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Part 1 Introduction to DNV-RP-C203
Fatigue Course DNV-RP-C203Fatigue Design of Offshore Steel Structures
Houston
September 26, 2013
Content
1. Basics of Fatigue
Fatigue failure modes
Low cycle and high cycle fatigue
Examples of fatigue in structures
2. Fatigue Design based on Nominal Stresses
Definition of S-N curves and stresses to be used
Validity of S-N curves
Effect of yield strength and environment
Fatigue limit and effect of variable amplitude loading
Thickness effects for different types of connections
S-N curves for welded structures of normalized steels and stainless steels,cast and forged joints
3. Stress Concentration Factors
Effect of fabrication tolerances on fatigue capacity
Stress concentration factors for plated structures, ship details, tubularjoints, pipelines
Det Norske Veritas AS. All rights reserved Slide 227 September 2013
THIS IS A PROPERTY OF DNV GL TRAINING,AND IS NOT TO BE DISTRIBUTED WITHOUT PERMISSION FROM DNV GL.
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Content - continued
4. Fillet Welds and Partial Penetration Welds
5. Fatigue Design Based on Hot Spot Stress Using Finite Element Analysis
Finite element modeling of hot spot region
Hot spot stress derivation
Hot spot stress S-N curve
6. Examples of Fatigue Analysis Based on Prescribed Long Term Distr ibuti onof Stress Ranges
7. Uncertainties in Fatigue Life Prediction and Selection of Design FatigueFactors
8. Improvement Techniques
Grinding
TIG dressing
Hammer peening
Det Norske Veritas AS. All rights reserved Slide 327 September 2013
Rules (Recommended Practice)
Norsok N-004 and DNV-RP-C203 developed at the same time (1997).Funded by the industry.
Hobbacher, A. (1996), Fatigue Design of Welded Joints andComponents. IIW. XIII-1539-96/ XV-845-96. Revised 2009.
DNV-RP-C203 Fatigue Design of Offshore Structures, revised 2011.Referred to in Norsok (www.standard.no).
http://exchange.dnv.com/publishing/Codes/ToC_edition.asp#Recommended Practices
DNV CN 30.7 Fatigue Strength Assessment of Ship Structures (Firstissued as DNV Report for fatigue assessment in 1993).Revised Jan 2009.
DNV-RP-C206 Fatigue Methodology for Offshore Ships. Issued in 2006.
ISO 19902 Design of Steel Offshore Structures. 2007.
API RP 2A Recommended practice for planning, designing andconstructing fixed offshore platforms.
Det Norske Veritas AS. All rights reserved Slide 527 September 2013
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Failure modes considered in DNV-RP-C203
1. Fatigue crack growth from the weld toe
2. Fatigue crack growth from a notch in the
base material
3. Fatigue crack growth from the weld root
into the plate below the fillet weld
4. Fatigue crack growth from the weld root
through the weld
Det Norske Veritas AS. All rights reserved Slide 627 September 2013
Fatigue history
15 October 1842:Fatigue failure ofshaft in train at
Versailles. 60 killed.
Testing of shafts:
Whler kurverafter 1850.
Several Cometcrashes in 1950s
due to fatiguecracks initiating from
corners of squarewindows.
At 18.30 27 March
1980: Fatigue failurein one of importantmembers of floating
platform Alexander L.Kielland
123 killed. Worstaccident in Norwegian
oil industry.
Det Norske Veritas AS. All rights reserved Slide 727 September 2013
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The Alexander L. Kielland platform
Built as drilling platform in
France in 1976
Used for accommodation in
the North Sea in 1980
Det Norske Veritas AS. All rights reserved Slide 827 September 2013
The Alexander L. Kielland accident
Det Norske Veritas AS. All rights reserved Slide 927 September 2013
D-6 failed
D was lost
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The Alexander L. Kielland accident
Det Norske Veritas AS. All rights reserved Slide 1027 September 2013
Hydrophone holderBrace
a
Fatigue crack that initiated
from the fillet welds
Failed member
Det Norske Veritas AS. All rights reserved Slide 1127 September 2013
The Alexander L. Kielland accident
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Fatigue crack growth in failed member
Crack initiation in filletwelds
Fatigue crack growth
around the brace
Final fracture in storm
Det Norske Veritas AS. All rights reserved Slide 1227 September 2013
Shuttle tanker operating for 9 years in North Sea
Det Norske Veritas AS. All rights reserved Slide 1327 September 2013
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Side plate shuttle tanker s/t = 825/16
Det Norske Veritas AS. All rights reserved Slide 1427 September 2013
9 years old double hull tanker for oil Cracks found at the intersection
between inner side and bulkhead.
The cracks were found in several
cargo tanks.
First time experienced this type of
cracking by DNV.
Owner has similar experience from
similar ships.
Det Norske Veritas AS. All rights reserved Slide 1527 September 2013
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Repair
Det Norske Veritas AS. All rights reserved Slide 1627 September 2013
Updates in Latest DNV-RP-C203
Det Norske Veritas AS. All rights reserved Slide 1727 September 2013
Addition of a new Section 2.3.6 on fillet welds at doubling plates.
Addition of text and figures in section 3.3.7 on stress concentration factors for tubular buttwelds.
Equations for stress concentration factors for tubular butt welds with machined sectionsare included.
Derivation of hot spot stress in tubular joints is included in Section 4.2.
Some more information of effect of hammer peening is included in Section 7.5. In addition anew Section D.15 on alternative S-N curves for improved welds by grinding and peening isadded in Appendix D Commentary.
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DNV-RP-C203 Fatigue Design of Offshore SteelStructures
Time Part
Topics
Headlines
0800-0830 Welcome Expectations and objectives
0830-0845 1
Introduction to RP-C203
Presentation of the content of RP-C203.
Definitions used in fatigue
0845-0930 2
S-N data
Definition of S-N data and stresses to be used
Nominal stress
Hot spot stress
Notch stress
Palmgren-Miner rule, constant amplitude fatigue limit,
variable amplitude loading
0930-0945 Coffee Break
0945-1015 2
S-N data
Cont.
1015-1145 3
Stress concentration
factors
Stress concentration factors for plated structures and tubular
joints
1145-1230 Lunch Det Norske Veritas AS. All rights reserved Slide 1827 September 2013
DNV-RP-C203 Fatigue Design of Offshore SteelStructures
1230 1315 4Fatigue analysis of local details
Modelling of details.Read out of hot spot stress.Hot spot stress S-N curve.
1315-1345 5Examples
Examples FE modelling, read out of hot spotstress and fatigue life calculation.
1345-1400 Coffee Break
1400-1430 6Simplified fatigue analysis
Weibull distribution of long term stress rangesClosed form equation for fatigue damageUse of design charts in fatigue design
1430-1500 7Uncertainties
Uncertainties in fatigue analysis and DFFs
1500-1530 8Fillet welds
Design of partial penetration and fillet welds.Examples
1530-1600 9Fabrication and improvement
Improvement of details by grinding and peening
1600 Wrap up and closure
Det Norske Veritas AS. All rights reserved Slide 1927 September 2013
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DNV-RP-C203Fatigue Design of OffshoreSteel Structures
Part 2 Basics of Fatigue
Houston
September 26, 2013
Det Norske Veritas AS. All rights reserved Slide 225 September 2013
Content
Introduction
Failure modes
Nominal stress S-N curves
Hot spot stress S-N curve
Fatigue damage accumulation:
Palmgren - Miner rule Effect of principal stress direction
Stress concentration factors
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Det Norske Veritas AS. All rights reserved Slide 325 September 2013
1. Fatigue crack growth from the weld toe
Det Norske Veritas AS. All rights reserved Slide 425 September 2013
2. Fatigue crack growth from a notch
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Det Norske Veritas AS. All rights reserved Slide 525 September 2013
3. Fatigue crack growth from the weld root
Det Norske Veritas AS. All rights reserved Slide 625 September 2013
Specimen 3 Fatigue crack into bulb
B-B
B
B
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Det Norske Veritas AS. All rights reserved Slide 725 September 2013
3. Fatigue from the weld root. Mitigation
Full penetration
L = 10 t
Det Norske Veritas AS. All rights reserved Slide 825 September 2013
4. Fatigue crack growth from the weld root
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Det Norske Veritas AS. All rights reserved Slide 925 September 2013
Definitions
Low cycle fatigue (LCF): N
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Det Norske Veritas AS. All rights reserved Slide 1125 September 2013
Specimen in testing machine
Det Norske Veritas AS. All rights reserved Slide 1225 September 2013
Specimen in testing machine
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Det Norske Veritas AS. All rights reserved Slide 1325 September 2013
Section through weld
Det Norske Veritas AS. All rights reserved Slide 1425 September 2013
Strain gauges on specimen
3 4
8765
21
w a2=20
a1=55
a2=20
a1=55
3 4
8765
21
w a2=20
a1=55
a2=20
a1=55
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Det Norske Veritas AS. All rights reserved Slide 1525 September 2013
Design S-N curve from fatigue test data
10
100
1000
10000 100000 1000000 10000000
Design S-N curve:
Mean 2 St. Dev.
