Dissertation2009 Trevon

i

Universit degli Studi

di Pavia

Istituto Universitario di Studi Superiori

ROSE SCHOOL

ASSESSMENT OF KINEMATIC EFFECTS ON OFFSHORE PILED FOUNDATIONS

A Dissertation Submitted in Partial Fulfillment of the Requirements for the Master Degree in

ENGINEERING SEISMOLOGY

by

TREVON JOSEPH

Supervisor: Dr. C.G. Lai Co-Supervisor: Dr. M. Corigliano

February, 2009

Index

ii

The dissertation entitled Assessment of Kinematic Effects on Offshore Piled Foundations, by Trevon Joseph, has been approved in partial fulfillment of the requirements for the Master Degree in Engineering Seismology.

Dr. C. Lai _______ ___ Dr. M. Corigliano ___

Abstract

iii

ABSTRACT This document outlines the effects on offshore piles due to kinematic action. The study initiates by firstly identifying the most advanced methods in the industry of offshore pile design and then determining if these methods include the consideration of kinematic effects and the effects of its omission in design. The main engineering document that was determined for offshore design was that of the American Petroleum Institute code, the API-RP2A (2000). This code though, was found to only consider the inertial effects of seismic platform foundation design. In another code studied, the Eurocode, explicit mention is given for the cases of seismic foundation design in that piles are to be designed for both inertial and kinematic effects. The selected case study was that of the Mango Platform, situated offshore Trinidad, in the West Indies. Here a comparative analysis was conducted between the actual pile design method and the effects of the inclusion of kinematic action. The study has found that it is best to include kinematic effects, and in so doing both the inertial and the kinematic loads need to be combined. Natural frequencies would thereby play a major role in the determination of the final loading parameters. It has been found that in general, offshore platforms have a longer natural period than that of the supporting soil and as such the superposition of the loads will always be such that the natural period of the ground, Tg < Tb, the natural period of the structure. This being the case then the superposition of the internal loads will call for the square-root-sum-squares method (SRSS). Extensive use is made of the KEOPE program that was primarily designed for the study of piles subjected to kinematic effects and embedded in layered soil. This program makes use of the beam on a non-linear Winkler foundation (BNWF) approach that uses springs, dashpots and contact elements and models them in a finite element environment in a seismic free-field ground motion analysis. It has been determined that if these effects are not considered then the pile is under-designed. For the platform studied in this thesis, the stresses in the piles increased so that the maximum unity checks increased from 0.32 to 0.88, a 175% increase in stresses, with the inclusion of the kinematic loads. It is customary to design piles that will allow initially low values of unity checks so as to add extra levels of safety to the design and for the longevity of the platform. Keywords: offshore piles; kinematic action; natural period; SRSS; KEOPE; non-linear; BNWF.

Acknowledgements

iv

ACKNOWLEDGEMENTS Special thanks to:

- Dr. Carlo Lai (dissertation supervisor) and Dr. Mirko Corigliano (dissertation co-

supervisor) of the European Centre for Training and Research in Earthquake

Engineering (EU Centre), Italy

- Carlos Cedeo, Elisa DeFreitas and Riaz Khan of the Atkins Trinidad Ltd., Trinidad

- Keith Wilson and Antoinette Seaton-Clarke of British Petroleum of Trinidad and

Tobago (bpTT), Trinidad

- Frank Puskar Energo Engineering Ltd., Texas

- Bob Gilbert University of Texas, Texas

- Tommy Laurendine University of Houston, Texas

- Bob Bea University of California, Berkley, California

- Robert May WS Atkins Geotechnical, Epson , Surrey

- Marshal Pounds, Jacob Chacko, Amalia Giannakou Fugro, Houston, Texas

- My Family.

- God.

Index

v

TABLE OF CONTENTS

Page

ABSTRACT................................................................................................................................... iii

ACKNOWLEDGEMENTS........................................................................................................... iv

TABLE OF CONTENTS................................................................................................................ v

LIST OF FIGURES ..................................................................................................................... viii

LIST OF TABLES....................................................................................................................... xiii

MOTIVATION ............................................................................................................................. xv

1 INTRODUCTION .................................................................................................................. 1

2 GENERAL REVIEW OF OFFSHORE STRUCTURES ....................................................... 6

2.1 Platform Types................................................................................................................ 6

2.1.1 Jacket Type Platforms............................................................................................. 6

2.1.2 Tension Leg Platforms ............................................................................................ 8

2.1.3 Gravity Platforms.................................................................................................... 8

2.1.4 Jackups.................................................................................................................... 9

2.1.5 FPSOs .................................................................................................................... 9

2.2 Offshore Design Criteria............................................................................................... 11

2.2.1 Wind...................................................................................................................... 12

2.2.2 Wave Action ......................................................................................................... 13

2.2.3 Current Loading .................................................................................................... 15

2.3 Seabed Instability.......................................................................................................... 15

3 LITERATURE REVIEW OF DESIGN OF OFFSHORE PILED FOUNDATIONS .......... 18

Index

vi

3.1 Codes Seismic Design Criteria .................................................................................. 18

3.1.1 API-RP2A ............................................................................................................. 18

3.1.2 Eurocode/ISO........................................................................................................ 22

3.2 Piles............................................................................................................................... 28

3.2.1 Static Pile Analysis ............................................................................................... 28

3.2.2 Dynamic Pile Analysis.......................................................................................... 42

3.2.3 Carbonate Soils ..................................................................................................... 60

3.2.4 Grouted vs. Ungrouted Piles ................................................................................. 60

3.2.5 Comparison of API-RP2A with other Codes for Offshore Pile Design ............... 62

3.3 Engineering Seismology Aspects.................................................................................. 67

3.3.1 Seismic Analysis ................................................................................................... 67

3.3.2 Dynamic Soil Properties ....................................................................................... 74

3.4 Seismic Soil-Pile-Structure Interaction (SSPSI)........................................................... 75

3.4.1 BEM...................................................................................................................... 76

3.4.2 FEM ...................................................................................................................... 76

3.4.3 BNWF ................................................................................................................... 76

3.4.4 Thin layer element method ................................................................................... 81

3.5 Kinematic vs. Inertial Effects ....................................................................................... 85

3.5.1 Kinematic Effects.................................................................................................. 86

3.5.2 Inertial Effects....................................................................................................... 87

3.5.3 Superposition of these Effects .............................................................................. 87

3.5.4 Pile-to-pile Interaction .......................................................................................... 90

4 CASE STUDY MANGO PLATFORM OFFSHORE TRINIDAD................................... 91

4.1 Platform Loading .......................................................................................................... 93

4.2 Geotechnical Characterization ...................................................................................... 97

4.3 Program Used by Consultant SACS ........................................................................ 107

4.4 Foundation Design ...................................................................................................... 109

4.4.1 Capacity of Piles ................................................................................................. 109

4.4.2 Final Design ........................................................................................................ 110

Index

vii

4.5 Seismic Analysis ......................................................................................................... 111

4.5.1 Seismic Sources .................................................................................................. 111

4.5.2 Strength Level Earthquake.................................................................................. 114

4.5.3 Ductility Level Earthquake ................................................................................. 117

4.6 Comparison Program KEOPE ................................................................................. 120

4.6.1 Outline................................................................................................................. 120

4.6.2 Input .................................................................................................................... 122

4.6.3 Output ................................................................................................................. 134

4.6.4 Results................................................................................................................. 135

4.6.5 Combination of Results....................................................................................... 143

5 DISCUSSION OF RESULTS............................................................................................. 154

6 CONCLUSIONS................................................................................................................. 160

REFERENCES ........................................................................................................................... 162

Appendix..................................................................................................................................... 166

