Pullout Behaviour of Suction Embedded Plate Anchors in Clay

280 Pages·2008·6.01 MB·English

Department of Civil Engineering

Pullout Behaviour of Suction Embedded Plate Anchors in Clay

Zhenhe Song

This thesis is presented for the Degree of
Doctor of Philosophy
of
Curtin University of Technology

August 2008
DECLARATION

To the best of my knowledge and belief this thesis contains no material previously
published by any other person except where due acknowledgement has been made.
This thesis contains no material which has been accepted for the award of any other
degree or diploma in any university.

Signature: ………………………………………….

Date: ……August 2008…………………...
Dedicated to
Mulin Jiang and my parents, for their love and support
ABSTRACT

In recent years oil and gas mining has moved into increasingly deeper water in
search of undeveloped fields. As water depths approach and exceed 3000 m
conventional offshore foundation systems become inefficient and ineffective in
stabilising platforms and floating production storage units. The trend of supporting
structure design in deep water has been to install catenary and taut leg mooring
systems. Consequently, many types of anchoring systems are being developed and
used in order to withstand large mooring forces. The SEPLA (Suction Embedded
Plate Anchor) is ideal for use in this situation. This project has employed advanced
numerical techniques and centrifuge testing to study pullout behaviour of plate
anchor foundations in different soil profiles and suction caisson installation effect
with the aim of generating a robust framework for design.

The behaviour of strip and circular plate anchors during vertical pullout in uniform
and normally consolidated clays has been studied by means of small strain and large
deformation finite element analyses. Both fully bonded (attached), and ‘vented’ (no
suction on rear face), anchors have been considered. The current numerical results
were compared with existing laboratory test data, finite element results and analytical
solutions. This study showed that the ultimate pullout capacity factors (N ) for deep
c
embedment were 11.6 and 11.7 for smooth and rough strip anchors and 13.1 and 13.7
for smooth and rough circular anchors respectively. When the anchor base was
vented, the soil stayed attached to the anchor base for deep embedment, and the
pullout capacity was therefore the same as for the attached anchor. The separation
depth ratio, H /B or H /D was found to increase linearly with the normalised strength
s s
ratio, s /γ'B or s /γ'D.
u u

Numerical simulation has been conducted to assess the bearing capacity for inclined
pullout plate anchors. This bearing capacity analysis was performed by embedding
the anchors in clay with different initial inclinations and different embedment ratios.
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Both the attached anchor base and vented base were evaluated. The results showed
that the bearing capacities of the inclined plate anchors were associated with the
inclination angles and base conditions. The separation depth of the plate anchors can
be assessed by a simple equation from vertically pulled out plate anchors.

Large deformation finite element analyses of plate anchor keying in clay has been
performed. The effects of anchor thickness, anchor padeye eccentricity, anchor-soil
interface roughness, soil shear strength, anchor submerged weight and soil
disturbance have been studied with anchors in uniform or normally consolidated
clays. The numerical results were compared with transparent soil test and existing
centrifuge test data. The study showed that the RITSS method works well in
simulating the anchor keying process. Anchor padeye eccentricity played an
important role in anchor keying. A normalised anchor geometry ratio was used to
estimate the loss in embedment during plate anchor’s keying.

Both finite element analysis and centrifuge tests have been conducted to study the
suction caisson installation effect. In finite element analysis, the soil disturbed zone
varied from 3 times the caisson wall thickness to a full area inside a caisson.
Centrifuge tests of suction embedded plate anchors were conducted in normally
consolidated kaolin clay and transparent uniform soil. It can be concluded that the
reduction in anchor capacity due to soil disturbance after suction caisson installation
depends on re-consolidation time and soil sensitivity. The soil disturbance also
reduced the loss of embedment during the anchor keying process

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ACKNOWLEDGEMENTS

I am indebted to Dr. Yuxia Hu for her interest, guidance and provision of financial
assistance during this research. Dr. Hu’s commitment and assistance was limitless
and greatly appreciated. I would also like to express my gratitude and appreciation to
Prof. Mark Randolph for his stimulating supervision, wonderful insight and
constructive advice. Sincere thanks are extended to Dr. Hamid Nikraz for his friendly
inspiration, pastoral care and for serving as co-supervisor.

Experiments could not have been performed without the support of the Centre for
Offshore Foundation Systems (COFS), University of Western Australia (UWA),
especially the centrifuge technicians Mr. Don Hurley and Mr. Bart Thompson. They
helped me to arrange the instrument, ramp up the centrifuge and prepare sample etc.
I would also like to thank Dr. Christopher Gaudin, centrifuge manager at COFS, for
his generous suggestions on this centrifuge study based on his experience. The
Suction Embedded Plate Anchor (SEPLA) model caisson was also first made by Dr.
Christophe Gaudin and Conleth O’Loughlin. Their work was essential in achieving
accurate measurements and is greatly appreciated. Assistance from Mr. Binaya
Bhattarai and Mrs. Clare Bearman, during the preparation of transparent soil samples,
are acknowledged. I would like to thank Mr. Gary Davies and his colleagues in the
Civil Engineering Workshop for their high efficiency and accuracy in the fabrication
work.