Number of cycles
Stress
range
Characteristic fatigue
strength (FAT class)
Det Norske Veritas AS. All rights reserved Slide 1625 September 2013
S-N curve The basic design S-N curve is given as
or:
Alternatively in air environment, the S-N curve can be defined by the
characteristic fatigue strength at 2106 cycles (FAT class)
logmalogNlog
where
N = predicted number of cycles to failure
for stress range = stress rangem = negative inverse slope of S-N curve
alog = intercept of log N-axis by S-N curve
maN
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Det Norske Veritas AS. All rights reserved Slide 1725 September 2013
Thickness effect
- For tubular joints tref
= 32 mm.
- For bolts tref= 25 mm.
- tref= 25 mm for welded connections.
- k: 0 - 0.30 for different details.
k
reft
tlogmalogNlog
tref = reference thickness
t = thickness through which a
crack will most likely grow.t = trefis used for thickness
less than tref.
k = thickness exponent onfatigue strength k
tref = reference thickness
t = thickness through which a
crack will most likely grow.t = trefis used for thickness
less than tref.
k = thickness exponent onfatigue strength k
Det Norske Veritas AS. All rights reserved Slide 1825 September 2013
S-N curves in DNV-RP-C203
10
100
1000
1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08
Number of cycles
Stressrange(MPa)
B1B2
C
C1
C2
DE
F
F1
F3G
W1
W2
W3
(FAT-class)Designation
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Det Norske Veritas AS. All rights reserved Slide 1925 September 2013
DNV-RP-C203
N107cyclesS-N curve
m1 1loga
N107cycles
2loga
m2 = 5.0
Fatigue limit at
107cycles *)
Thickness exponent k Stress concentration in the
S-N detail as derived by the
hot spot method
B1 4.0 15.117 17.146 106.97 0
B2 4.0 14.885 16.856 93.59 0
C 3.0 12.592 16.320 73.10 0.15
C1 3.0 12.449 16.081 65.50 0.15
C2 3.0 12.301 15.835 58.48 0.15
D 3.0 12.164 15.606 52.63 0.20 1.00
E 3.0 12.010 15.350 46.78 0.20 1.13
F 3.0 11.855 15.091 41.52 0.25 1.27
F1 3.0 11.699 14.832 36.84 0.25 1.43
F3 3.0 11.546 14.576 32.75 0.25 1.61
G 3.0 11.398 14.330 29.24 0.25 1.80
W1 3.0 11.261 14.101 26.32 0.25 2.00
W2 3.0 11.107 13.845 23.39 0.25 2.25
W3 3.0 10.970 13.617 21.05 0.25 2.50
T 3.0 12.164 15.606 52.63 0.25 for SCF 10.0
0.30 for SCF >10.0
1.00
*) see also section 2.10
Det Norske Veritas AS. All rights reserved Slide 2025 September 2013
Size effect
Increased weld length and increased possibility fordefects that can initiate to fatigue cracks.
Volume effect:
More flow of stress into a long/thick attachmentthan into a short.At tachment length:
The notch stress is increased with increasing platedimensions as the notch radius is not increasing inthe same proportion as the other geometry.
Thickness of plate:
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Det Norske Veritas AS. All rights reserved Slide 2125 September 2013
Influence of environment
ENVIRONMENTS
Air
SeawaterwithCathodic
Protection
Seawaterand Free
Corrosion
Det Norske Veritas AS. All rights reserved Slide 2225 September 2013
Basic S-N curve
10
100
1000
1.00E+04 1 .00E+05 1 .00E+06 1 .00E+07 1 .00E+08 1 .00E+09
Number of cycles
Stressrange(Mpa)
Air
Seawater with cathodic protection
Seawater free corrosion
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Det Norske Veritas AS. All rights reserved Slide 2325 September 2013
IIW/Eurocode 3
S-N curves in air
HSE/ISO Draft C
S-N curves in seawater withand without cathodicprotection
DNV- RP- C203
To be applied for structuresin air and seawater
Change in s lopemoved f rom N = 5*106
to N = 107
IIW/Eurocod e 3 S-N format
Eurocode 3 thicknesseffect
Derivation of S-N curves in DNV-RP-C203
February 2004
Det Norske Veritas AS. All rights reserved Slide 2425 September 2013
Illustration of stress at a bracket toe
Bracket toe
Fillet weld
AA
Hot spot
stress
Nominal stress
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Det Norske Veritas AS. All rights reserved Slide 2525 September 2013
Stress distribution at a bracket toe
Notch stress
t/2 3t/2
Stress
Hot spot stress
Surface stress
Notch stress
Hot spot stress
Membrane stress
Bracket toe
t
Fillet weld
Nominal stress
Section A-A:
Det Norske Veritas AS. All rights reserved Slide 2625 September 2013
S-N curve based on nominal stress
Nominal stress
Number of cycles
Stress
range
Nominal stress
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Det Norske Veritas AS. All rights reserved Slide 2725 September 2013
S-N curve based on hot spot stress
Hot spot
stress
Nominal stress
Number of cycles
Stress
range
Hot spot stress
Nominal stress
alNogspotHot K min
Det Norske Veritas AS. All rights reserved Slide 2825 September 2013
Different types of S-N curve
Notch stress
Hot spot stress
Nominal
stress
Number of cycles
Stress
range
Notch stressHot spot stress
Nominal stress
alNowgstressNotch KK min
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Det Norske Veritas AS. All rights reserved Slide 2925 September 2013
Definitions constant amplitude loading
Number of cycles
Stress
range
m = 3.01
Constant amplitude
fatigue limit
(recently doubted)
Det Norske Veritas AS. All rights reserved Slide 3025 September 2013
When is a detailed fatigue analysis required?
N
S
Fatigue limit
Stress cycling
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Det Norske Veritas AS. All rights reserved Slide 3125 September 2013
S-N curve for variable amplitude loading
Number of cycles
Stress
rangem = 3.0
1
m = 5.0
Haibachs correction:
m = 2m - 1
Det Norske Veritas AS. All rights reserved Slide 3225 September 2013
Definitions variable amplitude loading
N
S
Fatigue limit
Stress cyclinga
b
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Det Norske Veritas AS. All rights reserved Slide 3325 September 2013
na
1
N
nD
mk
1i
ii
k
1i i
i
S-N fatigue approach under the assumption
of linear cumulative damage
(Palmgren-Miner rule).
Palmgren - Miner Rule
Det Norske Veritas AS. All rights reserved Slide 3425 September 2013
Example Palmgren - Miner Rule
Number of cycles
Stress
range1
1N
1
1
N
nD
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Det Norske Veritas AS. All rights reserved Slide 3525 September 2013
Example Palmgren Miner Rule
Number of cycles
Stress
range1
2
1N 2N
2
2
1
1
N
n
N
nD
Det Norske Veritas AS. All rights reserved Slide 3625 September 2013
Relative fatigue damage in S-N curve
1.E+04 1.E+05 1.E+06 1.E+07 1.E+08 1.E+09 1.E+10 1.E+11
Load cycle in S-N curve
R
elativedamageratio
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Det Norske Veritas AS. All rights reserved Slide 3725 September 2013
Fatigue strength versus tensile strength
Note for welded structure, the fatigue
strength is independent of yield strength !
