Planning, Designing, and Constructing Tension Leg Platforms

API RECOMMENDED PRACTICE 2T THIRD EDITION, JULY 2010

Planning, Designing, and Constructing Tension Leg Platforms

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API RECOMMENDED PRACTICE 2T THIRD EDITION, JULY 2010

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Copyright © 2010 American Petroleum Institute

Foreword

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This recommended practice for planning, designing, and constructing tension leg platforms incorporates the many engineering disciplines that are involved with offshore installations, either floating or fixed. Defined herein are guidelines developed from the latest practices in tension leg platforms, and adapted from successful practices employed for related structural systems in the offshore and marine industries.

A tension leg platform (TLP) is a vertically moored, buoyant, compliant structural system wherein excess buoyancy of the platform (in excess of weight and riser loads) maintains tension in the mooring system. A TLP may be designed to serve a number of functional roles associated with offshore oil and gas exploitation. It is considered particularly suitable for deepwater applications. A TLP system consists of many components, each of which has a precedent in the offshore or marine industry. The uniqueness of a TLP is in the systematic influence of one component on another. Consequently, the design is a highly interactive process which should account for functional requirements, component size and proportion, equipment layout and space allocation, hydrodynamic reaction, structural detail, weight and centers of gravity, etc. All disciplines involved in the design process should anticipate several iterations to achieve proper balance of the design factors. This publication summarizes available information and guidance for the design, fabrication, and installation of a TLP system.

These recommendations are based on published literature and the work of many companies who are actively engaged in TLP design. As with earlier editions of this publication, it represents a snapshot of the state of the art and practice of TLP design. As new technology develops, this publication will be updated to reflect the latest accepted design and analysis methods.

Each section of this publication covers a specific aspect of tension leg platforms. The main text contains basic engineering design principles which are applicable to the design, construction, and operation. Equations for analyses are included where appropriate. In many cases these equations represent condensations of more complete analysis procedures, but they can be used for making reasonable and conservative predictions of motions, forces, or component strength. More detailed discussions of these engineering principles, describing the logic basis and advanced analytical concepts from which they were developed, are given in the commentary. The designer and operator are encouraged to use the most current analysis and testing methods available, and bring forth to the Institute any newfound principles or procedures for review and consideration.

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1. Scope 1

2. Normative References 1

3. Terms, Definitions, Acronyms, Abbreviations and Symbols 1

   1. Terms and Definitions 1

   2. Acronyms and Abbreviations 6

   3. Symbols 7

4. Planning 10

   1. General 10

   2. The Design Process 11

   3. Codes, Standards, and Regulations 13

   4. Operational Requirements 13

   5. Environmental Considerations 14

   6. Seafloor Characteristics 15

   7. Systems Design 16

   8. Fabrication and Installation 20

   9. Materials, Welding, and Corrosion Protection 21

   10. Safety and Reliability 22

   11. Operating and In-service Manuals 22

5. Design Criteria 25

   1. General 25

   2. Safety Categories 25

   3. Operational Requirements 26

   4. Stability Requirements 26

   5. Environmental Criteria 28

   6. Design Load Cases 30

6. Environmental Forces 36

   1. General 36

   2. Wind Forces 37

   3. Current Forces 41

   4. Vortex-induced Vibrations (VIVs) 42

   5. Wave Forces 45

   6. Ice Loads 50

   7. Wave Impact Forces 50

   8. Earthquakes 51

   9. Accidental Loads 51

   10. Fire and Blast Loading 51

7. Global Response 51

   1. Purpose and Scope 51

   2. System Modeling 52

   3. Static and Mean Response Analysis 52

   4. Equations of Motion and Solutions 55

   5. Frequency Domain Modeling and Solution 58

   6. Time Domain Modeling and Solutions 62

   7. Hydrodynamic Model Tests 66

   8. Global Performance Design Equations 68

   9. Responses for Fatigue Analysis 80

      v

      
      

8. Platform Structural Design 81

   1. Introduction 81

   2. General Structural Considerations 81

   3. Design Cases 84

   4. Hydrodynamic Loads for Hull Design 88

   5. Structural Analysis 92

   6. Structural Design 98

   7. Fabrication Tolerances 101

   8. Structural Materials 101

9. Tendon System Design 106

   1. General 106

   2. General Design 107

   3. Material Considerations 112

   4. Design Loads 117

   5. Load Analysis Methods 119

   6. Structural Design and Fabrication. 120

   7. Transportation, Handling and Installation Procedures 137

   8. Operational Procedures 138

   9. Corrosion Protection 138

10. Foundation Analysis and Design 138

    1. General 138

    2. Foundation Requirements and Site Investigations 140

    3. Loading 143

    4. Analysis Procedures 146

    5. Design of Piled Structures 147

    6. Design Of Piles 148

    7. Design Of Shallow Foundations 150

    8. Material Requirements 151

    9. Fabrication, Installation, and Surveys 151

11. Riser Systems 151

    1. General 151

    2. Riser System Types 152

    3. Design Considerations 153

    4. Riser Analysis 154

12. Facilities and Marine Systems 157

    1. General 157

    2. Considerations 157

    3. Drilling Specific Considerations 159

    4. Production Systems Considerations 160

    5. Hull System Considerations 163

    6. Personnel Safety Considerations 168

    7. Fire Protection Considerations 169

    8. Interacting (Interfacing) Checklists 171

    9. Interface Planning 175

    10. Volatile Fluid Storage [Flash Point  60 ° (140 °F)] 178

    11. Hull Piping 179

    12. Marine Monitoring Systems For TLPs 179

        
        

