Centrifuge model study on spudcan footprint interaction 1

CENTRIFUGE MODEL STUDY ON SPUDCAN-FOOTPRINT INTERACTIONGAN CHENG TI NATIONAL UNIVERSITY OF SINGAPORE 2009... 78 Chapter 4 Footprint Characteristics and Their Influence on Spudcan-footpri

CENTRIFUGE MODEL STUDY ON SPUDCAN-FOOTPRINT INTERACTION

GAN CHENG TI

NATIONAL UNIVERSITY OF SINGAPORE

2009

CENTRIFUGE MODEL STUDY ON

SPUDCAN-FOOTPRINT INTERACTION

GAN CHENG TI

(B.Eng (Hons.), UM)

A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

DEPARTMENT OF CIVIL ENGINEERING

NATIONAL UNIVERSITY OF SINGAPORE

2009

To my family

It has been a pleasurable experience to do my post-graduate study in NationalUniversity of Singapore First and foremost, I wish to express my deepestgratitude to my supervisors Prof Leung Chun Fai and Prof Chow Yean Khowfor generously giving me guidance and support throughout my study I deeplyappreciate you for the invaluable comments and the constructive suggestions.Thank you for offering me a room to learn, think and grow

Research scholarship awarded by National University of Singapore isgratefully acknowledged Special thanks to Mr Wong Chew Yuen for thefruitful discussions and the technical assistance for the centrifuge experiments

I wish to extend my thankfulness to all other officers of the geotechnicalengineering laboratory and the laboratory centrifuge laboratory, Mr Tan, DrShen, Shaja, Mr Choy, Mdm Jamilah and Mr Loo The laboratory tests couldnot be done without your assistances

Thanks to all my fellow friends of the offshore geotechnical group for themuch needed friendship and encouragements: Okky, Xiaoxian, Kar Lu, Xie Yi,Eddie, Kee Kiat, Sindhu and Xue Jing I sincerely wish you all the best Notforget to thank for the companion of Chee Wee, Pang, Kheng Ghee, Xiying,Yonggang, Xuemei, Simon F., Chong Han, Deep, Karma, Czhia Yheaw,Krishna, Sandi, Huawen and Wang Lei Without all of you, this part of my lifewould not be as enjoyable as it has been Great thanks to all of you

I wish to acknowledge Mr Colin Nelson and Mr Paul Handidjaja forunselfish sharing of their invaluable practical experience with me I truly hopethat the outcome of this study would clear some of your doubts A specialgratitude is extended to Prof Andrew C Palmer for his friendliness andgenerosity of sharing his experience in offshore engineering Your advice andencouragement given to me are truly an inspiration

A special thank to Prof Mark Cassidy for initializing an opportunity to me toperforming research in the Centre of Offshore Foundation Systems (COFS),The University of Western Australia It was truly an eye-opening visit.Working with him and Prof Christophe on the drum centrifuge model testingwas a pleasurable experience Thank you to Prof Cassidy once more forreviewing some of my result chapters My sincere gratitude is also given toProf Dave White and Prof M Randolph for the constructive discussions Icould not miss to thank Bart, Shane and Phil, who worked very hard to ensure

me a smooth experiment, Monica for arranging my temporary accommodation.Thanks also to my old friends: Kok Kuen, Ai Ling and Han Eng; and newfriends: Vickie, Edmond, Shinji, Hossain and An Jui Thank you for making

my stay in Perth a wonderful one

To my late-father who left me in the middle of my study, I will move on withyour everlasting love and support given to me To my family, I am indebted toall of you for the unconditional love, care and support For all of you, I couldnot thank you enough

