1. The document analyzes the influence of wall thickness on the remaining life of pigtails regarding creep and fatigue. 2. Finite element analysis was conducted on pigtails with wall thicknesses of 4.85mm, 4mm, and 3mm under internal pressure and angular displacement. 3. The analysis found equivalent plastic strains, creep strains, number of cycles until failure, and von Mises stresses at different time periods for each wall thickness. 4. Based on the results, the expected life of the original 4.85mm pigtail is about 6 years or 30 cycles. Reducing the wall thickness was found to have a negligible effect on life.

1. Creep fatigue
interaction in pigtails
By : Pascal Schreurs

2. Goal
• Assess the influence of the wall thickness of the
pigtail on the remaining life regarding creep and
fatigue
1 maart 2010 2

3. Action plan
• FEA analysis with plastic and creep properties
• Analysis of results of 2nd cycle
– Cyclic plasticity
– Creep
• Analysis of pigtal with wall thickness of
– 4.85mm (orginal)
– 4mm
– 3mm
• Analysis at lower temperature and pressure
• Analyse based on pigtail length 1000mm
• Agular displacement = 3,5°
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4. Model (1)
• Parametric model
• 1ste order Solid elements
– Sockolet
– Weldolet
– Pigtail
• Load
– Internal pressure
– Angular displacement (3.5°)
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5. ABAQUS Model (2)
• Material
– E-modulus @800°C
– yield @800°C
– Norton creep law
– Life assessment creep according ECCC-data sheets 2005
– Fatigue assessment curves from own tests
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6. Yieldstrength @800°C
140
120
Stress [MPa]
100
80
60
40
20
0
0 0.005 0.01 0.015
strain [-]
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7. Norton creeplaw
0.001
Omega methode
1 10 100 lower bound
0.0001 Omega methode
Strain rate [mm/mm.h]
mean
1E-05 diffusie kruip
Omegalowerbound
1E-06
ε = A ⋅σ n
&
1E-07
A = 6.157975 ⋅10-16
1E-08 n = 6.9865
Stress [MPa]
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8. Internal pressure
1 maart 2010
0
5
10
15
20
25
30
35
40
0
Loading
1
Internal pressure
2
Angle of rotation
creep for 100 hours
cooling down
Internal pressure
Angle of rotation
creep for 1000 hours
102 103 104 105 1105
0
1
2
3
4
0.5
1.5
2.5
3.5
Angular displacement
pressure
8
angle of rotation

9. Model
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10. Boundary conditions on manifold side
X
Kinematic coupling in cilindrical
coordinate system
Fixation of master-node
Ux, Uz, Rotx, Roty, Rotz Y
Axial force due to internal pressure Z
Symmetry boundary
conditions
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11. Randvoorwaarde aan reformer zijde
X
Kinematic coupling in cilindrical
coordinate system
Fixation of master-node
Ux, Uy, Uz, Rotx, Roty Y
Angular dicplacement RotZ (3.5°) Z
Symmetrie
randvoorwaarde
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12. Bending stress in pigtail after 1st operating
cycle
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13. Bending stress in pigtail after 100 uur
creep
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14. Plastic deformation after cooling down
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15. Plastic deformation after 2nd cycle
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16. kruiprek in pigtail na 1000 uur kruip
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17. Axial plastic deformation
200
150
Axial stress [MPa]
100
[
50
wall=4.85
0
wall=400
-50 wall=300
-100
-150
-0.005 0 0.005 0.01 0.015
Plastic axial strain [-]
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18. Equivalente plastische rek
200
Equivalente stress [MPa]
150
100
[
50
wall=4.85
0
wall=400
-50 wall=300
-100
-150
0 0.01 0.02 0.03 0.04
Equivalente plastic strain [-]
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19. Equivalente blijvende rek
200
Kruip
150 aandeel
Axiale stress [MPa]
100
[
50
wall=4.85
0
wall=400
-50 wall=300
-100
-150
0 0.01 0.02 0.03 0.04 0.05
Total non lineair axial strain [-]
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20. Fatigue analysis
• Assess the plastic strain and creepstrain after 10
hours
• Calculate the used lifefraction per cycle
Holdtime = 600 min
N = A⋅ε p
b
A = 0,13461
b = -1,43803083125054
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21. Fatigue data
1
holdtime 0min
Holdtime 30min
0.1 holdtime 600min
Plastic strain [-]
[
Holdtime = 600 min
0.01
N = A⋅ε p
b
0.001 A = 0,13461
b = -1,43803083125054
0.0001
10 100 1000 10000
Number cycles [-]
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22. Overview of equivalent strains
Wand- Tijdstip
dikte Druk Temp
[mm] [bar] [°C] 102 103 115 1105 ∆ε
485 35 800 1.44% 2.80% 4.04% 4.10% 1.24%
400 35 800 1.49% 2.89% 4.12% 4.19% 1.23%
300 35 800 1.56% 2.99% 4.17% 4.25% 1.18%
300 33 800 1.55% 2.96% 4.14% 4.21% 1.17%
300 33 780 1.53% 2.92% 4.08% 4.17% 1.16%
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23. Overview of number of cycles until failure
N
wall=485 pres=35 temp=800 75
wall=400 pres=35 temp=800 75
wall=300 pres=35 temp=800 80
wall=300 pres=33 temp=800 80
wall=300 pres=33 temp=780 82
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24. Creep analysis (1)
160
Von Mises spanning [MPa]
140
120
100
80
60
40
20
0
104 109 114 119
Tijd [uur]
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25. Creep analysis(2)
• Asses stress after 10 hours
• Use formula below to determine the creep relaxation
τ = (n − 1) ⋅ E ⋅ A ⋅ σ on −1
1
σ (t ) = 1,25 ⋅ σ o ⋅ (1 + τ ⋅ t )
1− n
• Material data according ECCC data-sheets
• Safetyfactor to ensure lower bound material data
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26. Creep analysis(3)
45
Von Mises stress [MPa]
40
35
[
30
25
20
15
10
5
0
115 615 1115 1615 2115
Time [hours]
[ ]
ln(tr ) = β 0 + β1 ⋅ log(σ (t )) + β 2 ⋅ log(σ (t )) 2 ⋅ (T − T0 ) + β 5
1 maart 2010 26

27. Creep analysis (4)
160
140
Von Mises stress [MPa]
120 FEA analyse
[
100
Analytisch
80
60
40
20
0
104 109 114 119
Time [Hours]
1 maart 2010 27

28. Sommation of creep and fatigue
• 5 cycles per year
• Creep assessment based on 1600 hour creeplife per
cycle
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29. Sommation of creep and fatigue
Levensduur bepaling
combinatie kruip vermoeiings
1
0.9
0.8
0.7
0.6
sommation rule 1991
Fatigue life fraction [-]
0.5
sommation rule 2009
0.4
4,85
0.3
4,00
0.2
3,00
0.1
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Creep life fraction [-]
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30. Conclusion
• Based on assessment line 2009 the expected life of
the pigtail is about 6 years (30 cycles);
• The effect of a smaller wall thickness is nihil
1 maart 2010 30