Det Norske Veritas AS. All rights reserved Slide 3825 September 2013
Different mean stresses (R-ratio)
Time
Dynamic
stress
amplitude
1. Static stress tension
2. Zero static stress (alternating stress)
3. Static stress compression
1
2
3
Tension
1.0R1R
max
min
R
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Det Norske Veritas AS. All rights reserved Slide 3925 September 2013
S-N curves and joint classification
For practical fatigue
design, welded joints are
divided into several
classes, each with a
corresponding design S-
N curve.
All tubular joints are
assumed to be class T.
Other types of joint are
depending upon:
- The geometrical arrangement of
the detail;
- The direction of the fluctuating
stress relative to the detail;
- The method of fabrication and
inspection of the detail.
Det Norske Veritas AS. All rights reserved Slide 4025 September 2013
Example: Cross joint
23
1
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Det Norske Veritas AS. All rights reserved Slide 4125 September 2013
From DNV RP-C203 Table A7:
Hot spot 1 and 2
l
6. Gusset plate welded tothe edge of a plate or
beam flange.
7. Flange welded to
another flange at
crossing joints.
6 and 7:The distance l is
governing detail category
for the stress directionshown in sketch. For
main stress in the other
beam the distance L will
govern detail category.
G l 150mm
W1 150 < l 300mm
W2 l > 300mm
Det Norske Veritas AS. All rights reserved Slide 4225 September 2013
Hot spot 1 and 2
From DNV RP-C-203 Table A7:
5.
E150mmr,
W
r
3
1
F
3
1
W
r
6
1
F1
6
1
W
r
10
1
F3
10
1
W
r
16
1
G
16
1
W
r
25
1
5. Gusset plate with aradius welded to theedge of a plate or beam
flange.
5. The specified radius tobe achieved by grinding.
Improved fabrication
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Det Norske Veritas AS. All rights reserved Slide 4325 September 2013
Hot spot 3
F 2. 2.Ends of continuouswelds at copeholes.
2.:
- Cope hole not to be
filled with weld
material.
From DNV RP-C-203 Table A4:
Det Norske Veritas AS. All rights reserved Slide 4425 September 2013
S-N Classification of Cross joint
Hot spot 1 and 2: Depends on length of weld
DNV RP-C-203 Table A7
l < 300 mm: W1
Hot spot 3: F DNV RP-C-203 Table A4
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Det Norske Veritas AS. All rights reserved Slide 4525 September 2013
Topside Support Stool - Fixed type
Principal stress direction
Force direction
Bracket toe
Welded attachment, l > 300mm: Table A7 Detail 1
Full Penetration weld or fillet weld
S-N curve: F3
Det Norske Veritas AS. All rights reserved Slide 4625 September 2013
Definition of local (nominal) stress
45 deg 45 deg
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Det Norske Veritas AS. All rights reserved Slide 4725 September 2013
Fatigue cracking along weld toe
Principal stress
direction
Weld
toe
Section
Fatigue crack
////
Principal stress
direction
Weld
toe
Section
Fatigue crack
////
//
//
Det Norske Veritas AS. All rights reserved Slide 4825 September 2013
Principal stress more parallel with weld toe
Principal stress
direction Weld
toe
Section
Fatigue crack
////
Principal stress
direction Weld
toe
Section
Fatigue crack
////
//
//
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Det Norske Veritas AS. All rights reserved Slide 4925 September 2013
F detail for stress normal to the weld
C2C2
F F
EEDD
Principal stress
direction
Weld
toe
Section
C2C2
F F
EEDD
Principal stress
direction
Weld
toe
Section
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DNV-RP-C203Fatigue Design of Offshore Steel Structures
Part 3 Stress Concentration Factors
Houston
September 26, 2013
Det Norske Veritas AS. All rights reserved Slide 227 September 2013
Definition of a Stress Concentration Factor:
Stress magnification at a structural detail due to the detail itself or
due to a fabrication tolerance with the nominal stress as reference
value
nominal
spothot
SCF
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2
Det Norske Veritas AS. All rights reserved Slide 327 September 2013
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
1.80
2.00
2.20
2.40
2.60
2.80
3.00
1.00 1.20 1.40 1.60 1.80 2.00 2.20 2.40 2.60 2.80 3.00 3.20 3.40 3.60 3.80 4.00
Relative distance from centre of hole x/r
Relativestress
r
Stress direction
x/r
Line for calculation of stress
Line for calculation
of stress
r
x
Stress Distribution at a Hole
Det Norske Veritas AS. All rights reserved Slide 427 September 2013
Fatigue Life?
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Det Norske Veritas AS. All rights reserved Slide 527 September 2013
Soft transitions, avoid stress concentrations
Crack in main deck
Fatigue Life?
Det Norske Veritas AS. All rights reserved Slide 627 September 2013
Notch region
t
a)
b)
Effect of Eccentricity
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Det Norske Veritas AS. All rights reserved Slide 727 September 2013
N
Static system:
Deflected shape:
N
N
Bending moment:
Axial + bending stress:
A - A
Effect of Eccentricity
Det Norske Veritas AS. All rights reserved Slide 827 September 2013
Single Side Welded Connections
m
Shift in neutral axis
-
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5
Det Norske Veritas AS. All rights reserved Slide 927 September 2013
Classical Beam Theory (Plate)
m
Neutral axisN
Moment distributionM = N*m/2
M = 0? DNV-RP-C203
M = N*m/2
Det Norske Veritas AS. All rights reserved Slide 1027 September 2013
Classical Beam Theory (Plate)
tt
N
t
N
W
M mammb
33
6/
2/22
m
Moment distributionM = N*m/2
tSCF m
31
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6
Det Norske Veritas AS. All rights reserved Slide 1127 September 2013
Stress Concentration Factors forButt Welds
5.1
5.1
0m
1
61SCF
T
tt
t
where
m = maximum misalignment
t = tT eccentricity due to change in thickness
0 = 0.1t is misalignment inherent in the S-N data for butt weldsT = thickness of thicker plate
t = thickness of thinner plate
1 9
44
3 0
Det Norske Veritas AS. All rights reserved Slide 1227 September 2013
Joint with Transition in Thickness
Neutral
axis
1
nominal
Tt
t
T = 40 mm
T = 25 mmt = 0.5 (40 -25) = 7.5 mm
m = 4 mm (due to fabrication)
0 = 0.1 t = 2.5 mm
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7
Det Norske Veritas AS. All rights reserved Slide 1327 September 2013
Calculation of SCF
5.1
5.1
0m
1
61SCF
t
Tt
t Equation 3.1.2
72.1
02.325
)9(61
25
40125
5.25.7461SCF
5.1
5.1
Det Norske Veritas AS. All rights reserved Slide 1427 September 2013
100 MPa
Transverse Frame
Side 1 and 2
ZY
X
End 1
End 2
Axial Stress
Buttweld
5.1
t
t
T1
1
t
)(61SCF
m
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
0 100 200 300 400
Position along weld measured from longitudinal (mm)
SCF
Analysis with cope hole
Analysis without cope hole
SCF equation (3)
Plate on Longitudinal Stiffeners
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8
Det Norske Veritas AS. All rights reserved Slide 1527 September 2013
SCFs for Welded Penetrations
H
tp
A A
tr
AA
r
tr
1.0
1.5
2.0
2.5
3.0
3.5
0.0 0.5 1.0 1.5 2.0
tr/tp
SCF
100
50
20
10
r/t p
H
tp
A A
tr
AA
r
tr
H
tp
A A
tr
AA
r
tr
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
SCF
tr/tp
r/tp
10
20
50
100
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
SCF
tr/tp
r/tp
10
20
50
100
Det Norske Veritas AS. All rights reserved Slide 1727 September 2013
SCFs for Scallops
SCF = 2.4 at point A (misalignment not included) SCF = 1.27 at point B
A
B
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9
Det Norske Veritas AS. All rights reserved Slide 1827 September 2013
Scallops
SCF = 1.27 at point A (misalignment not included)
SCF = 1.27 at point B
A
B35
120
Det Norske Veritas AS. All rights reserved Slide 1927 September 2013
Scallops
SCF = 1.17 at point A (misalignment not included)
SCF = 1.27 at point B
150
35
B
A
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10
Det Norske Veritas AS. All rights reserved Slide 2027 September 2013
Un-symmetrical stiffeners on laterally loaded panels, Kn
Neutral axis
nominal
Kn1nominalKn2nominal
Kn3nominal
Stress Concentration Factors LateralLoading