Index

viii

LIST OF FIGURES

Page

Figure 1-1. Map showing the locations worldwide of the locations of oil platforms [Dean,

(2006)].................................................................................................................................1

Figure 1-2. Map showing the locations of high seismicity worldwide,......................................2

Figure 1-3. Example Jacket Structures .......................................................................................3

Figure 2-1. Schematic of a jacket type platform with skirt piles being hammered into place

[Dean, (2006)].....................................................................................................................7

Figure 2-2. Topside module being placed on installed jacket [www.offshoreman.com]. ..........7

Figure 2-3 Typical examples of a Gravity-Type platform [Ehlers, (1982)]. ..............................8

Figure 2-4. Jackup platform in different stages of operation [McClelland et al, (1982)]...........9

Figure 2-5. FPSO type of offshore installation. [Dean, (2006)]. ..............................................10

Figure 2-6. Examples of designs for deeper water structures [Dean, (2006)]. .........................10

Figure 2-7. Loads on offshore platform foundations [Dean, (2008)]. ......................................11

Figure 2-8. Effect of wave loading and layered soil movement on ..........................................16

Figure 2-9. Effect of wave loading and layered soil movement on pile ...................................17

Figure 3-1. Standardized seismic acceleration spectrum for 5% damping [API-RP2A]..........21

Figure 3-2. Standardized seismic acceleration spectrum for 5% damping [ISO 19901-2] ......24

Figure 3-3. Defined attenuation curves from site specific study ..............................................24

Figure 3-4. Mean uniform hazard spectral accelerations [ISO 19901-2]. ................................25

Figure 3-5. Uniform Hazard Spectrum [ISO 19901-2].............................................................25

Figure 3-6. Derivation of the slope aR of the seismic hazard curve for T=Tdom [ISO 19901-2].

...........................................................................................................................................26

Index

ix

Figure 3-7. Derivation of spectral accelerations and probabilities for ALE and ELE [ISO

19901-2]. ...........................................................................................................................27

Figure 3-8. Schematic of a Laterally Loaded Pile Response, [Randolph et al, (2005)]. ..........33

Figure 3-9. Schematic of a Laterally Loaded Pile Response, [Randolph et al, (2005)]. ..........34

Figure 3-10. Damage to pile by ground displacement, [Niigata, (1964)].................................35

Figure 3-11. Bridge with failed approaches but with ...............................................................35

Figure 3-12. Distortion of pile by foundation by lateral soil displacement, [Finn & Thavaraj,

2001]. ................................................................................................................................36

Figure 3-13. Pilesoil relative displacements in fully liquefiable soil [MCEER/ATC, (2003)]

...........................................................................................................................................37

Figure 3-14. Pilesoil displacements with non-liquefiable crust [after MCEER/ATC (2003)].

...........................................................................................................................................37

Figure 3-15. Maximum moments along the pile; pile head free to rotate [Finn (1999)]..........37

Figure 3-16. Displacements of pile and free field in liquefiable soil, [Finn (1999)]. ...............38

Figure 3-17. Displacements of pile and free field with non-liquefiable soil above liquefiable

layer, [Finn (1999)]. ..........................................................................................................38

Figure 3-18. Bending-Moments of pile and free field in liquefiable soil, [Finn (1999)]. ........39

Figure 3-19. Bending-Moments of pile and free field with non-liquefiable soil above

liquefiable layer, [Finn (1999)].........................................................................................39

Figure 3-20. External area used in the calculation of skin-friction...........................................42

Figure 3-21. Recommended t-z curves, notice that the peak shear stress for clay ...................43

Figure 3-22. Recommended q-z curve, notice that the peak shear stress for clay, [API-RP2A].

...........................................................................................................................................44

Figure 3-23. The p-y curves for (a) soft clays (b) stiff clays (c) sand layers, [El Naggar et al.

(2005)]...............................................................................................................................46

Figure 3-24. Schematic of Dynamic p-y Analysis Model [Boulanger et al. 1999] ..................47

Figure 3-25. How piles are to be analyzed, [El Naggar & Bentley, (2000)]. ...........................48

Figure 3-26. Determination of stiffness from a generated static p-y curve to result ................50

Figure 3-27. Trend of decreasing stiffness with increased displacement, [El Naggar &

Bentley, (2000)]. ...............................................................................................................53

Figure 3-28. Comparison between [a] Analytical model and the use of static p-y data ...........53

Figure 3-29. p-y curves for single and a pile in a group. ..........................................................54

Figure 3-30. t-z curves for single and a pile in a group. ...........................................................54

Index

x

Figure 3-31. Dynamic Winkler computational .........................................................................55

Figure 3-32. The load-displacement curves for a 12-pile group, Mostafa and Naggar [2002].57

Figure 3-33. Quasi-3D model of soil-pile response, [Finn 1999].............................................59

Figure 3-34. Positions of connection types for grouted and ungrouted scenarios [adapted from

Honarvar et al, (2005)]......................................................................................................60

Figure 3-35. Frame strength and stiffness degradation in working load cycles [Honarvar et al,

(2008)]...............................................................................................................................62

Figure 3-36. Bending-Moments of pile and free field with non-liquefiable soil above

liquefiable layer, [Lehane et al. (2005a)]..........................................................................65

Figure 3-37. Simplified model of transmission of earthquake hypocenter to an offshore

platform [Dean, (2007)]. ...................................................................................................67

Figure 3-38. Probability Curve displaying effect on PS V for a 0.4sec period, [Crouse (1992)].

...........................................................................................................................................70

Figure 3-39. Probability Curve displaying effect on PSV for a 4.0sec period, [Crouse (1992)].

...........................................................................................................................................71

Figure 3-40. PSHA primary results, [Crouse (1992)]...............................................................72

Figure 3-41. PSHA final result, [Crouse (1992)]......................................................................72

Figure 3-42. Computed 1971 Holiday Inn Platform ..............................................................73

Figure 3-43. Typical cyclic response for a ductile soil, [Dean, (2008)]. ..................................75

Figure 3-44. Typical variations of shear modulus ratio G/Gmax and damping..........................75

Figure 3-45. Schematic of the beam on a non-linear Winkler foundation model for ...............77

Figure 3-46. Stress-Strain model used by Iwan [1976] and Mroz [1967]. ...............................80

Figure 3-47. Displacement response of a vertical wall in submerged soil, [Nogami & Kazama

(2001)]...............................................................................................................................82

Figure 3-48. The effect of water level on amplitude and frequency on a soil column, [Nogami

& Kazama (2001)]. ...........................................................................................................82

Figure 3-49. The effect of fluid on the Love-waves in a soil column [Nogami and Kazama,

(2001)]...............................................................................................................................83

Figure 3-50. Components of inertial and kinematic effects, [Ceci et Forcolin, (2007)]...........86

Figure 3-51. Combination of inertial and kinematic effects .....................................................89

Figure 3-52. Schematic of the pile-to-pile interaction between piles and the wave interference

through the differing layers from a head loaded active pile to the passive pile [Mylonakis

et al. (1997)]......................................................................................................................90

Index

xi

Figure 4-1. Model of Mango Platform, [MSL (2007)]. ............................................................93

Figure 4-2. Wave Directions for operating condition and ........................................................96

Figure 4-3. Wave Directions for the Operating retroflecting current condition .......................97

Figure 4-4. Undrained Shear Strength, [MSL (2007)]..............................................................98

Figure 4-5. Unit Weight Profile, [MSL (2007)]. ......................................................................98

Figure 4-6. Undrained Shear Strength, [MSL (2007)]..............................................................99

Figure 4-7. Unit Weight Profile, [MSL (2007)]. ......................................................................99

Figure 4-8. Borehole-Log at the Mango Platform Site, [Capital Signal, (2005)]...................101

Figure 4-9. Terms and Symbols used in the Borehole-Log for the ........................................102

Figure 4-10. Relationship between N-value and S- wave velocity, [Inazaki, (2006)]............104

Figure 4-11. Comparison of the Ohta and Goto (1978) and the Inazaki (2006).....................105

Figure 4-12. Relationship between P-wave and S-wave velocity, [Miller & Stewart, (1991)].