13. Corrosion 180

    1. General 180

    2. Antifouling 180

    3. Splash Zone 180

    4. Corrosion Protection of Internal Surfaces 181

    5. Corrosion Protecton of Hull External Submerged Surfaces 181

    6. Tendons 181

    7. Foundations 181

    8. Cathodic Protection (CP) Interaction 182

    9. Monitoring 182

14. Fabrication, Installation and Inspection 182

    1. Introduction 182

    2. Structural Fabrication 183

    3. Welding 185

    4. Platform Assembly 187

    5. Transportation 189

    6. Installation Operations 191

    7. Inspection and Testing 196

15. Surveys and Maintenance 199

    1. General 199

    2. Personnel 200

    3. Survey and Maintenance Planning and Recordkeeping 200

    4. Survey Frequency 201

    5. Survey Requirements 202

    6. Examination of Joints and Connections 205

    7. Requirements for Internal Examination 205

16. Assessment of Existing TLP’s Designed for Hurricanes 205

    1. Scope 205

    2. Assessment Indicators 206

    3. Assessment Conditions 206

    4. Assessment Process 206

    5. Acceptance Criteria 208

    6. Configuration Changes 209

    7. Marine Operations Manual 209

    8. General Requirements for all Existing TLP’s 209

Annex A (informative) Commentary on Global Response Analysis and Design Checks 212

Annex B (informative) Commentary on Design of Tendon Porches 222

Annex C (informative) Commentary on Tendon Fatigue 224

Annex D (informative) Commentary on Foundation Design 238

Annex E (informative) Drilling and Production Interacting Checklists 243

Annex F (informative) Regulations Governing TLPs 245

Bibliography 246

Figures

1. TLP Terminology 6

2. VIV of a Spring-supported, Damped Circular Cylinder 44

3. Wave Force Calculation Method and Guideline for Wave Forces on Cylindrical Members 48

4. Wave Force Regimes 49

5. TLP Restoring Force with Offset 55

6. Simple Model for TLP Response Analysis 58

7. Surge Motion Spectrum 73

8. Maximum Tension Components 75

9. Minimum Tension Components 77

10. Tendon Design Flow Chart 109

11. Local Stress Check at Tendon Section Transitions 123

12. Typical Section, Applied Loads, and Stress Distributions Through-thickness 124

13. Design, Fabrication, and Verification Process for Fracture-critical Tendon Welds 130

14. Components of an Integrated Template Foundation System 139

15. Components of an Independent Template Foundation System 139

16. Components for Directly Connecting the Pile to a Tendon 139

17. Components of a Shallow Foundation System 140

1. Helical Strakes 213

2. Short Fairing 213

3. Sample High-frequency TLP Tension Responses 217

1. Definition of Pipe and Notch Stresses 233

2. Relation Between Notch and Pipe Stresses 233

3. Transformation of Elastic Stress via Strain Energy 234

4. Calculated S-N Curves using Initiation Life for Various SCFs and Constant Mean Stress 235

D.1 Relative Residual Strength After Pile Overload 241

Tables

1. Project Design Load Cases 31

2. Return Period of Environmental Conditions 34

3. Loading Type Category Descriptions 36

4. Shape Coefficients for Perpendicular Wind Approach Angels 41

5. Environmental Parameters Influencing TLP Response 70

6. Components for Maximum Tension Determination 75

7. Allowable Stress Safety Factors 99

8. Safety Factors for Tension-collapse Check 125

9. Load and Resistance Factors for Tension-collapse Check 127

10. Local Pipe Strength Safety Factors 129

11. Connector Strength Safety Factors 135

12. Factors of Safety 144

13. Reference Standards for Design Tolerances 185

1. Structure Layout Interface Checklist 243

2. Utilities Interface Checklist 244

3. Rig Services Interface Checklist 244

Planning, Designing, and Constructing Tension Leg Platforms

1. Scope

   
   

   This recommended practice is a guide to the designer in organizing an efficient approach to the design of a tension leg platform (TLP). Emphasis is placed on participation of all engineering disciplines during each stage of planning, development, design, construction, installation, and inspection.

   
   

2. Normative References

   
   

   The following referenced documents are indispensible in the application of this standard. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

   
   

   API Specification 5L, Specification for Line Pipe

   
   

   DNV-RP-B401 1, Cathodic Protection Design

   NACE SP0176 2, Corrosion Control of Submerged Areas of Permanently Installed Steel Offshore Structures Associated With Petroleum Production

   
   

3. Terms, Definitions, Acronyms, Abbreviations and Symbols

3.1 Terms and Definitions

For purposes of this document, the following terms and definitions apply.

3.1.1

added mass

Effective addition to the system mass, which is proportional to the displaced mass of water.

3.1.2

bluff body

An opaque object located in a fluid flow stream and developing a high drag force because it lacks streamlining.

3.1.3

braces

Structural members that serve to stiffen the hull structure and provide deck support.

3.1.4

bulkhead

Stiffened vertical or horizontal load bearing diaphragm.

3.1.5

connector

A riser device used to latch and unlatch risers and lower marine riser packages to subsea equipment, or a tendon device used to latch and unlatch tendons to the foundation system and to connect the tendon to the platform.

1 Det Norske Veritas, Distribution Department, NO–1322 Høvik, Norway, e-mail: distribution@dnv.com.

2 NACE International, 1440 South Creek Drive, Houston, Texas, 77084-4906, www.nace.org.

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