Numerical Investigation of Scouring at the Base of a Circular Pile

Numerical Investigation of Scouring at the Base
of a Circular Pile in a Steady Tidal Current
By
Mark Donnelly-Orr
A thesis submitted to the University of Dublin, Trinity College, in partial fulfilment of the requirements for the degree of
MAI in Mechanical & Manufacturing Engineering
April 2015
Supervisor
Dr. Craig Meskell
Dept. of Mechanical and Manufacturing Engineering
Parsons Building
Trinity College Dublin
Dublin 2, Ireland

Mark Donnelly-Orr
i
Declaration
I declare that I am the sole author of this dissertation and that the work present in it, unless
otherwise referenced, is entirely my own. I also declare that the work has not been submitted, in
whole or in part, to any other university as an exercise for a degree or any other qualification.
I agree that the library of Trinity College Dublin may lend or copy this dissertation upon request.
Mark Donnelly-Orr
Date: 7th
April 2015

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Abstract
The flow around a cylindrical pile exposed to a steady current is numerically investigated and the
extent of scouring that occurs around the base is assessed.
The primary objectives of the report is to determine the extent of scouring around the base of a
wind turbine pylon installed as part of the Dublin Array on the Kish and Bray Banks in Dublin Bay,
Ireland. The effect different sea current speeds will have on the extent of the scour will also be
determined.
The secondary objective of the report is to investigate the scour mitigating effects of various
scour prevention devices designed in this report, but based of different ideas found in the
academic literature. Various collars and helical wires were considered and the effects of the
devices examined.
The methodology of the report involved investigating the types of sediment, marine conditions
and the physical nature of the turbine pylons expected in the Dublin Array; these parameters
where then implemented into ANSYS Fluent, a computational fluid dynamics (CFD) numerical
model, and a solution numerically calculated.
It was determined that localised clear-water scouring initially occurred at the base of the pile
once a sea current of 0.225-0.275m/s arose. As the sea current increased, the extent of the
scouring region increased from where it initially occurred. Once a sea current of 0.4-0.6m/s arose,
live-bed scouring was deemed to occur and the entire seabed was in motion. The sea current at
which these transitions occurred depended on the sediment size, which varied from 0.2-0.8mm
diameters on the Kish and Bray Sand Banks.
The scour prevention devices designed were shown to have a substantial effect on the flow
regime around the pile, disrupting the magnitude and momentum of the horseshoe vortices that
normally form around circular piles and cause scouring on the seabed. Irregularities in the scour
prevention device simulations results reduced the confidence of the conclusions made about the
devices designed.
The main conclusions drawn from the report is that scouring will occur around the wind turbine
pylon bases that are installed on the Kish and Bray Banks as part of the Dublin Array. But given
the self-nourishing aspect of the sand banks, the extent of scouring is deemed not to be a
permanent feature of the seabed around the wind turbine pylon bases, but will gradually increase
and decrease depending on the tidal conditions. A secondary conclusion is that the scour
prevention devices are effective at disrupting the horseshoe vortices that would otherwise occur
around a circular pile, and hence will reduce the effects of scouring.

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Keywords
Scour, Offshore Wind Turbine, Monopile, SST Transition Model, ANSYS Fluent, Sediment
Movement, Dublin Bay, Steady Current.
Acknowledgements
I would like to thank my Supervisor Dr. Craig Meskell for his support, teachings, enthusiasm, and
guidance throughout this project.
I would like to acknowledge the entire academic staff of the Department of Mechanical and
Manufacturing Engineering at Trinity College for their support and assistance during my time at
Trinity College Dublin.
In addition I would like to thank my friends Jaakko, Rupert, and Aaron for their support, advice
and welcomed distractions.
I would also like to thank my girlfriend Amy, who was always there for me.
Lastly, I would like to thank my family, especially my parents, Peter and Wendy, for their constant
encouragement and support throughout the duration of this thesis and my time at Trinity College
Dublin.

