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8/18/2019 Len Rogers; Acoustic Emission
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16th FPSO ForumThe Welding Institute - 25th October 2005
Crack Detection in Hull Structures byAcoustic Emission Monitoring
Len Rogers and Jack Still
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Benefits of an in-service Acoustic Emission
based inspection strategy
• Enhance safety and operational reliability by providing
100% volumetric inspection of the critical structuralelements predicted by the statutory Fatigue Design
Assessment (FDA)
• Detect crack initiation and rate of growth while the
vessel is in service• Intervene only when significant acoustic emission hot
spots are detected.
• Schedule remedial work to minimise service disruption
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There now exists:
• Fundamental understanding of the mechanicsof crack growth on a micro-scale, as the basisfor the interpretation of AE results.
• Industry standard intrinsically safe equipmentand distributed processing for cost effectiveinstallation
• AE detection algorithms with a proven recordof reliability on offshore installations.
• Standards for the measurement and
interpretation of results and the qualificationof personnel
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Comparison of Magnitude 4 events on the
Richter and AE Event Magnitude Scales.
Parameter Seismic Acoustic
event event
fracture event area ∼100m x 100m ∼ 100µm x 100µm
fracture velocity ∼ 500m/s ∼ 250m/s
characteristic time ∼ 0.2sec ∼ 0.4µs
characteristic freq. ∼ 2.5Hz ∼ 1.25MHz
wavelength (press.) ∼ 2km ∼ 4mm
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Microstructure of a fatigue crack in a
medium strength steel
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Threshold stress intensity factor for
crack growth Kth
If the initial defect can create a stress intensity at the crack tipsuch that σ = σ y (the yield strength) at r = l (the threshold plastic
zone size for local fracture instability), then the crack willpropagate in steps l given by
Kth σy√(π l ) E√(π d1).
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Calculated alternating stress intensity factor K as a
function of cycles to failure for a ferrite-pearlite steel
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Mechanics of fracture on a micro-scale
• Under cyclic stress clusters of atomic imperfections occur atintervals given by:
x ≤ 4m2cl2d1
3 /3h3 typically ≤10µm
• The strain hardened zone grows by this plastic deformationprocess to its threshold size given by:
l d1E2 / σy
2 typically 100µm
• At this point the crack advances suddenly through the
embrittled zone with velocity:vf √(σu /ρ) typically 250m/s
• Each crack jump is accompanied by acoustic emission withcharacteristic frequency given by:
ν
c = vf /2
l typically 1.25MHzand stress-wave amplitude given by:
ui(r)|max = χ(3π /64) Ρ / rciE
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Longitudinal (pressure) and transverse (shear)
wave lobes from a micro-fracture eventScruby and Wadley have produced the
following analytical solutions for the
displacement amplitudes of the transverse
and longitudinal stress-waves in a halfspace at distance ‘r’ from a micro-fracture
event of area ∆a :
ui(r)|max = χ (3π/64) Ρ/ r ci E
where P = σy ∆a vf is the Acoustic
Power of the ‘explosive’ micro-fractureevent (watts).
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Relationship between acoustic emission activity and
change in crack area for medium strength steel.
1.E-02
1.E-01
1.E+00
1.E+01
1.E+02
1.E+03
1.E+04
1.E+05
1.E+06
1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06
a (mm2)
Vβ
(e) (Volts)
Minimum Detectable
Fatigue Crack is typically
10mm × 1mm using a
detectability 'κ' of 30dB
( )2
n j
1
mminis∆awhere
25.0∑
=
=∆⋅=
j
j aeV β
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Determination of crack status from the change
in crack area estimated from the AE power.
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AE data base on fatigue in full scale node
joints simulating North Sea wave loading
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Typical acoustic emission signals at different
distances from the source event in a tubular
steel node joint
4th HIT SENSOR
1st HIT SENSOR
3rd HIT SENSOR
2nd HIT SENSOR
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AE amplitude distributions at
different stages of crack growth
Grading the sources of AE
according to signal amplitude.
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Industry standard ‘black box’ AE and Strain
data acquisition unit.
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Illustration of the use of coarse and fine resolution delta-T
space filters to resolve crack growth and fluid noise
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Location of AE sources from a fatigue crack in an
access window measured at different times (a) in
plan and (b) projected onto the weld line
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Why use Acoustic Emission Monitoring
on Offshore Structures?
C St d 1 f ti k d t ti i j k
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Case Study 1 - fatigue crack detection in jack-up
and floating production platforms
• Global surveillance ofcritical load path areas
e.g. complete leg segments,leg-hull locking supportsand leg-spud canconnections
• Monitoring during jacking,towing and operation
• An in-service measure of
physical damage in terms ofincrease in crack growtharea.
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Close-up of sub-sea AE sensor
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AE Installation on a Steel Tower Structure
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AE sensors attached to a node joint
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The installation of AE and strain sensors on the inside of a
subsea tubular brace of a floating production unit
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Typical tanker hull with acoustic emission
sensors installed at fatigue sensitive areas.Signals relatedto monitoringstation
Data acquisition unitlocated in wheel houseor control room
Satellite dishto relay data for further evaluation
Tanker hull structure
Bulkhead
Potential sites for fatigue cracks
Location of acousticemissions sensors
Moonpool
FPSO turretMooring system
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Case study 2: Sensor positions A, B, C and D
on a crane slewring bearing
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LR Innovative Technology
Enables you to:
• Hear cracks propagating anywhere in the structure.
• Determine their structural significance using real time information
supplied by the structure.
• Benefit from 20 years of experience in acoustic engineeringapplications.
• Remote non-invasive inspection
• Continuous global surveillance
• Response to fatigue and SCC cracks• Location and severity of cracks
• A means of reducing uncertainty in
crack life prediction• Ability to determine when to intervene
to minimise maintenance costs
In Conclusion Acoustic
Emission Monitoring Offers