installation and keying of follower embedded plate anchors

130 Pages·2007·3.42 MB·English

School of Civil and Resource Engineering
The University of Western Australia
INSTALLATION AND KEYING OF FOLLOWER EMBEDDED
PLATE ANCHORS

by
ADAM CHARLES LOWMASS

A thesis submitted for the degree of
MASTERS OF ENGINEERING SCIENCE
at
THE UNIVERSITY OF WESTERN AUSTRALIA

SCHOOL OF CIVIL AND RESOURCE ENGINEERING
2006
School of Civil and Resource Engineering
The University of Western Australia
ABSTRACT
The offshore oil and gas industry is moving into deeper water to meet the growing global
demand for hydrocarbons. Associated with this move to deep water is the need for more efficient
anchorage systems to moor floating facilities. Of the anchor concepts proposed in recent years,
the most promising utilise a follower to embed an initially vertical plate anchor, typically located
at the follower base. When the system has reached the design embedment depth, the plate anchor
mooring line is disengaged from the follower, leaving the follower free to be re-used for the next
installation. The mooring line attached to the vertically embedded plate anchor is tensioned
causing the plate anchor to rotate or ‘key’ to an orientation that is perpendicular to the direction
of loading. The offshore industry currently considers this keying process to be the main unknown in
relation to follower-embedded anchors.
This project contributes to the limited database of the behaviour of anchors during keying, in
particular quantifying the effects of eccentricity of loading from the plate on the vertical
displacement of the plate anchors during the keying process. Reduced scale model centrifuge
testing is used to facilitate the optical measurement of the rotation and displacement of the
various geometries of plate anchors through a soil/Perspex interface during keying. Additionally
the project has explored the effects of installation on the keying characteristics of Suction
Embedded Plate Anchors (SEPLA).
As well as the physical modelling aspect of the project, a numerical model is developed to
simulate the keying process, allowing accurate prediction of final embedment depth and anchor
orientation, and ultimately anchor load capacity.
This study has significantly enhanced the understanding of the keying process. In terms of the
practical application of embedded plates as anchors for floating offshore facilities, the influence
of padeye eccentricity ratio (e/B) on normalised embedment loss (∆z /B) resulting from keying is
e
possibly the most important finding of the study. It indicates that current guidelines, stating
embedment loss during keying is twice the anchor height (B) in cohesive soils, are extremely
conservative given typically padeye eccentricities (e/B < 0.5). These results have indicated that
for typical embedded plate anchors the embedment loss is < 0.3B.
School of Civil and Resource Engineering
The University of Western Australia
ACKNOWLEDGMENTS
I would firstly like to thank Conleth O’Loughlin who has gone out of his way over the past two
years to ensure I had every possible advantage in completing this Masters paper. Even though his
schedule was always busy, he made himself available to answer any questions I had and to offer,
much appreciated, guidance. Mark Randolph and Christophe Gaudin also provided notable
guidance during the course of the last two years.
Throughout the project, Don Herley and Bart Thompson provided much assistance, not only
during testing but also in other areas of the project. They provided a relaxed, yet productive work
environment that made the long hours in the laboratory welcome. The workshop staff, namely
Gary Davies, Neil McIntosh, Alby Kalajzich, David Jones, Frank Tan, John Breen, Shane De
Catania and Wayne Galbraith all contributed considerably with last minute modifications to
testing apparatus and the manufacture of testing apparatus and models. Wenge Liu also provided
great IT support.
Thanks must also go to, PhD candidate and friend, Mark Richardson who assisted me during the
busy times. He helped in completing centrifuge tests, as well as providing advice throughout the
project. Mark always made himself available and was happy for me to approach him at all times.
Friends and family have also assisted me greatly, providing the much need emotional support.
Special thanks must go to my cousin Mark Norwell who continually motivated and encouraged
me to complete the project.
Last, and probably most importantly, I would like to thank my parents, Anna and Peter. Both
provided the much need support during the stressful times of the project. They always made
themselves available to help and offer advice whenever necessary.
School of Civil and Resource Engineering
The University of Western Australia
TABLE OF CONTENTS
Chapter 1 Introduction.........................................................................................................1-1
1.1 Outline......................................................................................................................1-1
1.2 Mooring Systems......................................................................................................1-2
1.3 Anchoring Options....................................................................................................1-3
1.3.1 Gravity Anchors.......................................................................................1-3
1.3.2 Anchor Piles.............................................................................................1-3
1.3.3 Drag Embedment Anchors........................................................................1-4
1.3.4 Suction Caisson........................................................................................1-4
1.3.5 Torpedo and Deep Penetrating Anchor (DPA)..........................................1-5
1.3.6 Follower Embedded Anchors....................................................................1-6
1.3.7 Anchor Option Comparison......................................................................1-6
1.4 Research Objectives..................................................................................................1-8
1.5 Thesis Structure........................................................................................................1-8

