Physical modelling of the behaviour of vertically loaded ... · in deep sea sediments: laboratory,...
Transcript of Physical modelling of the behaviour of vertically loaded ... · in deep sea sediments: laboratory,...
BGA-CFMS Paris 25 November 2005 11
Physical modelling of the behaviour
of vertically loaded plate anchors in deep sea sediments: laboratory,
centrifuge and field tests• P.Foray (3S), S. Alhayari & E.Pons (SBM)• L. Thorel and N. Thetiot (LCPC Nantes)
• B. Souviat, S. Bale and E. Flavigny (3S)
SINGLE BUOY MOORINGS INC., Monaco
BGA-CFMS Paris 25 November 2005
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Outline
• Introduction• Research program• Laboratory model tests• Half-scale onshore field tests• Centrifuge tests• Numerical modelling• Conclusions
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Introduction• Plate anchors as an alternative to suction
caissons in deep sea sediments• VELPA developed by SBM for deepwater
Taut Moorings (S.Alhayari DOT Conf. Marseille 2003)
• Explore the possibility of installation• Evaluate the ultimate pullout capacity. Previous
work (Forest et al 1995): holding factors Nc =9 (long term) and 15 (short term)
• Effect of soil suction ?
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VErtically Loaded Plate Anchor (VELPA) forDeepwater Taut Moorings SBM, inc.
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Installation of the VELPA
IHC HydrohammerPyrodriver (combustion hammer)(Alhayari & Van Foeken 2003)
- self-penetration with follower- driving of the anchor- pretension and rotation
Prototype : 4, 8 and12m2
(4m in height x 3m in width for 12m2)
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Research Program• Physical modelling of the anchor combined with
numerical modelling• Reproduce the different phases of installation,
pretension and pullout of anchors• Reproduce the deep sea soil conditions• Laboratory tests (models at scale 1/15)• Field Tests (scale 1/6)• Centrifuge Tests (scale 1/100)• Numerical modelling
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Deep sea soil properties
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5
15
10
20
20 60 100 140
Plasticity Index PI, %
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Water content w, %
Gulf
of Guinea
1300 m
Gulf
of GuineaGulf of Mexico
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Typical deep sea soil profile
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Undrained shear strength Su, kPa
0 5 10 15 20
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Laboratory tests
•Laboratory tank:2m x 1m x1m
•Homogeneous clay:
1st Tank : Su = 1kPa2nd Tank: Su = 4 kPa3rd Tank: Su = 20 kPa
•6 to 9 tests in each tank
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Plate Anchor Scale Model
• Dimensions of the model plate:- height : 20 cm- width : 30 cm-thickness : 1.4 cm
•Efficient anchorage surface 6.10-2 m2.
•Scale: 1/15
•Instrumentation:inclinometerpore pressure
Front face
Rear face
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Pullout Tests
Sketch – Definition of the anchoring angle α
α
α
MUD LINE
ANCHORING LINE
PLATE ANCHOR
•Initial depth of the anchor between 40 cm and 70 cm•Anchoring angles varied from 25° to 90°•Loading rate: 4mm/min
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Pullout test 3 in lab tank n°1 – load and suction curvesInclination of the anchor : 60°
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0 20 40 60 80 100 120 140 160 180
Prescribed displacement (mm)
�������
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0,5
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1,5
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4,5
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LoadSuction
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Pullout Test 7 in Tank n°2. Load and Suction curves. Inclination of the anchor α = 45°
0,0
100,0
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400,0
500,0
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0,0 50,0 100,0 150,0 200,0 250,0 300,0Displacement (mm)
Forc
e (N
)
0,0
1,0
2,0
3,0
4,0
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6,0
7,0
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10,0
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Suc
tion
(kP
a)
Force (N)
Succion
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Summary of Laboratory Results
Suction Contribution
HoldingFactorNc
Ultimate Pullout capacity (N)
Su (kPa)Tank n°
20%6.2 – 9.57400 - 11600203
61%4 - 4.8917 - 11503.5-4.52
73%5.4 - 7.8300 - 4600.8-1.11
•Relatively low values of Nc•Drainage paths observed•Successful pretension and rotation phase
Ultimate Pullout Capacity = Su x Effective area x NcSuction contribution = (∆u x Effective area)/ Total Load
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Half scale onshore field tests
•Bourget du Lac site:Homogeneous clayover 6m deep
•Average shear strength:Su = 33 kPa
•Dimensions of the plate:Height: 0.675mWidth: 0.5mThickness: 3.3 cm
•Scale: 1/6
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DrivingFollower
•Driving of the plate to 4.5m deep
•Initial depth of middle pointafter pretension and rotation:
3.75m and 4.25 m
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Pullout Loading
•5 complete tests
•Inclination angles:35°, 38°, 40 °, 45°, 53°
•Unloading steps and strong changes in the pullout rate were applied to simulate storm conditions
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Typical pullout results
-120
-100
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-60
-40
-20
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0 100 200 300 400 500 600 700 800 900
tim e (s)
Po
re p
res
su
re (
kP
a)
-60
-40
-20
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120
Lo
ad
(k
N)
Pore pressure - Kyowa Pore pressure - Entran Load
-100
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-40
-20
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40
0 20 40 60 80 100 120
Displacement (cm) P
ore
pres
sure
(kPa
)
-40
-20
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Load
(kN
)
Pore pressure - Kyowa Pore pressure - Entran Load
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Summary of Field Results
• Ultimate pullout capacities: 80 to 100 kN• Holding capacity factors Nc = 7.5 to 9.3• Suction values up to 40 kPa, corresponding
to 15% to 20% of the total load, nearly constant (continuous loading or fast loading)
• Possible drainage paths• The anchoring depth may be not sufficient to
develop a complete deep failure mechanism• Technical success for all the phases of
installation, pretension/rotation and pullout.
