw orientation with respect to the direction of the sound wave. The flaw must be very near perpendicular to the sound-wave direction for a reflection to be received. Therefore, UT body wall inspection units incorporate numerous transducers at various angles to cover as much of the body wall as practical. P. 51^
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