Recent  Innovations  in  Flux  Leakage  Drill   Pipe  Inspection  Technology:   Quality  Control   Enhanced

This paper was presented at the Petroleum Industry Inspection Technology Topical held in Houston, Texas June 20 - 22, 1989 after selection by a program committee following the review of information submitted in an abstract.

Wade Edens M.Sc., B.Sc.
President, Oilfield Equipment Marketing, Inc. 4711 Dodge Street
San Antonio, Texas   78217   Tel: 210-657-7607   Fax: 210-657-3660
State University of New York; Syracuse University
LIFETIME MEMBER: American Society for Nondestructive Testing (ASNT)


Electromagnetic inspection of drill pipe has long been associated with a technology whose time is passing. As is true in so many inspection industries today, we in the tubular inspection arena are passing from perceived "tried and true" methods of pipe inspection to more advanced and versatile state-of-the-art methods. One of these advancements, introduced during the past 18 months, is the electromagnetic inspection systems (EMI) which use solid-state sensing devices to detect three dimensional flaws. When used correctly, these solid state sensors can advance pipe inspection technology and will positively affect quality control of drill pipe inspection.

It is easy to understand why the introduction of solid state sensors in this equipment is so important to those of us who are concerned daily with bringing a higher level of competency into the pipe inspection industry. The solid state sensor is a tool for the enhancement of today's quality control in tubular products and is a significant factor in bringing increased professionalism to the job title "pipe inspector".

Briefly, let me show you a typical used drill pipe inspection unit with its major component parts. The system is portable and is made up of a motorized drive unit which has an inspection scanner (often called a "head") attached to it. The head is encircled by a very strong active field DC electromagnet. This assembly is connected through several cables to the electronics recording console. As the drive unit moves along the length of the tubular, the head sends signals from suspected defect locations back to the chart recorder where they are displayed graphically on the chart. Upon passing over a discontinuity in the induced magnetic flux path, a wire search coil may be excited with a voltage in any of 8 shoes located in the head. Sounds simple enough. The problem is in displaying an accurate picture of the tubular's condition. This problem can be attributed to a large extent, to these 8 sensing devices commonly called "search coils". These coils of fine wire have several drawbacks. The most serious are: (1) great sensitivity to speed changes. Search coils require a constant velocity of the drive unit to maintain the precalibrated amplifier settings; (2) having a "most sensitive area" at the center of the coil which falls off in both directions to a "dead point" at the ends of the coil; (3) non-linearity of signal output. This means that the chart printout is not repeatable coil to coil or head size to head size. This lack of uniform sensitivity in search coils is a phenomenon which also makes small deviations in the flux path difficult to detect. The search coil adds the effect of all flux lines enclosed within its circumference. The primary defect signal is then blended with and added to the non-rejectable surface noise. The graphic output of this flaw is then masked, making evaluation difficult. A poor signal-to-noise ratio is the resultant outcome. On the other hand, solid state devices, due to their small size, can resolve small defects clearly.

Additionally, (4) search coils are fairly insensitive to low level changes in the magnetic flux field, such as we find in shallow bottom pitting. (5) Small output voltages are additionally reduced through the 70' of signal cable between the head and the chart recorder. (6) The search coil is not especially sensitive to off-axis defects and lastly, (7) due to the natural accumulation factor of flux by search coils, we see poor signal-to-noise (S/N) ratios.

All these factors present evaluation problem areas for inspectors, especially on used drill pipe. At present, search coils are the predominant method used in EMI of drill pipe tubular goods.

Currently, there is a solution available to inspection companies performing quality assurance inspection negating this list of short-comings produced when wire search coils are used in electro- magnetic inspection (EMI). This solution is the solid-state-sensor and its associated electronics.

Solid state sensing devices are packaged much like integrated circuit (IC) chips. They are hermetically sealed and are extremely durable. Sensors used in the device you are viewing have output voltages approximately 50 times greater than wire search coils and sense through much thicker cross sectional areas. Their small size makes them a better choice for detecting more localized defects found farther from the sensor.

The advantages of these devices are numerous and basically produce a list opposite of the drawbacks found in the typical search coil system. For instance, these solid state devices are not speed sensitive and thereby protect calibration. However, they are equally sensitive across the sensor shoe surface with no dead spots. The sensors will see off-axis defects. They are also linear in voltage output with much larger output voltages. Solid state detector signals are directly proportional to the actual magnitude of the magnetic field applied and have a uniform sensitivity over a wide frequency range. Search coils are much less sensitive to smaller deviations in the flux field and not uniformly sensitive to any defect. Solid state sensors are much more sensitive to low flux density changes and since 100 or more of these devices can be used together, their sensitivity to and resolution of localized defects is greatly enhanced.