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Centrifuge testing program
• Consolidation of kaolin « Speswhite clay » in order to reproduce the in-situ gradient in undrained shear strength Su = 0.8 z
• Soil properties control with in flight CPT tests• First series of tests on pre-embedded
anchors positioned at an inclination of 45°• Second series of complete tests (driving,
pretensioning at 80° and pullout at 45°• Third series of installation and pretension
tests (control of the plate rotation)
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Sample preparation
Sample consolidation
Installation of the pre-embedded anchors
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Container configuration for tests with pre-embedded anchors
Displacement sensors
Electric jack
520 mm
220 mm
210 mm
260 mm
900 mm
Cables
Force sensor
45°
Layer of sand
Displacement sensor
Anchor 2
Beam
Anchor 1
Centrifuge side Door Side
Figure 4 : Container configuration (Pullout tests)
Pore waterpressure sensors
Chain
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Container configuration for complete tests
Electricjack
520 mm
200 mm
900 mm
Cable
Force sensor
45°
Layer ofsand
Displacement sensor
Hydraulic jack
Chain
250 mm210 mm
180 mm
Fork
Follower
10°
Pretensioning position Pullout position
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Figure 9 : Container configuration (Complete tests)
Device for maintaining the anchor in place during consolidation (before being buried by hand down to -45mm)
Fork
Follower
Settlement sensor
Pulley for traction at 45°
Follower
Plan view Electric jack
Pulley for traction at 10°
Hydraulic jack
Penetrometer
Camera
Centrifuge arm
Settlement sensors
Water level sensor
Traction cable
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Pretension tests
85°
• Verification of the orientation of the plate after pretension.
•Final orientation determined by the orientation of the anchoring line
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Pretension tests• Control of the plate orientation after a pretension test
10° reversing Plate 3 (20.02.2003)
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Displacement (mm)
For
ce (
daN
)
10° reversing Plate 3 40°
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Pretension of the plate: displacement criteria
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Displacem ent from the first bending point (m m )
Plat
e in
clin
atio
n (°
)
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Pre-embedded anchorPullout test – inclination 45°
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Displacement (mm)
Pul
lout
For
ce (d
aN)
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50
Pre
sure
var
iatio
n (k
Pa)
P113
P103
Suction Pi2
Force
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Driven and pretensioned anchorfinal inclination 45°
0,2
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0 20 40 60 80 10 120Displacement (mm)
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-240
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-120
-80
-40
0
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Load
Suction
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Summary of centrifuge tests results
60
Suction (kPa)
Suction Contribution
HoldingFactorNc
Ultimate Pullout capacity (MN)
14%3124
Anchor 1: 6.6Anchor 2: 4.9
Driven and pretensioned anchors
28 (res.15)Anchor 1: 6.1Pre-embedded anchors
• High values of Nc, slightly lower for driven plates (effect of soil remoulding after installation ?)
• Peak/residual values for pre-embedded plates•« Plateau » values of UPC and suction for driven plates•Successful pretension and rotation phase
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Plaxis calculation- undrained conditions: evidence of succion effect
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Effect of embeddement depth on the failure mechanism
0.000
0.250
0.500
0.750
1.000
0.000
0.250
0.500
0.750
1.000
Low depth: heave of the soil surface
Large depth: deep failure mechanism
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Influence of load inclinationLoad-displacement curves as a function of plate inclination
-50
0
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300
0,00 0,01 0,01 0,02 0,02 0,03 0,03 0,04 0,04
D isplacement(m)
Pu
llou
t lo
ad (k
N/m
)
0°
15°
30°
45°
60°
75°
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��������������� ��� � �����
0 0,01 0 ,02 0 ,03 0 ,04 0 ,05-40
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160
Dis p lac ement [m]
Ex c es s PP [kN/m2]
C hart 2
7 5 °
6 0 °
4 5 °
3 0 °
1 5 °
0 °
D40
Unrealistic excess pore pressure
Pi-max
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Simulation of field conditions
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350
0 0,2 0,4 0,6 0,8 1 1,2 1,4
D isplacement ( m)
Pul
lout
Loa
d ( k
N)
75 kN ts les jours
50kN ts les jours
50kN ts les 2 jours
75kN ts les 2 jours
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Simulation of field conditions summary
30%100 kPa15 - 174 - 6Anchor at 2Om, 45°
Suction Contribution
Suction (kPa)
HoldingFactorsNc
Ultimate Pullout capacity (MN)
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Conclusions (1)• Pretension method using quasi-vertical
inclination of the anchoring line gave asatisfactory start of rotation
• Final inclination of the anchor controlled by the inclination of the anchoring line
• Suction contribution of 15% to 20% of the total capacity in most of the tests. This was confirmed by numerical analysis.
• Holding factors Nc higher than 15 were observed, provided the anchoring depth is sufficient to develop a deep failure mechanism
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Conclusions (2)• Interesting complementarity between:
- Laboratory tests- Field tests- Centrifuge tests- Numerical models
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Further research
• Effect of long term loading/dissipation of the suction
• Displacements under working load• Effect of cyclic or shock loading ?• Local setup effects ?• Full Scale tests in offshore conditions• 3D numerical analysis