Download - Fatigue of Offshore Structures
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Challenge the future
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
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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
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5
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9
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Question
.
What is fatigue?
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Fatigue
fatigue is
the progressive and localized
structural damage
of a material subjected to
cyclic loading
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Fatigue = cracks
Sourc
e
PhD
thesi
s G
aute
Sto
rhaug
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FPSO - bottom wing tank connection
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Question
.
Why is fatigue dangerous?
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Experiment
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Fatigue effects
Increases risk of fracture
Changes load paths
May cause leakage
May initiate domino effect
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Fatigue induced disaster
Alexander L. Kielland
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Capsized platform
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Crack
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Question
.
How to avoid fatigue?
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Structural design - general
Loading
Structure
Capacity
OK?
no
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Check of all possible failure mechanisms
Yielding Buckling Fracture
Delamination Fatigue Corrosion
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Experiment
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Instruction
0 45 90 135 180
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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
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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
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Fatigue damage
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SN - curve
Rule of Miner
i m
i
aN
S
i
i
nD
N miiSn
aD
1
Design lifetimeFatigue lifetime
D
Fatigue lifetime
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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
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Fatigue testing
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Test specimen with crack
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SN-curves of IIW - steel
2106
Class
Class
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Effect of welding detail on fatigue capacity
Class
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Important note!
In general, fatigue capacity of
structural details
does NOT depend on
steel grade and yield stress
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Effect of welding on fatigue lifetime
2 times shorter
2 times shorter
2 times shorter
2 times shorter
16 times shorter
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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
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Conclusion
You know
how to calculate
fatigue lifetime
of a structural detail
when fatigue loading is known
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Your enthusiasm!
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Repetition
to learn we need to repeat
To learn we need to repeat
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Question
.
What is fatigue loading?
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Question
.
What is fatigue capacity?
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Question
.
What is fatigue damage?
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Question
.
What is fatigue lifetime?
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Question
.
What is fatigue design criterion?
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Capacity
i
i
nD
N
Fatigue loading, capacity, damage, lifetime & design criterion
Design lifetimeFatigue lifetime
D
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Question
Where is the safety factor?
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Summary
Select detail
Select appropriate SN-curve
Define fatigue loading
Calculate fatigue capacity
Apply the rule of Miner
Calculate fatigue lifetime
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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
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SN - curve
Rule of Miner
i m
i
aN
S
i
i
nD
N
Design lifetimeFatigue lifetime
D
Formula
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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
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Next step
Select detail
Select appropriate SN-curve
Define fatigue loading
Calculate fatigue capacity
Apply the rule of Miner
Calculate fatigue lifetime
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Question
.
How to calculate fatigue loading?
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Conclusion
Fatigue induced by
quasi-static wave action
depends mainly on
moderate sea states
Non-linear response is less relevant
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Method
Spectral analysis
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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
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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
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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
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Keep stresses low! Preliminary design
Use proper details! Design/
Engineering
Allow only acceptable defects! Building
Monitor! Operation
Lifetime assurance - advice
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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
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Octopus - Monitas System
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Sensoren
Cabinet
DGPS
LBSGs
Strain gauges
FDSs
Motion sensor
Level gauge
Wave buoy
Wind sensor
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Benefits of Octopus-Monitas
Prevents lost of production
Prevents unexpected damage
More time for corrective measures
Rational lifetime extension
Feedback to design
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