.........................................................................................................................................106

Figure 4-13. The SACS System [EDI, (2007)].......................................................................109

Figure 4-14. Ultimate Pile Capacity for a 48in (1.2m) open pile as per the API-RP2A [MSL,

(2007)].............................................................................................................................110

Figure 4-15. Platform orientation and pile batter, [MSL (2007)]. ..........................................111

Figure 4-16. Present day plate structure of the Caribbean region. Directions and .................112

Figure 4-17. Reference vs. Soil Amplified Uniform Response Spectra for ...........................114

Figure 4-18. Seismic Response Spectra for SLE, [MSL (2007)]. ..........................................115

Figure 4-19. Seismic Response Spectra for DLE, [MSL, (2007)]..........................................118

Figure 4-20. USFOS model of Mango Platform, [MSL (2007)]. ...........................................119

Figure 4-21. Details of beam element, [Ceci & Forcolin (2007)]. .........................................121

Figure 4-22. Discretized model of pile in foundation, [Ceci & Forcolin (2007)]. .................121

Figure 4-23. Interface element of pile to soil, [Ceci & Forcolin (2007)]. ..............................121

Figure 4-24. Horizontal and Vertical Response Spectrum for the Strength Level Earthquake

.........................................................................................................................................122

Figure 4-25 Dataset 1..............................................................................................................123

Figure 4-26. Dataset 2.............................................................................................................123

Figure 4-27. Dataset 3.............................................................................................................123

Figure 4-28. Dataset 4.............................................................................................................123

Figure 4-29. Dataset 5.............................................................................................................124

Figure 4-30. Dataset 6.............................................................................................................124

Index

xii

Figure 4-31. Dataset 7.............................................................................................................124

Figure 4-32. Seismic input parameters window of the KEOPE Program...............................125

Figure 4-33. Site characterization input into KEOPE Program..............................................128

Figure 4-34. Shear Modulus and damping ratio for clay, [Ceci et al, (2006)]........................130

Figure 4-35. Shear Modulus and damping ratio for sand, [Ceci et al, (2006)].......................130

Figure 4-36. Shear Modulus and damping ratio for clay, [Ceci et al, (2006)]........................131

Figure 4-37. Pile characterization input into KEOPE Program..............................................134

Figure 4-38. Main Window of the KEOPE Program after running the analysis. ...................135

Figure 4-39. Bending Moment vs. Depth for Inertial Loading...............................................139

Figure 4-40. Shear Force vs. Depth for Inertial Loading........................................................139

Figure 4-41. Axial Force vs. Depth for Inertial Loading........................................................139

Figure 4-42. Determined internal pile forces, bending moment vs. depth .............................142

Figure 4-43. Determined internal pile forces, shear force vs. depth.......................................142

Figure 4-44. Transfer Functions from Linear and Non-Linear Approaches...........................143

Figure 4-45. Combined internal pile forces - bending moment, for the SPA pile group from

the kinematic analysis. ....................................................................................................146

Figure 4-46. Combined internal pile forces shear force, for the SPA pile group from the

kinematic analysis. ..........................................................................................................146

Figure 4-47. Combined internal pile forces - bending moment, for the SPB pile group from

the kinematic analysis. ....................................................................................................148

Figure 4-48. Combined internal pile forces shear force, for the SPB pile group from the

kinematic analysis. ..........................................................................................................148

Figure 4-49. Comparison of the new and old bending stresses. .............................................152

Figure 4-50. Comparison of the new and old shear stresses...................................................152

Figure 4-51. Comparison of the new and old axial stresses. ..................................................152

Figure 4-52. Comparison of the new and old combined stresses. ..........................................152

Figure 4-53. Comparison of the new and old bending stresses unity checks. ........................153

Figure 4-54. Comparison of the new and old shear stresses unity checks..............................153

Figure 4-55. Comparison of the new and old axial stresses unity checks. .............................153

Figure 4-56. Comparison of the new and old combined stresses unity checks. .....................153

Index

xiii

LIST OF TABLES

Page

Table 3-1 Determination of exposure level [API-RP2A] ........................................................ 20

Table 3-2. Determination of exposure level [ISO 19901-2] .................................................... 22

Table 3-3. Site Seismic Zone [ISO 19901-2]........................................................................... 23

Table 3-4. Seismic Risk Category [ISO 19901-2] ................................................................... 23

Table 3-5. Target annual probability of failure [ISO 19901-2]. .............................................. 25

Table 3-6. Correction Factor, Cc [ISO 19901-2]. .................................................................... 26

Table 3-7. Representative values of seismic reserve capacity, Cr [ISO 19901-2]................... 27

Table 3-8 Pile Design Parameters [ISO 19901-2, (2004)]....................................................... 29

Table 3-9 p-y curves for short-term static load [API-RP2A]................................................... 45

Table 3-10 Dynamic p-y curve parameter constants for a range of soil types......................... 52

Table 4-1. Mango Platform Platform Weights, [MSL (2007)]. ............................................ 92

Table 4-2. Operating conditions, [MSL (2007)]. ..................................................................... 94

Table 4-3. 100-year return period storm condition, [MSL (2007)]. ........................................ 94

Table 4-4. Operating retroflecting current condition, [MSL (2007)]. ..................................... 95

Table 4-5. 100-year return period retroflecting current condition, [MSL (2007)]. ................. 95

Table 4-6. 1000-Year Return Period Storm Condition. ........................................................... 95

Table 4-7 Mango Platform - Summary Soil Stratigraphy, [MSL (2007)]. .............................. 97

Table 4-8 Calculation Tables for Ohta and Goto method for Shear Wave Velocity of soil. . 103

Table 4-9 Values of Vs and Vp for the Mango Platform Site................................................. 107

Table 4-10. Ultimate Pile Capacity for a 48in (1.2m) open pile as per the API-RP2A [MSL,

(2007)]............................................................................................................................ 110

Index

xiv

Table 4-11. Ordinates of the SLE spectrum, [MSL (2007)]. ................................................. 115

Table 4-12. Load Combination for pile checks during seismic analysis, [MSL, (2007)]...... 116

Table 4-13. Pile unity check ratios, [MSL, (2007)]. .............................................................. 116

Table 4-14. Pile factor of safety, [MSL, (2007)]. .................................................................. 117

Table 4-15. Ordinates of the DLE spectrum, [MSL, (2007)]. ............................................... 117

Table 4-16. Load Factor Results, [MSL (2007)]. .................................................................. 120

Table 4-17. Time-History Responses compatible with the Reference Spectrum. ................. 123

Table 4-18. Input Parameters of Site Characterization for the KEOPE Program.................. 127

Table 4-19. Pile Input Parameters for the KEOPE Program.................................................. 133

Table 4-20. Inertial Pile Forces SPA [MSL Engineering, (2007)] ..................................... 137

Table 4-21. Inertial Pile Forces SPB [MSL Engineering, (2007)] ..................................... 138

Table 4-22. Internal pile forces determined from the KEOPE Program for the seven datasets.

........................................................................................................................................ 141

Table 4-23. Internal pile forces determined from the KEOPE Program for the seven datasets

........................................................................................................................................ 145

Table 4-24. Internal pile forces determined from the KEOPE Program for the seven datasets

........................................................................................................................................ 147

Table 4-25. Original set of maximum pile stresses from the inertial analysis....................... 150

Table 4-26. New set of maximum pile stresses after combination from the kinematic analysis.

