GEOTECHNICAL CONSIDERATIONS FOR OFFSHORE GRAVITY TYPE STRUCTURES WITH EMPHASIS ON FOUNDATION STABILITY UNDER STORM WAVE LOADING by THOMAS C. GAARD S., The University of California, Davis, 197 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF CIVIL ENGINEERING We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA April, 1982 O Thomas C. Gaard, 1982 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of Cl^\ ^^^.,1^ The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date 32/ ^ J z. % DE-6 (3/81) 11 ABSTRACT A thorough discussion of offshore gravity type structures presently being used, or considered for use in the near future by the oil industry, is presented, along with a brief summary of the major types of structures now used offshore. Factors affecting the stability of offshore gravity type structures are discussed, from the evaluation of a suitable site and the selection of soil parameters, through installation and short-term foundation safety. A case study of the Ekofisk tank is included to show how geotechnical concepts are applied offshore. A thorough description of wave loading on offshore gravity structures is presented, including a discussion on how the design storm is used in geotechnical analyses. Existing stability methods are reviewed. The merits and shortcomings of each method are discussed with respect to their application offshore. Procedures for analyzing the stability of offshore gravity type structures subjected to storm wave loading are developed based on the method of slices. Both Janbu's (1973) Generalized Procedure of Slices and Sarma's (1973) method are adapted for offshore analyses. The latter method is modified to perform pseudo-three-dimensional analyses. A computer program GRAVSTAB developed for this purpose is described and applied to several example problems. The versatility of the method of analysis is demonstrated and results are compared with existing methods. iii TABLE OF CONTENTS ABSTRACT ii TABLE OF CONTENTS .' iii LIST OF TABLES vi LIST OF FIGURES vii ACKNOWLEDGEMENTS x NOMENCLATURE xi CHAPTER 1 : INTRODUCTION 1 CHAPTER 2 : THE OFFSHORE GRAVITY TYPE STRUCTURE 11 2.1 General Characteristics 11 2.2 Platforms For General Offshore Development 14 2.2.1 Concrete Platforms 15 2.2.2 Steel Platforms 18 2.2.3 Hybrid Platforms 23 2.3 Platforms For Arctic Development 25 2.4 Deep Water Platforms And Other Structures 27 2.5 Sources Of New Platform Technology 30 CHAPTER 3 : DESIGN, CONSTRUCTION AND INSTALLATION 31 3.1 Preliminary Considerations 31 3.1.1 Sources Of Loading 31 3.1.1.1 Environmental Loads 31 3.1.1.2 Operational Loads 32 3.1.2 Environmental. Design Parameters 33 3.1.3 Site Selection And Soil Investigations 34 3.1.4 Selection Of Soil Parameters For Design 42 3.2 Platform Design 46 iv 3.2.1 Hydrodynamic Analyses 46 3.2.2 Geotechnical Analyses 47 3.2.3 Structural Requirements And Analyses 55 3.3 Platform Construction 57 3.4 Platform Installation 59 3.5 Platform Instrumentation 63 CHAPTER 4. : THE EKOFISK TANK - A CASE STUDY 67 CHAPTER 5 : CHARACTERISTICS OF WAVE LOADING 93 5.1 Ocean Waves 93 5.1.1 The Wave Climate 93 5.1.2 Wave Theories 94 5.1.3 Results Of Linear Wave Theory 97 5.2 Characterizing The Wave System 97 5.2.1 Obtaining The Design Storm 100 5.2.1.1 Statistical Description 100 5.2.1.2 Geotechnical Equivalent 101 5.2.2 Application Of The Design Storm 102 5.3 Wave Loads On The Foundation System 104 5.3.1 Wave Forces Acting On*The Structure 104 5.3.2 Wave Forces Acting On The Foundation 108 5.4 Effect Of Cyclic Loading On The Foundation System ...109 CHAPTER 6 : PROCEDURES FOR ANALYZING THE STABILITY OF OFFSHORE GRAVITY TYPE STRUCTURES 115 6.1 Fundamental Considerations 115 6.2 Modelling The Wave-Structure-Soil System 120 6.3 Loading Applied To The Foundation 126 6.4 Available Stability Methods 128 6.4.1 Classical Bearing Capacity Approach 128 V 6.4.2 Other Bearing Capacity Formulations 136 6.4.3 NGI Slip Surface Method 140 6.4.4 Method Of Slices 144 6.4.5 Finite Element Analyses 144 6.4.6 Model Tests 150 6.5 Summary 152 CHAPTER 7 : APPLICATION OF THE METHOD OF SLICES TO OFFSHORE GRAVITY STRUCTURE FOUNDATIONS 154 7.1 The Method Of Slices 156 7.2 Loading Applied To The Foundation 158 7.3 Treatment Of The Applied Horizontal Force 160 7.4 Modified Janbu Method 161 7.4.1 Assumptions 161 7.4.2 Derivation Of Equilibrium Equations .161 7.4.3 Working Formulas • 164 7.5 Modified Sarma Method 167 7.5.1 Assumptions 169 7.5.2 Derivation Of Equilibrium Equations 170 7.5.3 Working Formulas 173 CHAPTER 8 : EXAMPLES AND APPPLICATION OF ANALYSES 175 8.1 Description Of Computer Procedure 175 8.2 Example 1 - A Multi-layered Cohesive Deposit 177 8.3 Example 2 - A Cohesionless Deposit: Ekofisk Tank ....183 CHAPTER 9 : SUMMARY AND CONCLUSIONS 189 REFERENCES 194 LIST OF TABLES Table I - Comparison of Fixed Offshore Platforms 16 Table II - North Sea Concrete Gravity Platforms 19 Table III - Gravity Platforms in Other Parts of the World .. 