Mark Donnelly-Orr
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Table of Contents
Declaration .............................................................................................................................................................. i
Abstract ................................................................................................................................................................. iii
Keywords............................................................................................................................................................... iv
Acknowledgements............................................................................................................................................... iv
Table of Figures ...................................................................................................................................................... x
Table of Tables....................................................................................................................................................xviii
Nomenclature.......................................................................................................................................................xix
Abbreviations...................................................................................................................................................xix
Units.................................................................................................................................................................xix
1 Introduction................................................................................................................................................... 1
1.1 Background ........................................................................................................................................... 1
1.2 Problem Definition................................................................................................................................ 1
1.3 Objectives ............................................................................................................................................. 2
1.4 Methodology......................................................................................................................................... 2
1.5 Outline .................................................................................................................................................. 2
2 Context .......................................................................................................................................................... 4
2.1 Global Warming .................................................................................................................................... 4
2.2 EU Climate Goals................................................................................................................................... 4
2.3 Ireland’s Climate Goals ......................................................................................................................... 4
2.4 Wind Energy.......................................................................................................................................... 6
2.5 Offshore Wind Energy........................................................................................................................... 7
2.6 Successful Installations around the World.......................................................................................... 10
3 Literature Review......................................................................................................................................... 12
3.1 Site Data.............................................................................................................................................. 12
3.1.1 Site Layout.................................................................................................................................. 12
3.1.2 Sediment .................................................................................................................................... 13
3.1.3 Tidal Flows.................................................................................................................................. 21
3.1.4 Sea Water Properties ................................................................................................................. 22
3.1.5 Boundary Layer Formations ....................................................................................................... 23
3.1.6 Foundation Type ........................................................................................................................ 28
3.2 Scouring .............................................................................................................................................. 30
3.2.1 Fundamental Fluid Mechanics ................................................................................................... 30
3.2.2 Different Features ...................................................................................................................... 32
3.2.3 Current Based............................................................................................................................. 38
3.2.4 Maximum Scour Depth .............................................................................................................. 38

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3.2.5 Equilibrium Scour Depth.............................................................................................................39
3.2.6 Clear Water Scour vs. Live Bed Scour Criterion..........................................................................39
3.2.7 Scour Protection & Prevention Measures..................................................................................41
3.2.8 Shear Stresses Acting on the Seabed .........................................................................................49
3.2.9 Sediment Movement..................................................................................................................50
3.3 Computational Fluid Dynamics ...........................................................................................................58
3.3.1 Software Used ............................................................................................................................58
3.3.2 Turbulence Model Chosen .........................................................................................................58
3.3.3 Benefits of Chosen Model ..........................................................................................................59
3.3.4 Limits of Chosen Model..............................................................................................................59
3.3.5 Validation of Choice ...................................................................................................................59
3.4 Project Validation................................................................................................................................60
4 Methodology................................................................................................................................................61
4.1 Creating the 3D Model........................................................................................................................61
4.2 Meshing Development........................................................................................................................62
4.2.1 Basic Mesh..................................................................................................................................62
4.2.2 Initial Bias ...................................................................................................................................62
4.2.3 Symmetry ...................................................................................................................................64
4.2.4 Hex-Dominant Meshing..............................................................................................................64
4.2.5 Volume Meshing.........................................................................................................................66
4.2.6 Refined Volume Meshing ...........................................................................................................67
4.2.7 Inflation Layer Details & Issues ..................................................................................................70
4.2.8 Element Count Limits .................................................................................................................71
4.2.9 Final Meshing .............................................................................................................................71
4.3 ANSYS 3D Model Parameters and Boundary Conditions ....................................................................76
4.3.1 UDF.............................................................................................................................................76
4.3.2 Fluid Properties ..........................................................................................................................76
4.3.3 Inlet Velocity...............................................................................................................................77
4.3.4 Setup Options.............................................................................................................................77
4.3.5 ANSYS Model Used.....................................................................................................................78
4.3.6 Zero Shear on Walls....................................................................................................................78
4.4 Meshing Independence.......................................................................................................................80
4.4.1 Meshing Independence Tests Development..............................................................................80
4.4.2 Meshing Independence Tests Results ........................................................................................86
4.4.3 Refined Final Meshing ................................................................................................................92
4.5 Mesh Validation ..................................................................................................................................95
4.5.1 Mesh Validation Techniques ......................................................................................................95
4.5.2 Mesh Validation Results ...........................................................................................................103
4.6 Creating the Scour Prevention 3D Models........................................................................................110

