Len Rogers; Acoustic Emission

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

Documents

Len Rogers; Acoustic Emission