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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
Interlink Angle (degrees)
S11
in
OP
B l
ink
(MP
a)
Incremental S11
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
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
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
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
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)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
Interlink Angle (degrees)
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
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
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, 60%CBL to 94 ton, f=0.3
FEA 3-link, 80%CBL to 94 ton, f=0.3
FEA 3-link, 40%CBL to 94 ton, f=0.3
FEA 3-link, 80%CBL to 60 ton, f=0.3
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