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4.6.1 Rectangular Collar .................................................................................................................... 110
4.6.2 Triangular Collar....................................................................................................................... 111
4.6.3 Rounded Collar......................................................................................................................... 111
4.6.4 Helical Wires............................................................................................................................. 112
4.7 Meshing Development for Scour Prevention Models....................................................................... 115
4.7.1 Volume Meshing ...................................................................................................................... 115
4.7.2 Meshing Details........................................................................................................................ 115
4.7.3 Inflation Layer Details & Issues ................................................................................................ 116
4.7.4 Element Count, Scour Prevention Models ............................................................................... 116
4.7.5 Final Meshing ........................................................................................................................... 116
4.8 ANSYS Scour Prevention Model Parameters and Boundary Conditions........................................... 120
4.8.1 UDF........................................................................................................................................... 120
4.8.2 Setup Options........................................................................................................................... 120
4.8.3 Fluid Properties ........................................................................................................................ 120
4.8.4 Inlet Velocity ............................................................................................................................ 120
4.8.5 ANSYS Model Used................................................................................................................... 120
4.8.6 Zero Shear on Walls ................................................................................................................. 120
4.9 Determining if Scour will occur......................................................................................................... 121
4.9.1 Stresses on Seabed................................................................................................................... 121
4.9.2 Streamlines............................................................................................................................... 123
4.9.3 y-Velocity Component.............................................................................................................. 125
5 Results ....................................................................................................................................................... 126
5.1 3D Model .......................................................................................................................................... 126
5.1.1 Scour Regions of 3D Model ...................................................................................................... 126
5.1.2 Streamlines of 3D Models ........................................................................................................ 139
5.1.3 y-Velocity Component of 3D Model......................................................................................... 141
5.2 Scour Prevention Models.................................................................................................................. 143
5.2.1 Scour Regions of Scour Prevention Devices ............................................................................. 143
5.2.2 Streamlines of Scour Prevention Devices................................................................................. 146
5.2.3 y-Velocity Components of Scour Prevention Devices .............................................................. 150
6 Discussion .................................................................................................................................................. 153
6.1 3D Model .......................................................................................................................................... 153
6.1.1 Scour Region Shape.................................................................................................................. 153
6.1.2 Streamlines............................................................................................................................... 158
6.1.3 y-Velocity.................................................................................................................................. 159
6.2 Clear-Water/Live-Bed Criterion ........................................................................................................ 160
6.3 Downstream Vortices ....................................................................................................................... 162

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6.4 Scour Prevention Devices..................................................................................................................166
6.4.1 Scour Region.............................................................................................................................167
6.4.2 Streamlines...............................................................................................................................168
6.4.3 y-Velocity..................................................................................................................................170
6.4.4 Scour Prevention Device Logistical Factors ..............................................................................171
6.4.5 Best Choice...............................................................................................................................173
6.5 Meshing.............................................................................................................................................174
6.6 Mesh Validation ................................................................................................................................176
6.6.1 Coefficient of Pressure Distribution around Pile Wall..............................................................177
6.6.2 Coefficient of Pressure Distribution along Upstream Pile Wall................................................178
6.6.3 Coefficient of Pressure Distribution along Upstream Symmetry Line......................................178
6.6.4 Wall Shear Distribution along Upstream Symmetry Line.........................................................179
6.6.5 Boundary Layer Formation.......................................................................................................179
6.6.6 Viscous Sublayer.......................................................................................................................180
6.6.7 Mesh Validation Summary .......................................................................................................181
6.7 Meshing Independence.....................................................................................................................182
7 Recommendations.....................................................................................................................................183
7.1 3D Model Improvement....................................................................................................................183
7.2 Scour Prevention Models Improvement...........................................................................................183
7.3 General Improvements .....................................................................................................................184
8 Conclusion..................................................................................................................................................185
9 References .................................................................................................................................................186
10 Appendices ............................................................................................................................................190
10.1 A - Folk’s Classification System .........................................................................................................190
10.2 B - Definition of Phi ...........................................................................................................................193
10.3 C - y+
Definition.................................................................................................................................194
10.4 D - UDF Code.....................................................................................................................................195
10.5 E - Seabed Shear Stress Calculations.................................................................................................196
10.6 F - Seabed Shear Stress Calculations, Various Sediment Sizes..........................................................200
10.6.1 Wet Packed Sand; Sediment Density: 2082kg/m
3
....................................................................200
10.6.2 Sand, Water Filled; Sediment Density: 1922kg/m
3
..................................................................200
10.6.3 Sand with Gravel, wet; Sediment Density: 2020kg/m
3
............................................................200
10.7 G - Seabed Shear Stresses.................................................................................................................201
10.7.1 Varying Current Speed .............................................................................................................201
10.7.2 Varying Sediment Size ..............................................................................................................246

