Magnetic Wall Loss In Plain English

Electromagnetic inspection (EMI) methods used to detect wall loss defects in ferrous oil field pipe are important to personnel safety at the wellhead.  These EMI techniques have become more common but possibly not well understood by inspection companies and the clients they serve.  Magnetic wall loss detection is a nondestructive testing method which uses a magnetic field to locate areas of the pipe body wall which are thinner than the expected nominal wall thickness.  Historically, these defects are the ones not usually detectable by an induction search coil sensor.  The same methods which can locate thin areas of wall thickness can also be used to detect areas which are thicker than nominal.  It is important to note that the most commonly located defect in oil field tubulars is wall loss.

Initial portable electromagnetic wall loss systems introduced by OEM, Inc. at the 1990 Offshore Technology Conference in Houston used the “flux density” method to locate areas of localized wall loss.  But flux density is NOT flux leakage.  The term “flux leakage” describes an “Electromagnetic test technique for the detection and analysis of a surface discontinuity or near surface discontinuity using the flux that leaves a magnetically saturated, or nearly saturated, test object at a discontinuity”.1  “Flux density”, however, is defined as being the “Normal magnetic flux per unit area, measured in tesla (T)”.2  So flux leakage is magnetic flux leaving the location of a defect to be sensed by a detector device while flux density is the detectable applied magnetic field which is NOT leaking from the pipe’s body wall.  Flux density can be influenced by flux leakage but can also change on its own.  Flux density inspection, using an appropriate sensing device, is an independent method for nondestructive testing.

EMI systems in the pipe inspection service industry contain technology for locating defects considered to be wall loss; areas which are to be detected during the inspection process and quantified.  Quantified by methods OTHER than EMI!  It is generally accepted that magnetic wall loss methods cannot quantify the actual depth of the loss of metallic area (LMA) within the pipe body wall or determine the cross sectional area of the pipe.  The EMI methodology is a fast way to locate the defect area but actual measurement of the suspected defect needs to be accomplished with an ultrasonic thickness gage.  Also useful are dial depth gages and micrometers.

“Magnetic wall thickness measurement” has been publicized as a capability of EMI equipment for years, but the equipment has never been able to fulfill these claims.  An article published in CROSSTALK entitled “Magnetic Wall” states that one of the misconceptions or myths about magnetic wall loss inspection methods is that:  “EMI Wall systems measure pipe wall thickness”.  Yet the author cannot refrain from using the term “EMI Wall measurement” several times within that very same article.  The inability of magnetics to measure remaining wall thickness, defect depth or remaining cross sectional area is a function of physics and magnetics and is not reliant on the competency of the equipment performing the task.  Properly promoting magnetic inspection methods is paramount to the credibility it really deserves.

Consumers of EMI equipment and recipients of EMI services should pay attention to this incongruity.  Defects which are found with EMI wall loss inspection systems MUST be proven up. Only then will you know the TRUE condition of the pipe body wall at the defect location.  There are variables which can influence the methods used for wall loss inspection and the resulting output.  One important variable is volumetrics.  Simply stated, electromagnetic defect detection is a slave to the vagaries of defect size:  length, width and depth.  Sensors detecting wall loss areas are low level devices.  They cannot determine if the wall loss is deep yet small in area or shallow and broad in size.  Therefore, it is not reasonable to expect EMI equipment to perform the task of measurement.

Remaining cross sectional area measurement is also erroneously claimed as a capability of EMI wall loss detection systems.  These systems, however, do not display a number with respect to the actual cross sectional area of the pipe.  In fact, these systems are not actually “measuring” the remaining cross sectional area of the pipe.  Industry standards such as those published by API describe methods accepted for average cross sectional area calculations.  EMI “measurement” is not one of them.
Don’t misunderstand. EMI wall loss inspection systems are very capable inspection tools and perform an important task.  You can LOCATE lots of wall loss in pipe which simply can not be detected with a search coil unit and is easily missed with a gamma ray device.  And this wall loss can make the pipe unfit for service.  However, no one should promote the idea that the EMI unit can measure.

It is also a concern to see one of the more widely used inspection specifications include a section which states that the scope of the inspection procedure includes the DETERMINATION of the tube wall thickness with gamma ray, ultrasonics or magnetic wall monitoring equipment.  This section, relating to wall thickness inspection, contains percentages which calculate directly to real wall thickness values.  If you use EMI services, please pay attention!  Don’t expect your EMI service company to actually fulfill this requirement.  And if they say they can meet this type of specification, no one can complain if you laugh out loud.
We all, manufacturers and inspection service companies alike, must be concerned that the best inspection job possible is being performed.  The end result is reduced liability and a safer environment at the wellhead.  Additionally, we must sell products and services that are backed by integrity and honesty as well as quality.  After all, would you want to ride in a plane that had the engine parts nondestructively tested by someone who was only doing it for the money or did not understand the technique employed to test the parts?

Correctly understanding the principles of flux leakage and flux density allows us to locate detrimental defects; but within the limitations of these methods.  Prove up is essential. 

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1 & 2:  Nondestructive Testing HANDBOOK, Volume 5, Electromagnetic Testing, Third Edition,  Copyright 2004, American Society For Nondestructive Testing, Columbus, OH.