Breaking Wave Loads and Stress Analysis of Jacket Structures Supporting Offshore Wind Turbines

249 Pages·2012·9.25 MB·English

by  Louise Devaney

Breaking Wave Loads and Stress Analysis
of Jacket Structures Supporting Offshore
Wind Turbines

A thesis submitted to the University of Manchester for the degree of
Doctor of Engineering in the Faculty of Engineering and Physical
Sciences

2012

Louise Devaney

School of Mechanical, Aerospace and Civil Engineering
I. Contents
I. Contents ....................................................... 2
II. List of Figures .............................................. 8
III. List of Tables .............................................. 15
IV. List of Symbols and Abbreviations .......... 17
V. Abstract ...................................................... 20
VI. Declaration ................................................. 21
VII. Copyright Statement .................................. 21
VIII. Acknowledgements ................................... 22
Chapter 1 Introduction ................................................ 23
1.1 Motivation for Research ...................................................... 23
1.2 Aims of the Project .............................................................. 24
1.3 Industrial Relevance of the Project ..................................... 25
1.4 Outline of Thesis Structure ................................................. 26
Chapter 2 Design of Offshore Wind Support
Structures ..................................................................... 27
2.1 Introduction ......................................................................... 27
2.2 Overview of Support Structure Options for Offshore Wind
Turbines ...................................................................................... 27

2.2.1 Monopiles .............................................................................................................28

2.2.2 Jackets .................................................................................................................29

2.2.3 Tripods .................................................................................................................31

2.2.4 Gravity Bases .......................................................................................................32

2.2.5 Concepts from the Offshore Wind Accelerator competition .................................33

2.2.6 Summary ..............................................................................................................37
2.3 Preliminary Design of Support Structures ........................... 37

2.3.1 Introduction ...........................................................................................................37

2.3.2 Natural Frequency Check ....................................................................................38

2.3.3 Serviceability Limit State Design ..........................................................................40

2
I. Contents

2.3.4 Ultimate Limit State Design ..................................................................................40

2.3.5 Fatigue Limit State Design ...................................................................................41

2.3.6 Areas for Further Work .........................................................................................47
2.4 Summary ............................................................................ 48
Chapter 3 Wind and Wave Loading on Monopile
Foundations ..................................................................... 50
3.1 Introduction ......................................................................... 50
3.2 Overview of Wave Theory ................................................... 50

3.2.1 Linear Wave Theory .............................................................................................51

3.2.2 Stream Function and Transfer Function Methods................................................51

3.2.3 Calculation of wave forces with the Morison equation .........................................52
3.3 Investigation into Wave Loading on a Monopile Foundation 53

3.3.1 Overview of Investigation by Luck and Benoit .....................................................53

3.3.2 Overview of Investigation into Applicability of Linear and Nonlinear Wave

Theories .............................................................................................................................56

3.3.3 Methodology .........................................................................................................56

3.3.4 Results .................................................................................................................58

3.3.5 Conclusions ..........................................................................................................62
3.4 Experiments into the Impact of Breaking Waves on a
Monopile Foundation .................................................................. 63

3.4.1 Introduction ...........................................................................................................63

3.4.2 Review of work on breaking waves ......................................................................63

3.4.3 Methodology .........................................................................................................64

3.4.4 Results .................................................................................................................68

3.4.5 Conclusion ............................................................................................................71

3.4.6 Recommendations ...............................................................................................71
3.5 Investigation into Wind and Wave Loading on a Monopile for
Two Different Turbine Capacities ................................................ 72

3.5.1 Methodology .........................................................................................................72

3.5.2 Results .................................................................................................................73

3.5.3 Conclusion ............................................................................................................78

3.5.4 Recommendations ...............................................................................................78
3.6 Summary ............................................................................ 78

3
I. Contents

Chapter 4 Review of Stress Concentration Factors for
Uniplanar and Multiplanar Tubular Joints ..................... 80
4.1 Introduction ......................................................................... 80
4.2 Development of Parametric Equations for SCFs for Tubular
Joints .......................................................................................... 81

