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
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