Chain out of plane bending fatigue 1. Girassol failures and OPB mechanism 2. Phase 1: validation ...

Chain out of plane bending fatigue

1. Girassol failures and OPB mechanism2. Phase 1: validation

OPB stress measurement Analytical

3. Phase 2: Reduced scale chain OPB fatigue tests

4. Conclusions5. JIP proposal

1 - Story of Girassol failure eventsState 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

Chainhawse

Failed link

Chainhawse

Failed link

Chainhawse

Failed link

=>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.

ΔT

T

Δ

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 daysArea of max stress in Out of Plane Bending

MOPB

Crack propagation

1 - Interlinks locking modes

αi

αint

ri

r0

βNF

T

iOPB OPB

rM

I

OPB i frictionM r T

Bending stress:

Rolling

Sticking

Sliding:

i

iiOPB rr

rTrM

0int *sin**

int1 *2** i

aOPB rTkM

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 link

Indentation area

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

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

Tension : 20 t 94 t

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

2 - Experiments & analysis

),,( dTfOPB

2a– Quarter-Link Model

Fixed Link: Symmetric B.C. 2-3 plane: U1 = 0.0 3-direction: ? U3 = 0.0 (distributed coupling)

OPB Link: Link X-Section rotates with RP node, T/2 loading distributed via kinematic coupling.

OPB Link: Applied loading rotates with link rotation

T/2

Fixed Link and OPB Link: Symmetric B.C. 1-3 plane: U2 = 0.0

Surface Contact And friction

OPB Link: Constraint to enforce friction sliding (3-direction): ? U3 = 0.0 (distributed coupling)

1

2

3

1

2 3

T/2

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

Elastic material with contact and friction

+/- 2° amplitude

FEA Details:

Cases:

2a– Rolling

2a– Sliding

2a– Sticking-Sliding

2a– Sticking-Sliding vs. Rolling

Experiment and FEA124 mm Chain links

0

10

20

30

40

50

60

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Interlink angle (degrees)

Str

ess

amp

litu

de

(MP

a)

94 ton, test #30

85 ton, test #30

60 ton, test #30

80 ton, test #30

65 ton, test #30

Sticking-Sliding

Roilling

Sticking-Sliding

Rolling

2a– 3-Link Model

Experimental setup for 124mm links

2a– 3-Link Model

Ramberg-Osgood Stress-Strain Curve

0

200

400

600

800

1000

1200

0% 5% 10% 15%

Strain

Str

es

s (

MP

a)

Engineering

True

Yield 580.0 MPaUltimate 860.0 MPa

alpha = 0.71n = 10.3

eps ult = 12.0%

2a– 3-Link Model

2a– 3-Link Model Nonlinear vs. Elastic

f = 0.3, d = 150mm, elastic, T=94 ton

-40

-35

-30

-25

-20

-15

-10

-5

0

5

10

15

0 1 2 3 4 5

Interlink Angle (degrees)

S11

in

OP

B l

ink

(MP

a)Incremental S11

f = 0.3, d = 150mm, plasticity, T= 94 ton

-40

-30

-20

-10

0

10

20

30

0 1 2 3 4 5


S11

in

OP

B l

ink

(MP

a)

Incremental S11


0

10

20

30

40

50

60

0 0.2 0.4 0.6 0.8 1 1.2 1.4


Str

es

s a

mp

litu

de

(M

Pa

)

94 ton, test #30

85 ton, test #30

60 ton, test #30

80 ton, test #30

65 ton, test #30

FEA 3-link, 94 ton , nonlinear


0

10

20

30

40

50

60

0 0.2 0.4 0.6 0.8 1 1.2 1.4


Str

es

s a

mp

litu

de

(M

Pa

)

94 ton, test #30

85 ton, test #30

60 ton, test #30

80 ton, test #30

65 ton, test #30

FEA 3 link model, 94 ton, f=0.3

FEA 3 link model, 94 ton, f=0.3, cycle 2

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


0

10

20

30

40

50

60

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Interlink angle (degrees)S

tre

ss

am

pli

tud

e (

MP

a)

94 ton, test #30

85 ton, test #30

60 ton, test #30

80 ton, test #30

65 ton, test #30

FEA 3-link, 60%CBL to 94 ton, f=0.3

f = 0.3, d = 150mm, plasticity, T=878 ton (60% CBL) 94 ton

-50

-40

-30

-20

-10

0

10

20

30

40

50

0 1 2 3 4 5 6


S11

in

OP

B l

ink

(MP

a)

Incremental S11

2a– Link Intimacy

94 ton Load after 80% MBL94 ton Load with no Preload

Plastic Strains and Interlink Contact Intimacy for no-Preload vs. 80% CBL Preload

2a– Effect of Proof Load and Operating Tension


0

10

20

30

40

50

60

0 0.2 0.4 0.6 0.8 1 1.2 1.4


Str

es

s a

mp

litu

de

(M

Pa

)

94 ton, test #30

85 ton, test #30

60 ton, test #30

80 ton, test #30

65 ton, test #30





Better 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.

2 - Conclusion from the 1st test campaign

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

Pretension Lab results50t pitch A 13950050t + preload 94t Pitch A 10270035t Pitch A 609500

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:

tot ≈ 6.44°

Interlink angle variation on L5: int ≈ 4.9°

Bending stress range on L5: max≈ 380 MPa

Note: ,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: tot ≈ 2.70°

L2 Interlink angle variation: int ≈ 0.92°

L2 Mean stress range: 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

Summary of OPB tests results and determination of reduced curve

parameters

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Interlink angle (dg)

Average curves 81 mm 124 mmKuito reduced stressKuito Pitch B in water reduced stressGirassol reduced stressRolling reduced stresses for r0=30 mmRolling reduced stresses for r0=22.8 mmPoly. (Average curves 81 mm 124 mm)

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

S-N curve

1.8

2

2.2

2.4

2.6

2.8

4 4.5 5 5.5 6 6.5 7 7.5 8

log (N)

log

(

)

DNV RPC203 B1 free corrosion mean S-N curve

Curve chainhawse maximum stresses (failures points)

DNV RPC203 B1 with CP mean S-N curve

Straight chainhawse max stresses in water (non failures)

DNV RPC203 B1 in air mean S-N curve

BS7608 B in air mean S-N curve

4 - Conclusions Stress relationship

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 int but int a lot larger higher stresses than on the straight chainhawse

Previously obtained stress relationship function of int

• 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:

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

• 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 50000

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 20000

- Total Cost 500000

JIP Contribution 250000

SBM Contribution 250000

Questions?

Please Contact SBM MonacoLucile Rampi

[email protected]

Chain out of plane bending fatigue 1. Girassol failures and OPB mechanism 2. Phase 1: validation ...

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