20 Table IV - Environmental Design Criteria for Some Offshore Areas 35 Table V - Geotechnical Concerns for Offshore Gravity Type Platforms 48 Table VI - Example of the Accumulated Effect of a 100-year Storm 85 Table VII - Some Results of Linear Wave Theory 99 Table VIII - Comparison of Existing Stability Analyses 153 Table IX - Geometry and Loading Data for Example 1 177 Table X - Comparison of Computed Safety Factors for Example 1 179 Table XI - Coefficients for Estimating Undrained Strength from Triaxial Compression Data 182 Table XII - Effect of Shear Zone Representation on the Safety Factor 184 Table XIII - Geometry and Loading Data for Example 2 185 Table XIV - Effect of A-parameter on the Safety Factor 188 vi i LIST OF FIGURES Figure 1.1 - Steel Jacketed Platforms 2 Figure 1.2 - Mobile Platforms 2 Figure 1.3 - The Ekofisk Tank 5 Figure 2.1 - Components of an Offshore Gravity Type Platform 13 Figure 2.2 - North Sea Concrete Gravity Type Offshore Platforms 18 Figure 2.3 - Tecnomare Steel Gravity Type Offshore Platform 22 Figure 2.4 - Hybrid Gravity Type Offshore Platforms 24 Figure 2.5 - Arctic Platform Designs 27 Figure 2.6 - Proposed Deep-water Platforms 29 Figure 3.1 - Loads Acting on an Offshore Structure 32 Figure 3.2 - Plan of Survey Lines - Grid: Local Transverse , Mercator Spheroid 37 Figure 3.3 - Typical Soil Profile as Identified by Borehole, Cone Pentration Test and Gamma Ray Logging 43 Figure 3.4 - Comparison of Shear Strength Values from Sample Testing and from CPT 45 Figure 3.5 - Possible Failure Modes for an Offshore Gravity Structure Foundation 50 Figure 3.6 - Possible Modes of Sliding Failure 51 Figure 3.7 - Stability Diagram for a Raft Foundation 53 Figure 3.8 - Installation Sequence for a Gravity Platform .. 60 Figure 3.9 - Detail of CONDEEP Base Structure 61 Figure 3.10 - Maximum Dome Contact Pressures Observed During Installation of the "Beryl A" CONDEEP 64 Figure 4.1 - Detail of the Ekofisk Tank Bottom 69 Figure 4.2 - Loads on the Ekofisk Tank for the 100-Year Wave 71 vi i i Figure 4.3 - Design Storm Data for the Ekofisk Field 71 Figure 4.4 - Typical Geotechnical Profile from Ekofisk Field 72 Figure 4.5 - Shear Strength Data from Ekofisk 72 Figure 4.6 - Predicted Rocking Displacements for the Ekofisk Tank 76 Figure 4.7 - Load-Settlement Curve for Ekofisk Tank 76 Figure 4.8 - Ekofisk Settlement Data Relating Submerged Platform Weight and Storm Wave Data in the Early Months After Installation 79 Figure 4.9 - Settlement Data for Ekofisk Tank During Early Storms 79 Figure 4.10 - Location of Pressure Gauges and Piezometers Beneath Ekofisk Tank 82 Figure 4.11 - Pore Pressures Observed Under Ekofisk Tank During the First Major Storm 82 Figure 4.12 - Pore Pressure Rise per Cycle Observed in Undrained Simple Shear with Cyclic Loading for Samples Prepared with Relative Densities of 80% 85 Figure 4.13 - Theoretical Prediction of the Pore Water Pressure Distribution Beneath the Ekofisk Tank for Relative Densities of 77% and 85% .... 90 Figure 4.14 - Most Critical Failure Surface Found in Stability Analysis of Ekofisk Tank for Wave Loads Applied Under Undrained Conditions 92 Figure 5.1 - Regions of Validity for Various Wave Theories . 98 Figure 5.2 - Profile of an Airy Wave 99 Figure 5.3 - Forces Acting on the Foundation of an Offshore Gravity Structure 105 Figure 5.4 - Typical Design Storm Representation Used in Geotechnical Engineering 107 Figure 5.5 - Stress Path for a Foundation Element with Partial Drainage Subjected to Storm Wave Loading ill Figure 6.1 - Effective Stresses in Soil for Still Water Conditions (i.e. No Wave Loads) 118 Figure 6.2 - Definition Sketch of Effective Foundation 122 Figure 6.3 - Transformation of Loads, to Foundation Base ....123 Figure 6.4 - Theoretical Rupture Surface Geometry 129 Figure 6.5 - Comparison of Different Proposals for the Value of Nr 132 Figure 6.6 - Geometry of Rupture Surface Used for an Effective Stress Bearing Capacity Solution ....138 Figure 6.7 - Geometry of Sliding Body Used