Deepwater Flexible Risers and Pipelines
Description
The first volume in this series, published by Wiley-Scrivener, is Flexible Pipes, available at www.wiley.com. Laying the foundation for the series, it is a groundbreaking work, written by some of the world's foremost authorities on pipes and pipelines. Continuing in this series, the editors have compiled the second volume, equally as groundbreaking, expanding the scope to pipelines, risers, and umbilicals.
This is the most comprehensive and in-depth series on pipelines, covering not just the various materials and their aspects that make them different, but every process that goes into their installation, operation, and design. This is the future of pipelines, and it is an important breakthrough. A must-have for the veteran engineer and student alike, this volume is an important new advancement in the energy industry, a strong link in the chain of the world's energy production.
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Yong Bai, PhD, is the president of Offshore Pipelines & Risers Inc. in Houston, and is a professor and the director of the Offshore Engineering Research Center at Zhejiang University. He has previously taught at Stavanger University in Norway where he was a professor of offshore structures and has also worked with ABS as manager of the Offshore Technology Department, DNV as the JIP project manager and has also worked for Shell International E & P, JP Kenny, and MCS, where he was vice president of engineering. He is the co-author of two books on pipelines and over 100 papers on the design and installation of subsea pipelines and risers.
Content
Preface xix
Acknowledgment xxi
About the Author xxiii
Part 1: Local Analysis 1
1 Introduction 3
1.1 Flexible Pipelines Overview 3
1.2 Environmental Conditions 4
1.3 Flexible Pipeline Geometry 7
1.4 Base Case-Failure Modes and Design Criteria 9
1.5 Reinforcements 10
1.6 Project and Objectives 12
References 12
2 Structural Design of Flexible Pipes in Different Water Depth 15
2.1 Introduction 15
2.2 Theoretical Models 15
2.3 Comparison and Discussion 24
2.4 Conclusions 34
References 34
3 Structural Design of High Pressure Flexible Pipes of Different Internal Diameter 35
3.1 Introduction References 35
3.2 Analytical Models 37
3.2.1 Cylindrical Layers 37
3.2.2 Helix Layers 39
3.2.3 The Stiffness Matrix of Pipe as a Whole Helix Layers 40
3.2.4 Blasting Failure Criterion 41
3.3 FEA Modeling Description 42
3.4 Result and Discussion 46
3.5 Design 50
3.6 Conclusions 54
References 55
4 Tensile Behavior of Flexible Pipes 57
4.1 Introduction 57
4.2 Theoretical Models 58
4.2.1 Mechanical Model of Pressure Armor Layer 58
4.2.2 Mechanical Behavior of Tensile Armor Layer 61
4.2.3 Overall Mechanical Behavior 63
4.3 Numerical Model 64
4.3.1 Pressure Armor Stiffness 64
4.3.2 Full Pipe 69
4.4 Comparison and Discussion 71
4.5 Parametric Study 77
4.6 Conclusions 79
References 80
5 Design Case Study for Deep Water Risers 83
5.1 Abstract 83
5.2 Introduction 83
5.3 Cross-Sectional Design 85
5.4 Case Study 87
5.5 Design Result 94
5.6 Finite Elements Analysis 97
5.7 Conclusion 100
References 101
6 Unbonded Flexible Pipe Under Bending 103
6.1 Introduction 103
6.2 Helical Layer Within No-Slip Range 104
6.2.1 Geometry of Helical Layer 104
6.2.2 Bending Stiffness of Helical Layer 108
6.3 Helical Layer Within Slip Range 109
6.3.1 Critical Curvature 109
6.3.2 Axial Force in Helical Wire Within Slip Range 111
6.3.3 Axial Force in Helical Wire Within No-Slip Range 112
6.3.4 Bending Stiffness of Helical Layer 114
References 116
7 Coiling of Flexible Pipes 117
7.1 Introduction 117
