Page 361 - Pipelines and Risers
P. 361
332 Chapter 18
Magnetism
MFL pigs are equipped with large magnetic yokes to magnetize the pipe wall in the axial
direction. The magnetic yoke consists of a backing bar, permanent magnets, pole shoes and
brushes. The combination of the magnetic yoke and the pipe wall is called the magnetic
circuit. The magnetic resistance called reluctance, in the magnetic circuit should be
minimized in order to obtain a high magnetic flux density, also referred to as level of
magnetism, through the pipe wall. Minimization of the magnetic reluctance is achieved by
optimizing the design of the magnetic yoke and by using steels with a high magnetic
permeability. The magnetic power is given by the strength of the permanent magnets. The
strongest permanent magnets applied today are made of NdFeB. Alternatively, an
electromagnet can be applied as the magnetic power source instead of employing permanent
magnets.
Pipe wall magnetism is dependent on wall thickness, tool velocity, and pipe material, beside
the design of the magnetic yoke. The minimum pipe wall magnetism required in order to
obtain good flux leakage signals is 1.6 Tesla. Lower pipe wall magnetism levels will make the
measurement sensitive to all sorts of disturbances. The best performance is achieved at higher
magnetization levels, i.e. in excess of 1.7 Tesla. A magnetic field moving through a pipeline
will induce eddy currents in 'the pipe wall. At high velocities these eddy currents lead to a
lower pipe wall magnetization and a distorted MFL field from a defect. At thick walled pipe
and/or high tool speed there comes a point where the pipe wall is no longer sufficiently
magnetized.
Measurement errors can occur when the level of magnetization in the pipe wall deviates from
expectation. This has a higher probability to occur at lower D/t ratios @/t do), higher tool
velocities (above 3 ds) and lower steel grades.
Sensors and Resolution
Two types of sensors are applied to sense the magnetic flux leakage fields. In the past mostly
coil sensors were uscd since they could be shaped in all geometry's and do not need power.
Disadvantages are that they require a minimum tool speed and that a time differential signal
of the absolute flux leakage fields is obtained which requires integration
Nowadays more and more MFL pigging contractors apply Hall effect sensors which have the
advantage that they measure absolute magnetic field, are sensitive and small (i.e. make a point
measurement) and do not have a limit on minimum tool speed. The major disadvantage of
Hall effect sensors is that they require power.
A measurement grid is made over the pipeline, both in the circumferential and axial
directions. The resolution of the grid plays an important role on the detectability and sizing
performance of small defects; hence the best performance can only be obtained with a fine
grid. The grid spacing circumference is determined by the circumferential sensor spacing and
in the axial direction by the sampling frequency. The sensor spacing varies between 8 mm and
100 mm for the various MFL pigs. The axial sampling distance varies between 2.5 mm and 5