This paper introduces the research undertaken to improve the reliability of bolted flange connections in structural engineering, with offshore wind turbines being used as an example. The current problem is that bolts are often over- or under-loaded, leading to premature failure. This shows that the existing loading methods cannot be relied on to balance the strains, which is the root cause of the bolting problem. This work highlights the importance of good asset management of bolted flange connections in the safety aspects of bolted flange connections.

1. Combining ultrasound non-destructive testing
and bolting methods to improve reliability on
bolted flange connections in offshore wind
56th Annual British Conference on Non-Destructive Testing, UK
5 September 2017
Dominik Kujawski, Dr Shaun West and Henk van den Berg

2. 56th Annual British Conference on Non-Destructive Testing, UK | June 2016 |Kujawski, West & van den Berg
Introduction
The problem and the purpose of this paper
To highlight the importance of bolts monitoring in critical flanges
This paper introduces the
research undertaken to
improve the reliability of
bolted flange connections in
structural engineering on the
example of offshore wind
Research to improve the reliability of bolted flange connections
Purpose of this paper
Bolts are often over- or under-loaded and can lead to early failure
Problem

3. 56th Annual British Conference on Non-Destructive Testing, UK | June 2016 |Kujawski, West & van den Berg
Problem statement
Current practise in the industry – potential problems
What does the literature tell us?
- Offshore is exposed to high dynamic loads
- Inaccurate selections of bolting resistance
- Bolt cross torque results in loss of accuracy
- Loss of transparency in the applied preload
https://www.nrdc.org/stories/wind-farm-great-lakes-lets-give-it-twirl
The root cause of bolting problem is the imbalance of strains

4. 56th Annual British Conference on Non-Destructive Testing, UK | June 2016 |Kujawski, West & van den Berg
Introduction
Why bolting is important?
https://www.youtube.com/watch?v=A-FJI-U2chA
Key facts
- This is a critical aspect of the asset integrity
- Connected surfaces can withstand huge pressure
- For reliability bolts must be loaded correctly
The ability to transfer load from one component to another requires safe and efficient designs

5. 56th Annual British Conference on Non-Destructive Testing, UK | June 2016 |Kujawski, West & van den Berg
Background
Why ultrasound monitoring is valuable?
What does the literature tell us?
- Advantage of ultrasound testing is accuracy
- The bolt it stretches like a spring with applied load
- Applied load is measured by the stretch amount
The bolt elongation difficult to measure - ultrasound monitoring is considered as valuable

6. 56th Annual British Conference on Non-Destructive Testing, UK | June 2016 |Kujawski, West & van den Berg
Background
Current practise in the industry – traditional torque sequence
What does the literature tell us?
- Done in three stages using a quadrant protocol
- Intrinsic errors friction consumes 90% of the torque
- The errors of 50% is common
The sequence with the traditional torque technique increases the torque after completing every stage

7. 56th Annual British Conference on Non-Destructive Testing, UK | June 2016 |Kujawski, West & van den Berg
Methodology
A four step methodology was applied in this study
Traditional torque
Lab case 1
• Target load
400kN
• Torque wrench
• Ultrasound NDT
New method
Lab case 2
• Target load
400kN
• Combined
methods
• Ultrasound NDT
Statistical analyses
• Spread
• Distribution
• Cumulative
percentage
Box plot analysis
• Compare lab
results with real
project data
• Compare both
cases in a table

8. 56th Annual British Conference on Non-Destructive Testing, UK | June 2016 |Kujawski, West & van den Berg
Results and analysis
Flange chart comparison of the both lab cases
The crosstalk variations are one of the main reasons for the premature failure in the bolted connections
Traditional torque lab case 1 New method lab case 2

9. 56th Annual British Conference on Non-Destructive Testing, UK | June 2016 |Kujawski, West & van den Berg
Results and analysis
Spread chart comparison of the load target deviations for both lab cases
Traditional torque lab case 1 New method lab case 2
The targeted number of measurement results are 6% for lab case 1 and 28% for lab case 2

10. 56th Annual British Conference on Non-Destructive Testing, UK | June 2016 |Kujawski, West & van den Berg
Results and analysis
Distribution and cumulative percentage charts comparison for both lab cases
Traditional torque lab case 1 New method lab case 2
Lab case 2 has a higher number of measurements closer to the targeted value of 400kN

11. 56th Annual British Conference on Non-Destructive Testing, UK | June 2016 |Kujawski, West & van den Berg
Results and analysis
Percentage deviation comparison of both lab cases
Key findings
- The percentage deviation for lab case 1 is from 20%
above and more than 30% below
- The percentage deviation for lab case 2 is less than
10% above and below
Box plot chart provides a richer understanding of both lab cases and new method project application

12. 56th Annual British Conference on Non-Destructive Testing, UK | June 2016 |Kujawski, West & van den Berg
Results and analysis
Percentage deviation comparison of both lab cases and real project measurements
Key finding
- The percentage deviation for the real project
measurements is less than 10% both above and
below
Box plot chart provides a richer understanding of both lab cases and new method project application

13. 56th Annual British Conference on Non-Destructive Testing, UK | June 2016 |Kujawski, West & van den Berg
Results and analysis
Comparison of traditional and new method tightening techniques in the lab environment
Traditional torque New method
Strain balance 60% spread 10% spread
Time to complete a test flange 4 hours 2 hours
Employees needed Regular Regular
Transparency - Every bolt can be tracked
Long-term asset management -
Ultrasound testing provides quality
assurance
This table provides a direct comparison between the traditional and the new method

14. 56th Annual British Conference on Non-Destructive Testing, UK | June 2016 |Kujawski, West & van den Berg
Results and analysis
The key observations from the analysis were
Initial findings confirmed that the new method
increases the accuracy and reliability
- Increased strain balance
- Two times faster
- Performed by regular employees
- Increased transparency
The new method increases the availability and reliability, decreasing unscheduled maintenance

15. 56th Annual British Conference on Non-Destructive Testing, UK | June 2016 |Kujawski, West & van den Berg
Closing
Conclusions
This study of over 10,000
project data points confirms
that the new method of
tightening improves asset
integrity in comparison to
traditional approaches
Tightening is challenging due to the increasing size of constructions
The traditional tightening methods do not provide balanced loading
It is possible to combine NDT and bolt tightening
Confirmed by statistical analyses of 10,000 project data points

16. 56th Annual British Conference on Non-Destructive Testing, UK | June 2016 |Kujawski, West & van den Berg
Thanks for your time!