Effect of Best-estimate Geotechnical p-y Curves on Performance

of Offshore Structures

API TECHNICAL REPORT 2PY FIRST EDITION, FEBRUARY 2020

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Copyright © 2020 American Petroleum Institute

Foreword

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This document was produced under API standardization procedures that ensure appropriate notification and participation in the developmental process and is designated as an API standard. Questions concerning the interpretation of the content of this publication or comments and questions concerning the procedures under which this publication was developed should be directed in writing to the Director of Standards, American Petroleum Institute, 200 Massachusetts Avenue, Suite 1100, Washington, DC 20001. Requests for permission to reproduce or translate all or any part of the material published herein should also be addressed to the director.

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iii

Contents

Page

Executive Summary 1

Annex A (informative) SS-Caisson Structure Soil Data 38

Annex B (informative) SS-Platform Soil Data 51

Annex C (informative) EI-Tripod Platform Soil Data 58

Annex D (informative) Methodology Used to Derive p-y Curves in Clays 67

Bibliography 78

v

Contents

Figures

Page

1. Example Caisson Platform Leaning Damage 6

2. SS-Caisson Structural Detail 7

3. SS-Caisson Structure Model 8

4. SS-Caisson Soil DSS Strength Profile 9

5. SS-Caisson Response to Hurricane Andrew Wave Impact,

   Model with API 2GEO, 1st Edition p-y Curves 12

6. SS-Caisson Response to Hurricane Andrew Wave Impact,

   Model with Proposed API 2GEO, 2nd Edition (Draft) p-y Curves 12

7. SS-Caisson Damage to Hurricane ANdrew Wave Impact 14

8. SS-Caisson Hurricane Wave Load and Rotation 14

1. SS-Platform (After Hurricanes) 16

2. SS-Platform Structure Model and Design Detail 17

3. SS-Platform Damage After Hurricanes 18

4. SS-Platform Soil DSS Strength Profile 19

5. SS-Platform Response to Hurricane Ike Impact,

   Model with API 2GEO, 1st Edition p-y Curves 22

6. SS-Platform Response to Hurricane Ike Impact,

   Model with Proposed API 2GEO, 2nd Edition (Draft) p-y Curves 23

7. SS-Platform Damage to Hurricane Ike Wave Impact 25

8. SS-Platform Hurricane Damage, Field Observed vs. Model Predicted 26

9. SS-Platform Deformation Curves 27

1. EI-Tripod Platform 29

2. Platform Orientation and Coordinate System 29

3. EI-Tripod Platform Structure Model 30

4. EI-Tripod Platform Soil DSS Strength Profile 31

5. Main Deck Vibration Data Recording Samples 33

6. Processed Response Spectra 33

7. EI-Tripod Vibration Mode Shape Comparison 36

1. Mean Ratio of Measured Strength to Design Strength Versus Depth—from Cheon, et al. (2015) 39

2. SHANSEP Frame work—From Liedtke, et al. (2019) 40

3. Correlations with Water Content—From Liedtke, et al. (2019) 41

4. Proposed DSS Strength Profile 42

5. Soil Strength Profile for SS-Caisson 47

6. API 2GEO, 1st Edition Static vs. 2nd Edition Cyclic p-y Curves for SS-Caisson 48

7. API 2GEO, 2nd Edition Cyclic p-y Spring Response 49

8. API 2GEO, 1st Edition Static p-y Spring Response 50

1. Soil Strength Profile for SS-Platform 52

2. Soil Strength Profile Compared with Push Soil Sample Data from Adjacent Blocks 53

3. API 2GEO, 1st Edition Static vs. 2nd Edition Cyclic p-y Curves for SS-Platform 36-inch Piles 54

1. Soil Strength Profile 59

2. API 2GEO, 1st Edition Static vs. 2nd Edition Fatigue p-y Curves for EI-Tripod 60

1. Normalized p-y Curves for Monotonic Loads/Actions for Clay 70

2. Normalized p-y Curves for Monotonic Loads/Actions for Clay 71

3. Comparison of Total Lateral Bearing Capacity Factor Np

   According to RP 2GEO 1st and New Equations for Example 1 and Example 2 73

4. Cyclic Normalized Curves for Illustrative Clay Soil Profile with Ip > 30%, OCR < 2,

   and for GoM Cyclic Loading Conditions 74

   
   

   vi

   
   

5. Comparison of p-y Curve Recommendations Between RP 2GEO 1st

   and This Study at a Depth z = 30 m for Illustrative Clay Soil Profile for GoM Conditions:

   1. Full Scale and b) Low y/D Displacements 75

Tables

3.1 Hurricane Andrew Hindcast Metocean 10

4.1 Hurricane Ike Hindcast Metocean 20

1. EI-Tripod Measured Platform Natural Periods 34

2. Platform Natural Period Comparison, Model Predicted vs. Measured 34

1. Normalized p-y Curves for Monotonic Loads/Actions for Clay 69

2. Cyclic Modifiers for p-y Curves in Clays 71

Executive Summary

Fixed offshore platform pile foundation design and assessment in the Gulf of Mexico (GOM) has been based on API RP 2A (various editions) and API/ISO RP 2GEO, 1st Edition. Over the years, post-hurricane observations of the platform performance in the GOM have shown that these standards tend to underestimate soil stiffness and capacity. This underprediction does not necessarily lead to conservative structural design and assessment. For example, fatigue design requires a realistic estimate of structural dynamics and strength design with a clear definition of structural load path and failure mechanism.