Chapter 2 Plate Anchor and Keying Background...............................................................2-1
2.1 Previous Studies........................................................................................................2-1
2.1.1 SEPLA.....................................................................................................2-1
2.1.2 Keying......................................................................................................2-1
2.1.3 Plate Capacity...........................................................................................2-2

Chapter 3 Model Plate Anchor Testing................................................................................3-1
3.1 Centrifuge Testing Apparatus....................................................................................3-1
3.1.1 Principles of Centrifuge Testing................................................................3-1
3.1.2 The Geotechnical Beam Centrifuge..........................................................3-2
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The University of Western Australia
3.1.3 The Geotechnical Drum Centrifuge..........................................................3-4
3.2 Kaolin Clay...............................................................................................................3-5
3.2.1 Soil Properties..........................................................................................3-5
3.2.2 Sample Preparation...................................................................................3-6
3.3 Plate Anchor Keying Test Procedure.........................................................................3-7
3.3.1 Soil Characterisation Tests........................................................................3-7
3.3.2 SEPLA Beam Centrifuge Tests.................................................................3-8
3.3.3 Plate Keying Drum Centrifuge Tests........................................................3-11
3.3.4 Plate Keying Beam Centrifuge Tests........................................................3-14

Chapter 4 Experimental Results...........................................................................................4-1
4.1 SEPLA Tests............................................................................................................4-1
4.1.1 Soil Characterisation Tests........................................................................4-1
4.1.2 SEPLA Capacities....................................................................................4-2
4.2 Keying Tests.............................................................................................................4-4
4.2.1 Soil Characterisation Tests........................................................................4-5
4.2.2 Drum Keying Tests...................................................................................4-7
4.2.3 Beam Keying Tests...................................................................................4-8
4.2.4 Keying Test Summary.............................................................................4-10

Chapter 5 Analytical Simulation..........................................................................................5-1
5.1 Background...............................................................................................................5-1
5.2 Review of Numerical and Analytical Studies of Plate Anchors..................................5-1
5.3 Plasticity Concepts and the Yield Locus....................................................................5-2
5.4 Kinematic Anchor Analysis......................................................................................5-3
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The University of Western Australia
5.5 Results......................................................................................................................5-4

Chapter 6 Discussion of Theoretical and Experimental Results..........................................6-1
6.1 SEPLA Tests............................................................................................................6-1
6.2 Plate Keying.............................................................................................................6-2
6.2.1 Capacity...................................................................................................6-2
6.2.2 Keying......................................................................................................6-4
6.2.3 Comparison with Analytical Simulation....................................................6-7

Chapter 7 Conclusion and Further Research.......................................................................7-1
7.1 Experimental Findings..............................................................................................7-1
7.1.1 SEPLA Testing.........................................................................................7-1
7.1.2 Plate Keying Tests....................................................................................7-1
7.2 Recommendations for Future Development...............................................................7-2
7.3 Concluding Statement...............................................................................................7-3
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The University of Western Australia
NOMENCLATURE
A plate area
B plate height
b depth
f
C compression index
c
C swelling index
s
c local undrained shear
u
c coefficient of consolidation
v
d diameter
d padeye embedment loss
v
e loading eccentricity
F peak load
max
g gravitational acceleration
G specific gravity
s
H initial plate embedment / horizontal loading
k shear strength gradient
L length
L effective anchor length
e
L footing length
f
LL liquid limit
M mass / moment loading
N gravity scale factor
N non-dimensional T-bar factor
b
N , N , N non-dimensional breakout factor
c cy cp
P force per unit length
PL plastic limit
Q ultimate uplift capacity
u
s chain displacement
c
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The University of Western Australia
s local undrained shear
u
T and t time
T padeye load
a
T dimensionless time factor
v
v pullout rate
V normalised velocity / vertical loading
W submerged anchor weight
a
z depth
∆z anchor embedment loss
e

