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Corrosion Fatigue in SCRs and Armors in Flexibles

R. M. ThompsonChevron Energy Technology CompanySeptember 10, 2007

2© Chevron 2007

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

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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

Typical test setup

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Corrosion Fatigue of Armor Wires in Flexible Pipe

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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

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Corrosion Fatigue in SCRs and Armors in Flexibles

Questions/discussion…..