1. Chain out of plane bending fatigue
Girassol failures and OPB mechanism
Phase 1: validation
OPB stress measurement
Analytical
Phase 2: Reduced scale chain OPB fatigue tests
Conclusions
JIP proposal

2. 1 - Story of Girassol failure events
State of the art design
Girassol chains designed according to conventional fatigue assessment using API RP2SK T-N curves
API fatigue life >60 years (3 x design life)
According to industry best practice in 2001 Girassol mooring should not have failed!
Girassol events
Several chains broken in ~ 8 months
Failure on link 5
Bushing friction torque higher than interlink friction torque
Chain must bend before bushing rotates
=>New source of fatigue : Out of Plane Bending
SBM developed a methodology to assess performance of chains under this new fatigue mechanism

3. 1 - Story of Girassol failure events

4. 1 - OPB Failure mechanism
Tension fatigue is due to cyclic range of tension variations loading the chain.
OPB fatigue is due to range of interlink rotation under a certain tension.
Occurs predominantly in the first link after a link that is constrained against free rotational movement.
Failure can be fast.
Link Constraint provided by Chainhawse or Fairlead.
Mechanism aggravated by high pretensions and is generating critical cyclic stress loading

5. 1 - Failure mechanism Crack propagation initiated
at hot spot stress in bending
Crack initiation due to
corrosion pitting
Rupture in 235 days
Area of max stress in Out of Plane Bending
MOPB
Crack propagation

6. 1 - Interlinks locking modes
Bending stress:
Rolling
Sticking
Sliding: αi
αint ri r0 β N F T

7. 1 - Interlinks contact area
Flat contact area generated by the proof load test (> 66% MBL)
This indentation area may encourage “sticking mode” / “rolling mode”
Finite Element plastic analysis at proof load
Indentation area
Girassol recovered link

8. 2 – 1st test phase: OPB s measurement
SBM laboratory tests : measurement of bending
stresses in chains
Chain size (mm): 81, 107, 124, 146
Tension : 20 t  94 t

9. 2 - Experiments & analysis
Bending stress variation against interlink angle
Test campaign to measure OPB stress in “sticking” locking mode
Determine the influence of:
Tension
Diameter
Interlink angle
Derive an empirical law

10. 2a– Quarter-Link Model Cases:
94 ton tensile loading with zero friction
94 ton tensile loading with μfriction=0.25, 0.5
60% CBL (878 ton) with μfriction=0.5
94 ton tensile loading wth μfriction=0.1
94 ton tensile load with OPB link forced sliding μfriction=0.3
FEA Details:
Elastic material with contact and friction
+/- 2° amplitude

11. 2a– Rolling

12. 2a– Sliding

13. 2a– Sticking-Sliding

14. 2a– Sticking-Sliding vs. Rolling

15. 2a– 3-Link Model
Experimental setup for 124mm links

16. 2a– 3-Link Model

17. 2a– 3-Link Model

18. 2a– 3-Link Model Nonlinear vs. Elastic
94 ton tensile loading, rig shoe 150 mm, μfriction=0.3, elastic
94 ton tensile loading, rig shoe 150 mm, μfriction=0.3, von-Mises

19. 2a– 3-Link Model with Proof Loading

20. 2a– 3-Link Model with Proof Loading
60% MBL preload, 94 ton tensile loading, rig shoe 150 mm, μfriction=0.3, von-Mises

21. 2a– Link Intimacy
Plastic Strains and Interlink Contact Intimacy for no-Preload vs. 80% CBL Preload
94 ton Load with no Preload
94 ton Load after 80% MBL

22. 2a– Effect of Proof Load and Operating Tension

23. 2 - Conclusion from the 1st test campaign
Better understanding of the OPB phenomena
Empirical relationship to predict OPB stress
Redesigned Chain connection
Predictions have been done on other mooring chain with surprising results. Although traditionally neglected, OPB fatigue damage can be significant.
Further tests are still undergoing to determine more accurately the OPB stress relationship.