Example:
L340x12+150x15 -> Lif e = 15 years
T340x12+150x15 -> Life = 24 years
Det Norske Veritas AS. All rights reserved Slide 2127 September 2013
Tethers and Risers Subjected to Axial
Tension
kl
klN
tanh0
EI
Nk
N
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11
Det Norske Veritas AS. All rights reserved Slide 2227 September 2013
Concentricity
A A
Section A-Aa) Concentricity
t
t
m
Det Norske Veritas AS. All rights reserved Slide 2327 September 2013
Transition in Thickness
A A
b) Thickness Section A-A
T
t
(T-t)t
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12
Det Norske Veritas AS. All rights reserved Slide 2427 September 2013
Out of Roundness
A
Section A-Ac) Out of roundness
A
t
t
m
mm
Det Norske Veritas AS. All rights reserved Slide 2527 September 2013
Eccentricity
A A
Section A-Ad) Center eccentricity
t
t
mm
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13
Det Norske Veritas AS. All rights reserved Slide 2627 September 2013
2.5
t
T1
1
tD
1.82L
SCF at Thickness Transition
-t e
t
T1
1
t
)(61SCF
m
2
0.30.15.1
t
DLog
t
DLog
Det Norske Veritas AS. All rights reserved Slide 2727 September 2013
SCF at Thickness Transition
-t e
t
T1
1
t
)(61SCF
m
2
0.30.15.1
t
DLog
t
DLog
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
10 100 1000 10000
Diameter thickness ratio D/t
Exponent
1.0
1.1
1.2
1.3
1.4
1.5
1.6
10 100 1000 10000
Diameter thickness ratio D/t
SCF
SCFplates Eq. (3)
New proposal Eq. (6)
Shell theory Eq. (5)
Axisymmetric FE analysis
FE analysis calibrated D/t = 25 Eq. (4)
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14
Det Norske Veritas AS. All rights reserved Slide 2827 September 2013
SCF at Thickness Transition on Outside
-t e
t
T1
1t
)(61SCF
m
Thickness transition on outside is recommended!
(when SCF is considered together with S-N curve)
Det Norske Veritas AS. All rights reserved Slide 3027 September 2013
Ar 2r
.
t
Deformed shape
Hot spot
Ringstiffener
rA
rt1.56t1
insidefor the
0.541SCF
outsidefor the
0.541SCF
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15
Det Norske Veritas AS. All rights reserved Slide 3127 September 2013
tant
)t(tDt0.61SCF
2
cj
Conical Transition
Det Norske Veritas AS. All rights reserved Slide 3227 September 2013
Conical Transition with Ring Stiffeners
r
j
r
j
r
j
r
j
r
j
A
tDt1.101
junctiondiameterlargerinsidetheat
1tan
A
t0.91D0.541SCF
junctiondiameterlargeroutsidetheat
1tan
A
t0.91D0.541SCF
junctiondiametersmallerinsidetheat
1tan
A
t0.91D0.541SCF
junctiondiametersmalleroutsidetheat
1tan
A
t0.91D0.541SCF
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16
Det Norske Veritas AS. All rights reserved Slide 3327 September 2013
Cone with Eccentric Ring Stiffener
5)(
681
1tan3
13
csj
s
e
ttD
ItSCF
Ds
D L
ts
t L
tc
e
h
ts
b
tc
trh
ts
b
tc
tr
Det Norske Veritas AS. All rights reserved Slide 3427 September 2013
SCF in RP-C203 Appendix B SCFs for tubular joints
Appendix C SCFs for cut-outs
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17
Det Norske Veritas AS. All rights reserved Slide 3527 September 2013
Tubular Joints Definitions
Det Norske Veritas AS. All rights reserved Slide 3627 September 2013
Tubular Joint Hot Spot Locations
SCFAC: SCF from axial force at crown
SCFAS: SCF from axial force at saddle
SCFMIP: SCF from in-plane bending moment
SCFMOP: SCF from out-of-plane bending moment
SCFbrace SCFChord
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18
Det Norske Veritas AS. All rights reserved Slide 3727 September 2013
Axial load
zx y
1 2
34
56
7
8
In-plane Out-of-planebending moment bending moment
Tubular Joint Hot Spot Positions
Det Norske Veritas AS. All rights reserved Slide 3827 September 2013
Combination of Stresses
mzMOPmyMIPxASAC8
mzMOPxAS7
mzMOPmyMIPxASAC6
myMIPxAC5
mzMOPmyMIPxASAC4
mzMOPxAS3
mzMOPmyMIPxASAC2
myMIPxAC1
SCF22
1SCF2
2
1)SCF(SCF
2
1
SCFSCF
SCF22
1
SCF22
1
)SCF(SCF2
1
SCFSCF
SCF22
1SCF2
2
1)SCF(SCF
2
1
SCFSCF
SCF22
1SCF2
2
1)SCF(SCF
2
1
SCFSCF
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19
Det Norske Veritas AS. All rights reserved Slide 3927 September 2013
Use of Stress Concentration Factors
SCFs for Tubular joints see Appendix B of DNV-RP-C203 Equations developed by Efthymiou
SCFs applies for the outside of the tubular on the brace side and on the
chord side
Classification of tubular joints based on geometry and force flow through
the joint
SCFX > SCFY > SCFK
Det Norske Veritas AS. All rights reserved Slide 4027 September 2013
Definition of Joint Type, Examples
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20
Det Norske Veritas AS. All rights reserved Slide 4127 September 2013
Definition of Joint Type, Examples
Det Norske Veritas AS. All rights reserved Slide 4227 September 2013
Definition of Geometrical Parameters
saddleD
T
crown crown
L
t
d
D
d
D
2L
2T
D
T
t
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21
Det Norske Veritas AS. All rights reserved Slide 4327 September 2013
Definition of Geometrical Parameters
D
d AA
D
d BB
T
t AA
T
t BB
T
BRACE A
D
g
d
BRACE B
A
t A
Bt
dB
AB 2T
D
D
g
Det Norske Veritas AS. All rights reserved Slide 4427 September 2013
Definition of Geometrical Parameters
D
d AA D
d BB
D
d C
C
T
t AA T
t BB
T
t CC
AB
T
t
d
B
A t
BC
D
B
t
C
d
d
B
B
C
CA
A
A C
g g
2T
D
D
g ABAB
D
g BCBC
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22
Det Norske Veritas AS. All rights reserved Slide 4527 September 2013
Validity Range Of Equations for SCFs
0.2 1.0
0.2 1.0
8 32
4 40
20 90
sin
0.6 1.0
Det Norske Veritas AS. All rights reserved Slide 4627 September 2013
Fatigue Testing in Laboratory of Y-Joint
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23
Det Norske Veritas AS. All rights reserved Slide 4727 September 2013
Bending Moment in Chord
P
M = PL/8
L
P
M = PL/8
L End fixation described by C-parameterC = 0.5: Fixed
C = 1.0: Free
C= 0.7: Recommended
Alternative to use rotational springs
Det Norske Veritas AS. All rights reserved Slide 4827 September 2013
SCFs for T Joint
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24
Det Norske Veritas AS. All rights reserved Slide 4927 September 2013
Determination of Chord Length
To include the effect of the chord bending moment the length of the
chord to nearest joints needs to be determined
The stresses in the chord crown can be calculated as:
Nom
cbcb
SCF
brace
chord
AP
8W
PL
dtP
TD0.258
PL2
0.25
0.25
D
2L
D
d
T
t
P
M = PL/8
L
P
M = PL/8
L
Det Norske Veritas AS. All rights reserved Slide 5027 September 2013
lLe 3
2
P
M = PL/8
L
P
M = PL/8
L
Moment in Beam due to Local Pressure Loads
At supports: M= pl2/12Loading: p
CaissonSupport welded to the caisson
At supports: Q= pl
l
To get a correct bending moment
and SCF for the crown point
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25
Det Norske Veritas AS. All rights reserved Slide 5127 September 2013
Riser Supports in New Platforms
0.5P 0.67P 0.67P P P P
5941 6900 2200 3250 4750 3750 7292
0.5P 0.67P 0.67P P P P
5941 6900 2200 3250 4750 3750 7292
P
M
L
PP
For more loads:
Input to analysis: Lequivalent that gives M for load P
Det Norske Veritas AS. All rights reserved Slide 5227 September 2013
SCF as Function of Length to Diameter
0
5
10
15
20
25
30
0 20 40 60 80 100 120 140
ALFA
SCF
Chord saddle
Brace saddle
Chord crown
Brace crown
Chord Analysis
P
M = PL/8
L
P
M = PL/8
L
D
L2
Basis for Efhymious equations T-joints
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Det Norske Veritas AS. All rights reserved Slide 5327 September 2013
Pitfall using Influence Functions
l
pl2/12
Efthymious
equation
Actual bending moment
MWL
Det Norske Veritas AS. All rights reserved Slide 5427 September 2013
Proposed Modification of SCF Equation
+Mchord/(braceWchord)
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Det Norske Veritas AS. All rights reserved Slide 5527 September 2013
Tubular Joints with Gusset Plates
Geometry SCF
RHS 250x16 with favourablegeometry of gusset plate
2.9
RHS 250x16 with simple shape ofgusset plate
3.8
250x16 with favourable geometryof gusset plate
2.3
250x16 with simple shape ofgusset plate
3.0
10TYP.
