Page 363 - Pipelines and Risers

334 Chapter I8

relatively low MFL noise levels. In addition the quality of the line pipe steel in terms of the
number of non-metallic inclusions also influences magnetic noise levels.

The geometry of the defect plays an important role on defect detectability. Mainly the defect
depth and width, i.e. the cross sectional area of metal loss normal to the pipe axis, have a
strong influence on detectability. Defect length has a secondary effect on defect detectability.
In general, the detectability and sizing performance reduce for very short defects (pinhole
pitting, circumferential cracks) and for very long smooth defects (axial grooves, general
corrosion). Hall effect sensors that measure the absolute axial magnetic field are better suited
to measure smooth grooves than coil sensors.

Under optimal conditions, the MFL pigs can detect pits as small as 5% wall thickness loss
however, most MFL pigging contractors specify pit detectability between 10% and 40% wall
loss whereby the large influence of pipe wall magnetization and line pipe manufacturing
process has been taken into account.

Under optimal circumstances, the depth sizing accuracy of general and pitting defects will be
about 10% of the pipe wall thickness at 80% confidence.

Depth sizing of axial pits and grooves requires a good lengtwwidth correction factor on data
analysis and an accurate measurement of defect width. In general depth sizing of axial pits
will be less accurate. It has been found that the depth of defects with a length / width aspect
ratio above 2 and a width smaller than the sensor spacing can be severely undersized. Under
optimal conditions, the accuracy of depth sizing of axial pits will be +lo% and -20% of pipe
wall thickness at 80% confidence.

Depth sizing of circumferential pits and grooves requires a good lengthlwidth correction
factor on data analysis. Under optimal conditions the sizing accuracy can be as good as that of
general and pitting defects.

It should be realized that defect sizing of bottom-of-the-pipe corrosion whereby general and
localized corrosion interacts is more complex. Often only the localized defects are measured

Applicability
MFL pigs can be used under the following conditions:

All sorts of product.