........................................................................................................................................ 151

Motivation

xv

MOTIVATION

As growth and development continue new sources of energy need to be found and the trend for the

last number of years is to look beyond the shores for fossil fuels. Within this new region though

some different and in some cases larger forces arise compared to onshore forces and these must be

designed against. These offshore forces include wave loading, currents, boat impacts, seismic forces

and wind. Under the category of offshore installations are artificial islands, oil platform jackets,

compliant towers, tension leg platforms and floating production vessels, these are primarily

associated with the search and production of hydrocarbons. Other offshore structures include wind

and wave turbines.

This paper shall specifically focus on the seismic design of offshore piled foundations looking at the

most up-to-date methods, considering all the forces that impact on these piles and how they are

accounted for in design. For the seismic design of offshore structures this study shall focus on the

effects of kinematic and inertial loading on the piles to these structures. Kinematic effects are as a

result of seismic soil loading on the piles. Inertial effects result from seismic action on the platform

and the loading of the platform on the foundation piles and the interaction between the piles and the

soil. A brief focus shall also be on the other types of loading that are used in offshore design.

In many instances, especially for those platforms that are to be placed in regions that are not

seismically active, storm loadings are the critical loads. It has been acknowledged though that

seismic loading is considered to be one of the most critical loading and the effects of seismicity are

of paramount importance. Sub-soil layer differential motion increases the stresses in the piles.

Structural damage is caused by the vertically propagating shear-waves travelling along the platform

and cause increased stress to the platform joints and members.

Index

xvi

There has been much emphasis on platform design for wave action, but seismic action is also a load

to be considered. It is true though that most of the failures that occur offshore are as a result of

extreme storm conditions especially in the Gulf of Mexico, USA and in the North Sea. But in

regions such as offshore California, USA, and in the Caribbean Sea, which are seismic regions it is

mandatory that seismic loads are considered in design. It is thought by the author though, that not

enough emphasis is placed on this load in offshore design worldwide and in the event of failure by

these loads the environmental effects can be devastating. As such, a greater understanding is sought

of the seismic phenomenon and its impact on platforms. Also, one of the main leaders in platform

design recommendations, the API-RP2A, does not include considerations for the kinematic effects

in piles from seismic loading and the effects of such an omission, on a specific case will be studied.

Chapter 1: Introduction

1

1 INTRODUCTION

This study of offshore, piles and seismicity begins by looking into what is the offshore industry and

where this industry is prominent. It begins by an introduction into the locations worldwide where

offshore installations exist. From Figure 1-1, these regions can be seen to span the entire globe

along the borders of each continent. The most prominent locations are California and the Gulf of

Mexico in the United States, Mexico, Columbia, Venezuela, Trinidad and Tobago, Brazil, the North

Sea, Nigeria, the United Arab Emirates, Indonesia, Russia and Australia.

Figure 1-1. Map showing the locations worldwide of the locations of oil platforms [Dean, (2006)].

Figure 1-2 provides a map of the regions of the globe that are prone to seismic action. Overlaying

the two maps it can be intuitively concluded that those regions of the world where there exists

offshore installations in seismic areas, then these platforms are to be designed to resist earthquake

forces.


2

Figure 1-2. Map showing the locations of high seismicity worldwide,

the dots represent the locations of seismic events, [Bolt, (1988)].

The offshore structures themselves vary depending on purpose and location. The various types

range from simple jacket structures to large floating, production, storage and operating vessels.

These are to be designed to be as efficient and environmentally safe as possible. These shall all be

expanded on in later sections.

With a focus on offshore piles, these vary in strength, configuration and lengths. They vary in type

but are basically designed to be either compression piles or tension piles. In the jacket type

platform, its simplest configuration, a pile is hammered through each of its four legs. For larger

platforms additional skirt piles may be used around each of the legs through sleeves that are

attached to the legs.

As an example, in Figure 1-3, the North Rankine A Platform in the North-Western side of Australia

lies in 125m of water and has 8 piles at the base of each of its legs and its flare boom is supported

by a tripod type jacket with 2 piles per corner. The flare boom support structure also has additional

guys and box anchors that were added to compensate for the low shaft capacity of the driven piles

[Randolph, M et al. 2005].


3

These piles are designed to support not only the dead weight of the structure but, to an extent, also

the significant environmental loads to which these platforms are subjected relative to their onshore

counterparts. It was deemed economically unfeasible to design these piles to be depended on solely

to take the environmental loading which are normally cyclic in nature. And as such the guys were

installed to aid in this effort.

Figure 1-3. Example Jacket Structures the North Rankine A Platform [Randolph, M et al. 2005]

The objective of seismic design is to seek a balance between the cost and efficiency of the structure

in dealing with earthquakes. From the perspective of the structural engineer, he is able to design a

structure that will be impervious to damage from a large earthquake but as this is not efficient one

has to consider the small probability of occurrence of these phenomenon and factor this probability

into the design. He cannot also design a structure without considering the fact that an earthquake

can or cannot occur within the platforms life, the value of the structure, the effect on the

environment and the financial loss if failure was to occur.


4

Now the code from the American Petroleum Institute [API-RP2A] is to be considered the industry

leader in offshore design recommendations. The last published code was the API-RP2A

Recommended Practice for the Planning, Designing and Constructing Fixed Offshore Platforms

Working Stress Design [API-RP2A] which was published in 2000. Quite a bit of research has been

done over the last 8 years but the API-RP2A has not yet been updated. Newer codes have been

published that focus on offshore design such as the ISO and the Eurocodes. Other recommendations

have been published by such bodies as Fugro International, the University of Western Australia, the

Norwegian Geotechnical Institute and Imperial College, but these may focus on one aspect of

platform design instead of the entire platform as does the API-RP2A, ISO and Eurocodes. This

latter group also focuses on the design basis of platform that includes the procedures that are to be

followed to ensure proper design and safety requirements are met. These outlines are very vast and

thorough and cover all the areas of design that would apply to offshore. Some areas are more

extensively covered than others and it depends on the code. Specifically referring to that which

would affect the foundation design coupled with seismic factors would be the main focus of this

thesis. Also, many researchers worldwide have contributed to the development of this area of

engineering and their work is included in this thesis.

As a result of seismic action, which can be considered to be the propagation of S-waves, differing

phenomenon occurs that would directly affect a platform. Among these are soil liquefaction,

platform induced vibration, submarine slides and tsunamis. The location of the platforms with

respect to the location of faults should also be investigated along with the type of ground motion

that is to be expected. For seismically active regions a thorough site-specific investigation is be

done for each site and from this the expected value of seismic risk can be evaluated.

This study shall be initiated by an overview of the current key elements of platform design with

specific reference to the platform loads that will eventually be transferred to the plies and

effectively the soil. Efforts were made to have this literature review based on a number of recent

publications and codes some of which have not yet found themselves into offshore design offices

and some of which are the more popular methods of foundation design. With respect to proximity,

offshore piles can be designed, as in onshore piles as being single or group piles. Group piles are

normally those that find themselves at the bases of the main legs of the jacket type platform in

groups of skirt piles. Single pile design is normally attributed to the individual piles that are driven

through the main legs of the platforms.


5

This study shall be broken a series of sections. The first of these shall be a general review of the

different types of offshore structures. The next shall be that of a literature review focused on the

different codes and recommendations of offshore pile design. Introduced here shall be the

guidelines set out by the American code the API-RP2A and the Eurocode/ISO code and a

comparison is made here of their different methods of hazard determination. The engineering

seismology aspects of the detailed seismic hazard assessment shall also be expanded upon. Static

and dynamic design of single and group piles, carbonate soils and how they impact on pile design,

and the effects of grouting on piles, shall also be discussed.

