Centrifuge model study on spudcan footprint interaction 1

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 Acknowledgements It has been a pleasurable experience to my post-graduate study in National University of Singapore. First and foremost, I wish to express my deepest gratitude to my supervisors Prof. Leung Chun Fai and Prof. Chow Yean Khow for generously giving me guidance and support throughout my study. I deeply appreciate 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 is gratefully acknowledged. Special thanks to Mr Wong Chew Yuen for the fruitful discussions and the technical assistance for the centrifuge experiments. I wish to extend my thankfulness to all other officers of the geotechnical engineering laboratory and the laboratory centrifuge laboratory, Mr Tan, Dr Shen, Shaja, Mr Choy, Mdm. Jamilah and Mr Loo. The laboratory tests could not be done without your assistances. Thanks to all my fellow friends of the offshore geotechnical group for the much needed friendship and encouragements: Okky, Xiaoxian, Kar Lu, Xie Yi, Eddie, Kee Kiat, Sindhu and Xue Jing. I sincerely wish you all the best. Not forget 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 life would 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 for unselfish sharing of their invaluable practical experience with me. I truly hope that the outcome of this study would clear some of your doubts. A special gratitude is extended to Prof. Andrew C. Palmer for his friendliness and generosity of sharing his experience in offshore engineering. Your advice and encouragement given to me are truly an inspiration. i A special thank to Prof. Mark Cassidy for initializing an opportunity to me to performing 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 testing was a pleasurable experience. Thank you to Prof. Cassidy once more for reviewing some of my result chapters. My sincere gratitude is also given to Prof. Dave White and Prof. M. Randolph for the constructive discussions. I could 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 new friends: 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 with your everlasting love and support given to me. To my family, I am indebted to all of you for the unconditional love, care and support. For all of you, I could not thank you enough. ii Table of Contents Acknowledgement i Table of Contents iii Summary ix List of Tables xi List of Figures xii List of Symbols xxi Chapter Introduction 1.1 Jack-up unit . 1.2 1.3 Typical spudcan installation modes Explanation of footprint in the context of spudcan-footprint interaction 1.4 Problems concerning jack-up installation at site with old footprints . 1.5 Case histories and footprint-related reported incidents 1.6 SNAME guideline (2002) . 1.7 1.8 Needs for research Scope of study and outline of thesis . Chapter Literature Review 2.1 2.2 Introduction . 18 Guideline - SNAME 2002 18 2.3 Spudcan-footprint interaction . 20 2.3.1 2.3.2 A single leg 20 System behaviour 21 2.3.3 Experimental modelling 22 2.3.3.1 2.3.3.2 2.3.3.3 2.3.3.4 Effect of offset distance 22 Effect of leg stiffness 24 Effect of preload . 25 Effect of seabed irregularities . 25 iii 2.3.4 2.3.5 Numerical modelling . 26 2.3.4.1 Limit equilibrium method . 26 2.3.4.2 Finite element simulation . 27 Other preventive/mitigation measures . 29 2.3.5.1 Stomping . 29 2.3.5.2 2.3.5.3 Rack Phase Difference (RPD) monitoring . 30 Other methods 30 2.4 Spudcan foundation behaviour . 31 2.4.1 Spudcan penetration 31 2.4.2 Spudcan extraction 32 2.5 2.6 Shear strength profile 33 Shear strength measurement devices 34 2.7 Summary . 36 Chapter 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 3.3.2 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 3.3.1.4 Model spudcans 60 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 NUS centrifuge – half spudcan test . 67 3.3.2.1 3.3.3 3.4 3.3.2.2 Image processing technique 68 UWA drum centrifuge . 69 3.3.3.1 Model spudcan and leg . 70 Sample preparation . 71 3.4.1 3.4.2 3.5 Half-spudcan test setup . 67 NUS beam centrifuge 71 UWA drum centrifuge . 72 Shear strength measurement devices 73 3.5.1 Tests done in NUS . 73 iv 3.6 3.5.2 Tests done in UWA . 75 Experiment procedures . 76 3.6.1 3.6.2 Chapter Penetration rate 77 Boundary effect . 