1. SR 1 and 2 Hay Point Fatigue Damage Structural Risk

2. Background SR 1 and 2 have been in operation for around 38 years
They were commissioned in 1974 and have reclaimed 300 MT of coal

6. Structural Fatigue
Structural fatigue arises when a cyclic stress state causes a progressive failure of a material
In structural steelwork this typically occurs at welded joints or discontinuities

7. Structural Fatigue Design Standards
• Current fatigue design codes have details with a reliability of or a 1 in 45 probability of cracking at the end of their design life. This compares to structural strength design which has a reliability of Also the reliability decreases with age of the structure.
• Satisfactory control of fatigue damage relies on adequate
methods of fatigue crack detection and the ability to repair or replace the damaged component.

8. Structural Fatigue US Bridge Industry
• Some precedent from Bridge Industry in the US to operate up to 16% probability of failure for structures which can’t readily be replaced.
• Relies on thorough inspections

9. Fatigue Life Assessment of SR1 and 2
Fatigue assessment has been carried out on the machines which shows that they are at the end of their fatigue life
Fatigue cracking is now occurring requiring repairs to be made and regular inspections are being carried out to try and detect cracking

10. Interim Life Extension Retrofit
An Interim Life Extension Retrofit was proposed until SR1 & SR2 can be replaced
Rebuild lower luffing lug assembly
Replace counterweight pivot section
Replace back section of boom
Repairs to counterweight boom bracing connections
Inspection and weld repairs
SR1 and SR2 Risk Management

11. Results of Inspections SR2

12. Results of Inspections SR2

13. Results of Inspections SR2

14. Results of Inspections SR2

15. Fatigue Assessment SR2 Based on Citec data & Simulated loads
More joints examined in detail
Probability of Cracking calculated to BS 7608
Fracture mechanics crack growth curves developed

16. BS7608 Probability of Failure at 340MT Site Inspection Results
Fatigue Assessment SR2
Location
Weld Description
BS7608 Probability of Failure at 340MT
Site Inspection Results
CW07
diaphragm plate to web plate, counterweight boom, RHS
6.8%
Crack found
CW09
web plate (22mm) to pivot box plate (51mm), counterweight boom, LHS
3.0%
CW12a
web plate (22mm) to pivot box plate (51mm), top position, counterweight boom, RHS
2.6%
CW18a
web plate to top flange of counterweight boom, RHS
18.1%
CW35
gusset plate to top flange (along top flange), counterweight boom, LHS
27.2%
CW54
gusset plate to bottom flange, counterweight boom, LHS
13.8%
Cracks found

17. BS7608 Probability of Failure at 340MT Site Inspection Results
Fatigue Assessment SR2
Location
Weld Description
BS7608 Probability of Failure at 340MT
Site Inspection Results
CW08
diaphragm plate to web plate, counterweight boom, LHS
19.5%
CW13
web plate (22mm) to pivot box plate (51mm), middle position, counterweight boom, RHS
3.7%
CW17
top flange of drive support to top flange of counterweight boom, along top flange, LHS
5.1%
CW18
web plate to top flange of counterweight boom, LHS
18.2%
CW31
gusset plateto top flange (along diagonal brace), counterweight boom, LHS
23.5%
CW32
gusset plate to top flange (along diagonal brace), counterweight boom, RHS
37.5%
CW33
15.6%
CW34
9.3%
CW55b
web stiffener to web plate (outside), counterweight boom, RHS
5.3%