Mention is made of the very important aspect of soil-structure-interaction. A selection of the

various types of pile design shall be discussed with a focus on the main methods of SSI methods

that are used to model seismic response. These shall be the direct approaches of boundary element

method (BEM) and the finite element method (FEM). The other two that will be discussed are those

of the beam on a non-linear Winkler foundation (BNWF) and the thin-layered method. These latter

two are most used in the industry and as such shall be expanded upon considerably.

The effects of kinematic loading shall be discussed in the next section. This type of loading is as a

result of seismicity on platform piles. This is a dynamic analysis and can only be conducted by way

of a time-history analysis. This is a separate analysis to that of the inertial loading. Inertial loading

results from the loading on the piles from the motion of the superstructure due to seismic action.

This is usually conducted by way of a response spectrum analysis and is classified as a static

analysis. The effects of kinematic action shall be looked into with great detail for this loading type

has been found to be ignored in many offshore designs causing the possible under-design of piles.

The difficulty lies in the determination of accurate time-histories that would be applicable to the

site.

The next chapter of this study shall focus on the case study of the Mango Platform off the South-

eastern coast of Trinidad in the West Indies. Here an investigation into the seismic design is done

specifically with respect to the consideration of the kinematic effects on the platform and the effects

of the omission of loading. Consideration is given here to the recommendations of Eurocode 8: Part

5, Section 5.4.2. which specifically indicates that both kinematic and inertial loadings are to be

conducted on piles, but this is not indicated in the API-RP2A. A comparison of the results of

omitting this loading will be performed and its effects on the integrity of the platform.

Chapter 2: GENERAL REVIEW OF OFFSHORE STRUCTURES

6

2 GENERAL REVIEW OF OFFSHORE STRUCTURES

This chapter is designated to introduce the reader to the realm of offshore structures, their types and

functions. These offshore installations vary in type as they would each be designed on a basis of

purpose and location. Also to be looked at is the basis of design of these platforms. The seismic

design bases are dealt with in detail later section 3.1 with the analysis and comparison of the

applicable design codes. Here the various types of offshore platforms shall be looked at and their

structural design requirements shall be outlined. Also, as the main focus of the thesis shall be on the

seismic design of platforms with a specific focus on offshore piles primarily for jacket type

structures.

2.1 Platform Types

2.1.1 Jacket Type Platforms

These types of platforms are the most basic of types. These are called jacket type platforms as they

require that the actual platform be held in place (pinned) to the ocean floor by piles that are driven

from the surface of the water through the legs of the structure itself. The actual jacket itself after

approved design is constructed onshore and transported to the offshore site where it is then carefully

placed on a prepared seafloor location. The piles are then hammered into place. This platform type

is typical for smaller operations, but as the scope increases, so does the platform size. Larger

platforms require greater support from the ocean floor and at times it may be more economical to

introduce a series of pile groups at the base of the platforms to foster this. These piles are termed

skirt piles and are positioned from the surface by way of an extended hammer length as shown in

Figure 2-1. Upon proper leveling of the platform jacket structure the topside portion is placed atop

by the use of some of the worlds largest cranes positioned on large barges as depicted in Figure

2-2.


7

Figure 2-1. Schematic of a jacket type platform with skirt piles being hammered into place [Dean, (2006)].

Figure 2-2. Topside module being placed on installed jacket [www.offshoreman.com].


8

2.1.2 Tension Leg Platforms

Tension-leg platforms are those which consist of a buoyant hull which is attached to large diameter

piles [e.g. 3m in some cases] by steel pipes or tendons. These piles are permanently in tension and

calls for difficulty in design as their initial embedment depths are constantly being reduced and

hence their capacities. Also these piles are subjected to strain softening which is caused by the

smoothening of the soil during shearing resulting in reduced resistance during continued shearing.

This process continues as the platform is continuously being subjected to cyclic environmental

loads, namely wave loading, which would escalate during earthquake loading. An example of this

type of platform can be seen in Figure 2-6.

2.1.3 Gravity Platforms

These platform types rely on their weight to counteract the effects of design loading. These

platforms consist of a large base that is usually used as a storage area for extracted hydrocarbons.

These platforms are relatively bulky and require a large amount of concrete to be constructed. These

are very prominent in the colder climates where ice loading plays a major factor in design (Figure

2-3).

Figure 2-3 Typical examples of a Gravity-Type platform [Ehlers, (1982)].


9

2.1.4 Jackups

These are the type of platforms that are used in situations where temporary installations are

required. This type of platform can also be used in cases of drilling where the original structure is

unable to support the total additional loading that comes with drilling. This may be due to the

deterioration of the platform over time. It may be seen in some instances that the temporary use of a

jackup may be more cost effective than the remedial works to the platform to bring it to a state of

readiness for drilling. This type of platform is also used in cases where drilling to find oil is

necessary and no permanent structure is needed. These platform types are quite mobile and exist in

different stages of operation. While being transported, usually via tow, its legs are in the air. Upon

reaching the site it is necessary to lower the legs to the seabed and then loads are applied to the legs

to position them to the necessary depth. This stage is called preloading. The preload is then

removed once the required depth is achieved and then drilling commences. A pictorial

representation of the different operations can be viewed in Figure 2-4.

Figure 2-4. Jackup platform in different stages of operation [McClelland et al, (1982)].

2.1.5 FPSOs

These Floating, Production, Storage and Operating type platforms are specifically designed for

deepwater installations. These consist of a large boat that is fixed to a specific location offshore and

held in place by anchors. The FPSO is capable of drilling for hydrocarbons, processing the

extractions and the storage of the different hydrocarbon levels before transportation to an onshore

facility, (see Figure 2-5).


10

Figure 2-5. FPSO type of offshore installation. [Dean, (2006)].

The water depth on a location offshore where hydrocarbons are found plays a significant role in

determining the type of platform to be installed. Figure 2-6 gives an idea of the typical depth and

examples of the different types of platforms that can be used.

Figure 2-6. Examples of designs for deeper water structures [Dean, (2006)].


11

2.2 Offshore Design Criteria

There are various types of loading to which offshore installations are normally designed to

withstand in addition to seismic loading. These are incident on the installation generally as a

combination of the different loads that are of greater magnitude and complexity than onshore

structures, see Figure 2-7. The main supporting element of these are eventually the foundations, and

as such usually have to withstand the vertical loads from:

the self-weight of the structure - V any associated moment with the eccentric loading of the platform - MV lateral loads on platforms due to wind and currents LB & LC, the moment that is associated with these lateral loadings MB & MC cyclic loading due to waves - LW cyclic moment due to waves - MW seismic loads E

Figure 2-7. Loads on offshore platform foundations [Dean, (2008)].

These loads are predominantly resisted by the soil-pile-structure interaction. The loads are

transferred to the foundation through the load path for effective loading situations, although it is

understood that in limit state design there will be the presence of redundant members to cater for

emergencies, these loads eventually arise as tension, compression, bending moments, torsion and

shear forces in the piles. The capacities of the piles are determined from rational equations that have

been developed by researchers and the most popular of these are prescribed in the American


12

Petroleum Institute Recommended Practice for Planning, Designing and Constructing Fixed

Offshore Platforms Working Stress Design, 21st Edition, December 2000, hereby referred to as

the API-RP2A.

There is great complexity in the application of wind, wave and current loads on a platform. This

complexity is due to the incidence of these loads not all directly along a single path of the platform.

As such, these are applied in many different angles specified by a metocean study for a specific site.

