Chain out of plane bending fatigueGirassol failures and OPB mechanismPhase 1: validation OPB stress measurementAnalyticalPhase 2: Reduced scale chain OPB fatigue testsConclusionsJIP proposal

1 - Story of Girassol failure eventsState of the art designGirassol chains designed according to conventional fatigue assessment using API RP2SK T-N curvesAPI fatigue life >60 years (3 x design life) According to industry best practice in 2001 Girassol mooring should not have failed!Girassol eventsSeveral chains broken in ~ 8 monthsFailure on link 5Bushing friction torque higher than interlink friction torqueChain must bend before bushing rotates

=>New source of fatigue : Out of Plane BendingSBM developed a methodology to assess performance of chains under this new fatigue mechanism

1 - Story of Girassol failure events

1 - OPB Failure mechanismTension 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 mechanismCrack propagation initiatedat hot spot stress in bending

Crack initiation due to corrosion pitting

Rupture in 235 days

1 - Interlinks locking modes

Bending stress:

Rolling

Sticking

Sliding:

1 - Interlinks contact areaFlat 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

Girassol recovered linkIndentation area

2 1st test phase: OPB s measurementSBM laboratory tests : measurement of bending stresses in chains

Chain size (mm): 81, 107, 124, 146

Tension : 20 t 94 t

2 - Experiments & analysisBending stress variation against interlink angle

Test campaign to measure OPB stress in sticking locking modeDetermine the influence of:TensionDiameterInterlink angleDerive an empirical law

2a Quarter-Link Model 94 ton tensile loading with zero friction94 ton tensile loading with friction=0.25, 0.560% CBL (878 ton) with friction=0.594 ton tensile loading wth friction=0.194 ton tensile load with OPB link forced sliding friction=0.3Elastic material with contact and friction+/- 2 amplitude

FEA Details:Cases:

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, friction=0.3, elastic94 ton tensile loading, rig shoe 150 mm, friction=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, friction=0.3, von-Mises

2a Link Intimacy 94 ton Load after 80% MBL94 ton Load with no PreloadPlastic Strains and Interlink Contact Intimacy for no-Preload vs. 80% CBL Preload

2a Effect of Proof Load and Operating Tension

2 - Conclusion from the 1st test campaignBetter understanding of the OPB phenomena

Empirical relationship to predict OPB stressRedesigned 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 testingTest 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 designValidation of the stress relationship for smaller link .Obtain fatigue endurance data for OBP stresses

3 Phase 2: fatigue test campaign2 Chainhawse type tested

Fatigue test resultsGirassol design:

Kuito design:Pitch A: 1 million of cycles: no failurePitch B: 1.3 million of cycles : no failure

3 - Girassol resultsAngle 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 resultsAngle variation function of the strokePropagation : 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 angleKuito resultsPitch A, Pitch B in air and in seawater : quite good consistency

Stress relationshipKuito 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 curveStressesMaximum bending stresses are derived from measured stresses on link by multiplying by a SCF (1.08)S-N curveStraight chainhawse results: non failureFor high stress range, DNV in air mean curve gave a nice predictionCorrosion pitting at the end of the test may not be representative from long term offshore corrosionFor lower stress ranges, the predictions may be too conservative

Measured stress

4 - ConclusionsStress relationshipThe chainhawse geometry can affect the mode of interlink interactionThe curved chainhawse tend to concentrate the chain rotation to a single interlink angle rotationThe straight chainhawse tend to evenly spread out the chain rotation to several interlink anglesThe curved chainhawse exhibit lower stress as a function of aint but aint a lot larger higher stresses than on the straight chainhawsePreviously obtained stress relationship function of aint Matches initial slope for the straight chainhawse but then tend to underestimate the stressesOverestimate the stresses for the curved chainhawse (rolling?)S-N curveStandard S-N curve seem to give conservative predictionsThe trend seems to show a lower S-N curve slope (higher m value) compared to standard S-N curveA link in bending experiences significant shear at the OPB peak stress need of specific S-N curve for similar loading conditions

5 - JIP proposalFURTHER NEEDS: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.RPSCOPE 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 proposalJIP valueImprove 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

Questions?

Please Contact SBM MonacoLucile RampiLucile.rampi@singlebuoy.com00-33-92-05-86-24