SINGLE BUOY MOORINGS INC., Monaco
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) BGA-CFMS November BGA-CFMS Paris Paris 2525 November 2005 2005
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Outline • • • • • • •
Introduction Research program Laboratory model tests Half-scale onshore field tests Centrifuge tests Numerical modelling Conclusions BGA-CFMS Paris 25 November 2005
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Anchoring problems in deep offshore
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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 ? BGA-CFMS Paris 25 November 2005
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Suction anchors F
F
anchor suction L
Side frictiion D
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Plate anchors
VLA Anchors
SEPLA
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VErtically Loaded Plate Anchor (VELPA) for Deepwater Taut Moorings SBM, inc.
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Installation of the VELPA IHC Hydrohammer Pyrodriver (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) BGA-CFMS Paris 25 November 2005
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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 BGA-CFMS Paris 25 November 2005
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Deep sea soil properties Water content w, %
Plasticity Index PI, % 20
60
100
140
0
5
Gulf of Guinea
Gulf of Mexico 10
Gulf of Guinea
1300 m 15
20
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Typical deep sea soil profile 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 = 1kPa 2nd Tank: Su = 4 kPa 3rd 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
Front face
•Efficient anchorage surface 6.10-2 m2. •Scale: 1/15 •Instrumentation: inclinometer pore pressure
Rear face
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Driving of the plate + pretension at 80°
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Pullout Tests MUD LINE
α
ANCHORING LINE
α
PLATE ANCHOR
Sketch – Definition of the anchoring angle α
•Initial depth of the anchor between 40 cm and 70 cm •Anchoring angles varied from 25° to 90° •Loading rate: 4mm/min BGA-CFMS Paris 25 November 2005
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Pullout test 3 in lab tank n°1 – load and suction curves Inclination of the anchor : 60° 450
4,5
400
4
350
3,5
300
3
250
2,5
200
2
Load Suction
150
1,5
100
1
50
0,5
Prescribed displacement (mm) 0
0 0
20
40
60
80
100
120
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140
160
180
16
11,0
1000,0
10,0
900,0
9,0
800,0
8,0
700,0
7,0
Force (N)
1100,0
600,0
Force (N)
500,0
Succion
6,0 5,0
400,0
4,0
300,0
3,0
200,0
2,0
100,0
1,0
0,0
0,0 300,0
0,0
50,0
100,0
150,0
200,0
Displacement (mm)
250,0
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Suction (kPa)
Pullout Test 7 in Tank n°2. Load and Suction curves. Inclination of the anchor α = 45°
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Summary of Laboratory Results Tank n°
Su (kPa)
Ultimate Pullout capacity (N)
Holding Factor Nc
Suction Contribution
1
0.8-1.1
300 - 460
5.4 - 7.8
73%
2
3.5-4.5
917 - 1150
4 - 4.8
61%
3
20
7400 - 11600
6.2 – 9.5
20%
Ultimate Pullout Capacity = Su x Effective area x Nc Suction contribution = (∆u x Effective area)/ Total Load •Relatively low values of Nc •Drainage paths observed •Successful pretension and rotation phase BGA-CFMS Paris 25 November 2005
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Half scale onshore field tests •Bourget du Lac site: Homogeneous clay over 6m deep •Average shear strength: Su = 33 kPa •Dimensions of the plate: Height: 0.675m Width: 0.5m Thickness: 3.3 cm •Scale: 1/6 BGA-CFMS Paris 25 November 2005
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Anchor before installation
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Driving Follower
•Driving of the plate to 4.5m deep •Initial depth of middle point after 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 BGA-CFMS Paris 25 November 2005
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Anchor after complete pullout test
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Typical pullout results Load
-8 0
80
-6 0
60
-4 0
40
-2 0
20 0
100
200
300
400
500
600
700
800
0 tim e (s )
900 0
Pore pressure Kyowa Pore pressure Entran Load
-100
100
-80
100 80
-60
60
-40
40
-20
20 0
20
40
60
80
100
0
120 0
Displacement (cm) -2 0
20
-20
40
-4 0
40
-40
60
-6 0
20
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Load (kN)
P o re p re ss ure - E n tra n
Pore pressure (kPa)
P o re p re s s u re (k P a )
-1 0 0
120
P o re p re ss ure - K yo w a
L o a d (kN )
-1 2 0
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. BGA-CFMS Paris 25 November 2005
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Centrifuge Tests (LCPC Nantes)
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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) BGA-CFMS Paris 25 November 2005
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Sample preparation
Installation of the pre-embedded anchors
Sample consolidation BGA-CFMS Paris 25 November 2005
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Container configuration for tests with pre-embedded anchors Centrifuge side
Door Side
Chain
Beam
Force sensor
Displacement sensor 210 mm
Anchor 1
520 mm
45°
Cables
Anchor 2 Electric jack
220 mm
Layer of sand
260 mm
Displacement sensors 900 mm
Pore water pressure sensors
Figure 4 : Container configuration (Pullout tests)
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Container configuration for complete tests Pretensioning position
Pullout position
Hydraulic jack Follower
Chain Force sensor
Fork 10°
520 mm
210 mm 250 mm
45°
Cable
Displacement sensor Electric jack
180 mm Layer of sand
200 mm 900 mm BGA-CFMS Paris 25 November 2005
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Follower Hydraulic jack Penetrometer Settlement sensors
Pulley for traction at 45°
Camera
Water level sensor
Device for maintaining the anchor in place during consolidation (before being buried by hand down to -45mm)
Pulley for traction at 10°
Centrifuge arm
Traction cable
Plan view
Electric jack
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Figure 9 : Container configuration (Complete tests)
Settlement sensor
Follower
Fork
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Instrumented model plates
Dimensions: 4x 3x 0.4cm (scale 1/100) BGA-CFMS Paris 25 November 2005
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Pretension tests • Verification
of the orientation of the plate after pretension.