These angles are then applied relative to True North. To aid in the understanding of the angles of

loading for these platforms for these loads, reference is made to Figure 4-2 and Figure 4-3 which act

as schematics that are used to determine these angles. The metocean criteria for the site would

normally indicate the angular range from True North that the load is to be applied and usually four

angles within this range are applied for the specific load type. For the case of wind, a wind speed at

a certain height is usually found and applied at the different angles. For the case of waves a specific

wave height is stipulated and for the case of current loading, the speeds along a current profile is

given. In addition to this, for the different loading cases for the different operating cases, these wind

speeds would vary. These operating cases are usually: the normal operating case; the 100-year

design storm case; the 100-year extreme storm case; and the 10-year extreme storm case.

2.2.1 Wind

Wind loading is impacted upon the various components of the platform that would include the

members, the equipment, the facilities etc. These winds include steady forces and gust forces that

are to be rationally applied to the structure such as being made to act at a specific height and at a

specific duration such as one hour [API-RP2A]. It is to be noted that for conventional steel

structures the wind forces contribute usually less than 10% of the total global load. But in deeper

waters where compliant structures are found, it is found that the waves contribute a much larger

percentage. This is especially the case where the frequency of the wind is near to the natural

frequency of the platform so as to cause resonance. The wind force as determined by API-RP2A is

calculated by the following relationship:

(2.1)

where,

F = wind force [N],


13

= mass density of air [1.2kg/m],

u = wind speed [m/s],

CS = Shape coefficient,

A = area [m]

Now this force, LB from Figure 2-7, is assumed to act at a certain height above the water level and

in so doing creates a moment [MB] that is transferred to the piles.

2.2.2 Wave Action

Wave loading plays a large role in the design of platforms and is normally, in non-seismic zones,

the critical design load. It is difficult to determine the exact result of wave loading due to the

extreme randomness of the phenomenon. It is known that waves can be incident on a platform from

all directions especially during design storms. In addition to this, waves also affect the soil at the

base of the platforms especially if it is loose to medium dense material. The waves also contribute

to the creation of vortices around platform legs and at their base where it leads to scouring which

reduces the capacity of the pile unless it is designed against such.

Now the strength of a wave is calculated by its height, which is measured from the crest to the

trough. The waves impose a cyclic and buoyant force on the platforms and these are to be resisted

by the foundation piles. The effect of these waves on the platform is determined by the use of the

Morrisons equation:

(2.2)

where

F = hydrodynamic force vector per unit length acting normal to the axis of the member [N/m],

FD = drag force per unit length of the member [N/m],

FI = inertial force per unit length [N/m],

Cd = drag coefficient,

w = density of water,

U = component of velocity vector due to wave [m/s],

|U| = absolute value of U [m/sec],


14

Cm = inertia coefficient,

=t

U component of local acceleration vector of the water.

The wave crest should be positioned in such a manner, where upon incidence on the platform will

achieve the maximum overturning moments and shear forces. The stresses in members are due to

local hydrodynamic loadings and the stresses transferred from the other areas of the structure. Local

hydrodynamic loading not only includes the drag and inertia forces from the dynamic action of the

waves but also lift forces, axial Froude-Krylov forces [forces induced by the unsteady pressure field

generated by undisturbed waves], buoyancy and weight [API-RP2A]. Also included in this list is

that for members that are at the mean storm water level, will also be impacted by vertical slam

forces from wave action. These slam and buoyancy forces have a great impact on stresses. These

forces would all be supported by the piles.

In wave studies that have been done by Sumer et al. [1992], it has been proven that the effects of lee

wake vortices and the horseshoe vortex are primarily responsible for local scouring at the base of

platform legs. Further research [Carreiras et al. (2000)] has led to the upgrade of the initial

relationship. It has been determined that the depth of local scouring is related to the Keulegan-

Carpenter number, which is a function of the stroke of the horizontal excursion, 2 and the diameter

of the pile, D.

KC = D2

(2.3)

This parameter is then included in the equation for the local scour:

( )[ ]606.013.1 = KCeDS (2.4)

where,

S = the depth of local scour,

D = diameter of the pile.


15

Now, if the bearing stress of the pile weight coupled with the additional loading exceeds the bearing

stress of the soil below the tip and the shaft friction along the pile then the pile would move

downwards until a new bearing capacity is found that would be make greater the support forces. As

such the removal of soil around the perimeter of the pile may induce settlement. Also determined

from the study was that the roughness of the legs increased the level of scour.

2.2.3 Current Loading

Current loading is the vector sum of tidal currents, the circulatory currents and the storm generated

currents. The magnitude of these is most dependent on the location of the platform for the strengths

of the vectors increase with proximity to the shoreline. In platform design, the strength and

direction and profile of the current is important also for the consideration of deposits of inland and

oceanic material, and for the placement of berthing and docking equipment for boats. There is also

current loading that is associated with wave loading. To calculate the current force on members, for

no wave conditions, we use Equation 3.1 with 0=tu .

2.3 Seabed Instability

Seabed instability refers to the movement of the layers of the seabed due to the effect of the wave

pressures, earthquakes, soil self-weight, hydrates, shallow gas, faults and other geologic processes.

Under-compacted soils can be subjected to lateral movement during earthquakes and also to wave-

induced motion if wave loading is strong. This movement affects the platforms foundation by

adding additional stresses to the leg and pile members. Rapid sedimentation is to also be looked at

in the consideration of the seabed for pile and platform design as this would influence capacity.

Geological studies that employ historical rates of deposition are to be looked at to determine the rate

of loading. The effects are also greater if the adhesion between layers is low and the displacement

of the platform increases dramatically when the thickness of the sliding layer increases. Earthquake

induced forces can induce failure on seafloor slopes that are otherwise stable under the current self-

weight and wave loading [API-RP2A].

The effect of seabed instability is more critical to the effect of platform base and the pile response

along the shaft than the metocean parameters of severe winds, waves and currents on the platform.


16

These latter parameters are more critical to platform member response and developed stresses. The

combination of metocean and seabed instability is of critical importance to foundation design. This

is evident in Figure 2-8 where a prescribed soil movement of 0.5m is analyzed separately and

together with metocean loading and the results compared, on the effect on the top nodal response.

As can be seen also, for the case of the metocean loading only, the displacement is greater than for

seabed instability only. The combination of the loads leads to the most critical loading.

Figure 2-8. Effect of wave loading and layered soil movement on

top nodal tower response, [Mostapha and Naggar, (2005)].

Reports were published [Mostapha and Naggar, (2005)] on tests that were carried out to determine

the effects of seabed instability. Using dynamic soil parameters, the soil resistance was modeled and

the effect on platforms was evaluated. It was determined that the thickness of the sliding layer is

more critical than the non-linear pushover analysis established displacements. Also that the correct

bending moment calculated along a pile and the displacement that the pile head achieves is largely

dependent on the end fixity and the inclusion of the vertical loading in the pile.

Figure 2-9 displays that the most crucial loading is also that of the combination of both seabed

instability and metocean loading for both the bending moment and displacements of the platform

base. Also, as seen in this case, the larger loading is observed for the seabed instability than for

metocean. These prescribed instabilities are understood to be that of assumed displacements during

earthquakes.

When the thickness of the soil layer is relatively small [limiting thickness], then the failure of piles

is more by flow failure, where the soil flows pass the pile. But when the soil layer is large long-

pile failure mode is more dominant, where the maximum bending moment in the pile reaches the


17

yield moment of the pile before flow failure, translation of the pile or its rotation begins. The lateral

displacement of a layer of soil induces bending moments below the thicknesses of the mobile layers

and the locations of these maximum moments are critical in pile design as they would determine the

thickness of piles at these positions. It is common that in offshore design of piles and members,

much effort is undertaken to produce the most efficient design. This is due to issues of weight,

installation and transportation that all affect the cost of these structures.