Table of Contents

Table of Contents iii

List of Figures xii

List of Symbols xxi

Chapter 1 Introduction 1.1 Jack-up unit 1

1.2 Typical spudcan installation modes 2

1.3 Explanation of footprint in the context of spudcan-footprint interaction 3

1.4 Problems concerning jack-up installation at site with old footprints 4

1.5 Case histories and footprint-related reported incidents 6

1.6 SNAME guideline (2002) 7

1.7 Needs for research 8

1.8 Scope of study and outline of thesis 8

Chapter 2 Literature Review 2.1 Introduction 18

2.2 Guideline - SNAME 2002 18

2.3 Spudcan-footprint interaction 20

2.3.1 A single leg 20

2.3.2 System behaviour 21

2.3.3 Experimental modelling 22

2.3.3.1 Effect of offset distance 22

2.3.3.2 Effect of leg stiffness 24

2.3.3.3 Effect of preload 25

2.3.3.4 Effect of seabed irregularities 25

2.3.4 Numerical modelling 26

2.3.4.1 Limit equilibrium method 26

2.3.4.2 Finite element simulation 27

2.3.5 Other preventive/mitigation measures 29

2.3.5.1 Stomping 29

2.3.5.2 Rack Phase Difference (RPD) monitoring 30

2.3.5.3 Other methods 30

2.4 Spudcan foundation behaviour 31

2.4.1 Spudcan penetration 31

2.4.2 Spudcan extraction 32

2.5 Shear strength profile 33

2.6 Shear strength measurement devices 34

2.7 Summary 36

Chapter 3 Experimental Setup and Procedures 3.1 Introduction 54

3.2 Centrifuge modelling 54

3.2.1 Centrifuge scaling laws and model error 56

3.3 Experimental setup 57

3.3.1 NUS geotechnical centrifuge – Full spudcan test 57

3.3.1.1 Model container 58

3.3.1.2 Loading platform and actuators 58

3.3.1.3 Model spudcans 60

3.3.1.4 Model jack-up leg 60

3.3.1.5 Calibration of the strain gauges of the model leg 63

3.3.1.6 Derivation of the VHM acting at the spudcan 65

3.3.1.7 Instruments and transducers 66

3.3.2 NUS centrifuge – half spudcan test 67

3.3.2.1 Half-spudcan test setup 67

3.3.2.2 Image processing technique 68

3.3.3 UWA drum centrifuge 69

3.3.3.1 Model spudcan and leg 70

3.4 Sample preparation 71

3.4.1 NUS beam centrifuge 71

3.4.2 UWA drum centrifuge 72

3.5 Shear strength measurement devices 73

3.5.1 Tests done in NUS 73

3.5.2 Tests done in UWA 75

3.6 Experiment procedures 76

3.6.1 Penetration rate 77

3.6.2 Boundary effect 78

Chapter 4 Footprint Characteristics and Their Influence on Spudcan-footprint Interaction 4.1 Footprint definition 99

4.2 Test programme 100

4.2.1 Formation of a spudcan footprint 100

4.2.2 Evaluation of spudcan-footprint interaction 102

4.2.3 Evaluation of soil condition – Ball penetrometer test 102

4.2.4 Experiment procedure 103

4.3 Experimental results and discussions 103

4.3.1 Shear strength profiles and spudcan penetration depths 103

4.3.2 Characteristics and physical profile of footprint 105

4.3.3 Soil failure mechanism during spudcan penetration and extraction 106

4.3.3.1 Spudcan penetration in undisturbed ground–test CS_2A 107

4.3.3.2 Spudcan extraction – test CS_2A 108

4.3.3.3 Effect of spudcan extraction on footprint profile 109 4.3.4 Soil condition within and around a footprint 110

4.3.5 Spudcan-footprint interaction 112

4.3.5.1 Penetration in firm clay (do= 2 m) – test CS_1 112

4.3.5.2 Penetration in soft to firm clay (do = 5 m) – test CS_2 113

4.3.5.3 Penetration in soft to firm clay (do = 8 m) –test CS_3 114

4.3.5.4 Penetration in soft clay (do = 13 m) – test CS_4 114

4.4 Mechanisms of spudcan-footprint interaction – Soil response 115

4.4.1 Influence of footprint characteristics to off-centered spudcan installation and potential footprint mitigation methods 118

4.5 Concluding remarks 119

Chapter 5 Effect of Time on Spudcan-footprint

Interaction

5.1 Introduction 148

5.2 Test programme 149

5.2.1 Selection of jack-up operational period and elapsed time after a footprint is formed 149

5.2.2 Sample preparation 150

5.2.3 Model set-up and instrumentation 152

5.2.4 Shear strength measurement devices - Penetrometers 153

5.3 Effect of time on soil characteristics of a footprint 154

5.3.1 Normally consolidated clay 154

5.3.1.1 Comparison between soil conditions of a footprint formed with a very short (<1 day) and 2 year operation periods 156