7.2 Local Analysis 120
7.2.1 Dimensions and Material Characteristics 120
7.2.2 Tension Test 120
7.2.3 Bending Test 123
7.2.4 Summary 124
7.3 Global Analysis 126
7.3.1 Modeling 126
7.3.2 Interaction and Mesh 127
7.3.3 Load and Boundary Conditions 128
7.3.4 Discussion of the Results 128
7.4 Parametric Study 134
7.4.1 Diameter of the Coiling Drum 134
7.4.2 Sinking Distance of the Coiling Drum 135
7.4.3 Reeling Length 138
7.4.4 The Location of the Bearing Plate 139
7.5 Conclusions 142
References 143
Part 2: Riser Engineering 145
8 Flexible Risers and Flowlines 147
8.1 Introduction 147
8.2 Flexible Pipe Cross-Section 147
8.2.1 Carcass 149
8.2.2 Internal Polymer Sheath 150
8.2.3 Pressure Armor 150
8.2.4 Tensile Armor 151
8.2.5 External Polymer Sheath 151
8.2.6 Other Layers and Configurations 152
8.3 End Fitting and Annulus Venting Design 152
8.3.1 End Fitting Design and Top Stiffener (or Bellmouth) 152
8.3.2 Annulus Venting System 153
8.4 Flexible Riser Design 154
8.4.1 Design Analysis 154
8.4.2 Riser System Interface Design 155
8.4.3 Current Design Limitations 156
References 158
9 Lazy-Wave Static Analysis 159
9.1 Introduction 159
9.2 Fundamental Assumptions 162
9.3 Configuration Calculation 162
9.3.1 Cable Segment 163
9.3.1.1 Hang-Off Section 163
9.3.1.2 Buoyancy Section 166
9.3.1.3 Decline Section 166
9.3.2 Boundary-Layer Segment 167
9.3.3 Touchdown Segment 168
9.3.4 Boundary Conditions 170
9.4 Numerical Solution 171
9.5 Finite Element Model 174
9.5.1 Environment 175
9.5.2 Riser 175
9.5.3 Boundary Conditions 175
9.6 Comparison and Discussion 175
9.7 Parameter Analysis 180
9.7.1 Effect of Seabed Stiffness 180
9.7.2 Effect of Hang-Off Inclination Angle 182
9.7.3 Effect of Buoyancy Section Length 185
9.8 Conclusions 187
References 188
10 Steep-Wave Static Configuration 189
10.1 Introduction 189
10.2 Configuration Calculation 190
10.2.1 Touch-Down Segment 191
10.2.2 Buoyancy Segment 194
10.2.3 Hang-Off Segment 195
10.2.4 Boundary Conditions 195
10.3 Numerical Solution 196
10.4 Comparison and Discussion 198
10.5 Parametric Analysis 203
10.5.1 Effect of Buoyancy Segment's Equivalent Outer Diameter 203
10.5.2 Effect of Buoyancy Segment Length 205
10.5.3 Effect of Buoyancy Segment Location 207
10.5.4 Effect of Current Velocity 209
10.6 Conclusions 212
References 212
Contents ix
11 3D Rod Theory for Static and Dynamic Analysis 213
11.1 Introduction 213
11.2 Nomenclature 215
11.3 Mathematical Model 216
11.3.1 Governing Equations 216
11.3.2 Bending Hysteretic Behavior 220
11.3.3 Bend Stiffener Constraint 222
11.3.4 Pipe-Soil Interaction 224
11.4 Case Study 225
11.5 Results and Discussion 227
11.5.1 Static Analysis 227
11.5.2 Dynamic Analysis 231
11.5.2.1 Top-End Region 231
11.5.2.2 Touchdown Zone 233
11.5.3 Effect of Bend Stiffener Constraint 236
11.5.4 Effect of Bending Hysteretic Behavior 238
11.5.5 Effect of Top Angle Constraint 240
11.6 Conclusions 242
References 243
12 Dynamic Analysis of the Cable-Body of the Deep Underwater Towed System 247
12.1 Introduction 247
12.2 Establishment of Towed System Dynamic Model 248
12.3 Numerical Simulation and Analysis of Calculation Results 251
12.3.1 The Effect of Different Turning Radius 252
12.3.2 The Effect of Different Turning Speeds 253
12.3.3 Dynamic Analysis of the Towed System with the Change of the Parameters of the Cable 254
12.3.4 The Effect of the Diameters of the Towed Cable 257
12.3.5 The Effect of the Drag Coefficients of the Towed Cable 257
12.3.6 The Effect of the Added Mass Coefficient of the Towed Cable 261
12.4 Conclusions 263
Acknowledgments 264
References 264
13 Dynamic Analysis of Umbilical Cable Under Interference 267
