fatigue course dnv-rp-c203_fatigue design of offshore steel structures_tcm153-578957

7/27/2019 Fatigue Course DNV-RP-C203_Fatigue Design of Offshore Steel Structures_tcm153-578957

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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


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.



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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


through the weld


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.



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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


The Alexander L. Kielland accident


D-6 failed

D was lost


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Hydrophone holderBrace

a

Fatigue crack that initiated

from the fillet welds

Failed member




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Fatigue crack growth in failed member

Crack initiation in filletwelds

Fatigue crack growth

around the brace

Final fracture in storm


Shuttle tanker operating for 9 years in North Sea



7/288

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Side plate shuttle tanker s/t = 825/16


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.



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Repair


Updates in Latest DNV-RP-C203


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



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DNV-RP-C203Fatigue Design of OffshoreSteel Structures

Part 2 Basics of Fatigue

Houston

September 26, 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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1. Fatigue crack growth from the weld toe


2. Fatigue crack growth from a notch


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Specimen 3 Fatigue crack into bulb

B-B

B

B


13/288

4


3. Fatigue from the weld root. Mitigation

Full penetration

L = 10 t




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5


Definitions

Low cycle fatigue (LCF): N


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6


Specimen in testing machine


Specimen in testing machine


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7


Section through weld


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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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)


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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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


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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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


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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Influence of environment

ENVIRONMENTS

Air

SeawaterwithCathodic

Protection

Seawaterand Free

Corrosion


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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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


Illustration of stress at a bracket toe

Bracket toe

Fillet weld

AA

Hot spot

stress

Nominal stress


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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:


S-N curve based on nominal stress

Nominal stress

Number of cycles

Stress

range

Nominal stress


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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


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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Definitions constant amplitude loading

Number of cycles

Stress

range

m = 3.01

Constant amplitude

fatigue limit

(recently doubted)


When is a detailed fatigue analysis required?

N

S

Fatigue limit

Stress cycling


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S-N curve for variable amplitude loading

Number of cycles

Stress

rangem = 3.0

1

m = 5.0

Haibachs correction:

m = 2m - 1


Definitions variable amplitude loading

N

S

Fatigue limit

Stress cyclinga

b


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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


Example Palmgren - Miner Rule

Number of cycles

Stress

range1

1N

1

1

N

nD


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Example Palmgren Miner Rule

Number of cycles

Stress

range1

2

1N 2N

2

2

1

1

N

n

N

nD


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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Fatigue strength versus tensile strength

Note for welded structure, the fatigue

strength is independent of yield strength !


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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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.


Example: Cross joint

23

1


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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


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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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:


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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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


Definition of local (nominal) stress

45 deg 45 deg


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Fatigue cracking along weld toe

Principal stress

direction

Weld

toe

Section

Fatigue crack

////

Principal stress

direction

Weld

toe

Section

Fatigue crack

////

//

//


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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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


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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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


Fatigue Life?


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Soft transitions, avoid stress concentrations

Crack in main deck

Fatigue Life?


Notch region

t

a)

b)

Effect of Eccentricity


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N

Static system:

Deflected shape:

N

N

Bending moment:

Axial + bending stress:

A - A

Effect of Eccentricity


Single Side Welded Connections

m

Shift in neutral axis


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Classical Beam Theory (Plate)

m

Neutral axisN

Moment distributionM = N*m/2

M = 0? DNV-RP-C203

M = N*m/2


Classical Beam Theory (Plate)

tt

N

t

N

W

M mammb

33

6/

2/22

m

Moment distributionM = N*m/2

tSCF m

31


40/288

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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


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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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


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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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


SCFs for Scallops

SCF = 2.4 at point A (misalignment not included) SCF = 1.27 at point B

A

B


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Scallops

SCF = 1.27 at point A (misalignment not included)

SCF = 1.27 at point B

A

B35

120


Scallops

SCF = 1.17 at point A (misalignment not included)

SCF = 1.27 at point B

150

35

B

A


44/288

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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


Tethers and Risers Subjected to Axial

Tension

kl

klN

tanh0

EI

Nk

N


45/288

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Concentricity

A A

Section A-Aa) Concentricity

t

t

m


Transition in Thickness

A A

b) Thickness Section A-A

T

t

(T-t)t


46/288

12


Out of Roundness

A

Section A-Ac) Out of roundness

A

t

t

m

mm


Eccentricity

A A

Section A-Ad) Center eccentricity

t

t

mm


47/288

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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


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)


48/288

14


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)


Ar 2r

.

t

Deformed shape

Hot spot

Ringstiffener

rA

rt1.56t1

insidefor the

0.541SCF

outsidefor the

0.541SCF


49/288

15


tant

)t(tDt0.61SCF

2

cj

Conical Transition


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


50/288

16


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


SCF in RP-C203 Appendix B SCFs for tubular joints

Appendix C SCFs for cut-outs


51/288

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Tubular Joints Definitions


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


52/288

18


Axial load

zx y

1 2

34

56

7

8

In-plane Out-of-planebending moment bending moment

Tubular Joint Hot Spot Positions


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


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


Definition of Joint Type, Examples


54/288

20


Definition of Joint Type, Examples


Definition of Geometrical Parameters

saddleD

T

crown crown

L

t

d

D

d

D

2L

2T

D

T

t


55/288

21



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



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


56/288

22


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


Fatigue Testing in Laboratory of Y-Joint


57/288

23


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


SCFs for T Joint


58/288

24


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


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


59/288

25


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


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


60/288

26


Pitfall using Influence Functions

l

pl2/12

Efthymious

equation

Actual bending moment

MWL


Proposed Modification of SCF Equation

+Mchord/(braceWchord)


61/288

27


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.