4.2.1 SCFs for uniplanar tubular joints ..........................................................................81

4.2.2 SCFs for multiplanar joints ...................................................................................97
4.3 Development of FE Models ............................................... 103

4.3.1 Early FE models – using shells ..........................................................................103

4.3.2 Advanced FE models – solid elements ..............................................................105

4.3.3 Processing results ..............................................................................................106

4.3.4 Conclusions ........................................................................................................108
4.4 Summary .......................................................................... 108
Chapter 5 Finite Element Modelling of Uniplanar and
Multiplanar Tubular Joints ............................................ 111
5.1 Introduction ....................................................................... 111
5.2 Methodology ..................................................................... 112

5.2.1 Overview of Model Verification Process ............................................................112

5.2.2 Modelling Considerations ...................................................................................113

5.2.3 FE Model Details ................................................................................................114

5.2.4 Reporting of stresses .........................................................................................121

5.2.5 Model Verification Strategy ................................................................................122
5.3 Results of Model Verification ............................................. 122

5.3.1 Checks on the nominal stress ............................................................................122

5.3.2 Mesh Convergence Study ..................................................................................126

5.3.3 Efthymiou vs OTH ..............................................................................................128

5.3.4 FE models checked against OTH ......................................................................131

5.3.5 FE models checked against Efthymiou ..............................................................136

5.3.6 FE models checked against independent models .............................................137
5.4 Evaluation of Results ........................................................ 138

5.4.1 Summary of results ............................................................................................138

5.4.2 Potential inconsistencies in modelling approaches ...........................................139

5.4.3 Summary ............................................................................................................145

5.4.4 Conclusion ..........................................................................................................146

4
I. Contents

Chapter 6 Investigation into Effects of Loading of
Multiplanar Braces on Brace-Chord Intersection Stress
Distribution ................................................................... 147
6.1 Introduction ....................................................................... 147
6.2 Summary of SCF Calculations Using Efthymiou ............... 147
6.3 Investigation 1 – Comparison of Stresses in Uniplanar and
Multiplanar Tubular K Joints ...................................................... 150

6.3.1 Methodology .......................................................................................................150

6.3.2 Results ...............................................................................................................151

6.3.3 Conclusions ........................................................................................................156
6.4 Investigation 2 – Behaviour of Multiplanar Tubular Joints . 157

6.4.1 Introduction .........................................................................................................157

6.4.2 Methodology .......................................................................................................157

6.4.3 Results ...............................................................................................................158

6.4.4 Conclusions ........................................................................................................166
6.5 Investigation 3 – Carry-Over of Loads to Neighbouring
Braces for Multiplanar Joints ..................................................... 167

6.5.1 Introduction .........................................................................................................167

6.5.2 Methodology .......................................................................................................167

6.5.3 Results ...............................................................................................................168

6.5.4 Conclusions ........................................................................................................172
6.6 Overall Conclusion ............................................................ 172
Chapter 7 Parametric Study and Regression Analysis
to Develop Multiplanar Tubular Joint SCF Equations 174
7.1 Introduction ....................................................................... 174
7.2 Parameter Study Methodology .......................................... 175

7.2.1 Pre-processing ...................................................................................................175

7.2.2 Post-processing of data .....................................................................................178
7.3 Regression Analysis Methodology .................................... 179

7.3.1 Ranges of data for the analysis..........................................................................179

7.3.2 Modifying Efthymiou coefficients and equations ................................................182

7.3.3 Developing a new set of equations ....................................................................183

7.3.4 Validating the reliability of equations ..................................................................184

5
I. Contents

7.3.5 Development of equations for implementation ...................................................185
7.4 Results of Regression Analysis ......................................... 188

7.4.1 Selected equations .............................................................................................188

7.4.2 Factors for converting equations ........................................................................191
7.5 Comparison of final equations ........................................... 195