78 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 4.3 Evaluation of soil condition – Ball penetrometer test . 102 4.2.4 Experiment procedure 103 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 = m) – test CS_1 112 4.3.5.2 Penetration in soft to firm clay (do = m) – test CS_2 . 113 4.4 4.5 4.3.5.3 Penetration in soft to firm clay (do = m) –test CS_3 114 4.3.5.4 Penetration in soft clay (do = 13 m) – test CS_4 114 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 Concluding remarks 119 v Chapter 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 5.2.3 5.3 Sample preparation 150 Model set-up and instrumentation . 152 5.2.4 Shear strength measurement devices - Penetrometers . 153 Effect of time on soil characteristics of a footprint 154 5.3.1 Normally consolidated clay . 154 5.3.1.1 5.3.1.2 5.3.2 Comparison between soil conditions of a footprint formed with a very short (100 year after the footprint was formed for test NC (case OP 0) Fig. 5.8 The measured su profile of a footprint at various positions in NC clay for year operational period case (OP 2) Fig. 5.9 Rsu plots at 0.25, 0.5, 0.75, 1.0, 1.25, 1.5D from footprint centre at 1, 3, 5, and >100 year after the footprint was formed for test NC (case OP 2) Fig. 5.10 Contour map of Rsu for tests in NC clay Fig. 5.11 Schematic diagrams of simplified soil failure mechanisms during spudcan penetration and extraction Fig. 5.12 Excess pore pressure generated throughout the entire spudcan process Fig. 5.13 Measured ue at different stages Fig. 5.14 Schematic diagrams illustrating soil state changes during and after spudcan activity using critical state soil mechanics Fig. 5.15 Excess Pore pressure, ue dissipation Fig. 5.16 The measured su profile of a footprint at various positions in OC clay for immediate extraction case (OP 0) – test OC1 Fig. 5.17 The measured su profile of a footprint at various positions in OC clay for immediate extraction case (OP 2) – test OC2 xvii Fig. 5.18 Rsu plots at 0.25, 0.5, 0.75, 1.0, 1.25, 1.5D from footprint centre at 1, 3, 5, and 100 years after footprint was formed for Test OC (case OP 0) Fig. 5.19 Rsu plots at 0.25, 0.5, 0.75, 1.0, 1.25, 1.5D from footprint centre at 1, 3, 5, and 100 year after footprint was formed for Test OC (case OP 2) Fig. 5.20 Contour map of Rsu for OC clay Fig. 5.21 Model test set-up and PPT arrangement for test OC5 Fig. 5.22 Excess pore pressure generated throughout the entire spudcan process Fig. 5.23 Measured ue at different stages Fig. 5.24 Dissipation of ue for test OC5 Fig. 5.25 Shear induced pore pressures during spudcan penetration Fig. 5.26 su contour map of footprint created in NC clay Fig. 5.27 Load penetration curves for tests in NC clay Fig. 5.28 Induced horizontal force, H for tests in NC clay Fig. 5.29 Induced moment, M for tests in NC clay Fig. 5.30 su contour map of footprint created in OC clay Fig. 5.31 Load penetration curves for tests in OC clay Fig. 5.32 Induced horizontal force, H for tests in OC clay Fig. 5.33 Induced moment, M for tests in OC clay Fig. 6.1 Generalised soil condition of a footprint (for  < 0.002;  < 0.2) Fig. 6.2 Schematic diagram of test arrangement Fig. 6.3 Test results of P1 and P2 for study of different legs Fig. 6.4 VHM plots for test P3 Fig. 6.5 a) Load inclination angle,  and b) normalized eccentricity, e/D Fig. 6.6 VHM plots for tests P4 & P5 Fig. 6.7 Load inclination angle,  and b) normalized eccentricity, e/D xviii Fig. 6.8 Test results for tests P6 and P7 Fig. 6.9 a) Load inclination angle,  and b) normalized eccentricity, e/D Fig. 6.10 Plot of Hmax/suD2 versus de/D Fig. 6.11 Plot of Mmax/suD3 versus de/D Fig. 6.12 Plot of dh/D versus de/D Fig. 6.13 Plot of dm/D versus de/D Fig. 6.14 Plot of load inclination angle, h versus de/D Fig. 6.15 Plot of normalized load eccentricity em/D versus de/D Fig. 6.16 Test arrangement for tests OA1- Fig. 6.17 The undisturbed su profile of the soil samples Fig. 6.18 Preload pressure for the initial penetration and re-penetration at various offset distances Fig. 6.19 Percentage of V reduction for the re-penetration at various offset distances Fig. 6.20 Bearing capacity factor for the initial penetration, Nco for tests OA1 – OA6 Fig. 6.21 Bearing capacity factor for the re-penetration, Ncf for tests OA1 – OA6 Fig. 6.22 The induced horizontal force, H during the spudcan