13.1 Introduction 267
13.2 Dynamic Model of Umbilical Cable 269
13.2.1 Establishment of Mathematical Model 269
13.2.2 The Discrete Numerical Method for Solving the Lumped Mass Method 271
13.2.3 Calculation of the Clashing Force of Umbilical Cable 277
13.3 The Establishment of Dynamic Simulation Model in OrcaFlex 279
13.3.1 The Equivalent Calculation of the Stiffness of the Umbilical Cable 279
13.3.2 RAO of the Platform 281
13.3.3 The Choice of Wave Theory 281
13.3.4 Establishment of Model in OrcaFlex 282
13.4 The Calculation Results 283
13.4.1 The Clashing Force of Interference 283
13.4.2 The Variation of the Effective Tension Under Interference 285
13.4.3 The Variation of Bending Under Interference 287
13.5 Conclusion 291
References 294
14 Fatigue Analysis of Flexible Riser 295
14.1 Introduction 295
14.2 Fatigue Failure Mode of Flexible Riser 296
14.3 Global Model of Flexible Risers 297
14.3.1 Pipe Element 297
14.3.2 Bending Stiffener 298
14.3.3 Sea Condition 299
14.3.4 Platform Motion Response 300
14.3.5 Time Domain Simulation Analysis 301
14.4 Failure Mode and Design Criteria 302
14.4.1 Axisymmetric Load Model 302
14.4.2 Bending Load Model 303
14.5 Calculation Method of Fatigue Life of Flexible Riser 305
14.5.1 Rainflow Counting Method 305
14.5.2 S-N Curve 305
14.5.3 Miner's Linear Cumulative Damage Theory 307
14.5.4 Modification of Average Stress on Fatigue Damage 308
14.6 Example of Fatigue Life Analysis of Flexible Riser 309
References 314
15 Steel Tube Umbilical and Control Systems 317
15.1 Introduction 317
15.1.1 General 317
15.1.2 Feasibility Study 318
15.1.3 Detailed Design and Installation 319
15.1.4 Qualification Tests 320
15.2 Control Systems 320
15.2.1 General 320
15.2.2 Control Systems 321
15.2.3 Elements of Control System 322
15.2.4 Umbilical Technological Challenges and Solutions 323
15.3 Cross-Sectional Design of the Umbilical 326
15.4 Steel Tube Design Capacity Verification 327
15.4.1 Pressure Containment 328
15.4.2 Allowable Bending Radius 328
15.5 Extreme Wave Analysis 329
15.6 Manufacturing Fatigue Analysis 330
15.6.1 Accumulated Plastic Strain 330
15.6.2 Low Cycle Fatigue 331
15.7 In-Place Fatigue Analysis 331
15.7.1 Selection of Sea State Data From Wave Scatter Diagram 332
15.7.2 Analysis of Finite Element Static Model 332
15.8 Installation Analysis 332
15.9 Required On-Seabed Length for Stability 333
References 334
16 Stress and Fatigue of Umbilicals 337
16.1 Introduction 337
16.2 STU Fatigue Models 338
16.2.1 Simplified Model 339
16.2.1.1 Axial and Bending Stresses 339
16.2.1.2 Friction Stress 340
16.2.1.3 Simplified Approach: Combining Stresses 342
16.2.1.4 Simplified (Combining Stresses) Fatigue Damage 342
16.2.1.5 Simplified Model Assumptions 343
16.2.2 Enhanced Non-Linear Time Domain Fatigue Model 343
16.2.2.1 Friction Stresses 344
16.2.2.2 Effect of Multiple Tube Layers 344
16.2.2.3 Combined Friction Stresses 345
16.2.2.4 Axial and Bending Stresses 345
16.2.2.5 Combining Stresses 346
16.2.2.6 Fatigue Life 346
16.2.2.7 Benefits of Enhanced Non-Linear Time Domain Fatigue Model 347
16.3 Worked Example 348
16.3.1 Time Domain vs. Simplified Approaches 350
16.3.2 Effect of Friction on STU Fatigue 351
16.3.2.1 Influence of High Tube Friction on Umbilical Fatigue 352
16.3.2.2 Influence of Low Tube Friction on Umbilical Fatigue 352
16.3.2.3 Influence of Metal-to-Metal Friction vs. Metal-to-Plastic Contact on Umbilical Fatigue 352
16.3.3 Effect of Increasing Water Depth 353
16.3.4 Effect of Increasing the Tube Layer Radius 354
16.4 Conclusions 355
16.5 Recommendations 356
References 357