7.5.1 Summary of axial results ....................................................................................204

7.5.2 Summary of IPB results .....................................................................................205

7.5.3 Summary of OPB results ....................................................................................205
7.6 Evaluation of Converted Equations ................................... 206

7.6.1 Summary of results ............................................................................................208
7.7 Overall Summary, Conclusions and Future Work ............. 212

7.7.1 Overall Summary ...............................................................................................212

7.7.2 Conclusion ..........................................................................................................213

7.7.3 Further work .......................................................................................................214
Chapter 8 Design of Tubular Joints – Superposition
of SCFs to Determine Hot Spot Stresses for Multiplanar
Tubular Joints ................................................................ 216
8.1 Introduction ....................................................................... 216
8.2 Hot Spot Stress Prediction using Finite Element Analysis . 217
8.3 Hot Spot Stress Calculations ............................................ 218

8.3.1 Using the Efthymiou Equations ..........................................................................218

8.3.2 Using New Sets of Equations .............................................................................218
8.4 Results .............................................................................. 221

8.4.1 Analysis of results ..............................................................................................222

8.4.2 Overall discussion of results ..............................................................................230
8.5 Summary, Conclusions and Future Work .......................... 231

8.5.1 Summary ............................................................................................................231

8.5.2 Conclusions ........................................................................................................231

8.5.3 Future Work ........................................................................................................232
Chapter 9 Discussions, Conclusions and Future
Work ................................................................... 235
9.1 Introduction ....................................................................... 235
9.2 Wind and Wave Loading on Monopile Foundations .......... 235

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I. Contents

9.2.1 Summary ............................................................................................................235

9.2.2 Discussion ..........................................................................................................236

9.2.3 Conclusions ........................................................................................................237

9.2.4 Future Work ........................................................................................................237
9.3 Investigation into the Behaviour of Multiplanar Tubular Joints .
......................................................................................... 237

9.3.1 Summary ............................................................................................................237

9.3.2 Discussion ..........................................................................................................239

9.3.3 Conclusions ........................................................................................................241

9.3.4 Future Work ........................................................................................................242
Appendix A – Flow chart for preliminary design of
offshore wind turbine support structures ................... 243
References ................................................................... 245

Final word count (including footnotes and end notes): 60,690

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II. List of Figures
Figure 1-1: Offshore wind resource in Europe (taken from

http://www.wwindea.org/technology/ch02/imgs/2_2_2_img11.jpg, accessed 03.06.2012) .........23
Figure 2-1: Monopile foundation with its components and attachments (source: GL Garrad
Hassan available and online http://www.wind-energy-the-facts.org/en/part-i-technology/chapter-

5-offshore/wind-farm-design-offshore/offshore-support-structures.html) ....................................28
Figure 2-2: Diagram of jacket structure showing its features with appurtenances (source: GL
Garrad Hassan and available online http://www.wind-energy-the-facts.org/en/part-i-

technology/chapter-5-offshore/wind-farm-design-offshore/offshore-support-structures.html) .....29
Figure 2-3: Example of a cast joint taken from http://www.gusswerkstoff.de/221.jpg (accessed

25.02.2012) ..................................................................................................................................30
Figure 2-4: Diagram of tripod structure showing its features (source: GL Garrad Hassan and
available online http://www.wind-energy-the-facts.org/en/part-i-technology/chapter-5-

offshore/wind-farm-design-offshore/offshore-support-structures.html) ........................................31
Figure 2-5: Tripod used in the Global Tech 1 project in Germany taken from

http://www.owt.de/03_referenzen/pix/2009/01.jpg (accessed 19.02.2012) .................................32
Figure 2-6: Gravity base structure with appurtenances (source: GL Garrad Hassan and
available online http://www.wind-energy-the-facts.org/en/part-i-technology/chapter-5-