24. 3 – 2d test campaign: fatigue testing
Test program:
Monitoring of 40 mm chain links in 2 rescaled hawse (Girassol and Kuito)
Fatigue test with both hawses (in salt water)
Aim:
Investigate the interlink angle distribution in both hawses: influence of the chainhawse design
Validation of the stress relationship for smaller link Ø.
Obtain fatigue endurance data for OBP stresses

25. 3 – Phase 2: fatigue test campaign
2 Chainhawse type tested
Fatigue test results
Girassol design:
Kuito design:
Pitch A: 1 million of cycles: no failure
Pitch B: 1.3 million of cycles : no failure

26. 3 - Girassol results Angle variation function of the stroke
Propagation : p1 ≈ 80% for T=35t
Angle transmit by L4 larger than the induced hawse angle
Stress level at 35 t:
Total hawse angle variation: Datot ≈ 6.44°
Interlink angle variation on L5: Daint ≈ 4.9°
Bending stress range on L5: Ds max≈ 380 MPa
Note: s,NT ≈ 140 MPa

27. 3 - Kuito results Angle variation function of the stroke
Propagation : p1 ≈ 34% for T=35t
Stress level at 35 t:
Total hawse angle variation: Datot ≈ 2.70°
L2 Interlink angle variation: Daint ≈ 0.92°
L2 Mean stress range: Ds max≈ 280 MPa for T=35t

28. 3 - Stress function of interlink angle
Kuito results
Pitch A, Pitch B in air and in seawater : quite good consistency
Stress relationship
Kuito chainhawse : slope at origin matches old relationship, then higher stresses
Girassol chainhawse: stress level in between theoretical rolling stress and locking stress

29. 3 - Fatigue performance and S-N curve
Stresses
Maximum bending stresses are derived from measured stresses on link by multiplying by a SCF (1.08)
S-N curve
Straight chainhawse results: non failure
For high stress range, DNV in air mean curve gave a nice prediction
Corrosion pitting at the end of the test may not be representative from long term offshore corrosion
For lower stress ranges, the predictions may be too conservative
Measured stress

30. 4 - Conclusions Stress relationship S-N curve
The chainhawse geometry can affect the mode of interlink interaction
The curved chainhawse tend to concentrate the chain rotation to a single interlink angle rotation
The straight chainhawse tend to evenly spread out the chain rotation to several interlink angles
The curved chainhawse exhibit lower stress as a function of aint but aint a lot larger  higher stresses than on the straight chainhawse
Previously obtained stress relationship function of aint
Matches initial slope for the straight chainhawse but then tend to underestimate the stresses
Overestimate the stresses for the curved chainhawse (rolling?)
S-N curve
Standard S-N curve seem to give conservative predictions
The trend seems to show a lower S-N curve slope (higher m value) compared to standard S-N curve
A link in bending experiences significant shear at the OPB peak stress  need of specific S-N curve for similar loading conditions

31. 5 - JIP proposal FURTHER NEEDS: DELIVERABLES: SCOPE OF WORK :
Need OPB Stresses for higher tension levels (% MBL).
More endurance data for chain links subjected to OPB.
DELIVERABLES:
Improved Chain OPB stress relationships.
S-N curves to be used for OPB fatigue calculation.  RP
SCOPE OF WORK :
OPB stress measurements based on chain tests in the SBM laboratory (4 different chain size for 4 higher levels of tension).
Use FEA, in line with the work done by Chevron to calibrate the interlink stiffness and sliding threshold model by benchmarking tests results.
Develop a specific test rig for fatigue testing of chain-links in OPB.
S-N curve determination.
Develop RP for OPB fatigue prediction.

32. 5 - JIP proposal JIP value Budget
Improve the safety of deepwater mooring systems by providing a more accurate assessment method for OPB fatigue.
Added value: contribution of previous SBM and Chevron work (See 2005 OTC & 2006 OMAE papers)
Budget
Hrs
Cost$US
SBM chain test refurbishment for other chain size and higher loads
50000
Chain purchasing (4 different sizes)
20000
Tests of different chains (4) for (4) different tension levels
800
80000
Calibrate FEA interlink stiffness model / tests results
300
30000
Design a chain fatigue test rig
500
Construct fatigue test rig
150000
Fatigue test about 15 samples for S-N curve determination
1000
100000
Prepare design methodology for OPB fatigue determination for a Recommended Practice.
200
-  Total Cost
500000
JIP Contribution
250000
SBM Contribution

33. Please Contact SBM Monaco
Questions?
Please Contact SBM Monaco
Lucile Rampi