Greek
α soil adhesion factor;
β plate inclination to the vertical
γ weight of soil
θ plate inclination to the horizontal
θ inclination of the chain angle at anchor padeye
a
σ ’ effective vertical stress
v
φ friction angle
School of Civil and Resource Engineering
The University of Western Australia
LIST OF FIGURES
Figure 1.1: Offshore production facilities (Courtesy: Minerals Management Services)...........1-10
Figure 1.2: Offshore production facilities (Courtesy: Minerals Management Services)...........1-10
Figure 1.3: Mooring types (Vryhof, 2000)..............................................................................1-11
Figure 1.4: Drag anchors, sand and clay (Vryhof, 2000).........................................................1-11
Figure 1.5: Suction caisson.....................................................................................................1-12
Figure 1.6: DPA and Torpedo anchor and installation schematic (Lieng et al. 1999 and
Medeiros, 2001).................................................................................................1-12
Figure 1.7: SEPLA installation technique...............................................................................1-13
Figure 1.8: SEA installation technique....................................................................................1-13
Figure 2.1: The SEPLA concept: (cid:1) Suction installation, (cid:2) Caisson retrieval, (cid:3) Anchor
keying, (cid:4) Mobilised anchor................................................................................2-4
Figure 2.2: Failure mechanism of a plate (Merifield, 2002)......................................................2-4
Figure 2.3: Effect of overburden pressure on strip anchors (Merifield, 2002)...........................2-5
Figure 2.4: N comparison for Breakaway cases in weightless soil (Merifield, 2002)...............2-5
c
Figure 2.5: Upper Bound N values for horizontal anchors - inhomogeneous cohesive soil
c
(Merifield, 2002).................................................................................................2-6
Figure 3.1: Geotechnical beam centrifuge...............................................................................3-17
Figure 3.2: Beam strongbox....................................................................................................3-17
Figure 3.3: Motor driven actuator...........................................................................................3-18
Figure 3.4: T-bar penetrometer...............................................................................................3-18
Figure 3.5: Drum centrifuge with clamshell removed..............................................................3-19
Figure 3.6: Keying test setup in sample box, @ 1 g................................................................3-19
Figure 3.7: Digital camera cradle with trigger.........................................................................3-20
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The University of Western Australia
Figure 3.8: Tool table actuator................................................................................................3-20
Figure 3.9: Sub-Terrain Oil impregnated Multiple Pressure Instrument (STOMPI).................3-21
Figure 3.10: Mounted model SEPLA......................................................................................3-21
Figure 3.11: SEPLA test setup................................................................................................3-22
Figure 3.12: Experimental arrangement and test procedure (section view)..............................3-23
Figure 3.13: Caisson with attachments (pneumatic valve hidden behind caisson guide)..........3-24
Figure 3.14: SEPLA test, actuator setup..................................................................................3-24
Figure 3.15: Model plate anchors with various loading shafts attached (including two not
used in this study)...............................................................................................3-25
Figure 3.16: LCD attached to sample box...............................................................................3-25
Figure 3.17: Drum keying test, loading arm............................................................................3-26
Figure 3.18: Drum keying test layout (White, 2003)...............................................................3-26
Figure 3.19: Sample box held in place with brackets...............................................................3-27
Figure 3.20: Beam keying test configuration...........................................................................3-27
Figure 4.1: Clay, shear strength profile, SEPLA tests..............................................................4-12
Figure 4.2: Assumed load response during anchor keying and pullout for Test VE-ST1..........4-12
Figure 4.3: Dimensionless load displacement response for jacked SEPLA..............................4-13
Figure 4.4: Dimensionless load displacement response for suction embedded SEPLA............4-13
Figure 4.5: Loss of embedment as a function of padeye load inclination), e/B = 0.66..............4-14
Figure 4.6: Keying test load orientations.................................................................................4-14
Figure 4.7: Clay, shear strength profile, drum tests box 2........................................................4-15
Figure 4.8: Clay, shear strength profile, drum tests box 3........................................................4-15
Figure 4.9: Clay, shear strength profile, drum tests box 4........................................................4-16
Figure 4.10: Clay, shear strength profile, drum tests box 5......................................................4-16