17 Cross-Sectional Stiffness for Umbilicals 359
17.1 Introduction 359
17.2 Theoretical Model of Umbilicals 361
17.3 Bending Stiffness of Umbilicals 362
17.4 Tensile Stiffness of Umbilicals 366
17.5 Torsional Stiffness of Umbilicals 368
17.6 Ultimate Capacity of Umbilicals 368
17.6.1 Minimum Bending Curvature 368
17.6.2 Minimum Tensile Load 369
17.6.3 Tensile Capacity Curve 369
References 372
18 Umbilical Cross-Section Design 375
18.1 Introduction 375
18.1.1 General 375
18.1.2 Sectional Composition of the Umbilical Cable 375
18.1.3 Umbilical Cable Structure Features 376
18.2 Umbilicals Cross-Section Design Overview 377
18.2.1 Umbilical Cross-Section Design Flowchart 377
18.2.2 Load Analysis 378
18.3 Umbilical Cable Cross-Section Design 380
18.3.1 Umbilical Cable Cross-Section Layout Design 380
18.3.2 Tensile Performance Design 381
18.3.3 Bending Performance Design 382
References 384
Part 3: Fiber Glass Reinforced Deep Water Risers 385
19 Collapse Strength of Fiber Glass Reinforced Riser 387
19.1 Introduction 387
19.2 External Pressure Test 388
19.2.1 Testing Specimen 388
19.2.2 Testing System 389
19.2.3 Testing Results 389
19.3 Theoretical Analysis 390
19.3.1 Fundamental Assumptions 390
19.3.2 Constitutive Model of Materials 391
19.3.3 Establish the Equations of Motion 393
19.3.4 Establish Virtual Work Equations 394
19.4 Numerical Analysis 394
19.5 Finite Element Analysis 395
19.5.1 Establish the Finite Element Model 396
19.5.2 The Results of the Finite Element Analysis 397
19.6 Conclusion 401
References 402
20 Burst Strength of Fiber Glass Reinforced Riser 405
20.1 Introduction 405
20.2 Experiment 406
20.2.1 Dimensions and Material Properties of FGRFP 406
20.2.2 Experiment Device 407
20.2.3 Experiment Results 407
20.3 Numerical Simulations 407
20.3.1 Mesh and Interaction 407
20.3.2 Load and Boundary Conditions 408
20.3.3 Numerical Results 409
20.4 Analytical Solution 409
20.4.1 Basic Assumptions 409
20.4.2 Stress Analysis 411
20.4.3 Boundary Condition 414
20.5 Results and Discussion 416
20.6 Parametric Analysis 417
20.6.1 Winding Angle of Fiber Glass 417
20.6.2 Diameter-Thickness Ratio 418
20.7 Conclusions 419
References 419
21 Structural Analysis of Fiberglass Reinforced Bonded Flexible Pipe Subjected to Tension 421
21.1 Introduction 421
21.2 Experiment 423
21.2.1 Basic Assumptions 423
21.2.2 Material Characteristics 425
21.2.3 Experimental Results 426
21.3 Theoretical Solution 427
21.3.1 Basic Assumptions 429
21.3.2 Cross-Section Simplification 429
21.3.3 Fiber Deformation 430
21.3.4 Cross-Section Deformation 431
21.3.5 Equilibrium Equations 434
21.4 Finite Element Model 434
21.5 Comparison and Discussion 436
21.5.1 Tension-Extension Relation 436
21.5.2 Cross-Section Deformation 437
21.5.3 Fiberglass Stress 439
21.5.4 Contribution of Each Material 439
21.5.5 Summary 440
21.6 Parametric Study 442
21.6.1 Winding Angle 442
21.6.2 Fiberglass Amount 443
21.6.3 Diameter-Thickness Ratio 444
21.7 Conclusions 445
Acknowledgement 446
References 446
22 Fiberglass Reinforced Flexible Pipes Under Bending 449
22.1 Introduction 449
22.2 Experiment 451
22.2.1 Experimental Facility 451
22.2.2 Specimen 453
22.2.3 Experiment Process 453
22.2.4 Experimental Results 455
22.3 Analytical Solution 457
22.3.1 Fundamental Assumption 457
22.3.2 Kinematic Equation 457
22.3.3 Material Simplification 459
22.3.4 Constitutive Model 462
22.3.5 Principle of Virtual Work 464
22.3.6 Algorithm of Analytical Solutions 464
22.4 Finite Element Method 465
22.5 Result and Conclusion 466
22.6 Parametric Analysis 469
22.6.1 D/t Ratio 469
22.6.2 Initial Ovality 470
22.7 Conclusions 472
References 473
