Transcript SBM Whitebackground

Chain out of plane bending fatigue
1. Girassol failures and OPB mechanism
2. Phase 1: validation


OPB stress measurement
Analytical
3. Phase 2: Reduced scale chain OPB fatigue tests
4. Conclusions
5. 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

Failed link
Chainhawse
=>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
ΔT
T
Δ

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
Area of max stress
in Out of Plane
Bending
Rupture in 235 days
MOPB
Crack propagation
1 - Interlinks locking modes

Bending stress:
r0
 OPB  M OPB 
αint
ri
αi
N

β
Rolling
M OPB
F
T

ri
I

 ri

 ri * T * sin     int * 
 r0  ri

Sticking
M OPB  k * T a * 2ri  * int
1

Sliding:
MOPB  ri   friction  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  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
 OPB  f (T , d , )
2a– Quarter-Link Model
OPB Link:
Link X-Section rotates
with RP node, T/2 loading
distributed via kinematic
coupling.
Cases:
2


3
1

T/2
OPB Link:
Applied loading rotates
with link rotation

Surface Contact
And friction

Fixed Link:
Symmetric B.C. 2-3 plane: U1 = 0.0
3-direction: ? U3 = 0.0 (distributed
coupling)
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:
2
3

T/2

1
Fixed Link and OPB Link:
Symmetric B.C. 1-3 plane: U2 = 0.0
OPB Link:
Constraint to enforce friction sliding
(3-direction): ? U3 = 0.0 (distributed
coupling)
Elastic material with contact
and friction
+/- 2° amplitude
2a– Rolling
2a– Sliding
2a– Sticking-Sliding
2a– Sticking-Sliding vs. Rolling
Experiment and FEA
124 mm Chain links
60
Sticking-Sliding
Stress amplitude (MPa)
50
94 ton, test #30
40
85 ton, test #30
60 ton, test #30
30
80 ton, test #30
65 ton, test #30
Sticking-Sliding
20
Roilling
Rolling
10
0
0
0.2
0.4
0.6
0.8
1
Interlink angle (degrees)
1.2
1.4
2a– 3-Link Model
Experimental setup for 124mm links
2a– 3-Link Model
Ramberg-Osgood Stress-Strain Curve
1200
Stress (MPa)
1000
800
Engineering
True
600
Yield 580.0 MPa
Ultimate 860.0 MPa
eps ult = 12.0%
alpha = 0.71
n = 10.3
400
200
0
0%
5%
10%
Strain
15%
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
Incremental S11
Incremental S11
f = 0.3, d = 150mm, elastic, T=94 ton
f = 0.3, d = 150mm, plasticity, T= 94 ton
15
30
10
20
0
S11 in OPB link (MPa)
S11 in OPB link (MPa)
5
-5
-10
-15
-20
-25
-30
10
0
-10
-20
-30
-35
-40
-40
0
1
2
3
4
0
5
1
2
3
5
Experiment and FEA
Experiment and FEA
124 mm Chain links
124 mm Chain links
60
60
50
85 ton, test #30
60 ton, test #30
80 ton, test #30
40
65 ton, test #30
FEA 3 link model, 94 ton, f=0.3
30
FEA 3 link model, 94 ton, f=0.3, cycle 2
20
Stress amplitude (MPa)
94 ton, test #30
Stress amplitude (MPa)
4
Interlink Angle (degrees)
Interlink Angle (degrees)
94 ton, test #30
50
85 ton, test #30
60 ton, test #30
40
80 ton, test #30
65 ton, test #30
FEA 3-link, 94 ton , nonlinear
30
20
10
10
0
0
0
0.2
0.4
0.6
0.8
1
Interlink angle (degrees)
1.2
1.4
0
0.2
0.4
0.6
0.8
1
Interlink angle (degrees)
1.2
1.4
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
Incremental S11
Experiment and FEA
f = 0.3, d = 150mm, plasticity, T=878 ton (60% CBL) 94 ton
124 mm Chain links
50
60
40
Stress amplitude (MPa)
S11 in OPB link (MPa)
30
20
10
0
-10
-20
-30
50
40
94 ton, test #30
30
85 ton, test #30
60 ton, test #30
20
80 ton, test #30
65 ton, test #30
10
FEA 3-link, 60%CBL to 94 ton, f=0.3
-40
0
-50
0
1
2
3
Interlink Angle (degrees)
4
5
6
0
0.2
0.4
0.6
0.8
1
Interlink angle (degrees)
1.2
1.4
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
Experiment and FEA
124 mm Chain links
60
Stress amplitude (MPa)
50
40
94 ton, test #30
85 ton, test #30
30
60 ton, test #30
80 ton, test #30
20
65 ton, test #30
FEA 3-link, 60%CBL to 94 ton, f=0.3
10
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
0
0
0.2
0.4
0.6
0.8
1
Interlink angle (degrees)
1.2
1.4
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:
Pretension
50t pitch A
50t + preload 94t Pitch A
35t Pitch A
Lab results
139500
102700
609500
• 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: Dtot ≈ 6.44°

Interlink angle variation on
L5: Dint ≈ 4.9°

Bending stress range on
L5: D 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: Dtot ≈ 2.70°

L2 Interlink angle
variation: Dint ≈ 0.92°

L2 Mean stress range:
D 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
0.7
Summary of OPB tests results and determination of reduced curve
parameters
Average curves 81 mm 124 mm
Kuito reduced stress
Kuito Pitch B in water reduced stress
Girassol reduced stress
Rolling reduced stresses for r0=30 mm
Rolling reduced stresses for r0=22.8 mm
Poly. (Average curves 81 mm 124 mm)
0.6

Girassol chainhawse:
stress level in
between theoretical
rolling stress and
locking stress
0.5
0.4
0.3
0.2
0.1
Interlink angle (dg)
0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
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
2.8
Corrosion pitting at the
end of the test may not 2.6
be representative from
2.4
long term offshore
corrosion
2.2
For lower stress ranges,
the predictions may be 2
too conservative
S-N curve
log (D)

Measured
stress
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
log (N)
BS7608 B in air mean S-N curve
1.8
4
4.5
5
5.5
6
6.5
7
7.5
8
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 Monaco
Lucile Rampi
[email protected]
00-33-92-05-86-24