Modelling of Spudcan and Suction Caisson Performance M.J.

Modelling of Spudcan and Suction Caisson Performance M.J.

Modelling of Spudcan and Suction Caisson Performance
M.J. Cassidy, M.F. Randolph (University of Western Australia) and B.W. Byrne (Oxford University)
Project: An intensive study of shallow foundation behaviour
Purpose: Make a comparative assessment of the response of similarly sized spudcans and caisson.
Motivation: Investigation of shallow foundation alternatives for the long-term deployment of mobile offshore drilling units

Investigating Spudcan and Caisson Foundation
Response in the UWA Drum Centrifuge

Analysis of jack-up at the Sunrise site
Based on the experimental results (and existing theory) a force-resultant model for describing the
behaviour of caissons in clays has been developed. Though plasticity models depicting spudcan
behaviour exist they have been limited to the traditional inverted conical spudcan footings shape and the
extension to the caisson configuration here represents a significant advance. As the model is developed
within strain-hardening plasticity theory it allows it to be implemented with structural analysis codes.
(1) The model has been verified by retrospective simulation of the centrifuge experiments.
(2) Its practical application shown by a series of example analyses of the proposed production jack-up at
the Sunrise site.
(3) The benefits of the caisson configuration compared with typical spudcan shapes have been shown.

Drum Centrifuge installed 1998
1.2m diameter; soil sample channel
300mm high (vertically) and 200mm
radial depth

SOIL

max acceleration 485g
(after Stewart, Boyle & Randolph, Centrifuge 98)

Caisson

2

Loading legs used in the experiments

Vertical Load, V (MN)

1.5

Caisson L/D=0.5

Model spudcan and caisson footings

1

2

-1

0

-0.4
-1.5

0

-0.5

0.5

V/V u

1.5

-1
-1.5

1

2

3
4
Vertical Penetration, w (m)

Vert. stress (kPa)
-7.1 10.6 28.3 46.0 63.7 81.3 99.0
0.8
28.3

0.8

0.7

24.8

0.7

21.2

0.6

swipe 1
swipe 2
swipe 3
swipe 4
swipe 5
swipe 6

0.6

1

-0.2

0

50

100

1.2

0

50

100

D30 L15
D30 L10
Spudcan V0=204.39MN

1
0.8
0.6
0.4
0.2
-50

time (s)

D = 30m

5

Simplified model of proposed Sunrise jack-up and footing options

6

Deck displacement response for 100-year conditions for
different footing options

-0.5

0.5

V/V u

All normalised results against theoretical yield surface

1.5

0.5

17.7

0.5

24.8
21.2
17.7

0.4

14.1

0.3

10.6

0.2

7.1

3.5

0.1

3.5

0

0.0

0

0.0

-0.1
-0.2

-3.5

-0.1
-0.2

-3.5

0.4

14.1

0.3

10.6

0.2

7.1

0.1

0.8

1.8

Vert. Load (MN)

2.8

0

0.2
0.4
Hor. Disp. (m)

Project Conclusions and Benefits

28.3

swipe 1
swipe 2
swipe 3
swipe 4
swipe 5
swipe 6

0.6

Example combined loading data of a caisson

Hor. Stress (kPa)

H/Asu

M/DAsu

0.2

1.4

-100

Hor. Load (MN)

3

0.4

Single leg

-0.5

Hor. stress (kPa)

4

0.6

-50

Example vertical installation data

Hor. Load (MN)

0.8

Windward Legs
Leeward Leg

0

5

Caisson
Spudcan
Theory

1.6

D = 30m

120m

8
6
4
2
0
-2
-4
-6
-8
-100
1.8

0

0

1

Two legs

0.5

-1.5

1.2

L = 15m

39.1m

"poppet" valve

Spudcan

3.375

Hor. Hull Disp. (m)

8 mm thread

11.625

L=10m

8 mm thread

33.9m

Mean water depth 143.1m

Spudcan

Leeward
Wave elev. (m)

The drum centrifuge at UWA has been used to comparatively assess the performance of spudcan and
caisson foundations in both normally consolidated kaolin clay and normally consolidated calcareous
silt (from the Gorgon area). The tests were designed to investigate footing behaviour under combined
loading, and to provide data that can be used to validate existing as well as develop new plasticity models. The
novelty of the testing was to compare the response of skirted and non-skirted foundations and therefore
assess whether improvements in performance could be obtained by the addition of a skirt. In order to apply
appropriate loading combinations to the model scale footings in a centrifuge, a hinged loading arm was
developed. This allowed higher ratios of moment to horizontal load to be applied to the footings than the more
conventional fixed arm approach.

84.375m

4.35 2.7

Windward

1. The caisson in the centrifuge tests displayed increased moment stiffness, increased horizontal capacity and
larger combined load capacity under tension, but surprisingly no significant improvement in moment capacity.
2. A simple strain-hardening plasticity model for the analysis of caisson was developed and implemented in
UWAs jack-up analysis package.
3. For the Woodsides proposed Sunrise site, (a) optimum caisson geometries (b) penetration and relative
movement of the foundations, including permanent deformation in extreme events (c) comparative behaviour
to equivalent sized spudcan footings, were studied and the advantages of caisson foundations quantified.

For more details visit us at www.cofs.uwa.edu.au

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