ELECTROMAGNETIC  INSPECTION:   Wall  Loss  and  Flaw  Location  in  Oil   Country  Tubular  Goods

This paper was presented at the Petroleum Industry Inspection Technology II Topical Conference held in Houston, Texas, June 25-27, 1991 after selection by a program committee following the review of information submitted in an abstract.

Oil Country Tubular Goods (OCTG) inspection companies are hampered with less than critical electromagnetic inspection techniques to locate service induced FLAWS, including WALL LOSS in used drill pipe and used tubing. The pipe inspector tries to discover cracks, seams and pits, formation wear, off-axis defects, rod wear and gradual loss of metallic area (both ID and OD). These defects can result in a physical separation of the tubular's body wall during the drilling or production process. The problem is that the inspector is often using the same electromagnetic inspection equipment that was available 15 years ago. This equipment, which uses the search coil technique, leaves much to be desired in both sensitivity to and resolution of three dimensional flaws. The draw-backs to using the search coil, an oval multi-wrap of hair-like wire, for testing OCTG have been enumerated in many periodicals and texts. There are serious limitations to their use in pipe inspection equipment which is employed to qualify tubular products for critical use.


Performance Testing:
There are two major areas of concern. The first is in the ability to separate nominal service induced flaws from man-made test notches. The American Society for Non- Destructive Testing (ASNT) suggests that reference standards be "...free of discontinuities and (be) of the same nominal alloy, heat treatment and dimensions as the tubular test objects". ¹ In reality the test pieces presented to EMI pipe inspection companies are not often new pieces of pipe. Some of the test standards offered have been in prior service. Typical search coil systems have difficulty separating the nominal service induced flaws from the man-made test notches to be located and identified by the inspector.

The second area of concern centers around wall loss. In a used drill pipe or used tubing performance test standard, wall loss areas (to which traditionally used search coils are not sensitive) may also be included. Because of this several pipe inspection companies have tried to examine used drill pipe or used tubing with a 4 function EMI system. Unfortunately, these EMI systems employ a rotating spool-like fixture on which is mounted a gamma radiation device that bombards the body wall and calculates the wall loss. This device tests the pipe in an 18" barber pole spiral helix. A small percentage of the body wall is actually examined, certainly not 100%.


New Technology:
To improve his situation the inspector may now take advantage of a recent advancement in electronic inspection technology. This step forward greatly decreases the difficulty of locating flaws, including reduced wall thickness. At the heart of this innovation is the solid- state, small area, diverted flux detector. Wall reduction areas, often previously left undetected, now are detectable since the introduction of these solid-state devices. Their increased sensitivity to flaws, over traditionally used inductive coils, provides tubular inspection personnel with a better opportunity to locate such three dimensional defects.

These small area solid state sensor semiconductors are packaged as IC's and provide signal amplification at the defect location. Packaging them as IC's also provides durability and hence longevity for the devices. The sensors have specific qualities which allow them to be used as both extremely accurate flaw detectors and wall loss indicators (loss of metallic area). These small area sensors can be incorporated into new or existing mechanical devices generically referred to as pipe inspection equipment.

Using semiconductor sensors to locate wall loss provides 100% coverage of the tubular under inspection. The sensors are extremely sensitive to the change in magnetic field due to loss of metallic area. That portion of the pipe under observation needs only to have a 5% to 10% reduction of wall thickness to provide a significant signal to the chart recorder. The diameter of the defect needs only to be the approximate size of a quarter. Suspected rod wear in tubing or formation wear in drill pipe may be detected from upset to upset.


Electronics:
In conjunction with the use of these localized flux sensing devices for flaw or wall loss is state-of-the-art upstream electronics. This sophisticated electronics processes incoming signals from the sensor arrays of both the flaw and wall loss portions of the system. Both the flaw and wall loss inspection systems locate areas of suspected flaw or wall reduction, simultaneously. Unique signal processing characteristics of this equipment allow normally noisy tubulars to be quieted. This clarifies the signal output and provides an extremely good signal-to-noise ratio. A graphic chart recorder depicts the area and an indicator light alerts the inspector to the clock position where the signal originated. Properly calibrated, both flaw and wall loss renditions are characterized by linear outputs. Defect prove-up is accomplished by traditional non-destructive testing methods, using magnetic particles and ultrasonic wall measurement device. The chart is then interpreted by the inspector.


Repeatability:
Consistency of defect rendition is another important QA consideration. Since the semiconductor IC's are capable of identical output signals, the suspected flaw is basically rendered on the chart at the same amplitude no matter where the defect encounters the active transducer surface.


