Effect of Sweet CO2 Environments on Fracture Toughness (FT) and

Effect of Sweet CO2 Environments on
Fracture Toughness (FT) and
Fatigue Crack Growth Rate (FCGR)
by
J.R. Gordon and H. Yin
Microalloying International
Presented at
AWS PIPELINES CONFERENCE
HOUSTON, TEXAS
MARCH 2014
1
Presentation Outline
• Overview of Engineering Critical Assessment (ECA)
• Flaw Acceptance Criteria for Girth Welds
• Offshore pipeline Sweet CO2 and Sour H2S Service conditions
• Fracture Mechanics based Testings of X65/X70 in environments
• Fracture Toughness in sweet CO2 environments • Fatigue Crack Growth Rate (FCGR) in sweet CO2
environments
• On‐going research program
2
Overview of ECA
•
Most pipeline codes include flaw acceptance criteria to ensure that flaws introduced during fabrication do not impact the performance of the pipeline during installation or operation.
•
Historically the flaw acceptance criteria in pipeline codes have been based on a combination of good workmanship principles in conjunction with RT (radiography) inspection.
•
Over the last 20 years there has been an increasing trend to adopt alternative fitness‐for‐service concepts for critical pipeline applications.
•
The concept of fitness‐for‐service has led to the development of alternative flaw acceptance criteria that are determined by performing an Engineering Critical Assessment (ECA) of the pipeline. •
Codes: BS7910; API 579; DnV OS F101 Appendix A; API 1104 Appendix A; CSA Z662 Annex K; etc.
3
Overview of ECA (cont.)
•
It is now standard practice to perform an Engineering Critical Assessment (ECA) for fracture and fatigue analyses of onshore and offshore pipelines. •
The adoption of ECA methods is ideally suited to modern construction methods that use mechanized welding in combination with automated ultrasonic testing (AUT).
•
Although the adoption of ECA concepts for pipelines may result in relaxed flaw acceptance criteria it will lead to improved integrity since the emphasis is placed on the most important aspects of overall quality:
– Material Selection
– Procedure Qualification
– Improved Inspection Technology
4
Pipeline Construction/Inspection
The combination of Mechanized Welding and Automated Ultrasonic Testing (AUT) Inspection is ideally suited for ECA Analysis. (AUT has the ability to size flaw height and Length)
5
Pipeline Construction/Inspection
Stress
Pipeline Stress Analysis
Flaw Acceptance Criteria for AUT
Correct for NDE Sizing Inaccuracy
Calculate using ECA Procedure
Flaw Size
Fracture Mechanics
Toughness
WPQ CTOD Tests
6
Pipeline Construction/Inspection (cont.)
• If you know 2 of the 3 parameters you can calculate the limiting value of the 3rd parameter:
– Determine limiting flaw size
– Determine maximum allowable stress
– Develop minimum required toughness
• Alternatively if you know all three parameters you can determine overall structural integrity and the margin of safety.
• In general, pipeline ECAs determine limiting flaw size based on Applied Stress (calculated) and Material Toughness (measured).
7
BENEFITS of ECA / FFS
• Ensure sound pipeline integrity (with reliable NDE inspection)
• Reduce construction cost by minimizing unnecessary repairs
• Required by code and standards for new constructions • Acceptance by regulatory agencies
8
Example of ECA Result: On‐Shore Pipeline
API 1104 Appendix A ECA Flaw Acceptance Criteria 36" OD
8
Tolerable Flaw Plot API 1104 0.625 inch wall
7
Tolerable Flaw Plot 0.625 inch wall Fatigue and Fracture
Proposed Flaw Acceptance Criteria
Flaw Height (mm)
6
5
4
3
Fracture Limit State (Kmat)
2
Fatigue Limit (FCGR)
1
0
0
50
100
150
200
250
300
350
400
450
Flaw Length (mm)
9
Offshore risers and flowlines
•
•
A number of fields in offshore development are initially sweet (Region 0), souring (pH2S greater than 0.05psia) occurs late in life.
Both sweet and sour environments affect material properties (Kmat and FCGR laws)
10
Kmat and FCGR Testing in Sweet CO2 Environments
• Pipe Grade : API X70 / X65
• Pipe Size:
12.75” OD x 1.377” WT (35mm)
• Test Environments: – Solution = 5% NaCl
– CO2 > 14.7 psia
– pH = 4 – 6 (buffered with NaHCO3)
11
Fracture Toughness Specimen – B x 2B SENB
B = 0.72” (18.3 mm), W = 1.44”, a/W = 0.5
In air testing, Load rate: dK/dt = 16 N.mm^‐1.5 (a test takes minutes)
In CO2 environment testing, load rate : dK/dt = 0.05 N.mm^‐1.5 (a test takes days)
(Slow rate rising displacement J‐integral or CTOD testing)
12
In‐Air, CO2 and H2S J R‐curve Results
1800
-3/2
1500
In‐Air
K-rate:0.05Nmm
5wt% NaCl
RT
CO2
J (N/mm)
1200
/s
CO2 J R‐curve results are intermediate to in‐air and H2S results
PP - Air
HAZ - Air
WCL - Air
PP - pH = 6, pH2S = 0.46psia
PP - pH = 6, pH2S = 1.1psia
900
PP - pH = 5.5, pH2S = 1.1psia
WCL - pH = 6, pH2S = 1.1psia
H2S
WCL - pH = 6, pH2S = 0.46psia
WCL - pH = 5.5, pH2S = 1.1psia
HAZ - pH = 6, pH2S = 1.1psia
600
HAZ - pH = 6, pH2S = 0.4psia
HAZ - pH = 5.5, pH2S = 1.1psia
PP pH = 6, pH2S = 0psia
PP pH = 5, pH2S = 0psia
300
PP pH = 6, pH2S = 0psia, 140F
0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
a (mm)
13
Fracture toughness in sweet CO2 environments
Notch Location
Parent Pipe
HAZ
WCL
•
Air
x
x
x
pH
pH2S
pCO2
Temp ( C )
20
20
20
PP Fracture toughness influenced by pH and temperature in pure CO2. Reduction is over 50% of in‐air values
Toughness properties at RT are lower than that at 140F.