re-penetration at various offset distances Fig. 6.23 The induced moment, M during the spudcan re-penetration at various offset distances Fig. 6.24 The load inclination angle,  for re-penetration at various offset distances Fig. 6.25 The normalized load eccentricity, e/D for re-penetration at various offset distances Fig. 6.26 Plot of Hmax/suDs2 versus Rd/Df Fig. 6.27 Plot of Mmax/suDs3 versus Rd/Df Fig. 6.28 Plot of h versus Rd/Df xix Fig. 6.29 Plot of em/Ds versus Rd/Df Fig. 6.30 Test arrangements for tests OA7 – OA12 Fig. 6.31 Preload pressure measured during the initial penetration and repenetration for tests OA7 – 10 Fig. 6.32 The induced H for tests OA7 – 10 Fig. 6.33 The induced M for tests OA7 – 10 Fig. 6.34 Preload pressure measured during the initial penetration and repenetration for tests OA11 and OA12 Fig. 6.35 The induced H for tests OA11 – 12 Fig. 6.36 The induced M for tests OA11 – 12 Fig. 6.37 Effect of Ds/Df on Hmax/suDs2 at various offset distances Fig. 6.38 Effect of Ds/Df on Mmax/suDs3 at various offset distances Fig. A1 Instrumented spudcan leg xx List of Symbols Roman A Area of spudcan AL Axial area of leg cv Coefficient of consolidation c Adjusted coefficient of consolidation D Diameter of spudcan Df Diameter of spudcan used to create footprint Ds Diameter of spudcan used for re-penetration d Penetration depth of spudcan Penetration depth to L.R.P. of initial spudcan installation de plus thickness of spudcan conical base dp Depth at pore pressure transducer level E Young’s modulus e Load eccentricity EA Axial rigidity EI Flexural rigidity Gs Specific gravity g Earth gravitational acceleration (9.81 ms-2) H Horizontal force h Thickness of soil layer I Area moment of Inertial K Coefficient of permeability k Gradient of shear strength profile L Length of leg N Ratio of centrifugal acceleration to gravitational acceleration M Moment M’ Critical state frictional constant m Exponent factor for shear strength increase Nball Resistance factor for ball xxi Nco Bearing capacity factor for penetration in undisturbed soil Ncf Bearing capacity factor for spudcan re-penetration NT-bar Resistance factor for T-bar OCR Over-consolidated ratio Po Desired preload q Preload pressure qball Ball penetration resistance qo Maximum preload pressure qT-bar T-bar penetration resistance R Radius Rd Radial distance from spudcan centre to footprint centre Rsu Shear strength ratio s Undrained shear strength ratio for normally consolidated clay su Undrained shear strength of soil suo su of undisturbed soil at spudcan L.P.R. sum Undrained shear strength at mudline Tv Time factor for soil consolidation t Time for soil consolidation ue Excess pore pressure V Vertical force v Penetration/extraction velocity V Normalized penetration rate wL Liquid limit wp Plastic limit z Depth from mudline Greek   Interface friction ratio a Axial strain b Bending strain ' Angle of internal friction xxii  Slope of swelling line  Slope of normal consolidation line  Load inclination angle o’ Initial effective vertical stress p’ Effective pre-consolidation stress v’ Effective vertical stress  Adjusted time factor for soil consolidation Subscripts ball value related to or measured by ball penetrometer bearing due to bearing failure footprint value related to footprint h value related to Hmax m value related to Mmax M model scale max maximum value p prototype scale slide due to sliding failure T-bar value related to or measured by T-bar undisturbed value related to undisturbed soil Abbreviations L.R.P. Load reference point NUS National University of Singapore OCR Overconsolidation ratio PIV Particle image velocimetry PPT Pore pressure transducer RPD Rack phase difference SFI Spudcan-footprint interaction SNAME Society of Naval Architects and Marine Engineers UWA The University of Western Australia VA Vertical actuator xxiii . 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.),. time 16 5 5.5 Effect of time on spudcan-footprint interaction 16 6 5.5 .1 Normally consolidated clay 16 8 5.5.2 Over-consolidated clay 17 1 5.6 Practical implications – Spudcan-footprint interaction. .10 9 4.3.4 Soil condition within and around a footprint 11 0 4.3.5 Spudcan-footprint interaction 11 2 4.3.5 .1 Penetration in firm clay (d o = 2 m) – test CS _1 112 4.3.5.2 Penetration in soft to firm