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© Chevron 2006
Corrosion Fatigue in SCRs and Armors in Flexibles
R. M. ThompsonChevron Energy Technology CompanySeptember 10, 2007
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Corrosion Fatigue in SCRs and Armors in Flexibles
Summary
Discussion limited to corrosion fatigue testing and evaluation
Will not cover metallurgical, welding, inspection issues
First review SCRs, then flexible pipe
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SCR Corrosion Fatigue Testing Overview
Sour service has been primary focus of corrosion fatigue test activity
What is the current testing strategy?
Performing sour service testing• Fatigue (S-N) testing
• Fatigue crack growth testing
Key issues with sour service testing• Environment (pH, H2S%)
• Test frequency
• Project driven
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SCR Corrosion Fatigue Test StrategyAccount for both Size Effects and Environment
2) Small vs. LargeScale Data with Shift2) Small vs. Large
Scale Data with Shift
Air
Sour Brine
Stre
ss R
ange
1) Small Scale Data To Define Environmental Shift
Cycles to Failure Cycles to Failure
Shift
Small Scale Scatterband
Large Scale DataSt
ress
Ran
ge
Shift
Corrected for Environment
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SCR Corrosion Fatigue Testing OverviewS-N Fatigue Test Specimen
Dog-bone, uniaxial tension
Coated to reduced exposed surface within chamber
ID exposed to environment
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SCR Corrosion Fatigue Testing Fatigue Results – Published Data
Buitrago, J. and Weir, M. S., “Experimental Fatigue Evaluation of Deepwater Risers in Mild Sour Service,” Deep Offshore Technology Conference, New Orleans, Nov. 2002.
Fatigue Life (cycles)
103 104 105 106 107 108
Stre
ss R
ange
, Δσ
(ksi
)
10
20
30
40
506070
Lab AirH2S/N2 (0.33 and 0.01 Hz)
Buitrago and Weir (DOT Conf. 2002)Lab AirLab Air (full-scale)H2S (0.33 Hz)
Runout
B-curve (mean)
Phase I - Flowline
X'/40 curve
X'/20 curve
X' curve (mean)
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SCR Corrosion Fatigue Testing OverviewFatigue Crack Growth Testing
Extracted from welded pipe
Radial (shown) or Circumferential crack growth
Precracked in air then test in environment
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SCR Corrosion Fatigue Testing Fatigue Crack Growth Results
As removed
After cleaning
Material:Environment:
Test Type:Test Parameter:
) K, ksioin1 10 100
Fatig
ue C
rack
Gro
wth
Rat
e, d
a/dN
, in
ch/c
ycle
10-9
10-8
10-7
10-6
10-5
10-4
10-3
lab air (tests 20C & 15E)H2S (tests 20D & 20H)CO2 (test 15A)
steelLA and H2Sconstant KmaxKmax = 33.3 ksioin
Delta K ksi√in
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SCR Corrosion Fatigue Testing IssuesFrequency Effects - Crack Growth
Buitrago, J., Weir, M. S., Kan, C. K., Hudak, S. J., and McMaster, F. J., “Effect of Loading Frequency on Fatigue Performance of Risers in Sour Environment,” Offshore Mechanics and Arctic Engineering Conference, Vancouver, 2004.
How slow do we need to test?
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SCR Corrosion Fatigue Testing IssuespH Effects – Fatigue (S-N)
pH = 5.0
pH = 3.5
What pH: Project specific or standardized?
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Corrosion Fatigue of Armor Wires in Flexible Pipe
Diffusion of gas through polymer pressure sheath can create corrosive conditions for steel armor wires
Also potential of seawater intrusion from leaks in the outer sheath layer
Qualification testing for corrosion fatigue required by industry standards API 17B/17J
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Corrosion Fatigue of Armor Wires in Flexible Pipe
No standard testing procedures included in API
Each pipe manufacturer has developed their own detailed procedures, stress analysis techniques also different
Appropriate safety factors to apply to fatigue curves?
Are notches required (recommended by API)?
Opportunities for standardization
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Corrosion Fatigue of Armor Wires in Flexible Pipe
Known effects of variables based on published data:
• Oxygen in solution – major decrease in fatigue life
• pH – limited info, should there be buffering?
• Temp - fatigue life shorter at lower temp (20C)
• Gas mix – limited info, H2S/CO2 worse
• Test frequency – higher improves fatigue life, but hard to test at realistic frequency. Keep at 0.5 Hz or less.
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Corrosion Fatigue of Armor Wires in Flexible Pipe
Selection of test environment
• Technip has published several papers proposing saturation or supersaturation of electrolyte in annulus with iron
• Are there localized areas that could have lower pH?
• What about lubricants used on the wires?
• Is there enough time to initiate pitting damage that could decrease time for fatigue initiation?
Some operators accept this concept and allow testing at higher pH, others do not
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SCR Corrosion Fatigue Testing Sour Service Testing Issues
Testing is time/cost intensive
Majority of work is project driven• Test data required to design facilities
Limited opportunity to undertake research• Questions can remain unanswered
However, the same unanswered questions or environmental unknowns present themselves in future work
Standardization of design parameters would be useful – eliminate expensive testing
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SCR Corrosion Fatigue Testing Testing Issues
Effect of testing variables
• Frequency, stress range
• Environment (H2S concentration, pH, water chemistry, temperature)
• Sweet service – sometimes better than air sometimes worse ???
Corrosion Inhibition – is it safe to test with inhibitor?
High Strength Materials (New work in RPSEA/Deepstar)
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Corrosion fatigue of Armor Wires in Flexible Pipe
Confinement and effect on annulus environment
• Some remaining questions regarding selection of pH for testing
• Currently worked on a project-by project basis
Testing methodology
• Opportunity for standardization
• Can standard design curves be developed