40TYP.
Det Norske Veritas AS. All rights reserved Slide 5627 September 2013
Pipelines
Welded Pipes
Seamless Pipes
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Det Norske Veritas AS. All rights reserved Slide 5727 September 2013
Pipelines
dtm e
t
SCF /31
Det Norske Veritas AS. All rights reserved Slide 5827 September 2013
Stresses at Weld
t
D
m
A
A B
B
A -A
-
+
B -B
+
-
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Det Norske Veritas AS. All rights reserved Slide 5927 September 2013
Classification of Welds in Pipelines
Description
Welding Geometry and hot
spot
Tolerance requirement S-Ncurve Thicknessexponent kSCF
min(0.15t, 3 mm) F1 0.00 1.0Single side
Hot spot min(0.15t, 3 mm) F3 0.00 1.0
min(0.1t, 2 mm) F 0.00 1.0Single side
on backing
Hot spot min(0.1t, 2 mm) F1 0.00 1.0
Det Norske Veritas AS. All rights reserved Slide 6027 September 2013
Single side D 0.15 Eq. (2.9.1)
Double side D 0.15 Eq. (2.9.1)
Classification of Welds in Pipelines
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Det Norske Veritas AS. All rights reserved Slide 6127 September 2013
Seamless Pipes
where
Thickness = (tmax tmin)/2
Ovality = Dmax - Dmin if the pipes are supported such that flush outside at one pointis achieved (no pipe centralising)
Ovality = (Dmax - Dmin)/2 if the pipes are centralised during construction
Ovality = (Dmax - Dmin)/4 if the pipes are centralised during construction androtated until a good fit around the circumference is achieved
22
OvalityThicknessTot
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1
DNV-RP-C203
Fatigue Design of OffshoreSteel Structures
Part 4 Finite Element Analysis and Hot Spot Stress
Houston
September 26, 2013
Version Slide 227 September 2013
Finite Elements and the Hot Spot Stress MethodDefinition of Stresses
Nominal Stress
Hot Spot Stress
Notch Stress
Link between different stresses and S-N curves
Background for hot spot S-N curve
Recommendations on finite element modeling
Read out of hot spot stress
Effect of stress gradient through thickness
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2
Version Slide 327 September 2013
The Hot Spot Stress Concept
Long term di stribution of Sea states
Structural model of FPSO
Long term distribution o f nominal stress ranges
Structural stressconcentrationfactor Kg forconsidered detail
hot spot=Kg*nominal
Finite elementmodel ofconsidered detail
hot spot
Hot spot S-N curve
Calculated fatigue life
Scope of FPSO -FatigueCapacity JIP
Long term di stribution of Sea states
Structural model
Long term distribution o f nominal stress ranges
Structural stressconcentrationfactor Kg forconsidered detail
hot spot=Kg*nominal
Finite elementmodel ofconsidered detail
hot spot
Hot spot S-N curve
Calculated fatigue life
Version Slide 427 September 2013
Structural Analysis
Global model
Meshrefinement
Coarsemesh
Finemesh
Intermediatesub-model
Sub-modelSCF-model, orlocal model, orfine meshmodel
=
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3
Version Slide 527 September 2013
Illustration of Stress at a Bracket Toe
Bracket toe
Fillet weld
AA
Hot spot
stress
Nominal stress
Version Slide 627 September 2013
Different Types of S-N Curve
Notch stressHot spot stress
Nominal stress
Number of cycles
Stress
range
Notch stress
Hot spot stress
Nominal stress
alNowgstressNotch KK min
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4
Version Slide 727 September 2013
Method A
Shell element FE model:- 4 node and 8 node elements- Size t x t
- Extrapolation of surface stress from 0.5t and 1.5t to the intersection line
Solid FE model
- 20-node isoparametric elements
- Size t x t
- Extrapolation of surface stress from 0.5t and 1.5t to the weld toe
Version Slide 827 September 2013
Method B
Stress 0.5t from the intersection line in shell FE models
Stress 0.5t from the weld toe in solid FE models
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5
Version Slide 927 September 2013
Test Specimens from HHI
1
54
32
Version Slide 1027 September 2013
Link Between Nominal and Hot Spot Stress
alnogstressspotHot K min
Specimen
no
1
2
3
4
5
K-factor resulting
from fatigue S-N data
from the HHI tests
1.320
1.958
1.334
1.641
1.689
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6
Version Slide 1127 September 2013
Hot Spot Stress S-N Data from the HHI Tests
10
100
1000
10000 100000 1000000 10000000
Number of cycles
Hotspotstressrange(MPa)
No 1
No 2
No 3
No 4
No 5
Mean
Mean minus 2 std
FAT 90
Version Slide 1327 September 2013
Derivation of Hot Spot Stress
t/2 3t/2Distance
t/2 3t/2
Stress
Extrapolated geometric stress(Method A)
Direct calculated geometric stress(Method B)
Weld toe or int ersection line
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7
Version Slide 1427 September 2013
Hotspot
Intersectionline
Element for stressextrapolation
Main stressdirection
End ofbracket
Plate
8 node shellelement
t
t
Example of FE Modeling 8-node Shell Elements
Version Slide 1527 September 2013
Derivation of Hot Spot Stress
Intersection
line
Hot
spot
1
4 3
2
A
0.5 t
Gaussian integration
point
1
4 3
2
1.5 t
Extrapolated
hot spot stress
B
Intersection
line
Hot
spot
1
4 3
2
A
0.5 t
Gaussian integration
point
1
4 3
2
1.5 t
Extrapolated
hot spot stress
B
Intersection
line
Hot
spot
1
4 3
2
A
0.5 t
Gaussian integration
point
1
4 3
2
1.5 t
Extrapolated
hot spot stress
B
Intersection
line
Hot
spot
1
4 3
2
A
0.5 t
Gaussian integration
point
1
4 3
2
1.5 t
Extrapolated
hot spot stress
B
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Version Slide 1827 September 2013
Mesh Size using 4-Node Shell Elements
Recommended mesh size 0.5t x 0.5t to 2t x 2t.
Larger mesh sizes at the hot spot region may provide non-conservative results.
Method A:Hot spot stress based on linear extrapolation of stresses at 0.5t and1.5t when linked to D - curve.
