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


Outline

• Introduction• Research program• Laboratory model tests• Half-scale onshore field tests• Centrifuge tests• Numerical modelling• Conclusions


Anchoring problems in deep offshore


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 ?


Suction anchors

anchorsuction

Side frictiion

F

F

D

L


Plate anchors

VLA Anchors

SEPLA


VErtically Loaded Plate Anchor (VELPA) forDeepwater Taut Moorings SBM, inc.


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)


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


Deep sea soil properties

0

5

15

10

20

20 60 100 140

Plasticity Index PI, %

��

Water content w, %

Gulf

of Guinea

1300 m

Gulf

of GuineaGulf of Mexico


Typical deep sea soil profile

��

Undrained shear strength Su, kPa

0 5 10 15 20


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


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


Driving of the plate + pretension at 80°


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


Pullout test 3 in lab tank n°1 – load and suction curvesInclination of the anchor : 60°

0

50

100

150

200

250

300

350

400

450

0 20 40 60 80 100 120 140 160 180

Prescribed displacement (mm)

��

0

0,5

1

1,5

2

2,5

3

3,5

4

4,5

��

LoadSuction


Pullout Test 7 in Tank n°2. Load and Suction curves. Inclination of the anchor α = 45°

0,0

100,0

200,0

300,0

400,0

500,0

600,0

700,0

800,0

900,0

1000,0

1100,0

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

5,0

6,0

7,0

8,0

9,0

10,0

11,0

Suc

tion

(kP

a)

Force (N)

Succion


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


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


Anchor before installation


DrivingFollower

•Driving of the plate to 4.5m deep

•Initial depth of middle pointafter pretension and rotation:

3.75m and 4.25 m


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


Anchor after complete pullout test


Typical pullout results

-120

-100

-80

-60

-40

-20

0

20

40

60

0 100 200 300 400 500 600 700 800 900

tim e (s)

Po

re p

res

su

re (

kP

a)

-60

-40

-20

0

20

40

60

80

100

120

Lo

ad

(k

N)

Pore pressure - Kyowa Pore pressure - Entran Load

-100

-80

-60

-40

-20

0

20

40

0 20 40 60 80 100 120

Displacement (cm) P

ore

pres

sure

(kPa

)

-40

-20

0

20

40

60

80

100

Load

(kN

)

Pore pressure - Kyowa Pore pressure - Entran Load


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.


Centrifuge Tests (LCPC Nantes)


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)


Sample preparation

Sample consolidation

Installation of the pre-embedded anchors


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


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


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


Instrumented model plates

Dimensions: 4x 3x 0.4cm (scale 1/100)


Pretension tests

85°

• Verification of the orientation of the plate after pretension.

•Final orientation determined by the orientation of the anchoring line


Pretension tests• Control of the plate orientation after a pretension test

10° reversing Plate 3 (20.02.2003)

0

2

4

6

8

10

12

0 5 10 15 20 25 30 35 40

Displacement (mm)

For

ce (

daN

)

10° reversing Plate 3 40°


Pretension of the plate: displacement criteria

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25 30 35 40 45 50

Displacem ent from the first bending point (m m )

Plat

e in

clin

atio

n (°

)


Pre-embedded anchorPullout test – inclination 45°

0

20

40

60

80

100

120

0 50 100 150

Displacement (mm)

Pul

lout

For

ce (d

aN)

-250

-200

-150

-100

-50

0

50

Pre

sure

var

iatio

n (k

Pa)

P113

P103

Suction Pi2

Force


Driven and pretensioned anchorfinal inclination 45°

0,2

0,4

0,6

0,8

1

1,2

0 20 40 60 80 10 120Displacement (mm)

��

-240

-200

-160

-120

-80

-40

0

��

Load

Suction


Summary of centrifuge tests results

60

Suction (kPa)


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


Numerical modelling: Plaxis

x

y

A

A

0 1

23

4

5

α


Plaxis calculation- undrained conditions: evidence of succion effect


Plaxis calculation_Anchor at 90°


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


Influence of load inclinationLoad-displacement curves as a function of plate inclination

-50

0

50

100

150

200

250

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°


��

0 0,01 0 ,02 0 ,03 0 ,04 0 ,05-40

0

40

80

120

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

��


Simulation of field conditions

0

50

100

150

200

250

300

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

� ��

��

��


Simulation of field conditions summary

30%100 kPa15 - 174 - 6Anchor at 2Om, 45°


Suction (kPa)

HoldingFactorsNc

Ultimate Pullout capacity (MN)


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


Conclusions (2)• Interesting complementarity between:

- Laboratory tests- Field tests- Centrifuge tests- Numerical models


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


Thank you for your attention

physical modelling of the behaviour of vertically loaded ... · in deep sea sediments: laboratory,...

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