Research work, consisting of small- and large-scale pile tests, laboratory testing, and numerical analyses, has been carried out for years to understand this conservatism and define best-estimate geotechnical criteria, i.e. best-estimate soil lateral reaction for pile foundation design. These efforts culminated in the proposed draft of API/ISO RP 2GEO, 2nd Edition, which systematically re-defines the clay soil lateral reaction formulations for piles under monotonic, cyclic, and fatigue loads.

The goal of this study, sponsored by API SC2, was to perform structural analyses using soil models developed by 2GEO, 1st Edition criteria and 2GEO, 2nd Edition draft criteria to determine the effect of the new clay soil p-y formulations on the structural responses of these platforms. The predicted responses were then compared with measured or observed platform performance in field.

Three (3) featured fixed offshore platforms in the GOM, i.e. a free-standing caisson structure, a jacket type platform, and a tripod platform, were selected as representative structures for the study. The selected platforms are actual platforms in operation in the GOM. The caisson structure and jacket platform were damaged in Hurricanes Andrew and Ike, respectively. Platform vibration of the tripod platform was measured and recorded in the field to verify the tripod structural integrity. All relevant platform design, operation, and maintenance data of these structures are sufficient and available for the study.

Results of the analyses performed in this study demonstrate that structural models using the soil reaction formulations of the proposed 2GEO, 2nd Edition (draft), in combination with the best-estimate soil profiles interpreted based on the standard direct simple shear (DSS) tests, adequately predicted the platform responses observed in hurricanes and in normal sea conditions.

The clay soil p-y reaction formulations (monotonic, cyclic, and fatigue), when used with best-estimate DSS shear strength profiles as proposed in the draft of 2GEO, 2nd Edition, show significant improvement over the formulations in 2GEO, 1st Edition and older RP 2A editions (e.g. the so-called “Matlock curves”), and are endorsed and recommended for pile foundation design and assessment. In particular, the use of the new cyclic p-y curves is recommended for platform structural analysis under extreme environmental conditions.

2 API TECHNICAL REPORT 2PY

1 Introduction

1.1 Background

Post-hurricane observations of fixed offshore platform performance in the Gulf of Mexico (GOM) often demonstrate platform structural damage or failure rather than foundation failure. However, platform analysis and assessment by API RP 2A (various editions) and API/ISO RP 2GEO, 1st Edition (Ref. 1, 2) foundation design approach are frequently governed by the foundation capacity and predict failure in the piles rather than the jacket structure.

Subsea wellhead and conductor fatigue design and analysis also predict unrealistic fatigue life expectancy due to the deviated dynamics caused by underestimated soil stiffness near the mudline. These have led to questions of unknown reliability and level of conservatism in the current API recommended pile foundation design approach.

Unrealistic conservatism in pile foundation design may have significant impact on new platform design and existing platform assessment. For new platform design, it may lead to excessively larger, deeper pile design associated with higher cost. Fatigue life can also be miscalculated due to inadequate estimates of platform dynamics (e.g. natural periods). For existing platform assessment, it may lead to misjudgment of platform actual capacity (due to pre-mature foundation failure in analysis) and locations of critical structural components (for platform repair and strengthening).

Research work and studies on fixed offshore platform performance in GOM hurricanes, funded by MMS/BSEE, API, and industries, have been carried out for decades (Ref. 5-12, 14-15, 23-26). A general perception indicates that pile foundations appear to be stronger and stiffer than calculated. Pushover analysis for jacket structures to their ultimate capacities commonly indicates that the foundation governs the capacity of the structural system. However, there are few, if any, observed pile foundation failures that have occurred in these hurricanes.

Discrepancy between predicted and observed performance of platform foundation system may be in part due to:

• underestimation of soil stiffness and capacity on the soil-structure interaction curves, especially the pile foundation soil lateral p-y curves for clays;

  
  

• interpretation of geotechnical soil boring and test data;

  
  

• loading rate and cyclic effect on pile capacity under short term wave loading conditions;

  
  

• aging effect of increased pile capacity for years or decades after installation;

  
  

• platform computer modeling inaccuracies and over-simplifications;

  
  

• disproportionate redundancies associated with structure components and foundation system in existing design practice;

  
  

• incomplete or misguided structural data and information.

Geotechnical research work, consisting of small- and large-scale pile tests, laboratory testing, and numerical analyses, has been performed for years to check the pile foundation design and define the best- estimate geotechnical criteria (Ref. 16-18, 20, 29-33). This culminated in the proposed draft of API/ISO RP 2GEO, 2nd Edition (Ref. 3, Annex D), which systematically re-defined the soil lateral reaction formulations, especially for clays.