Method B:Hot spot stress from 0.5t should be linked to Ecurve(or stress to be multiplied with 1.12 and Dcurve).
Version Slide 1927 September 2013
Mesh Size using 8-Node Shell Elements
Recommended mesh size from t x t up to 2t x 2t.
Smaller and larger mesh sizes at the hot spot region may provide
non-conservative results.
Method A :
Hot spot stress based on linear extrapolation of stresses 0.5t and
1.5t when linked to D - curve.
Method B :
Hot spot stress based on read out points 0.5t when linked to E
curve
(or stress to be multiplied with 1.12 and Dcurve)
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Version Slide 2027 September 2013
Mesh Size using Solid Elements
For 20-node hexahedral elements it is sufficient with one element overthe thickness to pick up a linear stress distribution.
For simple 8-node brick elements at least 4 elements are required for
the same purpose.
Width length ratio within 1:4.
Modelling of a fillet weld will likely limit the size of the mesh at the hot
spot region.
In order to capture St Venant torsion it is recommended to use several
elements for modelling of a bulb section.
Version Slide 2627 September 2013
Method A
= 0.90 if the detail is classified as C2 with stress parallel to the weld, ref. Table A-3.
= 0.80 if the detail is classified as C1 with stress parallel to the weld, ref. Table A-3.
= 0.72 if the detail is classified as C with stress parallel to the weld, ref. Table A-3.
The effective hot spot stress is derived as
Principal stress
direction
Fatigue crack
//
//
Principal stress
direction
Fatigue crack
//
//
//
//
Principal stress
direction
Fatigue crack
//
//
Principal stress
direction
Fatigue crack
//
//
//
//
2
1
2
//
2 81.0
max
Eff
Read stress from 0.5 t and 1.5t and
extrapolate to intersection line
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Version Slide 2927 September 2013
Method B
= 0.90 if the detail is classified as C2 with stress parallel to the weld, ref. Table A-3.
= 0.80 if the detail is classified as C1 with stress parallel to the weld, ref. Table A-3.
= 0.72 if the detail is classified as C with stress parallel to the weld, ref. Table A-3.
The effective hot spot stress is derived as
Principal stress
direction
Fatigue crack
//
//
Principal stress
direction
Fatigue crack
//
//
//
//
Principal stress
direction
Fatigue crack
////
Principal stress
direction
Fatigue crack
////
//
//
2
1
2
//
2
12.1
12.1
81.012.1
max
Eff
Read stress directly from 0.5 t
Version Slide 3327 September 2013
Derivation of Effective Hot Spot Stressfrom FE Analysis
At hot spots with significant plate bending one might derive an effectivehot spot stress for fatigue assessment based on the following equation:
Only to be used at hot spots with possibility for redistribution of stresses
spotbspotaspote ,,, 60.0
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Version Slide 3427 September 2013
Stress Extrapolation in a Three-dimensional Model
Warning on Presented Procedure
Should not be used for single sided butt welds (Nominal S-N curve is W3,
F3 or F, G if welded on backing)
Should not be used for cruciform joints as calculated KG = 1.0 from FEA
with shell or 3D analysis model (Nominal S-N curve is E, F)
Can not be used for fatigue cracking of the root in fillet welds
Version Slide 3527 September 2013
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DNV-RP-C203
Part 5a Case Study Hot Spot Stress Method
Houston
September 26, 2013
Case Study Hot Spot Stress
Stochastic (spectral) fatigue analysis of typical detail (longitudinal
stiffener to transverse bulkhead connection)
Two configurations of the connection analyzed
Full penetration welding (only weld toe cracking under consideration)
Steps of the analysis
- Global and local FE modeling (SESAM PatranPre)
- Load transfer (HydroD / Wadam)
- Submodeling (SubMOD)
- Structural analysis (SESTRA)
- Fatigue calculations (STOFAT)
- Presentation of results (Xtract)
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Global and Local Models
Global model
Local model
Analyzed Configurations
Configuration #1 Original
Configuration #2 - Improved
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Loads
Load transfer (pressure and inertia loads) Prescribed displacements
Calculation Method
Method A for hot spot stress calculation
Omni directional scatter diagram for GoM
S-N Curve: D in Air and SW with CP used
20 years design life
DFF = 1.0
Membrane Stress at Gaussian points used for visualization purposes
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Calculated Fatigue Damage (Unfactored)
Original configuration, S-N Curve D in SW with CP
Calculated Fatigue Damage (Unfactored)
Improved configuration, S-N Curve D in SW with CP
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Derivation of Hot Spot Stress
Method A (Curve D)
Hot Spot Stress Fatigue Results
Calculated Fatigue Damage (Unfactored) for considered hot spots
Note: The same No. of cycles for both configurations, with different stress range
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DNV-RP-C203
Part 6 Simplified Analysis Method
Houston
September 26, 2013
Version Slide 227 September 2013
Long Term Distribution of Stresses
Deterministic Fatigue Analysis
Weibull Distribution
Closed form Fatigue Damage
Allowable Extreme Stress
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2
Det Norske Veritas AS. All rights reserved Slide 327 September 2013
Example Miner Palmgren
Number of cycles
Stress
range1
2
1N 2N
3
2
1
1
N
n
N
nD
Version Slide 427 September 2013
Norsok N-004 Annex K Deterministic fatigue
H0
H1
H2
H3
H
LOGNn1n2n3
123
i
ni
H S
N
LOGN
N1N2N3 Ni
Waveexceedancediagram
Long- termdistributionof hot- spot stresses S-NCurve
1 2
3
456
7
8
D=100
i=1
niNi
H0H1H2H3
Member endhot- spots
Calculationin eachwave action
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Version Slide 527 September 2013
Simplified Fatigue Assessment
For fatigue assessment in conceptualdesign phase
For mass dominated structures such asSemisubmersibles, Ships, FPSOs andTLPs
Less appropriate for drag dominatedstructures such as jackets and truss towerswith slender tubular members
Version Slide 627 September 2013
Weibull Distribution
h
q
exp)Q(
where
Q = probability for exceedance of the stress range
h = Weibull shape parameter
q = Weibull scale parameter is defined from the stress range level,0, as
1/h
0
0
)n(lnq
0 is the largest stress range out of n0 cycles.
