dissertation2009 trevon
DESCRIPTION
ASSESSMENT OF KINEMATIC EFFECTS ON OFFSHORE PILED FOUNDATIONSTRANSCRIPT
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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
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Index
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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 ___
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Abstract
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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.
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Acknowledgements
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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.
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Index
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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
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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
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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
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Index
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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
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Index
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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
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Index
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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
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Index
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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
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Index
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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
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Index
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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
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Index
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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
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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.
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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.
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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.
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Chapter 1: Introduction
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].
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Chapter 1: Introduction
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.
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Chapter 1: Introduction
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.
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Chapter 1: Introduction
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.
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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.
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Chapter 2: GENERAL REVIEW OF OFFSHORE STRUCTURES
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].
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Chapter 2: GENERAL REVIEW OF OFFSHORE STRUCTURES
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)].
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Chapter 2: GENERAL REVIEW OF OFFSHORE STRUCTURES
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).
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Chapter 2: GENERAL REVIEW OF OFFSHORE STRUCTURES
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)].
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Chapter 2: GENERAL REVIEW OF OFFSHORE STRUCTURES
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
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Chapter 2: GENERAL REVIEW OF OFFSHORE STRUCTURES
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],
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Chapter 2: GENERAL REVIEW OF OFFSHORE STRUCTURES
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],
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Chapter 2: GENERAL REVIEW OF OFFSHORE STRUCTURES
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.
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Chapter 2: GENERAL REVIEW OF OFFSHORE STRUCTURES
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.
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Chapter 2: GENERAL REVIEW OF OFFSHORE STRUCTURES
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
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Chapter 2: GENERAL REVIEW OF OFFSHORE STRUCTURES
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)].
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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
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Chapter 3: LITERATURE REVIEW OF DESIGN OF OFFSHORE PILED FOUNDATIONS
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.
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Chapter 3: LITERATURE REVIEW OF DESIGN OF OFFSHORE PILED FOUNDATIONS
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]
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Chapter 3: LITERATURE REVIEW OF DESIGN OF OFFSHORE PILED FOUNDATIONS
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
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Chapter 3: LITERATURE REVIEW OF DESIGN OF OFFSHORE PILED FOUNDATIONS
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
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Chapter 3: LITERATURE REVIEW OF DESIGN OF OFFSHORE PILED FOUNDATIONS
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.
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Chapter 3: LITERATURE REVIEW OF DESIGN OF OFFSHORE PILED FOUNDATIONS
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].
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Chapter 3: LITERATURE REVIEW OF DESIGN OF OFFSHORE PILED FOUNDATIONS
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
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Chapter 3: LITERATURE REVIEW OF DESIGN OF OFFSHORE PILED FOUNDATIONS
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)
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Chapter 3: LITERATURE REVIEW OF DESIGN OF OFFSHORE PILED FOUNDATIONS
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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Chapter 3: LITERATURE REVIEW OF DESIGN OF OFFSHORE PILED FOUNDATIONS
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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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Chapter 3: LITERATURE REVIEW OF DESIGN OF OFFSHORE PILED FOUNDATIONS
29
(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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Chapter 3: LITERATURE REVIEW OF DESIGN OF OFFSHORE PILED FOUNDATIONS
30
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