23 Fiberglass Reinforced Flexible Pipes Under Torsion 475
23.1 Introduction 475
23.2 Experiments 477
23.3 Experimental Results 478
23.4 Analytical Solution 481
23.4.1 Coordinate Systems 481
23.4.2 Elastic Constants of Reinforced Layers (k = 2, 3 ... (n - 1)) 483
23.4.3 Reinforced Layers Stiffness Matrix k = 2, 3...(n - 1) 484
23.4.4 Inner Layer and Outer Layer Stiffness Matrix (k = 1, n) 486
23.4.5 Stress and Deformation Analysis 487
23.4.6 Boundary Conditions 491
23.4.7 Interface Conditions 492
23.4.8 Geometric Nonlinearity 493
23.5 Numerical Simulations 494
23.6 Results and Discussions 496
23.7 Parametric Analysis 498
23.7.1 Effect of Winding Angle 498
23.7.2 Effect of Thickness of Reinforced Layers 498
23.8 Conclusions 499
Acknowledgments 500
References 501
24 Cross-Section Design of Fiberglass Reinforced Riser 503
24.1 Introduction 503
24.2 Nomenclature 503
24.3 Basic Structure of Pipe 505
24.3.1 Overall Structure 505
24.3.2 Material 506
24.4 Strength Failure Design Criteria 506
24.4.1 Burst Pressure 506
24.4.2 Burst Pressure Under Internal Pressure Bending Moment 508
24.4.3 Yield Tension 508
24.5 Failure Criteria for Instability Design 510
24.5.1 Minimum Bending Radius 510
24.5.2 External Pressure Instability Pressure 510
24.6 Design Criteria for Leakage Failure 511
References 511
25 Fatigue Life Assessment of Fiberglass Reinforced Flexible Pipes 513
25.1 Introduction 513
25.2 Global Analysis 515
25.3 Rain Flow Method 517
25.4 Local Analysis 519
25.5 Modeling 519
25.6 Result Discussion 520
25.7 Sensitivity Analysis 524
25.8 Fatigue Life Assessment 527
25.9 Conclusion 528
References 529
Part 4: Ancillary Equipments for Flexibles and Umbilicals 531
26 Typical Connector Design for Risers 533
26.1 Introduction 533
26.2 Carcass 534
26.3 Typical Connector 535
26.4 Seal System 536
26.5 Termination of the Carcass 537
26.6 Smooth Bore Pipe 539
26.7 Rough Bore Pipe 540
26.8 Discussion 542
26.9 Conclusions 544
References 545
27 Bend Stiffener and Restrictor Design 547
27.1 Introduction 547
27.2 Response Model 548
27.3 Extreme Load Description 549
27.4 General Optimization Scheme 550
27.5 Application Example 552
27.6 Non-Dimensional Bend Stiffener Design 553
27.7 Alternative Non-Dimensional Parameters 556
27.8 Conclusions 558
References 558
28 End Termination Design for Umbilicals 561
28.1 Introduction 561
28.2 Umbilical Termination Assembly 561
28.2.1 General 561
28.2.2 UTA Design 562
28.2.3 UTA Structural Design Basis 565
28.3 Subsea Termination Interface 566
References 568
29 Mechanical Properties of Glass Fibre Reinforced Pipeline During the Laying Process 569
29.1 Introduction 569
29.2 Theoretical Analysis 570
29.2.1 Wave Load 570
29.2.2 Motion Response of the Vessel 572
29.2.3 Dynamic Numerical Solution 573
29.3 Static Analysis 575
29.4 Dynamic Characteristic Analysis 579
29.4.1 Influence of the Wave Direction 579
29.4.2 Influencing of Different Lay Angle 582
29.4.3 Influencing Submerged Weight 584
29.5 Conclusions 584
References 586
Index 589
1
Introduction
1.1 Flexible Pipelines Overview
The oil and gas industry proclaims pipelines as the most economical device of large scale overland conveyance for crude oil and natural gas, compared to truck and rail transportation. These can continuously carry large quantities of fluids and they are considered by far to be the most reliable. The rapid development of the petroleum industry leads to high financial investments in engineering research, which has quickly conducted to achievement of remarkable effects, so that recently steel homogeneous pipes have been replaced by flexible pipelines, widely used by oil and gas industry for both onshore and offshore purposes. Mostly, they are employed offshore as transportation of well products such as oil, gas, and condensate, well control lines, injection of water and gases, and export of processed product, but also as flowlines connecting subsea wells, well-head platforms, templates or loading terminals, processing platforms, and jumper lines connecting fixed platforms to support vessels.