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10.8 H - 3D Model Streamlines ................................................................................................................. 296
10.8.1 0.2m/s Streamlines .................................................................................................................. 296
10.8.2 0.7m/s Current Speed .............................................................................................................. 300
10.8.3 1.42m/s Current Speed ............................................................................................................ 303
10.9 I - Meshing Independence Results Tables......................................................................................... 307
10.10 J - Scour Prevention Models Streamlines ..................................................................................... 309
10.10.1 Basic Model Streamlines...................................................................................................... 309
10.10.2 Rectangular Collar Model Streamlines ................................................................................ 313
10.10.3 Triangular Collar Model Streamlines ................................................................................... 316
10.10.4 Rounded Collar Model Streamlines ..................................................................................... 320
10.10.5 Helical Wire (Half Wire) Model Streamlines........................................................................ 323
10.10.6 Helical Wire (Full Wire) Model Streamlines......................................................................... 327
10.11 K - Finite Length Pile Model Results ............................................................................................. 331
10.11.1 Model Geometry and Meshing............................................................................................ 331
10.11.2 Finite Length Pile Model Streamlines .................................................................................. 333

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Table of Figures
Figure 2-1 - Renewable Electricity Growth to 2010 [8] ................................................................................................... 5
Figure 2-2 - Electricity Generated by Wind (GWh) 1990-2012 [9] .................................................................................. 5
Figure 2-3 - Installed Wind Generating Capacity 2000 – 2012 [9]................................................................................... 6
Figure 2-4 Viewshed of Dublin Array on the Surrounding Area [23]............................................................................... 9
Figure 2-5 Location of offshore wind turbine arrays. [24] ............................................................................................ 11
Figure 3-1 Site Layout on the Kish and Bray Banks, wind turbines indicated by the black dots ................................... 12
Figure 3-2 Location of Site Investigation Boreholes [25]............................................................................................... 14
Figure 3-3 Sediment Distribution Results for Offshore Borehole 1 into Kish Sand Bank [25] ....................................... 14
Figure 3-4 Sediment Distribution Results for Offshore Borehole 2 into Kish Sand Bank [25]....................................... 15
Figure 3-5 Sediment Distribution Results for Offshore Borehole 3 into Kish Sand Bank [25] ....................................... 15
Figure 3-6 Sediment Type Classification based on percentage sand, mud and gravel (after Folk [28]) [27] ................ 17
Figure 3-7 Spatial Distribution of Derived Sediment Types [27] ................................................................................... 18
Figure 3-8 Variation in Mean Particle Size [27] ............................................................................................................. 19
Figure 3-9 Location of Biological Trawls Traversing Proposed Development and Samples Presented in ([27]) [29].... 20
Figure 3-10 Location of Recording Stations in Kish Banks [32]...................................................................................... 21
Figure 3-11 Development of a boundary layer as it progresses along a flat plate and the distortion of a fluid
particle as it flows within the boundary layer. [38]....................................................................................................... 23
Figure 3-12 a) Velocity profile for turbulent water flow plotted using a linear scale for both the horizontal and
vertical axis. b) The same velocity data as in a), plotted using a log10 vertical scale and linear horizontal scale. [39] . 24
Figure 3-13 Velocity Profile for water flow using a Power Law. Both axis are linear scale. .......................................... 24
Figure 3-14 Velocity Profile for water flow using a Power Law. The vertical axis using a log10 scale, and the