One of the main problems facing the petroleum industry as it tries to maintain quality control in drill pipe tubular inspection is that ... "Most electromagnetic flux leakage detectors are limited to the outside pipe surface. This problem is particularly severe for the extra thick-walled tubulars that are specified for certain critical wells..." "This situation becomes more acute when pipe wall thickness approaches one half inch".1 It has been documented that "Failure of drill-string tubulars are costly to the oil industry in the form of lost rig time, damaged tubular goods, and abandoned or side-tracked wells. Based on drilling records, costs associated with a downhole separation of the drill string averaged about $106,000 per occurrence and have been estimated to occur on about 14 percent of all wells".2 "A single catastrophic failure can cost anywhere from 1/2 million to over 2 million dollars".3

So the economic benefit of better inspection cannot be overlooked. Equipment using solid state detection in the hands of professional pipe inspectors will provide the next higher level of competency in tubular inspection. Since most electro-magnetic inspection (EMI) units today use search coils, with often times dire consequences centering around the tubular's actual physical integrity, this advancement in drill pipe tubular inspection has found its time.

To further combat the difficulties of inspecting drill pipe with search coils and the errant chart signals which result, there needed to be changes made in defect signal processing. Generally when using search coils to inspect used drill pipe, the inspector can look forward to a chart which is difficult to interpret because the reduced signal-to-noise ratio inherent in search coils often masks true defects. Remember, search coils add up all the magnetic flux leakage contained in the circumference of the coil and present this accumulation of various electronic frequencies as one gross signal. Without any additional adjustment of the in-coming signal, used drill pipe, especially, presents an interpretation problem for the inspector, or for anyone else who would oversee the inspection process.

A major oil company has stated the inspection equipment available has certain limits which must be improved. As a result "Since corrosion pitting and poor surface condition are common to many strings of drill pipe presently in use by the oil industry, improvements to signal-flaw discrimination and internal surface sensitivity are needed to effectively inspect the drill pipe body to reduce down hole failures".4

The solid state sensor drill pipe inspection units provide updated, state-of-the-art defect signal processing. There have been two major advancements in defect signal processing associated with these units. The first is adjustable threshold of incoming signals which allows the inspector to subtract certain frequencies which produce unwanted chart noise. This new signal processing improves the S/N ratio and chart readability. The second is pulse stretching.  Pulse stretching of short, fast signals from cracks in the body wall is also available in the newest of solid-state-sensor drill pipe inspection units. Pulse stretching helps to overcome the signal filtering effects of the chart's pen galvanometers.

This pulse stretching allows the graphic printout of tight, short fatigue cracks in a more representative way. These two signal modifications in flux leakage inspection units are required to present a signal-to-noise ratio on the chart which is more easily interpreted.

Test standards have appeared on the scene recently which stretch the capabilities of older search coil technology. In drill pipe inspection, the repeatability of the system is often checked through a test commonly known as the "hands of the clock". The drill pipe test standard is rotated 4 times, a quarter turn each time. This verifies the sensitivity of the shoe to the defect and the repeatability of the system. Search coil drill pipe units often show the test defect at various amplitudes, or in the worse case, miss it completely in one quadrant or more. Conversely, the solid state sensor drill pipe unit, when used in conjunction with the other signal modifying techniques mentioned, sees the defect notch clearly, equally and repeatably in each quadrant.

Solid state sensor drill pipe inspection equipment is not a lab curiosity, and is currently being used by inspection companies in Texas, Louisiana, and outside the continental USA. Solid state sensors and their associated electronics are available to be retrofitted into older EMI systems, by replacing the existing circuitry and search coil type shoes. Owners of a typical EMI tubular inspection system can often afford to purchase a retro-fit for existing equipment more easily than to buy an entire new system.

It is reasonable to assume that a quality control program for drill pipe will be enhanced with solid state sensors used in conjunction with advanced signal processing techniques.


  1. American Society for Nondestructive Testing, Inc. Nondestructive Testing Handbook, Second Edition; Vol. four,(1986).
  2. Dale, B.A., and Moyer, M.C., "Sensitivity and Reliability of Commercial Drill-String Inspection Services", (February 2-5, 1988).
  3. Moyer, M.C., and Dale, B.A., "Methods for Evaluating the Quality of Oilfield Tubular Inspections", (1984).
  4. Dale, B.A., "An Experimental Investigation of Fatigue Crack Growth in Drillstring Tubulars", (October 5-8, 1986).
  5. Moyer, M.C., Peterson, C.W., Carr, B.A., "The Importance of Quality Tubular Inspections", Technology (April 13, 1981), Oil & Gas Journal, pg. 103-106.
  6. Reynolds, B.W., Gill Jr., L.O., "A Practical Evaluation of Non-Destructive Inspection Units: Methods and Results", (1987).  



#1 Reference #5 above

#2 Reference #4 above

#3 Reference #5 above

#4 Reference #2 above.