5.3.1.2 Pore pressure monitoring in NC clay 159

5.3.2 Over-consolidated clay 162

5.3.2.1 Pore pressure monitoring in OC clay 163

5.3.2.2 Comparison between shear induced pore pressure in NC and OC clays 164

5.4 Effect of stress history on the soil strength changes with time 165

5.5 Effect of time on spudcan-footprint interaction 166

5.5.1 Normally consolidated clay 168

5.5.2 Over-consolidated clay 171

5.6 Practical implications – Spudcan-footprint interaction in NC and OC clays 172

5.7 Concluding Remarks 174

Chapter 6 Dimensional Analysis of Spudcan-footprint Interaction 6.1 Introduction 206

6.2 Footprint condition prior to future spudcan installation 207

6.3 Effect of jack-up rig configuration 210

6.3.1 Leg flexural rigidity 210

6.3.2 Effect of spudcan diameter 212

6.3.2.1 Tests P2 (D = 10 m) versus Test P3 (D = 6 m) 213

6.3.2.2 Test P4 (D = 10 m) versus Test P5 (D = 6 m) 214

6.3.3 Effect of preload pressure 215

6.4 Dimensional analyses 216

6.4.1 Normalisation of Hmax and Mmax 216

6.4.2 Depth of occurrence of Hmax and Mmax 219

6.4.3 Load inclination anglehand normalised load eccentricity em/D 220

6.5 Effect of offset distance 220

6.5.1 Spudcan of the same diameter 221

6.5.1.1 Vertical response, V 221

6.5.1.2 Bearing capacity factor 222

6.5.1.3 Induced H and M 223

6.5.1.4 Load inclination angle, and normalised load eccentricity, e/Ds 224

6.5.1.5 Dimensionless forms: Hmax/suD2and Mmax/suD3 224

6.5.1.6 Effect of de/Df on load inclination angle,h and normalised load eccentricity, e/Ds 226

6.5.2 Spudcans of different sizes 227

6.5.2.1 8 m footprint with 6 m spudcan re-penetration 227

6.5.2.2 8 m footprint with 10 m spudcan re-penetration 229

6.5.2.3 Effect of Df/Ds 229

6.5.2.4 Practical implications 230

6.6 Concluding remarks 231

Chapter 7 Conclusions 7.1 Introduction 265

7.2 Main findings 266

7.2.1 Characteristics of spudcan footprint and its influence on spudcan-footprint interaction 266

7.2.2 Effect of time on spudcan-footprint interaction 267

7.2.3 Parametric studies and dimensional analysis for spudcan-footprint interaction 269

7.2.4 Limitations 270

7.3 Recommendations for future research 271

7.3.1 Soil behaviour and V-H-M response………271

7.3.2 Footprint mitigation methods 271

7.3.3 Partially restrained leg-hull connection 273

7.3.4 Numerical simulation of spudcan-footprint interaction 273

References 275

Mobile jack-up units are commonly used for offshore oil and gas exploration inrelatively shallow waters as a jack-up unit can be moved from one location toanother with relative ease Owing to its mobile nature, a jack-up unit may beinstalled at a site with existence of footprints left by the previous rig for drillingadditional wells or enhancing production of existing wells If a jack-up isinstalled in the vicinity of footprints, the spudcans have a tendency of slidingtowards the footprints and that could result in overstressing of the structure,unsatisfactory rig position and uncontrolled leg penetration through softerremoulded soils

To date, relatively few studies have been conducted to investigate thisproblem and no guidelines are available to ensure a safe installation This hashighlighted the need for a more thorough and systematic study An extensivecentrifuge model study was conducted to investigate spudcan-footprintinteraction Herein, a footprint is referred to a seabed condition with changes inphysical profile (depression) and soil properties underneath it First part of thisstudy aims to investigate the footprint characteristics and its influence onspudcan re-installation in clays of various strengths In softer clays, spudcan-footprint interaction is found to be dominated by the shear strength variation ofthe soil beneath the depression In firmer clays, the interaction is dominated bythe physical profile of the depression The ability to identify the dominantfactor in the interaction is beneficial in order to devise effective mitigationmeasures to overcome the problem of spudcan sliding towards footprint.Simplified failure mechanisms on spudcan-footprint interaction at differentdepths for re-penetration at an offset of a half spudcan diameter are also

reported in this study Soil bearing failure is found to be dominant forinteraction in surface penetration The soil failure mechanism transforms to asliding failure in deep penetration.