Flexible pipelines are relatively new kinds of profiles which have developed since the late 1970s; initially, they were used in few offshore areas until being employed in many projects, thanks to the applicability in water depths up to 8,000 ft, pressures up to 10,000 psi, and high temperatures up to 150°, beside the high adaptability to different environmental conditions and large vessels motion [1].
Nowadays, even if many researchers are employed to enhance knowledge about flexible pipelines, there are no books available that systematically introduce the design procedures and analysis criteria that are always valid for the wide spectrum of these structures. On the other hand, it is possible to find many reference sources which can streamline the issues.
Moreover, they are widely used also because of their easy and cheap transport and installation, due to the possibility of being prefabricated onshore in long lengths and stored in limited size on reels; in fact, as most relevant structural property, these pipes show very low bending stiffness in comparison to axial tensile stiffness. Besides, the economic benefits, they are considered to be technically proficient due to their easy and fast laying procedure, durability, and recoverability. In the light of the above, the petroleum industry turns into flexible structures, allowing for permanent connection between the subsea system and any facility at the water surface with large motions.
Being flexible pipes, crucial elements for the right operational oil and gas spill in terms of both performance and pollution, at the same time, a deep finite element model (FEM) is highly recommended in order to verify the reliability of the design. This method called DTA (Design Through Analysis) involves both the abovementioned procedures and the two-step process is used in complementary way in order to reach less conservative outcomes in designing, thus minimizing the project CAPEX (Capital Expenditure) and OPEX (Operating Expenditure). In some cases, it shows that codes and regulations are over conservative, and the real behavior can be captured through a FE simulation when the input parameters are well defined, if not data are statistically modeled in order to produce a reliable distribution for a range of loads and effects.
1.2 Environmental Conditions
The wide expansion of these structures during the last decades has been made possible thanks to the costs-design optimization. It considers a deep understanding of the environmental conditions in which the pipeline will be installed and operated. Some of the drivers are: water depth and oceanographic data, chemical composition and flow phase of the extracted fluid, quantity of salt in the surrounding water, and operating internal and external temperatures and pressure. In addition, transport and installation circumstances must be considered due to the fact that in some cases, extreme loading conditions are shown during these phases. Most of the time, combination of loads needs to be taken into account. The analysis of the surrounding conditions, for example, temperature and corrosion, cannot be under estimation because of limiting capacity of the structures.