18. BS7608 Probability of Failure at 340MT Site Inspection Results
Fatigue Assessment SR2
Location
Weld Description
BS Probability of Failure at 340MT
Site Inspection Results
CW04
web to flange, counterweight boom, RHS
<0.1%
Crack found
CW05
web to flange, counterweight boom, LHS
CW06a
web plate (22mm) to pivot box plate (51mm), counterweight boom, RHS
CW10
Internal horizontal stiffener to internal vertical stiffener of slew cone wall
0.90%
CW11a
0.10%
CW14
web plate (22mm) to pivot box plate (51mm), bottom position, counterweight boom, LHS
CW15
web plate (22mm) to pivot box plate (51mm), top position, counterweight boom, LHS
CW15a
0.70%
CW15c
horizontal stiffener on diaphragm plate to web plate, counterweight boom, RHS
CW19a
web stiffener plate to web (along the stiffener), counterweight boom, RHS
CW37
gusset plate to top flange (along diagonal brace), counterweight boom, RHS
0.80%
CW39
Cracks found
CW51a
gusset plate to bottom flange , counterweight boom, LHS
0.30%
CW52
gusset plate to bottom flange of boom (along diagonal brace), counterweight boom, RHS

19. BS7608 Probability of Failure at 340MT Site Inspection Results
Fatigue Assessment SR2
Location
Weld Description
BS7608 Probability of Failure at 340MT
Site Inspection Results
CR01-1
cone stiffener to top plate of cone
5.9%
Cracks found
CR01-2
48.0%
CR01-3a
Stiffener plate of lug to bottom flange of RHS girder
28.0%

20. BS7608 Probability of Failure at 340MT Site Inspection Results
Fatigue Assessment SR2
Location
Weld Description
BS7608 Probability of Failure at 340MT
Site Inspection Results
BWB01
Boom wall to lug, luff end, bucketwheel boom, RHS
35.1%
Crack previously found
BWB02
Diaphragm plate to boom wall, luff end, bucketwheel boom, LHS
42.8%
BWB05
Longitudinal stiffener to boom wall, bucketwheel boom, LHS
2.4%

21. Results of Inspections SR1

22. Results of Inspections SR1

23. Results of Inspections SR1

24. Results of Inspections SR1

25. Results of Inspections SR1

26. Results of Inspections SR1

27. Results of Inspections SR1

28. Results of Inspections SR1

29. Results of Inspections SR1

30. Results of Inspections SR1

31. Results of Inspections SR1

32. Crack growth – fracture mechanics
Crack growth will increase rapidly and become critical within a given period for the detail under consideration.
Inspection intervals at are set to attempt to detect cracks as they become critical.

33. Crack growth – fracture mechanics

34. Crack growth – fracture mechanics

35. Implications of the assessment
Design Curve has been used to set inspection intervals – allows for a safety factor over the mean curve

36. Crack detection
There is a chance inspections may not detect cracks particularly in difficult to access locations.
Reliability of detection poor without paint removal
Crack Depth mm

37. Redundancy
Redundancy is characterised by alternative load path or residual capacity to take overload
Counterweight boom – critical load for bracing expected to be horizontal load from collision or non-permanent dynamics – some redundancy under working loads
Counterweight pivot box - no redundancy under working loads
Bucket-wheel Boom – no redundancy under working loads
Cone – expect some redundancy under working loads

38. Probability of structural failure
The factors involved in determining probability of a fatality per annum will include.
Probability of Cracking (PC)
Redundancy Factor (RF)
Number of inspections in critical period (NI)
Critical period as a fraction of 12 months (CPF)
Probability of Non Detection of a crack (PND)
Exposure factor as a fraction of 12 months (EXP)

39. Tolerable Risk

40. Tolerable Risk
Upper limit of tolerable region 1 in annual probability of fatality
ALARP
Upper limit of broadly acceptable region
1 in annual probability of fatality

41. Tolerable Risk

42. Main Issues for fatigue management
Cracking found by inspection not correlating with fatigue analysis (possible contributors: - fabrication defects, corrosion, dynamic effects)
Amount of inspection required (cost & downtime)
Exposure of personnel

43. Fatigue Risk Management
Probability of Cracking (PC) – can’t change
Redundancy Factor (RF) – some effect on counterweight and cone, not on boom or cw pivot – can make an allowance but can’t change
Number of inspections in critical period (NI) – can change
Critical period (CPF) – stack only
Probability of Non Detection of a crack (PND) – remove paint, good access
Exposure factor as a fraction of 12 months (EXP) – reduce exposure of personnel