horizontal axis using a linear scale. ............................................................................................................................... 25
Figure 3-15 Boundary Layer Velocity Profile [38].......................................................................................................... 26
Figure 3-16 Viscous Sublayer Velocity Profile [40]........................................................................................................ 27
Figure 3-17 Share of Substructure Types for Online Wind Farms End 2011 [43].......................................................... 28
Figure 3-18 Monopile Foundation [44] ......................................................................................................................... 29
Figure 3-19 Flow around a cylindrical pile, Isometric View [42].................................................................................... 30
Figure 3-20 Flow around a pile, Side View [55]............................................................................................................. 31
Figure 3-21 Formation of Horseshoe Vortices [56] ....................................................................................................... 31
Figure 3-22 Flow around a cylindrical object, Top View [57] ........................................................................................ 31
Figure 3-23 Separation Distance Xs/D as function of δ/D. [42] ..................................................................................... 32
Figure 3-24 Ultimate Scour Depth (Suc) as a function of diameter of obstruction [60]................................................. 33
Figure 3-25 Scour Depth vs. Time Curves for Pier Shape Effects Test [61].................................................................... 34
Figure 3-26 Separation Distance Xs/D as function of δ/D. [42] ..................................................................................... 35
Figure 3-27 Influence of the pile Reynolds number (a) Separation distance Xs/D. (b) Maximum bed shear stress
amplification under the horseshoe vortex on the upstream symmetry line. [42] ........................................................ 36
Figure 3-28 Suc/D as a function of flow Froude number for different model sizes. [60] ............................................... 37
Figure 3-29 Equilibrium Scour Depth as a Function of Mean Approach Flow Velocity [71].......................................... 40
Figure 3-30 Flexible Scour Protection around a Circular Pile [76]................................................................................. 41
Figure 3-31 Flow around a Monopile with Bed Protection. [77]................................................................................... 42
Figure 3-32 Bed Degradation Erosion around Pile with Riprap, white arrow indicates current flow direction [76].... 42
Figure 3-33 Scour Prevention Mats, before and after installation [78]......................................................................... 44
Figure 3-34 Description of how the Scour Prevention Mats work. [78]........................................................................ 45
Figure 3-35 Three Dimensional Bathymetric Surveys of the seabed around a monopile foundation before and
after Scour Prevention Mat installation. [79]................................................................................................................ 46
Figure 3-36 Threaded Pile (Helical Wires or Cables wrapped spirally on the pile to form thread [80]......................... 46
Figure 3-37 Vortex flow fields at the upstream plane of symmetry of an unprotected pile[80] .................................. 47
Figure 3-38 Vortex flow fields at the upstream plane of symmetry of a triple threaded pile [80] ............................... 47
Figure 3-39 Scour around an Unprotected Pile (current only) [83]............................................................................... 48
Figure 3-40 Edge Scour at the pile protected by a small collar (current only) [83]....................................................... 49
Figure 3-41 Scour at the pile protected by a large collar (current only) [83]................................................................ 49
Figure 3-42 Modes of Sediment Transport [39]............................................................................................................ 50
Figure 3-43 Diagram showing the range of current speeds at which sediment particles of different sizes are
eroded and their form of transportation. [39].............................................................................................................. 51
Figure 3-44 The Hjulström curve [87]............................................................................................................................ 52
Figure 3-45 Forces acting on a sediment particle resting on a bed of similar particles. [88] ........................................ 52
Figure 3-46 Forces acting on a stationary sediment grain resting on a bed of similar grains in a flow. [39] ................ 53

