1. 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

2. Fundamental Magnetiser Designs
Magnetiser Bar
Solid Body Bristle

3. Mechanical Review Solid Core Bristle Design / “Sweep’s Brush”
Simple and robust
Maintains good coupling with pipe-wall at all times
Bristles absorb the impact of in pipe obstacles
Sensors contact intrados/extrados of bends
Predictable drag forces
Compressibility limited by solid core and poles
Magnetiser Bar / “Magbar” Design
Possibility of extreme compressibility
Mechanically more complex design
Poles & sensors experience lift-off in bends
Large mass of magnetiser bar accelerated at pipeline obstacles
Large clamping forces
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.

4. Magbar Issues with Bend Inspection
Discrimination
Sensors
Main Corrosion
3 of 4 external defects (20mm x 40%) visible. No signal from defect on bend extrados
[Extrados]
[Intrados]
SWEEP’S BRUSH
MAGNETIZER BAR
POF are now considering including bend inspection performance in their required specification

5. 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
Operating below the ‘knee’ of the curve
Sensitive to material variation, stress/strain etc.
Poor defect detection & sizing
Above the ‘knee’
Pipe-steel is in saturation
Sensitive only to metal loss and wall thickness variation
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”

6. Regions of high current density, J
Eddy currents
Faraday’s Law
Changing magnetic flux dB/dt induces electric current in conductor
Lenz’s law
Current generated by changing magnetic field will produce a magnetic field in opposition to that which generated it (induced field).
ε = EMF
J = Current Density
σ = 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

7. Velocity Effects
Axial field (-Hz) contour plot for pipe section between poles
Low velocity (<2m/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
High fields migrate further toward rear bristle stack on inner wall

13. 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

14.  Short pole-spacing 110mm   Long pole-spacing 230mm 
Inner/Outer Pipe-wall Fields
Direction of Motion
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
 Short pole-spacing 110mm 
 Long pole-spacing 230mm 
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”

15. Magbar vs. Sweeps Brush 12mm
0m/s
(static)
Sweeps Brush
Magbar
5m/s

16. Magbar vs. Sweeps Brush 18mm
0m/s
(static)
Sweeps Brush
Magbar
5m/s

17. Magbar vs. Sweeps Brush Sweeps Brush Magbar
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

18. Axial distance along pipe
Magnetiser & Sensor Lift-off
Axial distance along pipe
Negligible drop in pipe-wall field
at sensor position
Sensor will not measure drop
~15% drop in pipe-wall field

19. Magnetics Review Solid Core Bristle Design / “Sweep’s 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

20. Thank you for listening
Questions?