(a) (b)

Figure 2-9. Effect of wave loading and layered soil movement on pile

(a) displacement (b) bending moment, [Mostapha and Naggar, (2005)].

Chapter 3: LITERATURE REVIEW OF DESIGN OF OFFSHORE PILED FOUNDATIONS

18

3 LITERATURE REVIEW OF DESIGN OF OFFSHORE PILED FOUNDATIONS

Here shall be detailed the most current in design procedures in the offshore industry for the design

of piled foundations with an emphasis on seismic design.

3.1 Codes Seismic Design Criteria

The API-RP2A has outlined specific guidelines that are to be used in the planning, design and

construction of offshore platforms and their peripherals, including foundation and additional

constructs that aid in support. Another series of codes of similar design are those of the Eurocode 8,

Eurocode 5, (International Organization for Standardization) ISO 19901-2:2004, ISO 19901-4:2003

and ISO 19902:2007, all accepted as British Codes, incorporated under the European Committee for

Standardization (CEN). For the purposes of this study the aforementioned shall be referred to as the

Eurocode unless specifically stated. Both of these codes, the API-RP2A and the Eurocode/ISO,

have addressed the seismic design of the platforms and their components and they shall be

individually looked at.

3.1.1 API-RP2A

The latest version of this code was published in 2000 but the recommendations are still heavily

implemented in offshore design. On the issue of the earthquake guidelines the code advocates that

the design should be based on the historic seismicity of an area. This is the basis of ground motion

prediction and as such is to be adhered to. Before installation, the area is to be investigated for

seismicity and geologic processes that will cause ground motion. The seismic design of the platform

is to be based on the level of expected ground motion, the risk involved and the damage that is


19

deemed acceptable to the platform depending on its intended operation. The siting of platforms in

direct proximity of faults is to be avoided, but if this cannot be then the platform is to be designed to

be able to withstand the predicted ground motion.

The first step in the determination of the seismic level is to determine the exposure category

intended for the platform. These are based on the consideration of life safety and consequences of

failure. Life safety relates to the presence of human life that is to be present on platforms during an

event, and the consequences of failure relates to the expected damage to operation and environment

in the event of failure. The latter relates to such areas as the impact on the environment and the cost

of platform down-time to all interested parties. For life-safety there exists three categories: manned-

nonevacuated, manned-evacuated, and unmanned. For consequences of failure there are also three

categories: high consequences of failure, medium consequences of failure, and low consequences of

failure.

Manned-nonevacuated (L-1) platforms refer to those that must have a human presence onboard at

all times even at times when a design natural event is expected, for these there will be permanent

living quarters that will be included in the original design. A manned-evacuated (L-2) platform

will be freed from all human presence in the expectation of a design natural event, as these would

also contain living quarters. And an unmanned (L-3) platform is one in which no living quarters

exists for human accommodation.

High consequences of failure (L-1) is refered to major platforms that will cause large impacts on

the environment and the profit of the operator. These platforms are considered those that do not

possess the ability to shut off valves in the event of a design loading and which has wells that would

be able to flow freely upon rupture, also these platforms are in depths of water greater than 122m.

The closing of valves is normally impractical in some cases, such as with those that are in the

vicinity of seismic faults, where no prediction is given. For medium consequences of failure (L-2)

platforms, these refer to those that do posses wells that would flow freely under rupture but can be

closed off in wake of predicted design loading. The low consequences of failure (L-3) category

refers to minimal platforms where production is also shut in during design events and valves are

closed, also these platforms are in water depths of less than 30m.


20

With respect to seismicity it is then necessary to determine the type of zone to which the platform

would fall. This would be either seismically active zones or areas of low seismicity. Platforms that

are designed to withstand earthquake forces are to fulfill two requirements, strength requirements

and ductility requirements. Strength requirements are to ensure that the platform has sufficient

strength to withstand the design earthquake (in this case that which will most likely not be exceeded

throughout the life of the platform), and not suffer any significant structural damage. Ductility

requirements are to ensure that if the design earthquake is exceeded, and structural damage is

caused then the platform will not collapse totally but rather possess sufficient reserve capacity.

When the area intended for the platform installation is known to produce earthquakes a specific site

study is to be done to determine the level of ground motion expected. This detailed site study is

done by way of a Probabilistic Seismic Hazard Assessment (PSHA) and Deterministic Seismic

Hazard Assessment (DSHA) which shall be addressed later, although the API-RP2A does give a

thorough overview of the most important concepts of the methods. For the areas of low seismicity

the critical loading, as seen in the Gulf of Mexico, is storm or other environmental loading. Now the

API-RP2A has produced charts (see appendix) that give guidelines on the level of earthquake in

which to design. Zones 1 and 2 are described as zones of low seismic activity and 3-5 are of high

seismicity and the detailed site studies are to be done. Table 3-1 below displays the typical values of

the zone, Z, to the ratio of horizontal ground acceleration to gravitational acceleration, G, to be used

for the purpose of preliminary designs and in the simplified procedure. This approach is the

response spectrum approach and incorporates the use of Figure 3-1.

Table 3-1 Determination of exposure level [API-RP2A]


21

Figure 3-1. Standardized seismic acceleration spectrum for 5% damping [API-RP2A]

It is required that all platforms be designed for a Strength Level Earthquake (SLE). For this design,

it is to be ensured that serviceability requirements are met and that adequate ductility has been

provided in the structure. The return interval of these earthquakes is normally in the range 100 to

500 years.

Platforms are also further required to be designed for a Ductility Level Earthquake (DLE). Here,

having displayed adequate ductility for the SLE, it is to be displayed that during a large event the


22

platform would have sufficient reserve capacity to prevent collapse, (limit state design). The return

interval of these earthquakes is normally in the range 1,000 to 10,000 years.

The API-RP2A was found to give a thorough overview of the methodology behind the

determination of the factors that influence the seismic hazard assessment process, but no detailed

design methods are provided.

3.1.2 Eurocode/ISO

The latest version of this code that addresses seismicity was published in 2004. This code is

published by the British Standards Institute and in incorporated in both the Eurocode 8 and

International Organization for Standardization, ISO 19901-2 (Seismic design procedures and

criteria) and ISO 19902 (Fixed Offshore Steel Structures). The ISO code has been found to be the

most recent of all the codes with ISO 19902 (Petroleum and natural gas industries Fixed steel

offshore structures) being published in 2007.

The methods for the determination of the exposure level and life safety in the ISO are the same as

for the API-RP2A in that it is also categorized. It is again dependent on the presence of manpower

on the platform, this is displayed in Table 3-2.

Table 3-2. Determination of exposure level [ISO 19901-2]

Also, as the case of the API-RP2A, identified is two levels of earthquakes that need to be addressed

during platform seismic design. These are the Extreme Level Earthquake and the Abnormal Level

Earthquake. Here, two methods are identified for the determination of the design ground response,

the simplified method and a detailed method. The ISO has proven to give a better outline of the

procedure of seismic design than the API-RP2A, and as such is more user friendly. This is done by


23

use of the seismic hazard charts (see appendix for an excerpt from the ISO code for offshore North

America) for a 1.0s and 0.2s oscillator. The ISO has displayed also that it is designed to be used

internationally as the spectral accelerations that are outlined in this document are for countries and

locations around the world that produce petroleum and natural gas. The API-RP2A is only focused

on American territory. The first step, in this approach, is to determine the exposure level for the site.

The site seismic zone is then determined from Table 3-3.

Table 3-3. Site Seismic Zone [ISO 19901-2]

The sites seismic risk category, SRC, is then determined depending on the exposure level, Table

3-4.