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4
Version Slide 727 September 2013
Weibull Distribution. Example h = 0.70
0
50
100
150
200
250
300
0.1 1 10 100 1000 10000 100000 100000010000000100000000
Stressrange(MPa)
Log n
Version Slide 827 September 2013
Example h = 0.70
0
50
100
150
200
250
300
1 10 100 1000 10000 100000 1000000 1000000
0
1E+08
Log n
Stressrange(MPa
h
nLog
nLog/1
20
20 1
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Version Slide 927 September 2013
Weibull Long Term Stress Range Distribution
o
no Log n
h > 1
h < 1
1/no 1 Q()Probability o fexceedance
h = 1
Integrated Fatigue Damage
Version Slide 1027 September 2013
Damage calculated based on One-slope SN curve
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Version Slide 1127 September 2013
Integrated Fatigue Damage
h
0
h
0
1h
0h),q(
Sexp
h),q(
Shh),f(S,
1/h
0
00
))(ln(nh),q(
dS(S)N
h),f(S,TvdS
(S)N
h),f(S,TvD
0
1
1
S 1
0d0
S
0 2
0d0
Damage calculated based on Two-slope SN curve
Version Slide 1227 September 2013
0
100
200
300
400
500
600
700
800
0.5 0.6 0.7 0.8 0.9 1 1.1 1.2
Weibull shpe parameter h
AllowablestressrangeinMPa
E D C2 C1 C B2 B1
F
F1
F3
G
W1
W2
W3
Allowable Extreme Stress Range
during 108
cycles
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Version Slide 1327 September 2013
Allowable extreme stress range in MPa during 108
cycles for components in air
Weibull shape parameter h
S-N curves
0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20
B1 1449.3 1092.2 861.2 704.7 594.1 512.9 451.4 403.6
B2 1268.1 955.7 753.6 616.6 519.7 448.7 394.9 353.1
C 1319.3 919.6 688.1 542.8 445.5 377.2 326.9 289.0
C1 1182.0 824.0 616.5 486.2 399.2 337.8 292.9 258.9
C2 1055.3 735.6 550.3 434.1 356.3 301.6 261.5 231.1
D and T 949.9 662.1 495.4 390.7 320.8 271.5 235.4 208.1
E 843.9 588.3 440.2 347.2 284.9 241.2 209.2 184.9
F 749.2 522.3 390.8 308.2 253.0 214.1 185.6 164.1
F1 664.8 463.4 346.7 273.5 224.5 190.0 164.7 145.6
F3 591.1 412.0 308.3 243.2 199.6 169.0 146.5 129.4
G 527.6 367.8 275.2 217.1 178.2 150.8 130.8 115.6
W1 475.0 331.0 247.8 195.4 160.4 135.8 117.7 104.0
W2 422.1 294.1 220.1 173.6 142.5 120.6 104.6 92.5
W3 379.9 264.8 198.2 156.0 128.2 108.6 94.2 83.2
Allowable Extreme Stress Range
Version Slide 1427 September 2013
Utilisation factors as function of design life and Design
Fatigue Factor
Design life in yearsDFF
20 25 30
1 1.00 0.80 0.67
2 0.50 0.40 0.33
3 0.33 0.27 0.22
5 0.20 0.16 0.13
10 0.10 0.08 0.07
Design Charts (Different Design Lives and DFFs)
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Version Slide 1527 September 2013
Reduction factor on stress to correspond with utilisation factor for C W3 curves in air environmentWeibull shape parameter hFatigue
damage
utilisation 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20
0.10 0.497 0.511 0.526 0.540 0.552 0.563 0.573 0.582
0.20 0.609 0.620 0.632 0.642 0.652 0.661 0.670 0.677
0.22 0.627 0.899 0.852 0.822 0.802 0.789 0.781 0.775
0.27 0.661 0.676 0.686 0.695 0.703 0.711 0.719 0.725
0.30 0.688 0.697 0.706 0.715 0.723 0.730 0.737 0.743
0.33 0.708 0.717 0.725 0.733 0.741 0.748 0.754 0.760
0.40 0.751 0.758 0.765 0.772 0.779 0.785 0.790 0.795
0.50 0.805 0.810 0.816 0.821 0.826 0.831 0.835 0.839
0.60 0.852 0.856 0.860 0.864 0.868 0.871 0.875 0.878
0.67 0.882 0.885 0.888 0.891 0.894 0.897 0.900 0.902
0.70 0.894 0.897 0.900 0.902 0.905 0.908 0.910 0.912
0.80 0.932 0.934 0.936 0.938 0.939 0.941 0.942 0.944
1.00 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000
Reduction Factor on Stress for Lower Utilisation
Version Slide 1627 September 2013
Example Use of Design Charts
Detail on deck of FPSO in air environment classified as F3
Design life 25 years and DFF = 2
Thickness t = 35 mm
Weibull shape parameter from CN 30.7 h = 0.97
Allowable stress range?
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Version Slide 1727 September 2013
Topside Support Stool - Fixed Type
Principal stress direction
Force direction
Bracket toe
Welded attachment, l > 300mm: Table A7.1
Full Penetration weld or fillet weld
S-N curve: F3
Version Slide 1827 September 2013
Bending Moments at 10-8 Probability Level
Mwo,s = - 0.11kwmCwL2 B ( CB + 0.7 ) (kNm)
Mwo,h = 0.19 kwm CwL2B CB (kNm)
hwoswo MMM ,,20
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Version Slide 1927 September 2013
Bending Moments at 10-8 Probabilty Level
Mwo,s = - 0.11kwmCwL2 B ( CB + 0.7 ) (kNm)
Mwo,h = 0.19 kwm CwL2B CB (kNm)
hwoswo MMM ,,20 d
Z
M2020
Version Slide 2027 September 2013
Allowable Stress Range for 20 Years
The tables for allowable stress ranges in DNV-RP-C203
presents values only for selected values of the Weibull shape
parameter h.
Therefore, interpolation in the tables may be required.
Example of interpolation using values from Table 5-2 for 20
years and DFF = 1 for t = 25 mm:
This gives allowable stress range for h = 0.97 by interpolation
in h :
199.6-(199.6-169.0) (0.97-0.90)/(1.0-0.90) = 178.18 MPa
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Version Slide 2127 September 2013
Example Use of Design Charts
25 years and DFF = 2 gives = 0.40
Reduction factor = 0.779+(0.785-0.779)(0.97-0.90)/(1.0-0.90) = 0.783
178.18*0.783 = 139.55 MPa
Allowable stress range 139.55*(25/35)0.25 = 128.9 MPa
Version Slide 2227 September 2013
Example Fatigue Assessment of Ship Side
s = 800 mm
t
p = 85.3kN/m2
12
2spM
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12
Version Slide 2327 September 2013
Required Plate Thickness DNV-RP-C203
Maximum stress range is calculated as2
222
26/
12 t
sptsp
W
M
Calculate the pressure from another DNV document: CN 30.7 Fatigue
Assessment of Ship Structures.
Pressure in CN 30.7 is given at 10-4probability level.
h
rf/15.0
where h = Weibull shape parameter
To get the stress on 10-8probability level the stress at 10-4probability level
is divided by a factor
Calculate h = 1.0 from equations in CN 30.7.
Version Slide 2427 September 2013
Required Plate Thickness DNV-RP-C203
From simplified fatigue assessment the maximum stress
range during 108 cycles is 215.3 MPa for detail E,
Weibull parameter 1.0
5.02/1
*2
*5.0/
spt
h
mmt 163.215*2
800.0*1000*5.0/3.85
5.020.1/1
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Fatigue Course DNV-RP-C203Fatigue Design of Offshore Steel Structures
Part 7 Uncertainties in Fatigue Life Calculations and Design Fatigue Factors (DFF)
Houston
September 26, 2013
Det Norske Veritas AS. All rights reserved Slide 227 September 2013
Uncertainties In Fatigue Life Calculation
Factors contributing to uncertainty
Failure probability
Design Fatigue Factors
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2
Det Norske Veritas AS. All rights reserved Slide 327 September 2013
Uncertainties In Fatigue Life Predictions
Environment
Load modelling
Error in estimating number of load cycles
Structural model for response analysis (Transfer-function for
stress)
Stress concentration factors
S-N data
Miner Palmgren damage accumulation
Fabrication tolerances
Workmanship
Corrosion protection effectiveness over the life of the structure
(including maintenance).
Uncertainties in SN Data
Det Norske Veritas AS. All rights reserved Slide 427 September 2013
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3
Det Norske Veritas AS. All rights reserved Slide 627 September 2013
20 Years Design Life and Statistical Scatter inS-N Data Only
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
0 20 40 60 80 100 120 140 160 180 200
Time in service (years )
Accumulatedprobability
Det Norske Veritas AS. All rights reserved Slide 827 September 2013
Accumulated Probability of Fatigue Failure
0.0000001
0.000001
0.00001
0.0001
0.001
0.01
0.1
1
0 2 4 6 8 10 12 14 16 18 20
Time in service (years)
Accu
mulatedprobabilityoffatiguefailure
Unc. in S-N curveonly
Unc. in S-N, Miner,
CoVnom = 0.15,CoVhs = 0.05
Unc. in S-N, Miner,CoVnom = 0.20,
CoVhs = 0.05
Unc. in S-N, Miner,
CoVnom = 0.15,CoVhs = 0.10
Unc. in S-N, Miner,
CoVnom = 0.20,CoVhs = 0.10
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4
Det Norske Veritas AS. All rights reserved Slide 927 September 2013
Failure Probability
Det Norske Veritas AS. All rights reserved Slide 1027 September 2013
Design Fatigue Factors (DFF) Norsok
Access for inspection and repair
Accessible
Classificationof structuralcomponentsbased ondamageconsequence
No access orin the splashzone Below splash
zone
Above splash
zone orinternal
Substantialconsequences 10 3 2
Withoutsubstantialconsequences
3 2 1
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5
Det Norske Veritas AS. All rights reserved Slide 1127 September 2013
Design Fatigue Factors FPSOs
Structural Elements Class FMS
Internal structures, accessible and not welded
directly to the submerged part of the shell plate1 2
Internal structure, accessible and welded directly to
the submerged part of the shell plate2 3
External structure above lowest inspection
waterline, accessible for inspection and repair1 2
External structure below lowest inspection
waterline, accessible for inspection by divers2 3
External structure below lowest inspection
waterline, inaccessible for inspection by divers3 10
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1
DNV-RP-C203
Fatigue Design of OffshoreSteel Structures
Part 8 Partial Penetration and Fillet Welds
Houston
September 26, 2013
Version Slide 827 September 2013
Partial penetration/Fillet weld
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2
Version Slide 927 September 2013
0
0.2
0.4
0.6
0.8
1
1.2
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
2ai/tp
h/tp
tp = 50 mm
tp = 25 mm
tp = 12 mm
tp = 6mm
Weld toe failure
Weld root failure
Failure from the weld root?
