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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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