Second part of this study is to investigate the degree of soil strengthvariation changes with time as excess pore pressures generated by the previousspudcan installation dissipate This has significant implications to the spudcan-footprint interaction as the soil property changes with time The results revealedthat the soil gains strength with time from the remoulded stage due to thedisturbance created by the previous spudcan installation and extraction Thestrength gain is significant in normally-consolidated clay as compared to that inover-consolidated clay No unique horizontal load and moment profiles areobtained during spudcan re-penetrations reflecting the influence of the soilbehaviour at different times due to re-consolidation The findings suggest thatspudcan-footprint interaction is not only time dependent, but also soil stress-history dependent

In the final part of this study, dimensional analyses are performed togeneralize this complex problem The experimental results have successfullyidentified some critical parameters such as spudcan size, preload pressure,offset distance and spudcan diameter ratio, which affect spudcan-footprintinteraction and need to be considered for future rig deployment

Keywords: Spudcan-footprint interaction, centrifuge test, clay, horizontal

force, moment, shear strength, depression

List of Tables

Table 1.1 Example of early jack-up units

Table 2.1 Summary of the literature review on spudcan-footprint interaction

(S-F-I)

Table 3.1 Scaling relations (after Leung et al., 1991)

Table 3.2 Model jack-up unit used in University of Western Australia (UWA)

and Cambridge University

Table 3.3 The real rig parameters (Global Maritime, 2003)

Table 3.4 NUS model jack-up leg

Table 3.5 Properties of Malaysia kaolin clay (after Goh, 2003)

Table 3.6 Engineering properties of UWA kaolin clay (after Stewart, 1992)Table 4.1 Summary of centrifuge model test details and results (tests done in

NUS)

Table 5.1 Summary of all tests done in UWA

Table 5.2 Summary of the test results for spudcan-footprint interaction (tests

done in UWA)

Table 6.1 Summary of test details for footprint soil condition

Table 6.2 Summary of test details

Table 6.3 Summary of tests results

Table 6.4 Summary of test details for study of offset distance

Table 6.5 Summary of test results for study of offset distance

List of Figures

Fig 1.1 Two basic types of jack-up (courtesy of Salen Offshore Drilling Co.)Fig 1.2 Jackup supported by spudcans (after Reardon, 1986)

Fig 1.3 Examples of spudcan footings (after McClelland et al., 1981)

Fig 1.4 A rack and pinion consists of a rack and two pinions that mesh with

the rack (after Bennett et al., 2005)

Fig 1.5 Representative leg-hull connection (after SNAME, 2002b)

Fig 1.6 General installation modes for jack-up rig (after Purwana, 2007)Fig 1.7 Record jack-up spudcan bearing pressures from 1979 to 2008 (after

Osborne et al., 2008)

Fig 1.8 Bathymetry of an established site

Fig 1.9 Comparing recorded geotechnical jackup incidents for 1979 to 1988

and 1996 to 2005 (after Osborne, 2005)

Fig 1.10 Case histories classified according to cause of failure (after MSL,

2004)

Fig 2.1 Predicted movement of spudcan penetration with two extreme jackup

leg connections (after Stewart & Finnie, 2001)

Fig 2.2 Horizontal load-displacement response (after Stewart & Finnie, 2001)Fig 2.3 Aspects of resisting forces from soil for footings moving vertically

with strong/weak lateral soil variability (after Dean & Serra, 2004)Fig 2.4 Measured vertical load at various offset distances Offsets are

expressed as multiple of spudcan diameter (after Stewart & Finnie,2001)

Fig 2.5 a) Measured horizontal load at various offset distances; b) Summary

of peak horizontal forces (after Stewart & Finnie, 2001)

Fig 2.6 Maximum horizontal and moment load on Mod V spudcan during

reinstallation (after Cassidy et al., 2009)

Fig 2.7 Equivalent jack-up stiffness over leg length (after Foo et al., 2003)