•
At RT and 140F minimum toughness properties are at pH = 4.
•
At pH = 4, the Jmaxload is 200 N/mm and 350N/mm at RT and 140F respectively.
/s)
16
16
16
Jm axload (N/mm) J1m m (N/mm) J0.2 (N/mm)
1319
861
622
1006
942
705
500
PP in CO2
5wt% NaCl
-3/2
K-rate: 0.05Nmm /s
400
J1mm - RT
Jmaxload - RT
J0.2mm - RT
300
J1mm - 140F
Jmaxload - 140F
J0.2mm - 140F
200
100
0
3
4
5
pH
14
215
225
166
600
J (N/mm)
•
-3/2
K-rate (Nmm
Environment
6
Fracture Toughness Comparison: Sweet CO2 vs. Mild H2S, for pH = 5 ‐ 6
In‐Air J_max Load = 622 N/mm WCL
861 N/mm HAZ
1319 N/mm PP
J Values (N/mm)
600
J @ max load ‐ All data
J @ 1.0mm crack growth
J @ 0.2mm crack growth
500
400
300
200
100
0
0
0.2
0.4
0.6
pH2S (psia)
0.8
1
1.2
15
FCGR Specimen – CT, 1/2T
B = 0.75” (19 mm), W = 2”, a/W = 0.45
* FCGR Testing: frequency scan da/dN testing, load frequency effect on da/dN
* Knockdown Factor (KDF) = FCGRenv / FCGRin‐air
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FCGR in Sweet CO2 Environments
Kmax = 1150Nmm3/2
3/2
K = 1000Nmm
Karl, pH = 5, RT,
-3/2
1000Nmm
R = 0.13
5wt% NaCl
OMAE 2008
-3/2
pH = 4.0, RT, 800Nmm
KDF BS7910-Mean
Increase in FCGR
10
DNV - Internal
-3/2
pH = 6.5, RT, 1000Nmm
pH = 6.1, No H2S,w/ Inhibitor - 140F, WCL
pH = 6.1, No H2S,w/ Inhibitor - 140F, PP
OMAE 2008
Corrosion 2005
DNV - Internal
1
17
1E-3
0.01
f (Hz)
0.1
1
• Frequency scan behaviour is similar to sour service behaviour i.e. increasing FCGR with decreasing frequency and reaching a plateau.
• Data available in literature has mainly focused on riser frequencies of ~0.1Hz.
• Limited data available on FCGR as a function of frequency.
KDF vs Test Frequency for X65 and X70 (Literature + DnV Data)
1000
H2S Mean with +/‐ SD Error Bar
H2S Mean+2SD
H2S Literature Data
DnV Data ‐ Thunder Horse
CO2 Sweet Env.
KDF (BS7910 in‐air mean, R<0.5)
100
10
1
0.1
0.0001
ΔK = 1000 ‐ 1150 N/mm^3/2, pH = 5 ‐ 6, pH2S = 0.12 ‐ 1.46 psia (Region 1); pCO2 = 14.48 ‐ 13.14 psia Temp = RT to 60 C
0.001
0.01
Frequency (Hz)
0.1
1
18
FCGR Comparisons: CO2 vs. H2S & f = 0.001Hz vs. 0.1 Hz
1.00E+00
Flowline, 0.001Hz, CO2
Flowline, 0.001Hz, H2S
1.00E‐01
SCR, 0.1 Hz, CO2
SCR, 0.1 Hz, H2S
BS7910 in‐air, Mean, R < 0.5
1.00E‐02
BS7910 in‐air, Mean+2SD, R<0.5
FCGR ‐ da/dN (mm/cycle)
BS7910 in‐air, Mean, R>0.5
STAGE B
BS7910 in‐air, Mean+2D, R>0.5
1.00E‐03
1.00E‐04
1.00E‐05
STAGE A
1.00E‐06
1.00E‐07
1.00E‐08
10
100
1000
ΔK (N.mm^‐3/2)
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Summary
• Sweet CO2 fracture toughness (FT) tests show CO2 environments degraded fracture toughness significantly.
• FCGR in sweet CO2 run faster than in air (i.e., KDF greater than 1.0).
– KDF increases with decrease of loading frequency
– KDF for sweet CO2 is lower than KDF for sour H2S • Operation ECA analyses of pipelines in Sweet CO2 services should consider the negative effects of CO2 environments on fracture toughness and FCGR laws.
• JIP testing is on‐going to research on key variables:
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Summary cont.
•
•
•
•
Environmental Variables on FT and FCGR
– pH
– pCO2
– Temperature
– Inhibitors
Loading Variables on FCGR
– ΔK, R‐ratio
– f, fatigue loading frequency
– S‐N fatigue Behaviour
Sample Geometry
– Shallow notch vs. Deep notch
Microstructure
– Parent Pipe/HAZ/WCL
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