•Final orientation determined by the orientation of the anchoring line
85°
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Pretension tests • Control
of the plate orientation after a pretension test 10° reversing Plate 3 (20.02.2003) 12
10
40°
10° reversing Plate 3
Force (daN)
8
6
4
2
0 0
5
10
15
20
25
30
35
40
Displacement (mm)
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Pretension of the plate: displacement criteria 100 90
P late inclination (°)
80 70 60 50 40 30 20 10 0 0
5
10
15
20
25
30
35
40
45
50
D isplacem ent from the first bending point (m m ) BGA-CFMS Paris 25 November 2005
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Pre-embedded anchor Pullout test – inclination 45° 120
50 P113 P103
80
0 -50
Suction Pi2
60
-100 Force
40
-150
20
-200
0
-250 150
0
50
100
Presure variation (kPa)
Pullout Force (daN)
100
Displacement (mm) BGA-CFMS Paris 25 November 2005
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Driven and pretensioned anchor final inclination 45° 1,2
-240
1
-200 -160
0,8
-120
Load
0,6 0,4
-80
Suction
-40
0,2
0
20
40 60 80 10 Displacement (mm)
120
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Summary of centrifuge tests results Ultimate Pullout capacity (MN) Preembedded anchors
Holding Factor Nc
Suction (kPa)
Suction Contribution
Anchor 1: 6.1 28 (res.15)
Driven and Anchor 1: 6.6 pretensioned Anchor 2: 4.9 anchors
31 24
60
14%
• 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 BGA-CFMS Paris 25 November 2005
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Numerical modelling: Plaxis 3
2
α 4 A
5 A
y
0
x
1
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Plaxis calculation- undrained conditions: evidence of succion effect
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Plaxis calculation_Anchor at 90°
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Effect of embeddement depth on the failure mechanism 1 .0 00
Low depth: heave of the soil surface
0 .7 50
0 .5 00
0 .2 50
0 .0 00
1.000
0.750
0.500
Large depth: deep failure mechanism
0.250
0.000
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Influence of load inclination L o ad -d isp lacem en t cu rves as a fu n ctio n o f p late in clin atio n 300
P u llo u t lo ad (kN /m )
250
200 0° 15°
150
30° 45°
100
60° 75°
50
0 0,00
0,01
0,01
0,02
0,02
0,03
0,03
0,04
0,04
-50
D isp lacem en t(m )
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C hart 2 Ex c es s PP [kN/m2] 160
75°
Pi-max
60° 45°
120
30°
Unrealistic excess pore pressure
80
15° 0°
40
0
D40
-40 0
0,01
0,02
0,03
0,04
0,05
Dis plac ement [m]
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Simulation of field conditions 350
P ullout Load ( kN )
300 250 75 kN ts les jours
200
50kN ts les jours 50kN ts les 2 jours
150
75kN ts les 2 jours
100 50 0 0
0,2
0,4
0,6
0,8
1
1,2
1,4
D isplacem ent ( m )
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Simulation of field conditions summary Ultimate Pullout capacity (MN) Anchor at 2Om, 45°
4-6
Holding Factors Nc 15 - 17
Suction (kPa)
100 kPa
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Suction Contribution
30%
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Conclusions (1) • Pretension method using quasi-vertical inclination of the anchoring line gave a satisfactory 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 BGA-CFMS Paris 25 November 2005
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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 BGA-CFMS Paris 25 November 2005
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Thank you for your attention
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