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Delft University of Technology

Fatigue of offshore structures OE4606 Introduction to Offshore Engineering

Prof.dr.ir. Miroslaw Lech (Mirek) Kaminski

3mE - Ship & Offshore Structures

17 September 2014

2

Course schedule

# Date Subject Lecturer

1 10-9-2014 Introduction Mirek Kaminski

2 11-9-2014 Loads on Offshore structures Mirek Kaminski

3 17-9-2014 Fatigue of Offshore Structures Mirek Kaminski

4

5

6

7

8

9

3

Question

.

What is fatigue?

4

Fatigue

fatigue is

the progressive and localized

structural damage

of a material subjected to

cyclic loading

5

Fatigue = cracks

Sourc

e

PhD

thesi

s G

aute

Sto

rhaug

6

FPSO - bottom wing tank connection

7

Question

.

Why is fatigue dangerous?

8

Experiment

9

Fatigue effects

Increases risk of fracture

Changes load paths

May cause leakage

May initiate domino effect

10

Fatigue induced disaster

Alexander L. Kielland

11

Capsized platform

12

Crack

13

Question

.

How to avoid fatigue?

14

Structural design - general

Loading

Structure

Capacity

OK?

no

15

Check of all possible failure mechanisms

Yielding Buckling Fracture

Delamination Fatigue Corrosion

16

Experiment

17

Instruction

0 45 90 135 180

18

mS

aN

)()()( SLogmaLogNLog

m

aLogNLog

mSLog

)()(

1)(

SN-curve = fatigue capacity (resistance)

S N

45 225

45 183

45 150

45 175

90 30

90 20

90 25

90 22

135 5

135 8

135 7

135 6

180 2

180 5

180 3

180 4 -1/m -0.3333 m= 3.00

Log(a)/m 2.412 a (design) = 12206435

19

Fatigue damage

S

45

90

135

180

Stress

range

n

3

9

2

0

Loading

cycles

N

134

17

5

2

Capacity

cycles

n/N

0.022

0.538

0.404

0.000

Damage

0.964Total damage

20

Fatigue damage

21

SN - curve

Rule of Miner

i m

i

aN

S

i

i

nD

N miiSn

aD

1

Design lifetimeFatigue lifetime

D

Fatigue lifetime

22

Damage ~ (stress range)3

Damage ~ number of stress cycles

20 years = 100 millions of wave (stress) cycles

Damage is cumulative

Moderate stresses cycles are important

Fatigue - general

23

Fatigue testing

24

Test specimen with crack

25

SN-curves of IIW - steel

2106

Class

Class

26

Effect of welding detail on fatigue capacity

Class

27

Important note!

In general, fatigue capacity of

structural details

does NOT depend on

steel grade and yield stress

28

Effect of welding on fatigue lifetime

2 times shorter

2 times shorter

2 times shorter

2 times shorter

16 times shorter

29

Avoid stress concentrations no stiffness jumps

Place welds away of stress concentration areas

Use adequate class of details

Consider weld dressing

Protect against corrosion

Design tips for structural details

30

Conclusion

You know

how to calculate

fatigue lifetime

of a structural detail

when fatigue loading is known

31

Your enthusiasm!

32

Repetition

to learn we need to repeat

To learn we need to repeat

33

Question

.

What is fatigue loading?

34

Question

.

What is fatigue capacity?

35

Question

.

What is fatigue damage?

36

Question

.

What is fatigue lifetime?

37

Question

.

What is fatigue design criterion?

38

Capacity

i

i

nD

N

Fatigue loading, capacity, damage, lifetime & design criterion

Design lifetimeFatigue lifetime

D

39

Question

Where is the safety factor?

40

Summary

Select detail

Select appropriate SN-curve

Define fatigue loading

Calculate fatigue capacity

Apply the rule of Miner

Calculate fatigue lifetime

41

Exercise 1 calculate fatigue lifetime

SN - curve = 1 1073

= 3

Design lifetime 20 year

S n

10 150

20 200

50 2

100 0

Stress

MPa

range

Loading

number of

stress

cycles

42

SN - curve

Rule of Miner

i m

i

aN

S

i

i

nD

N

Design lifetimeFatigue lifetime

D

Formula

43

Solution

S n N n/N

10 150 10000 0.015

20 200 1250 0.160

50 2 80 0.025

100 0 10 0.000

Total damage 0.200

Lifetime 100 year

Stress

MPa

range

Loading

number of

stress

cycles

Capacity

number of

cycles

stress

Damage

44

Next step

Select detail

Select appropriate SN-curve

Define fatigue loading

Calculate fatigue capacity

Apply the rule of Miner

Calculate fatigue lifetime

45

Question

.

How to calculate fatigue loading?

46

Conclusion

Fatigue induced by

quasi-static wave action

depends mainly on

moderate sea states

Non-linear response is less relevant

47

Method

Spectral analysis

48

Steps of fatigue analysis

1.E+01

1.E+03

1.E+05

1.E+07

1.E+09

10 30 50 70 90 110 130 150

Stress range (s) [MPa]

Nu

mb

er

of

cycle

s

0.0

0.1

0.2

0.3

0.4

Dam

ag

e

Loading (n) Resistance (N) Damage (D)

Metocean analysis

waves Hydrodynamic analysis

Motions & pressures Stress

analysis

Stresses Fatigue analysis

Lifetime

49

Steps of spectral fatigue analysis

Short Term Statistics

Sea state

Wave Spectrum (WS)

Operational Profile

Heading & draft

Short Term stress distribution

Rayleigh distribution

defined by SS

Long Term Statistics

Scatter Diagram

Long-Term stress distribution

Weibull distribution

defined by Rayleigh

Fatigue Loading loop

Fatigue Capacity SN curve

Miners rule

Fatigue damage & lifetime

Stress Spectrum (SS)

SS = WS x RAO2

Hydro-Structural Analysis

Stress (RAO)

Response Amplitude Operator

per unit wave amplitude

for each wave frequency, heading & draft

50

First principles, empirical methods, experience Keep the overall stress level low

Preliminary design

Rules and direct calculations methods Avoid stress risers, use adequate details

Design/

Engineering

Fabrication procedures and quality assurance Remove unacceptable defects

Construction

Inspection, repair & maintenance procedures Use advisory monitoring systems

Operation

Lifetime assurance - methods

51

Keep stresses low! Preliminary design

Use proper details! Design/

Engineering

Allow only acceptable defects! Building

Monitor! Operation

Lifetime assurance - advice

52

Based on stresses calculated using design tool and measured data

Based on stresses calculated using design

tool and design data

Based on measured stresses

Time

Life

tim

e s

hort

enin

g

53

Octopus - Monitas System

54

Sensoren

Cabinet

DGPS

LBSGs

Strain gauges

FDSs

Motion sensor

Level gauge

Wave buoy

Wind sensor

55

56

Benefits of Octopus-Monitas

Prevents lost of production

Prevents unexpected damage

More time for corrective measures

Rational lifetime extension

Feedback to design

57

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