Pipelines

Welded Pipes

Seamless Pipes


62/288

28


Pipelines

dtm e

t

SCF /31


Stresses at Weld

t

D

m

A

A B

B

A -A

-

+

B -B

+

-


63/288

29


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


Single side D 0.15 Eq. (2.9.1)

Double side D 0.15 Eq. (2.9.1)

Classification of Welds in Pipelines


64/288

30


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


65/288

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


66/288

2


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


Structural Analysis

Global model

Meshrefinement

Coarsemesh

Finemesh

Intermediatesub-model

Sub-modelSCF-model, orlocal model, orfine meshmodel

=


67/288

3


Illustration of Stress at a Bracket Toe

Bracket toe

Fillet weld

AA

Hot spot

stress

Nominal stress


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


68/288

4


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


Method B

Stress 0.5t from the intersection line in shell FE models

Stress 0.5t from the weld toe in solid FE models


69/288

5


Test Specimens from HHI

1

54

32


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


70/288

6


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


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


71/288

7


Hotspot

Intersectionline

Element for stressextrapolation

Main stressdirection

End ofbracket

Plate

8 node shellelement

t

t

Example of FE Modeling 8-node Shell Elements



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


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


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


point

1

4 3

2

1.5 t

Extrapolated

hot spot stress

B


72/288

8


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).


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)


73/288

9


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.


Method A

= 0.90 if the detail is classified as C2 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


74/288

10


Method B



= 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


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


75/288

11


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



76/288

1

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)


77/288

2

Global and Local Models

Global model

Local model

Analyzed Configurations

Configuration #1 Original

Configuration #2 - Improved


78/288

3

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


79/288

4

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


80/288

5


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


81/288

1

DNV-RP-C203

Part 6 Simplified Analysis Method

Houston

September 26, 2013


Long Term Distribution of Stresses

Deterministic Fatigue Analysis

Weibull Distribution

Closed form Fatigue Damage

Allowable Extreme Stress


82/288

2


Example Miner Palmgren

Number of cycles

Stress

range1

2

1N 2N

3

2

1

1

N

n

N

nD


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 hotspot stresses S-NCurve

1 2

3

456

7

8

D=100

i=1

niNi

H0H1H2H3

Member endhot- spots

Calculationin eachwave action


83/288

3


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


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.


84/288

4


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


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


85/288

5


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


Damage calculated based on One-slope SN curve


86/288

6


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


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


87/288

7


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


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)


88/288

8


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


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?


89/288

9


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


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


90/288

10


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


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


91/288

11


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


Example Fatigue Assessment of Ship Side

s = 800 mm

t

p = 85.3kN/m2

12

2spM


92/288

12


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.


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


93/288

1

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


Uncertainties In Fatigue Life Calculation

Factors contributing to uncertainty

Failure probability

Design Fatigue Factors


94/288

2


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



95/288

3


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


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,


Unc. in S-N, Miner,



96/288

4


Failure Probability


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


97/288

5


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


98/288

1

DNV-RP-C203

Fatigue Design of OffshoreSteel Structures

Part 8 Partial Penetration and Fillet Welds

Houston

September 26, 2013


Partial penetration/Fillet weld


99/288

2


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?


Fillet weld connection with symbols

Th

t

a

L

PT

h

t

a

L

P

Throatsection


100/288

3


Throatsection

2

//

22

w 0.2

Stress components in fillet welds


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


101/288

1

DNV-RP-C203

Part 9 Fabrication and Improvement

Houston

September 26, 2013


Methods for fatigue life improvement

Grinding

TIG dressing

Hammer peening


102/288

2


Example of joints suitable for improvements


Example unsuitable for improvement


103/288

3


Pneumatic grinders and burrs


The weld toe burr grinding technique


104/288

4


Grinding of weld toe


Correctly ground weld toe


105/288

5


Incorrectly ground weld toe


Gauge for measuring weld groove


106/288

6


Grinding

Depth of grinding shouldbe 0.5mm below bottomof any visible undercut.BA

T


Weld ProfilingT

R

Weld profiling by grinding


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7


Weld ProfilingT

R

5.025.0 )/()(tan17.047.0 RT

BendingMembranereducedLocal

5.025.0)/()(tan13.060.0 RT

BendingMembraneLocal

Weld profiling by grinding


TIG dressing of welds


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8


Fillet weld before and after TIG dressing


Hammer peening


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9


Hammer peening operation


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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10


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


Hammer peening3) Less than 350 MPa 0.011f y


Improvement on Fatigue Life


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


112/288

Det Norske Veritas AS October 2012

Any comments may be sent by e-mail to [email protected]

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FOREWORD

DNV is a global provider of knowledge for managing risk. Today, safe and responsible business conduct is both a licenseto operate and a competitive advantage. Our core competence is to identify, assess, and advise on risk management. Fromour leading position in certification, classification, verification, and training, we develop and apply standards and best

practices. This helps our customers safely and responsibly improve their business performance. DNV is an independentorganisation with dedicated risk professionals in more than 100 countries, with the purpose of safeguarding life, propertyand the environment.

DNV service documents consist of among others the following types of documents:

Service Specifications. Procedural requirements. Standards. Technical requirements. Recommended Practices. Guidance.

The Standards and Recommended Practices are offered within the following areas:A) Qualification, Quality and Safety MethodologyB) Materials TechnologyC) StructuresD) SystemsE) Special FacilitiesF) Pipelines and RisersG) Asset Operation

H) Marine OperationsJ) Cleaner Energy

O) Subsea SystemsU) Unconventional Oil & Gas


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


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

fatigue course dnv-rp-c203_fatigue design of offshore steel structures_tcm153-578957

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