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

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

1 - Story of Girassol failure events

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

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

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

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

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

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

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

2a– Rolling

2a– Sliding

2a– Sticking-Sliding

2a– Sticking-Sliding vs. Rolling

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

2a– 3-Link Model

2a– 3-Link Model

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

2a– 3-Link Model with Proof Loading

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

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

2a– Effect of Proof Load and Operating Tension

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.

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

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

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

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

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

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

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

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.

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

Please Contact SBM Monaco Questions? Please Contact SBM Monaco Lucile Rampi Lucile.rampi@singlebuoy.com 00-33-92-05-86-24