Dr. Jun Liu, of Dalian University of Technology, and Mrs. Zarnaz Mehryer, of
Curtin University, both helped me in becoming familiar with AFENA FE code when
I arrived here 4 years ago; I am grateful to them all. And I would like to thank Dr.
Dong Wang for the open discussion of plate anchors behaviour.

The research presented here is supported by the Australian Research Council through
the Large ARC discovery scheme (DP0344019). In addition, I received financial
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support from Curtin International Research Tuition Scholarship (CIRTS). This
support is gratefully acknowledged.

Mrs. Diane Garth and Mrs. Sucy Leong, of Curtin University of Technology and Mrs.
Monica Mackman, of UWA cordially assisted me with patience. I am happy to see
that they are always smiling. Thank you. Thanks are also due to the IT support staff
in the Faculty of Engineering and Computing.

Finally, thank you to my wife Mulin Jiang and my family for their support and
encouragement throughout the period of my studies.
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PUBLICATIONS

Journal:

Song, Z., Hu, Y. and Randolph, M.F. (2007) “Numerical simulation of vertical
pullout of plate anchors in clay”, accepted by Journal of Geotechnical and
Geoenvironmental Engineering.

Song, Z., Hu, Y., O'Loughlin, C. and Randolph, M.F. (2007) “Loss in anchor
embedment during plate anchor keying in clay”, to be submitted

Song, Z., Hu, Y., and Randolph, M.F. (2007) “Bearing capacities of inclined pullout
plate anchors in Clay”, to be submitted

Wang, D., Hu, Y., Song, Z. (2007) “Analysis of uplift capacity of circular plate
anchors in uniform clay”, Rock and Soil Mechanics, 28(6), 179-183 (In Chinese)

Conference:

Song, Z. and Hu, Y., (2008) “Bearing capacity and keying of plate anchor in
normally consolidated clay”, Proceedings of the 18th (2008) Annual International
Offshore and Polar Engineering Conference - ISOPE-2008, Vancouver, BC, Canada,
July 6-11, 2008, In press.

Hu, Y. and Song, Z., (2008) “Large deformation FE analysis of plate anchor keying
in clay”, Proceedings of the 12th International Conference of International
Association for Computer Methods and Advances in Geomechanics (IACMAG), Goa,
India, 1-6 October, 2008, In press.

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Song, Z. and Hu, Y., (2007) “Suction embedded plate anchor test in centrifuge”,
Proceedings of 10th Australia New Zealand Conference on Geomechanics, Brisbane
Australia, October 21 - 24, 2007, pp. 562-567.

Wang, D. Hu, Y., Song, Z. (2007) “Large deformation analysis of rectangular plate
anchors in normally consolidated clay”, Proceedings of 10th Australia New Zealand
Conference on Geomechanics, Brisbane Australia, October 21 - 24, 2007, pp. 268-
273.

Song, Z., Hu, Y., and Gaudin, C. (2007) “The influence of disturbed zone on
capacity of suction embedded plate anchors”, Proceedings of the 17th (2007) Annual
International Offshore and Polar Engineering Conference - ISOPE-2007, Lisbon,
Portugal, July 1-6, 2007, pp. 1340 -1346.

Song, Z., Hu, Y., Wang, D. and O'Loughlin, C. (2006). “Pullout capacity and
rotational behaviour of square anchors in kaolin clay and transparent soil”.
International Conference on Physical Modelling in Geotechnics, Hong Kong, P.R.
China, August 4-6, pp. 1325-1331.

Song, Z. and Hu, Y. and O'Loughlin, C. (2006). “Anchor and chain reaction during
inclined pullout in clay”. 25th International Conference on Offshore Mechanics and
Arctic Engineering, June 4-9, 2006, Hamburg, Germany, OMAE2006-92411

Song, Z. (2006) “Design issues for SEPLAs in uniform clay”, Proceeding of 7th
young geotechnical professionals conference, October18-21, 2006, Adelaide, pp.
199-204.

Song, Z., Hu, Y. and Randolph, M.F. (2005). “Pullout behaviour of inclined plate
anchors in clay”, Proceeding of the 11th Conference of the International Association
of Computer Methods and Advances in Geomechanics (IACMAG), June 19-24, 2005,
Turin, Italy, pp. 715-722.

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Song, Z. and Hu, Y. (2005), “Vertical pullout behaviour of plate anchors in uniform
clay”, Frontiers in Offshore Geotechnics: ISFOG 2005, Perth, Western Australia,
Australia, September 19-21, 2005, pp. 205-211.
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