Temperature can affect the correct purpose of the pipe, in some cases, very high or low temperature of the fluids leads to the need of extra thermal insulation design, such as pipe-in-pipe or wet insulation, which considers the different thermal responses of materials. Polymeric materials exhibit lower thermal conductivity and higher thermal expansion coefficients compared to steel, so that the plastic layers govern the temperature profile through the pipe wall. Operating temperature is the foremost principle for selecting the polymer material in order to ensure the correct mechanical behavior of the pipe. In fact, for a given material, as the temperature rises the magnitude of the yield stress and Young modulus decreases, so the ultimate strain increases and vice versa, leading to various mechanical response properties during the service life of the structures. Generally, temperature profile impacts most of the design parameters, it is also influenced by the water depth outwardly and by the reservoir formation internally; thus, it must be carefully considered for both service limit state and ultimate limit state.
Cross-sectional integrity needs to be mentioned for a remarkable pipeline design, which is of relevant importance especially for deep water. In fact, here hydro-static pressure is high, and the cross-section must be dimensioned against local buckling failure which is extremely influenced by imperfections. Not only, any other steel components provide structural support against axial, bending, and torsional loads, and their integrity is essential. Corrosion and cracks damages are hard to detect, being the structural profile made by different layers. Considering the conservative but actual hypothesis of failure of polymeric materials, steel corrosion is caused by interaction with salt water, air, or internal acid fluids or combination of them, which chemically alter materials. Besides the mechanical properties and price, the main driver for the selection of steel materials is the corrosion resistance to operating environment. The annulus conditions are continuously tested and monitored, and it is common practice to include deterioration protections such as coating, corrosion inhibitors, application of special materials and cathodic protection, in addition to lubrication oil distributed along the pipe during the manufacturing.
During the years, the availability of reservoirs onshore and in shallow water has decreased, and the need of petroleum pushed the industry to open new challenging offshore campaigns. The employment of flexible pipes in subsea brings researchers to focus on the estimation of the structural behavior in deep waters. Here, the environmental conditions are tougher, and reinforcements are necessary. Hydro-static pressure rises as water depth increases, which leads to considerable hoop stresses and buckling issues. For the whole pipe, this problem is managed making use of devices that enhance the strength in this sense, but at the same time, they bring additional structural weight and increasing gravity loads. Now, it can be deduced that as the water column grows, the magnitude of the tensile load rises. Dynamic tension is amplified by existing drag forces if considering the substantial pipe length in deep water, which is directly reflected on the end fitting, with consequently fatigue damages at this point due to constraint effects that may take place.
What being said, the challenge of the engineering is trying to make the best design, keeping the costs as low as possible, moving from local to global analysis. Any structure needs a particular design for the specific field and for precise environmental conditions, at the same time taking advantages from the experiences about other projects. The selection of the most suitable setup for any flexible structure is founded on practice and engineering judgment, in Figure 1.1, some examples of possible riser configurations using flexible pipes for which the choice is based on the abovementioned principles [2-4].
Figure 1.1 Employment of flexible pipelines as risers.
1.3 Flexible Pipeline Geometry
Even if flexible pipelines have many applications, the common factor is to provide large flexural deformations while they are subjected to other loading forces, strictly necessary during installation and transport. Mostly, this property depends on their geometrical configuration, which is made by many layers. Generally, flexible pipes are compound by many main layers of different materials and functions:
- Internal polymeric sealing layer provides insulation from internal fluids, prevents corrosion and leakage due to extract materials,
- Helical armoring layers provide the required strength against different loading conditions, the number of layers and wires per each layer is variable and depends on the design;
- Outer polymeric layer prevents seawater from interacting with the armor layers.
They can be characterized in two main groups: the metal-based flexible pipes designed to withstand high loads see Figure 1.2 and the composite-based flexible pipes (FCP), which are much simpler and employed for lower functional requirements. These can be further divided in two more groups: bonded and unbonded structures.
In the bonded structures, all the elements are fused together in the surrounding matrix through a vulcanization process.