Version Slide 1027 September 2013
Fillet weld connection with symbols
Th
t
a
L
PT
h
t
a
L
P
Throatsection
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3
Version Slide 1127 September 2013
Throatsection
2
//
22
w 0.2
Stress components in fillet welds
Version Slide 1227 September 2013
Design S-N curve: W3
10
100
1000
1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08
Number of cycles
Stressrange(MPa)
B1B2
C
C1
C2
DE
F
F1
F3G
W1
W2
W3
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DNV-RP-C203
Part 9 Fabrication and Improvement
Houston
September 26, 2013
Version Slide 227 September 2013
Methods for fatigue life improvement
Grinding
TIG dressing
Hammer peening
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Version Slide 327 September 2013
Example of joints suitable for improvements
Version Slide 427 September 2013
Example unsuitable for improvement
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Version Slide 527 September 2013
Pneumatic grinders and burrs
Version Slide 627 September 2013
The weld toe burr grinding technique
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Version Slide 727 September 2013
Grinding of weld toe
Version Slide 927 September 2013
Correctly ground weld toe
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Version Slide 1027 September 2013
Incorrectly ground weld toe
Version Slide 1127 September 2013
Gauge for measuring weld groove
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Version Slide 1227 September 2013
Grinding
Depth of grinding shouldbe 0.5mm below bottomof any visible undercut.BA
T
Version Slide 1327 September 2013
Weld ProfilingT
R
Weld profiling by grinding
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Version Slide 1427 September 2013
Weld ProfilingT
R
5.025.0 )/()(tan17.047.0 RT
BendingMembranereducedLocal
5.025.0)/()(tan13.060.0 RT
BendingMembraneLocal
Weld profiling by grinding
Version Slide 1527 September 2013
TIG dressing of welds
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Version Slide 1627 September 2013
Fillet weld before and after TIG dressing
Version Slide 1727 September 2013
Hammer peening
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Version Slide 1827 September 2013
Hammer peening operation
Version Slide 2127 September 2013
Warnings for hammer peening Hammer peening should only be used on members where failure
will be without substantial consequences
Overload in compression must be avoided, because the residual
stress set up by hammer peening will be destroyed. The same may
occur with spectrum loading.
Peening tip must be small enough to reach weld toe.
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Version Slide 2727 September 2013
Improvement on fatigue life by different methods
Improvement method Minimum specified yieldstrength Increase in fatigue life(factor on life)1)
Grinding Less than 350 MPa 0.01f y
Higher than 350 MPa 3.5
TIG dressing Less than 350 MPa 0.01f y
Higher than 350 MPa 3.5
Hammer peening3) Less than 350 MPa 0.011f y
Higher than 350 MPa 4.0
Improvement on Fatigue Life
Version Slide 2827 September 2013
Commentary D.15
Factor for improvement or new S-N
curve should likely be a function of
considered connection
The maximum improvement should also
be a function of fabrication and NDT
Testing is recommended
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RECOMMENDED PRACTICE
DETNORSKEVERITASAS
The electronic pdf version of this document found through http://www.dnv.com is the officially binding version
DNV-RP-C203
Fatigue Design ofOffshore Steel Structures
OCTOBER 2012
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Det Norske Veritas AS October 2012
Any comments may be sent by e-mail to [email protected]
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7/27/2019 Fatigue Course DNV-RP-C203_Fatigue Design of Offshore Steel Structures_tcm153-578957
113/288DET NORSKE VERITAS AS
Recommended Practice DNV-RP-C203, October 2012
Changes Page 3
CHANGES
General
This document supersedes DNV-RP-C203, October 2011.
Text affected by the main changes in this edition is highlighted in red colour. However, if the changes involve
a whole chapter, section or sub-section, normally only the title will be in red colour.
Main changes
General
A number of editorial corrections have been made.
Sec.3
3.3.7.3: new section Stress concentration factors for butt welds between members with equal thickness. 3.3.12 Stress concentration factors for joints with gusset plates: Added recommendations for applicable
S-N curves at inside of tubulars.
Sec.4 4.2 Tubular joints: correction of equation. 4.3.7: insertion of a new paragraph regarding simple cruciform joints.
Sec.10
References /88/ and /89/ added.
App.C
Changes in Figure C-19 and Figure C-23.
App.D
5 S-N curves (B1, B2, C, C1 and C2) deleted in D.15 Table D-9.
In addition to the above stated main changes, editorial corrections may have been made.
Editorial Corrections
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7/27/2019 Fatigue Course DNV-RP-C203_Fatigue Design of Offshore Steel Structures_tcm153-578957
114/288DET NORSKE VERITAS AS
Recommended Practice DNV-RP-C203, October 2012
Contents Page 4
CONTENTS
1. INTRODUCTION ................................................................................................................. 7
1.1 General...................................................................................................................................................7
1.2 Validity of standard ..............................................................................................................................71.2.1 Material...................................................................................................................................................................71.2.2 Temperature............................................................................................................................................................71.2.3 Low cycle and high cycle fatigue........................................................................................................................... 7
1.3 Methods for fatigue analysis ................................................................................................................7
1.4 Definitions..............................................................................................................................................8
1.5 Symbols ..................................................................................................................................................9
2. FATIGUE ANALYSIS BASED ON S-N DATA............................................................... 10
2.1 Introduction.........................................................................................................................................10
2.2 Fatigue damage accumulation...........................................................................................................12
2.3 Fatigue analysis methodology and calculation of Stresses ..............................................................122.3.1 General..................................................................................................................................................................12
2.3.2 Plated structures using nominal stress S-N curves ...............................................................................................122.3.3 Plated structures using hot spot stress S-N curves ...............................................................................................132.3.4 Tubular joints .......................................................................................................................................................132.3.5 Fillet welds at cruciform joints.............................................................................................................................152.3.6 Fillet welds at doubling plates ..............................................................................................................................152.3.7 Fillet welded bearing supports..............................................................................................................................16
2.4 S-N curves............................................................................................................................................162.4.1 General..................................................................................................................................................................162.4.2 Failure criterion inherent the S-N curves ............................................................................................................. 162.4.3 S-N curves and joint classification ....................................................................................................................... 162.4.4 S-N curves in air ................................................................................................................................................... 182.4.5 S-N curves in seawater with cathodic protection .................................................................................................192.4.6 S-N curves for tubular joints ................................................................................................................................202.4.7 S-N curves for cast nodes.....................................................................................................................................20
2.4.8 S-N curves for forged nodes.................................................................................................................................202.4.9 S-N curves for free corrosion ...............................................................................................................................212.4.10 S-N curves for base material of high strength steel.............................................................................................. 212.4.11 S-N curves for stainless steel................................................................................................................................222.4.12 S-N curves for small diameter umbilicals ............................................................................................................222.4.13 Qualification of new S-N curves based on fatigue test data.................................................................................23
2.5 Mean stress influence for non welded structures.............................................................................23
2.6 Effect of fabrication tolerances..........................................................................................................23
2.7 Requirements to NDE and acceptance criteria................................................................................24
2.8 Design chart for fillet and partial penetratio