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Figure 3-47 Lift and Drag on a bed sediment particle. [88, 89] .....................................................................................53
Figure 3-48 The Shields Diagram ...................................................................................................................................55
Figure 3-49 A modified and updated version of Shields Diagram. [88, 92] ...................................................................56
Figure 3-50 Updated Shields Diagram, recast in terms of shear velocity, U*, and particle diameter, D........................57
Figure 4-1 3D Model, Isometric View ............................................................................................................................61
Figure 4-2 Bias Meshing, viewed from below................................................................................................................63
Figure 4-3 Bias meshing and edge sizing at base of pylon, view from below. ...............................................................63
Figure 4-4 3D Model with Symmetry Applied, Isometric View......................................................................................64
Figure 4-5 Tetrahedral Meshing Structure, Isometric view ...........................................................................................65
Figure 4-6 Hexahedral Meshing Structure, Isometric view............................................................................................65
Figure 4-7 Initial Volume Meshing, isometric view........................................................................................................66
Figure 4-8 Initial Volume Meshing, Reserve Isometric View .........................................................................................67
Figure 4-9 Revised Volume Meshing..............................................................................................................................68
Figure 4-10 Initial Volume Meshing around pile............................................................................................................69
Figure 4-11 Revised Volume Meshing around pile ........................................................................................................69
Figure 4-12 y+ value on pile wall...................................................................................................................................71
Figure 4-13 Overview of Final Meshing .........................................................................................................................72
Figure 4-14 Close-Up Overview of Final Meshing..........................................................................................................72
Figure 4-15 Reverse View Close-Up of Final Meshing....................................................................................................73
Figure 4-16 Close-Up of Pile Meshing............................................................................................................................73
Figure 4-17 Close-Up of Inflation Layer .........................................................................................................................74
Figure 4-18 Named Blocks .............................................................................................................................................74
Figure 4-19 ANSYS Fluent Setup Launcher Options .......................................................................................................77
Figure 4-20 Surfaces with zero shear stress...................................................................................................................79
Figure 4-21 Monitoring Points, Top View ......................................................................................................................81
Figure 4-22 Monitoring Points, Top View, Close Up ......................................................................................................81
Figure 4-23 Monitoring Points, Side View, Close Up......................................................................................................82
Figure 4-24 Meshing, Independence Meshing Test Model 1.........................................................................................83
Figure 4-27 Monitoring Points 1-8.................................................................................................................................85
Figure 4-28 Monitoring Points 9-16...............................................................................................................................85
Figure 4-29 Plotted Monitor Points, Pressure Values, Points 1-8..................................................................................88
Figure 4-30 Plotted Monitor Points, Pressure Values, Points 9-16................................................................................88
Figure 4-31 Plotted Monitor Points, Wall Shear Values, Points 1-8...............................................................................89
Figure 4-32 Plotted Monitor Points, Velocity Values, Points 9-16.................................................................................89
Figure 4-33 Seabed Shear Stress, Basic Model ..............................................................................................................90
Figure 4-34 Seabed Shear Stress, Meshing Independence Test 1 .................................................................................90
Figure 4-37 Overview of Refined Final Meshing, Isometric View ..................................................................................93
Figure 4-38 Overview of Refined Final Meshing, Isometric View, Close Up ..................................................................93
Figure 4-39 Overview of Refined Final Meshing, Reverse Isometric View....................................................................94
Figure 4-40 Overview of Refined Final Meshing, Reverse Isometric View, Close Up.....................................................94
Figure 4-41 Mean Pressure Distribution on the Pile [37]...............................................................................................95
Figure 4-42 Pressure Distribution around Pile Wall Data Source ..................................................................................96
Figure 4-43 Pressure Coefficient Distribution along the length of the upstream edge of the pile, [42]........................98
Figure 4-44 Pressure Distribution along Upstream Edge of Pile Data Source................................................................99
Figure 4-45 Coefficient of Pressure Distribution on the Seabed along the upstream symmetry line. Note: the
pressure coefficient is normalized by the pressure coefficient at the toe of the pile, [42].........................................100
Figure 4-46 Pressure Distribution along Upstream Symmetry Line Data Source ........................................................101
Figure 4-47 Seabed Shear Stress amplification along upstream symmetry line [42]...................................................102
Figure 4-48 Boundary Layer Data Line, Isometric view................................................................................................103
Figure 4-49 Boundary Layer Data Line, Z-axis view......................................................................................................103
Figure 4-50 Comparison of Pressure Distribution around Pile Wall Data....................................................................104
Figure 4-51 Comparison of Pressure Distribution Data along Upstream Pile Wall......................................................105
Figure 4-52 Comparison of Pressure Distribution Data along Upstream Symmetry Line............................................106
Figure 4-53 Comparison of Wall Shear Data along Upstream Symmetry Line.............................................................107
Figure 4-54 Boundary Layer Formation Check, 0.2m/s................................................................................................108