offshore/wind-farm-design-offshore/offshore-support-structures.html) ........................................33
Figure 2-7: Suction bucket monopile from Universal Foundations taken from
http://www.carbontrust.co.uk/emerging-technologies/current-focus-areas/offshore-

wind/pages/foundation-design.aspx (accessed 19.02.2012) .......................................................34
Figure 2-8: Twisted jacket concept from Keystone taken from
http://www.carbontrust.co.uk/emerging-technologies/current-focus-areas/offshore-

wind/pages/foundation-design.aspx (accessed 19.02.2012) .......................................................35
Figure 2-9: Gravity base structure from Gifford, BMT and Freyssinet taken from
http://www.carbontrust.co.uk/emerging-technologies/current-focus-areas/offshore-

wind/pages/foundation-design.aspx (accessed 19.02.2012) .......................................................36
Figure 2-10: Tribucket design by SPT Offshore & Wood Group taken from
http://www.carbontrust.co.uk/emerging-technologies/current-focus-areas/offshore-

wind/pages/foundation-design.aspx (accessed 19.02.2012) .......................................................36
Figure 2-11: Soft to stiff frequency intervals of a three bladed, constant rotational speed wind
turbine with frequency on the x-axis and dynamic amplification factor (DAF) displayed on the y-

axis ...............................................................................................................................................38

Figure 2-12: Variation of DAF with frequency ratio, taken from Barltrop and Adams [21] ...........39

Figure 2-13: Section through weld for fabrication mismatch, taken from DNV-RP- C203 [19] ....43

Figure 2-14: Section through weld for changes in thickness, taken from DNV-RP- C203 [19] ...43

Figure 2-15: Classification of simple planar joints taken from DNV-RP-C203 [19] .....................44

Figure 2-16: Geometries for different types of joint .....................................................................45

8
II. List of Figures

Figure 2-17: Definition of geometrical parameters taken from DNV-RP-C203 [19] .....................46
Figure 3-1: Definition of wave parameters for a sinusoidal wave taken from Novak et al, 2001

[26] ...............................................................................................................................................51
Figure 3-2: Schematic for determining forced exerted on a cylinder submerged in water depth, d

.....................................................................................................................................................52
Figure 3-3: Diagram of flume with cylinder tested in the laboratories taken from Luck and Benoit,

2004 [25] ......................................................................................................................................54
Figure 3-4: Maximum and minimum measured forces on the monopile according to wave height

H, water depth d, gradient m and period T taken from Luck and Benoit, 2004 [25] ....................55
Figure 3-5: Calculated force vs. measured force (maximum and minimum) taken from Luck and

Benoit, 2004 [25]. Lines show gradients of 1, 1/2 and 1/4 ..........................................................56
Figure 3-6: Calculated force from nonlinear wave theory against measured force by Luck and

Benoit [25] ....................................................................................................................................58
Figure 3-7: Measured and calculated forces vs. wave height for water depth d = 0.3m with

maximum theoretical force represented by blue curve ................................................................60
Figure 3-8: Measured and calculated forces vs. wave height for water depth d =0.4m with

maximum theoretical force represented by blue curve ................................................................60
Figure 3-9: Measured and calculated forces vs. wave height for water depth d = 0.6m with

maximum theoretical force represented by blue curve ................................................................61
Figure 3-10: Measured and calculated forces vs. wave height for water depth d = 0.8m with

maximum theoretical force given as blue curve ...........................................................................61

Figure 3-11: Experimental setup – section through flume with cylinder and equipment .............64

Figure 3-12: Experimental setup – plan view of flume with cylinder and equipment ...................64

Figure 3-13: Experimental setup – section through flume with cylinder, ramp and equipment ...65
Figure 3-14: Section through cylinder showing wave forces applied and its equivalent static

system ..........................................................................................................................................66
Figure 3-15: Picoscope trace for wave gauges placed at two positions in the flume (see Figure

