5.2 reliability assessment of monitored jacket support structures of
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Reliability assessment of monitored
jacket support structures of offshore
wind turbines
Ronald Schneider & Falk Hille
14.10.2015
Fatigue of jacket structures
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Linked picture: http://www.windkraft-journal.de/wp-content/uploads/2013/11/alpha-ventus.jpg
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Reliability of jacket structures
subjected to fatigue
Deterioration model
• Fatigue deterioration is
modeled at hotspot level using
fracture mechanics
• A welded connection fails as
soon as a fatigue crack grows
beyond a critical size
• A structural element fails as
soon as a welded connection
fails
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Rouhan and Schoefs (2003)
Deterioration model
Fatigue model parameters 𝐗 are
uncertain
• Random fatigue loading
• Material parameters
• Initial conditions
• Model uncertainties
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Rouhan and Schoefs (2003)
Deterioration model
• System deterioration state 𝐃𝑡
in terms of failed elements
• 𝐃𝑡 is a function of uncertain
fatigue model parameters 𝐗
𝐃𝑡 = ℎ 𝐗, 𝑡
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failed elements
Structural model
• Failure probability of weakened
system
• Remove failed elements from
structural model according to 𝐝𝑡
• Compute conditional system
failure probability Pr 𝐹𝑡|𝐃𝑡 = 𝐝𝑡
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failed elements
Coupling of deterioration and structural model
System failure probability
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Pr 𝐹𝑡 = Pr 𝐹𝑡|𝐃𝑡 = ℎ 𝐱, 𝑡 𝑓𝐗 𝐱 𝑑𝐱𝐱
= Pr 𝐹𝑡|𝐗 = 𝐱 𝑓𝐗 𝐱 𝑑𝐱𝐱
failed elements
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Reliability updating with damage
detection information
Subspace-based damage detection
• Suitable for detecting fatigue
damage in jacket structures
• Utilizes dynamic structural
response without extracting
modal parameters
• Dynamic response signals are
processed by subspace-based
damage detection algorithm
• Compares reference state with
current state
14.10.2015 Reliability assessment of monitored jacket support structures of offshore wind turbines
0 5 10 15 20 25 30 35 40
105
106
107
108
109
Test Number
2-T
est V
alu
e
Sensor Setup B
undamaged
5 nodes
9 nodes
13 nodes
17 nodes
21 nodes
25 nodes
Threshold
10
1.3 1.35 1.4 1.45 1.5 1.55
x 104
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
Acceleration signals
Damage indicator (detection/no detection)
Subspace-based damage detection algorithm
Possible system configuration
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0 5 10 15 20 25 30 35 40
1e6
2e6
3e6
4e6
5e6
6e6
Test Number
2-T
est V
alu
e
Sensor Setup A
undamaged
5 nodes
9 nodes
13 nodes
17 nodes
21 nodes
25 nodes
Threshold
Detectable damage: 22% reduction in bending stiffness
Results from simulation
Probability of detection
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Probability of detection
Pr 𝑍|𝐃𝑡 = 𝐝𝑡 describes quality of
detection system
Detection system is not perfect
due to many uncertain factors:
• Environmental conditions
• Measurement uncertainties of
sensor and amplifier system
• Uncertainties in damage
detection algorithm
Likelihood function
Link between monitoring outcome and model parameters
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Pr 𝑍 𝐃𝑡 = 𝐝𝑡 Pr 𝑍 𝐃𝑡 = ℎ 𝐱, 𝑡 Pr 𝑍 𝐗 = 𝐱
Bayes theorem
Updated system failure probability
Pr 𝐹𝑡|𝑍 =Pr 𝐹𝑡 ∩ 𝑍
Pr 𝑍= Pr 𝐹𝑡|𝐗 = 𝐱 Pr 𝑍|𝐗 = 𝐱 𝑓𝐗 𝐱 𝑑𝐱𝐱
Pr 𝑍|𝐗 = 𝐱 𝑓𝐗 𝐱 𝑑𝐱𝐱
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Numerical example
2D jacket-type structure
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Prior system failure probability
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Posterior system failure probability
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Monitoring in year 10
Damage detection
Monitoring once a year
No damage detection
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Conclusions
Conclusions
• Reliability assessment of monitored jacket structures
• Damage detection system
• Probability of detection (active field of research!)
• Approach forms the basis for decisions on maintenance
actions and future inspection efforts
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References
• Hille F., Petryna Y., Rücker W. (2014): Subspace-based
detection of fatigue damage on jacket support structures
of offshore wind turbines, 7th European Workshop on
Structural Health Monitoring, Nates France
• Schneider R., Thöns S. and Straub D. (submitted to
Structural Safety, July 2015): Reliability analysis and
updating of deteriorating structural systems with subset
simulation.
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