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Figure 4-55 Boundary Layer Formation Check, 1.42m/s ............................................................................................. 108
Figure 4-56 Velocity Contour Plane, 0.2m/s................................................................................................................ 109
Figure 4-57 Velocity Contour Plane, 1.42m/s.............................................................................................................. 109
Figure 4-58 Rectangular Collar, Side and Isometric View............................................................................................ 110
Figure 4-59 Triangular Collar, Side and Isometric View............................................................................................... 111
Figure 4-60 Rounded Collar, Side and Isometric View ................................................................................................ 111
Figure 4-61 Helical Wire (Full Wire), Side and Isometric View.................................................................................... 112
Figure 4-62 Helical Wire (Full Wire), Filleted, Side and Isometric View ...................................................................... 113
Figure 4-63 Helical Wire (Half Wire), Side and Isometric View ................................................................................... 113
Figure 4-64 Helical Wire (Half Wire), Filleted, Side and Isometric View ..................................................................... 114
Figure 4-65 Volume Meshing for Scour Prevention Devices ....................................................................................... 115
Figure 4-66 Named Blocks........................................................................................................................................... 116
Figure 4-67 Rectangular Collar, Final Meshing............................................................................................................ 117
Figure 4-68 Triangular Collar, Final Meshing............................................................................................................... 117
Figure 4-69 Rounded Collar, Final Meshing................................................................................................................. 118
Figure 4-70 Helical Wire (Full Wire), Final Meshing .................................................................................................... 118
Figure 4-71 Helical Wire (Half Wire), Final Meshing ................................................................................................... 119
Figure 4-72 Shields Diagram with Various Sediment Sizes.......................................................................................... 122
Figure 4-73, Streamlines Source Plane, Close Up Isometric View ............................................................................... 124
Figure 4-74 Streamlines Source Plane, Source Points................................................................................................. 124
Figure 4-75 y-Velocity Component Source Plane, Isometric View .............................................................................. 125
Figure 5-1 Seabed Shear Stresses, Current Speed: 0.1m/s, Sediment Size: 0.0005m................................................. 127
Figure 5-3 Seabed Shear Stresses, Current Speed: 0.25m/s, Sediment Size: 0.0005m............................................... 128
Figure 5-15 Seabed Shear Stresses, Current Speed: 1.42m/s, Sediment Size: 0.0005m............................................. 134
Figure 5-23 Streamlines for 0.2m/s current, Isometric View ...................................................................................... 139
Figure 5-24 Streamlines for 0.7m/s current, Isometric View ...................................................................................... 140
Figure 5-25 Streamlines for 1.42m/s current, Isometric View .................................................................................... 140
Figure 5-26 y-Velocity Component, 0.2m/s Current, Symmetry Wall Plane ............................................................... 141
Figure 5-27 y-Velocity Component, 0.7m/s Current, Symmetry Wall Plane ............................................................... 141
Figure 5-28 y-Velocity Component, 1.42m/s Current, Symmetry Wall Plane ............................................................. 142
Figure 5-29 Seabed Shear Stress, Basic Model, Current Speed: 0.5m/s, Sediment Size: 0.0008m............................. 143
Figure 5-30 Seabed Shear Stress, Rectangular Collar, Current Speed: 0.5m/s, Sediment Size: 0.0008m ................... 144
Figure 5-31 Seabed Shear Stress, Triangular Collar, Current Speed: 0.5m/s, Sediment Size: 0.0008m...................... 144
Figure 5-32 Seabed Shear Stress, Rounded Collar, Current Speed: 0.5m/s, Sediment Size: 0.0008m........................ 145
Figure 5-33 Seabed Shear Stress, Helical Wire (Half Wire), Current Speed: 0.5m/s, Sediment Size: 0.0008m........... 145
Figure 5-34 Seabed Shear Stress, Helical Wire (Full Wire), Current Speed: 0.5m/s, Sediment Size: 0.0008m ........... 146
Figure 5-35 Streamlines for Basic Model, 0.5m/s current, Isometric View................................................................. 147
Figure 5-36 Streamlines for Rectangular Collar Model, 0.5m/s current, Isometric View............................................ 147
Figure 5-37 Streamlines for Triangular Collar Model, 0.5m/s current, Isometric View .............................................. 148
Figure 5-38 Streamlines for Rounded Collar Model, 0.5m/s current, Isometric View ................................................ 148

