Securing the Best Performance Entitlement from MFL Technology

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Securing the Best Performance Entitlement from
MFL Technology

• Ian Mullin
• GE Oil Gas, PII Pipeline Solutions

• Introduction to Magnetiser Design
• Mechanical Review
• Required Saturation Fields
• Velocity Effects
• Pole Spacing
• Magnetiser Bar vs. Solid Body Bristle

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Fundamental Magnetiser Designs
Magnetiser Bar
Solid Body Bristle
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Mechanical Review
Solid core bristle design is mechanically more
robust and suitable for most pipeline
environments. Magnetiser bar designs can be more
suitable for multi-diameter lines if compromises
are made elsewhere.
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Magbar Issues with Bend Inspection
SWEEPS BRUSH
MAGNETIZER BAR
POF are now considering including bend inspection
performance in their required specification
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Required Saturation Field Levels
Saturation - That degree of magnetization where a
further increase in magnetization force produces
no significant increase in the magnetic flux
density (permeability) in a specimen.

• - Same vehicle used in half/full magnet build
• Same EXTERNAL defect detected and sized
• Same run speed
• Defect sized exactly the same

• Above the knee
• Pipe-steel is in saturation
• Sensitive only to metal loss and wall thickness
  variation

• Operating below the knee of the curve
• Sensitive to material variation, stress/strain
  etc.
  
• Poor defect detection sizing

There are several sources of noise during
inspection (magnetic, sensor, dynamics,
electronics) and all of these must be addressed
in order to obtain the best signal to noise
ratios. Designing solely to achieve the highest
fields possible will result in a sub-optimal
design.
ASTM, Standard Terminology of Symbols and
Definitions Relating to Magnetic Testing
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Eddy currents

• Faradays Law
• Changing magnetic flux dB/dt induces electric
  current in conductor
• Lenzs law
• Current generated by changing magnetic field will
  produce a magnetic field in opposition to that
  which generated it (induced field).

e EMF J Current Density s Electrical
Conductivity
Regions of high current density, J

• Result of pig moving through pipeline
• Eddy currents generated in pipe (good electrical
  conductor) predominantly at points of pole
  contact
• Opposing induced fields attenuate field levels
  across the pipe-wall
• Field is concentrated onto inner pipe-wall

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Velocity Effects

• Axial field (-Hz) contour plot for pipe section
  between poles
• Low velocity (lt2m/s)
• Axial field profile demonstrates good uniformity
  across wall thickness and axially across the
  sensor position
• High field levels at sensor position throughout
  the wall thickness
• With increasing velocity
• Axial fields attenuated across wall thickness
• Field levels drop on outer wall

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Pole Spacing
Axial field levels measured 90 into pipe-wall
(OUTER)

• Short Pole-Spacing
• Poor performance across speed range (0-5m/s)
• Very sensitive to sensor positioning vibration
  of sensor during inspection will produce noise on
  data
• Little room for sensor positioning
• Very high fields possible at low velocity
• Long Pole-Spacing
• Field levels lower than short pole-spacing
  design
• Maintains field levels from 0-5m/s
• Relatively insensitive to sensor positioning
  hence also less noise due to sensor vibration
• More room for optimal positioning of sensor

Pole Spacing
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Inner/Outer Pipe-wall Fields

• Field levels are predominantly much higher on the
  inner pipe-wall
• Ideally the sensor should be placed in or around
  the crossover point (red circles)
• Short pole-spacing
• - optimum sensor positioning possible?
• large field gradients
• inner wall field levels can be over 2x outer
  wall
• Long pole-spacing
• room for optimum sensor positioning
• smaller field gradients
• inner wall field levels can still be over 2x
  outer wall

When field levels are quoted for performance
comparison it is crucial that they are OUTER wall
levels, as these will be the minimum values (POF
standards). However, it is not possible to
directly measure outer-wall fields on-board
during an inspection run.
Pipeline Operators Forum, Specifications and
Requirements for Intelligent Pig Inspection of
Pipelines
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Magbar vs. Sweeps Brush 12mm
0m/s (static)
Sweeps Brush
Magbar
5m/s
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Magbar vs. Sweeps Brush 18mm
0m/s (static)
Sweeps Brush
Magbar
5m/s
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Magbar vs. Sweeps Brush

• Sweeps Brush
• Median pole spacing (150mm) maintains field
  across full speed range in 12mm/18mm wall
• In 22mm wall fields have collapsed beyond 3m/s
• Shorter pole-spacing gives
• higher fields at low velocities
• less speed stability
• Longer pole-spacing gives
• lower peak fields
• better speed stability
• less peaky field profiles

• Magbar
• Median pole spacing (110mm) shows poor speed
  stability but high fields at low velocity
• Shorter pole spacing gives
• higher fields at low velocities
• variations in pole spacing has little influence
  on speed performance
• Longer pole-spacing gives
• lower peak fields
• some improvement in speed stability

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Magnetiser Sensor Lift-off
Negligible drop in pipe-wall field at sensor
position
Sensor will not measure drop
15 drop in pipe-wall field
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Magnetics Review

• Solid Core Bristle Design / Sweeps Brush
• - Suits long pole-spacing
• Speed stable magnetic performance
• Low sensitivity to lift-off
• Uniform field profiles
• Lower peak field levels at low velocity relative
  to magbar
• - Good in realistic pipeline environment across a
  range of speeds

• Magnetiser Bar / Magbar Design
• - Suits shorter pole spacing
• High peak field levels at low velocity
• Poor magnetic performance at high speed
• Sensitive to speed variations
• Peaky field profiles
• - Good in ideal environment at low controlled
  speed

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Thank you for listening Questions?