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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