3-11) .............................................................................................................................................67
Figure 3-16: Picoscope trace for strain gauges at the top and bottom of the cylinder for non-

breaking waves ............................................................................................................................67
Figure 3-17: Picoscope trace for strain gauges at the top and bottom of the cylinder for breaking

waves ...........................................................................................................................................68

Figure 3-18: Comparison of experimental and calculated results for non-breaking waves .........69

Figure 3-19: Comparison of experimental and calculated results for breaking waves ................69
Figure 3-20: Comparison of data determined from experimental results (LCD+PKS) and the

results from Luck and Benoit (ML+MB) using linear wave theory ...............................................70
Figure 3-21: Comparison of data determined from experimental results (LCD+PKS) and the

results from Luck and Benoit (ML+MB) using nonlinear wave theory .........................................70
Figure 3-22: Graph of a comparison of hub loading experienced during operation by a 2MW and

5MW turbine from cut-in speed of 4m/s to cut-out speed of 25m/s .............................................74

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II. List of Figures

Figure 3-23: Graph of the moments due to wind and wave loading for a 2MW and 5MW turbine

with wind speed U = 8m/s ............................................................................................................75
Figure 3-24: Graph of the moments due to wind and wave loading for a 2MW and 5MW turbine

with wind speed U = 10m/s ..........................................................................................................75
Figure 3-25: Graph of the moments due to wind and wave loading for a 2MW and 5MW turbine

with wind speed U = 12m/s ..........................................................................................................76
Figure 3-26: Graph of the moments due to wind and wave loading for a 2MW and 5MW turbine

with wind speed U = 14m/s ..........................................................................................................76
Figure 3-27: Graph of the moments due to wind and wave loading for a 2MW and 5MW turbine

with wind speed U = 20m/s ..........................................................................................................77
Figure 3-28: Graph of the moments due to wind and wave loading for a 2MW and 5MW turbine

with wind speed U = 25m/s ..........................................................................................................77

Figure 4-1: Chord saddle SCF variation with β ratio taken from Lalani et al [46] ........................89
Figure 4-2: OPB on K-joints at brace saddle locations – comparison with FE results and SCFs

determined from simplified formulae (taken from Karamanos et al [53]) .....................................95

Figure 4-3: Configuration of KK-joints tested – note geometries – taken from Romeijn [57] ......98
Figure 4-4: Comparison between numerical and experimental hot spot strains taken from

Romeijn [57] .................................................................................................................................99
Figure 4-5: Direction of positive multiplanar modes used in van Wingerde’s analysis taken from

van Wingerde et al [54] ..............................................................................................................100

Figure 4-6: Composition of meshes in Reber’s model taken from Reber [43] ...........................104
Figure 4-7: Example of shell model using combination of 8-node and 16-node elements with

weld modelled ............................................................................................................................105
Figure 4-8: Typical weld details modelled in FE software using brick elements for tubulars and

welds taken from Choo and Qian [63] ........................................................................................106
Figure 4-9: Definition of the geometric stress zone in tubular joints taken from DNV-OS-J101

[12] .............................................................................................................................................107
Figure 4-10: Location of weld singularity for hot spot stress extrapolation dependent on element

types used in tubular joint FE models taken from DNV-OS-J101 [12] .......................................108

Figure 5-1: Weld geometry details for different brace inclinations from GL Garrad Hassan .....115
Figure 5-2: Stress extrapolation regions defined on both brace and chord of T joint in ANSYS

Workbench .................................................................................................................................115
Figure 5-3: Divisions on a uniplanar T joint to allow for different mesh density generation and

mesh symmetry taken from ANSYS Workbench .......................................................................116

Figure 5-4: Definition of stresses in welded structures taken from DNV-OS-J101 (2010) [12] .117

Figure 5-5: Meshing controls assigned to a uniplanar T joint taken from ANSYS Workbench .118
Figure 5-6: Variation in mesh sizes for different regions in the joint taken from ANSYS

Workbench .................................................................................................................................120

Figure 5-7: Types of elements available in ANSYS Workbench................................................121

10