Page 46 - Pipeline Pigging Technology

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magnet (Fig.6). To retain its power, the magnet is fitted with a "keeper". This
is simply a metal bar which carries the flux from one pole to the other. If the
cross-sectional area of the keeper at any point is insufficient to contain the
flux, then leakage will occur.
Similarly, the MFLILI tools use magnets to induce a flux into the pipe wall
(Fig.7). Sensors are mounted between the "poles" to detect any leakage which
occurs due to thinning, or "metal loss".
Clearly it is important to induce a sufficient flux density into the pipe wall,
and this requires very powerful, and often fairly-large, magnets. This has
proven to be a limiting factor with respect to the use of MFL in heavy-wall
pipe, as well as to the development of the smaller-size tools.
The early MFL tools suffered particularly from the lack of suitably-powerful
magnets. To deal with this problem, Tuboscope, who introduced the first
commercial ILI tool in 1967, chose to utilize electro-magnets. All other MFL
tools have since resorted to permanent magnets, and it is here that one of the
most significant developments has taken place.
British Gas, who developed what is now generally regarded as a second-
generation or 'advanced' ILI tool, commented in a recent paper [4] that one
of the greatest benefits during the latter stages of its development programme
came from the improvements in magnetic materials. For example, Neodym-
ium-Iron-Boron magnets have ten times the strength in energy per unit
volume than the Alcomax magnets used in the early 1970s.
Another development which has contributed to the success of the British
Gas tool is the design of the sensor system. Early sensor designs tended to be
very large, giving rise to loss of contact with the pipe wall under various
dynamic and geometric conditions. This particularly affected inspection in
the girth weld area. The current system is now so sophisticated that metal loss
in the weld itself can be detected. It can also determine whether the loss is
internal or external, and can be adapted to determine absolute wall thickness
if required.
British Gas once described the rate of data gathering as being equivalent
to reading the Bible every six seconds. At the end of a run which may last many
hours there is obviously a vast amount of data to be analyzed. The accurate
identification, sizing and location of defects is fundamental requirement, but
it is also important to ensure that the information is presented to the operator
in an understandable and usable format. Not surprisingly, therefore, a great
deal of work has gone into this aspect as well.
It is probably true to say that the successful development and introduction
of the advanced MFL tool has contributed more to the industry's acceptance
of ILI as a reliable method of inspection than any other single factor.

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