Case History:
For example, there have been instances of relocating a specific tubular test standard. The artificial defects had not been ground out after initial calibration of the electronics. Even though the original inspector was not running the inspection unit, the previously cut notches provided a rendition that was unmistakable on the chart since the SOP (standard operating procedure) calibration was the same. Even after use in three wells, the tubular test standard's fingerprint was recognizable. Repeatability is achievable using semiconductor sensors. Semiconductor "...signals are directly proportional to the actual magnitudes of the magnetic flux density and have uniform sensitivity over a wide frequency range". ² Conversely, search coils (used in older EMI equipment) respond differently for each frequency component they encounter.

These small area, semiconductor transducers "...can reveal each local portion of the distorted magnetic field distribution in detail... (and) they can resolve these local differences better (than search coils). These small detectors provide better resolution of small discontinuities (defects), as any small sensor would". ³

With search coils loss of sensitivity also occurs as portions of the winding are raised from the test material surface. Conversely, the semiconductor sensor can enhance the electromagnetic coupling of the field because of its small, thin profile which "...can be placed flat upon the test material surface, whereas the larger coil pickups usually extend farther away from the surface." ⁴

"Some inspection systems employ ...(solid-state) sensors in both their transverse and longitudinal inspection heads:-

a) Transverse Heads
Such heads traditionally have 8 or 16 coils encircling the tube. The sensitivity therefore goes down as the OD of the tube is increased. i.e. the longer coil used in the larger OD heads will have a poorer signal to noise ratio than the shorter coils used in the smaller OD heads, for the same size of pit or fatigue crack. With (semiconductor IC) elements, where there may be over 100 around the circumference of the tube, this problem does not exist.

b) Rotating Heads
Much existing equipment employs coil arrays in which the coils are 0.5 - 0.75 inches long. (Semiconductor)... elements are much smaller than this, and so two advantages arise.

(i) in systems which count the number of sensors which detect MFL, (magnetic flux leakage) a more accurate indication of the length of the defect can be obtained.

(ii) the largest indication from the array may be better related to the depth of the defect than is the case with a flat coil array". ⁵


HARD TO LOCATE DEFECTS:
A major complaint often is heard concerning the search coil inspection unit's performance especially for defect location of off-axis cracks of up to 45 degrees. Conventional units have extreme difficulty locating defects which are over 10 degrees off perpendicular to the magnetic field. When locating these defects at all, the DC current level on the magnetizing coil and amplifier settings are so extreme that the background noise masks the defect and can reduce greatly the signal-to-noise ratio; often no better than "2 to 1" initially. Conversely, small area semiconductors can resolve these defects up to 45 degrees left or right hand at normal current and amplifier settings.

Another advantage to the pipe inspection company is not having a radioactive source acting as a wall loss device. The elimination of government intervention into a business, not needing to provide radiation badges, pay annual fees or keep current records of individual's exposure to gamma radiation reduces the cost burden on any company.


CONCLUSION:
Difficulties in flaw detection including wall loss location have led to state-of-the-art advancements in electromagnetic inspection (EMI) systems using sophisticated semiconductor sensors. Inspection companies should note that these sensors are available in new equipment or as a retrofit for existing pipe inspection equipment.

Small area solid-state sensors are now being used in inspection equipment world-wide to provide a less costly and more efficient way in which to identify specific suspected defects or wall loss, electromagnetically, i.e. (without the use of a radioactive device).

When used in a standard pipe inspection unit these sensors can reveal localized portions of a diverted flux field more accurately than search coils. In addition these small area sensors are capable of revealing 5% to 10% wall loss areas, the size of a quarter. This state-of-the-art pipe inspection equipment aids the inspector in providing a pedigree for the tubular products they inspect.

Installed in a typical EMI inspection system, these sensors replace the less efficient search coil and gamma radiation tools normally used to inspect used drill pipe and used tubing. The results of this application of semiconductors as flux sensors afford greater quality control of used tubular products. The ultimate benefit is in providing increased safety at the well-head.



REFERENCES:

1. McMaster, Robert C., Nondestructive Testing Handbook, Second Edition; Volume Four, 1986.
2. American Society for Nondestructive Testing, Nondestructive Testing Handbook, Second Edition; 1989, Editors: Schmidt, J. Thomas and Skeie, Kermit.
3. Stanley, Roderic, Magnetism & Mythology which Surrounds It; Second Edition.
4. Hull, Barry & John, Vernon, Non-Destructive Testing; First Edition, 1988.
5. McMaster, Robert C., Nondestructive Testing Handbook, Volume II; 1959.



FOOTNOTES:

#1 Reference #1 above, pg. 252

#2 Reference #1 above, pg. 322

#3 Reference #1 above, pg. 323

#4 Reference #1 above, pg. 323

#5 Reference #3 above, section 10, pg. 3