Table 3-4. Seismic Risk Category [ISO 19901-2]

If the site is deemed to be SRC 1 then there is no need to conduct a seismic evaluation and for SRC

2 the simplified method is used. This simplified method uses a standard procedure that calls for the

adaptation of a standardized seismic hazard graph, Figure 3-2. In the case of SRC 3 either the

simplified or the detailed method may be used at this point but the simplified method calls for the

use of seismic hazard maps, but depending on the seismic risk category this method may produce a

conservative design that is greater than that of the detailed design and in these cases it is best to use

the detailed method. The detailed method is also used for the case of SRC 4.


24

Figure 3-2. Standardized seismic acceleration spectrum for 5% damping [ISO 19901-2]

Use of the detailed method of the ISO refers to clause 8. This would include a site specific study

that is done via a PSHA and a DSHA to determine the seismic hazard attenuation curves. The

curves (Figure 3-3) are used in the

consideration of the randomness of the

attenuation of seismic waves that would

travel from the sources to site,

specifically these would be for spectral

accelerations, Sa, over a range of periods,

T1TN. The summation over individual

probabilities from different sources

would provide the total annual

probability of exceedance for a given

level of peak ground acceleration (PGA) or spectral acceleration (SA), (Figure 3-4). Spectral

response varies with the natural period of the oscillator and as such a family of curves are produced.

These curves are used to construct the uniform hazard spectrum (Figure 3-5), where all the points

Figure 3-3. Defined attenuation curves from site specific study for spectral accelerations at periods T1 TN [ ISO 19901-2].


25

are defined for one annual probability of exceedance. There exists an inverse relationship between

the return period of a uniform hazard spectrum and the targeted annual probability of exceedance.

Figure 3-4. Mean uniform hazard spectral accelerations [ISO 19901-2].

Figure 3-5. Uniform Hazard Spectrum [ISO 19901-2].

The seismic action procedure follows after the determination of the Uniform Hazard Spectra for the

site. The exposure level of the platform is used to find the targeted annual probability of failure, Pf,

from Table 3-5. The ALE spectral accelerations are found from the derived hazard curve for the site

and the value of Pf.

Table 3-5. Target annual probability of failure [ISO 19901-2].

The dominant modal period of the structure to be designed, Tdom, shall correspond to a probability

distribution of the parameter, Sa. As in figure 8, the site specific mean uniform hazard spectral

accelerations is plotted but on a logarithmic scale, for Tdom. The new plot, figure 10, is now used to

determine the site-specific spectral acceleration at Pf, Sa,Pf (Tdom). The procedure that follows is that

the slope of the tangent at Pf, termed aR, defined as the ratio of the spectral accelerations


26

corresponding to two probabilities that are on either side of Pf and which are one order of

magnitude apart, P1 & P2, as seen in Figure 3-6.

Figure 3-6. Derivation of the slope aR of the seismic hazard curve for T=Tdom [ISO 19901-2].

A correction factor, Cc, is then applied to aR to account for the uncertainties that are not depicted in

the seismic hazard curve. The values of Cc are found in Table 3-6.

Table 3-6. Correction Factor, Cc [ISO 19901-2].

The value for the ALE spectral acceleration is determined by the application of the correction

factor, Cc to Sa,Pf(Tdom), the site specific spectral acceleration at the required Pf and the dominant

structural period, Tdom, using the following relationship:

( ) ( )domPfacdomALEa TSCTS ,, = (3.1) For some structures the reserve capacity and ductility are known, the ELE can be determined from

the following relationship:

( ) ( )r

domALEadomELEa C

TSTS ,, = (3.2)


27

where, Cr is the seismic reserve capacity. These values are to be computed as best as possible

during the initial design stages to be able to be applied to this stage before the final design is made.

ISO19902 provides some guidelines for these factors as displayed in Table 3-7.

Table 3-7. Representative values of seismic reserve capacity, Cr [ISO 19901-2].

The ALE and the ELE are then directly read off the logarithmic seismic hazard curve, and the

results are as displayed in Figure 3-7.

Figure 3-7. Derivation of spectral accelerations and probabilities for ALE and ELE [ISO 19901-2].


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The effects of the local soil at the platform and a dynamic site response analysis is performed, using

linear or non-linear soil properties and the results are used to modify the result.

The effects of seismicity include seafloor slip, liquefaction of a layer of supporting stratum,

tsunamis and ground-motion that would inherently affect the stresses in the platform. The intention

would be to obtain a code that would give direction as to exactly what is the necessary procedure in

obtaining as accurate as possible a prediction of the seismic motion on a given site. Seismic actions

for a site are predicted by a thorough investigation of the subsurface of the soils. The investigation

is to include the determination of liquefiable potential of the soil, the presence of submarine slides

that can be triggered by earthquakes, the proximity of the site to seismogenic faults, the

characteristics of the ground motion expected through-out the life of the structure, and the intended

risk for the type of platform intended.

3.2 Piles

This section shall focus on the actual piling aspect of offshore installations. The aim here is to

identify the state-of-the-art methods of seismic pile design that have been developed.

3.2.1 Static Pile Analysis

There has been a great deal of research done in the area of axial capacity of offshore piling. The

basis of development has been the use of the Cone Penetration Test [CPT] in the determination of

methods of design. This is predominantly used as it is readily available and easy to manipulate. In

light of this though many of the developed methods are still empirically based due to studies done

on tests in the centrifuge. This data is still to be extrapolated from smaller diameter piles to model

the response of actual piles in actual design scenarios. More of these studies have been done, as

before mentioned, further to that of the API-RP2A.

3.2.1.1 Static Single

3.2.1.1.1 Ultimate Capacity

As per the API-RP2A, the governing equation for the ultimate static capacity, Qd, of offshore piles

is determined by the following equation:


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(3.3)

where,

Qf = skin friction resistance (kN),

Qp = total end bearing (kN),

f = unit skin friction (kPa),

As = side surface area of pile (m),

q = end unit bearing capacity (kPa),

Ap = gross end area of pile (m).

3.2.1.1.2 End Bearing Resistance

For the API-RP2A and ISO guidelines it has been adopted the following equation for the limited

bearing pressure, qbu.

max' = buvoqbu qNq (3.4) Where the value of Nq is taken to vary from 12 to 50 and this depends on the grain size and the

relative density of the material. The different values for the sand and silt and other appropriate pile

parameters are provided in Table 3-8. The limiting values for qbu are also provided.

Table 3-8 Pile Design Parameters [ISO 19901-2, (2004)]

The cone penetrometer is to be seen as a model pile. But due to its small size adjustments are to be

made to the cone so as to have its penetration resistance values simulate the effect that a much

larger penetrator would have. The adjustments are allowed by way of an adjustment factor that is to


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be applied to the raw values of qu. The reasoning behind this lies in the displacement of an actual

pile to mobilise the full cone resistance and the embedment depth of the pile in the bearing layer.

Of importance here is the ability of a pile to act as plugged or unplugged. Under static loading open

ended piles will act as plugged, but drive as unplugged. This is in comparison to dynamic piling

which increases the inertial effect on the soil within the pile and results in open-ended driving.

During static loading the soil within the pile would move with the pile. Data from tests on piles

has led to the following equation for limiting bearing pressure for piles in sand from Jardine and

Chow, 1996. This method is known as the Marine Technology Directorate approach (MTD).

(3.5)

where,

d = diameter of the pile,

dcone = diameter of the CPT cone,

= area ratio.

This relationship is for piles of diameter less than 0.02(Dr 30)m.

This MTD approach was found to produce very conservative results for typical offshore piles of

diameter >2m and = 0.1. In a recent report to the API-RP2A, Fugro in a 2004 report has

recommended the following equation for the bearing capacity of driven open-ended piles. This

equation models the cone penetration test as a miniature pile and applies a fraction of its

resistance to determine the

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