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Figure 5-39 Streamlines for Helical Wire (Half Wire) Model, 0.5m/s current, Isometric View....................................149
Figure 5-40 Streamlines for Helical Wire (Full Wire) Model, 0.5m/s current, Isometric View ....................................149
Figure 5-41 y-Velocity Component, Basic Model, 0.5m/s Current, Symmetry Wall Plane ..........................................150
Figure 5-42 y-Velocity Component, Rectangular Collar Model, 0.5m/s Current, Symmetry Wall Plane .....................151
Figure 5-43 y-Velocity Component, Triangular Collar Model, 0.5m/s Current, Symmetry Wall Plane........................151
Figure 5-44 y-Velocity Component, Rounded Collar Model, 0.5m/s Current, Symmetry Wall Plane..........................152
Figure 5-45 y-Velocity Component, Helical Wire (Half Wire) Collar Model, 0.5m/s Current, Symmetry Wall Plane ..152
Figure 5-46 y-Velocity Component, Helical Wire (Full Wire) Model, 0.5m/s Current, Symmetry Wall Plane .............153
Figure 6-1 Seabed Shear Stress Amplification. (a) Numerical Model Results, Published Study [42]. (b) Experimental
Results, Published Study [86].......................................................................................................................................154
Figure 6-2 Seabed Shear Stress Amplification, CFD Results, Current Study.................................................................154
Figure 6-3 Overview of flow around a Wall Mounted Cylindrical Pile, [110]...............................................................163
Figure 6-4 Mean Arched Vortices visualised by streamlines [108] ..............................................................................163
Figure 6-5 Mean Arched Vortices visualised by streamlines, Finite length Pile Model ...............................................165
Figure 6-6 Downward Trailing Vortices visualised by Streamlines, Finite length Pile Model, Side View.....................165
Figure 6-7 Meshing Irregularities.................................................................................................................................174
Figure 6-8 Comparison of the CFD Velocity Profile results with theoretical Linear Velocity profile and the Velocity
profile developed in the UDF code ..............................................................................................................................181
Figure 6-9 Meshing Independence Test Irregularity....................................................................................................182
Figure 10-1 The 15 major textural groups [28]............................................................................................................190
Figure 10-2 Textural Names of Classifications seen in Figure 82 [28]..........................................................................191
Figure 10-3 Expansion of the bottom tier of Figure 82 [28] ........................................................................................192
Figure 10-4 Particle Size shown in Phi and mm, and related to the Wentworth and Folk's Classification Schemes
[27]...............................................................................................................................................................................193
Figure 10-5 Shields Diagram showing the data line for 0.0002m sediment vs. the Shields Curve ..............................198
Figure 10-6 Seabed Shear Stresses, Current Speed: 0.1m/s, Sediment Size: 0.0002m ...............................................201
Figure 10-8 Seabed Shear Stresses, Current Speed: 0.25m/s, Sediment Size: 0.0002m .............................................202
Figure 10-10 Seabed Shear Stresses, Current Speed: 0.35m/s, Sediment Size: 0.0002m ...........................................203

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Figure 10-40 Seabed Shear Stresses, Current Speed: 0.35m/s, Sediment Size: 0.0004m........................................... 218
Figure 10-98 Seabed Shear Stresses, Current Speed: 0.1m/s, Sediment Size: 0.0004m ............................................ 248

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Figure 10-99 Seabed Shear Stresses, Current Speed: 0.1m/s, Sediment Size: 0.0005m.............................................248
Figure 10-110 Seabed Shear Stresses, Current Speed: 0.25m/s, Sediment Size: 0.0002m .........................................254
Figure 10-112 Seabed Shear Stresses, Current Speed: 0.25m/s, Sediment Size: 0.0004m.........................................255

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Figure 10-187 Seabed Shear Stresses, Current Speed: 1.42m/s, Sediment Size: 0.0002m......................................... 292
Figure 10-194 Streamlines for 0.2m/s current, Isometric View .................................................................................. 296
Figure 10-195 Streamlines for 0.2m/s current, X+ View ............................................................................................. 297
Figure 10-196 Streamlines for 0.2m/s current, X- View.............................................................................................. 297
Figure 10-197 Streamlines for 0.2m/s current, Y+ View ............................................................................................. 298
Figure 10-198 Streamlines for 0.2m/s current, Y- View.............................................................................................. 298
Figure 10-199 Streamlines for 0.2m/s current, Z+ View ............................................................................................. 299
Figure 10-200 Streamlines for 0.2m/s current, Z- View .............................................................................................. 299
Figure 10-201 Streamlines for 0.7m/s current, Isometric View .................................................................................. 300
Figure 10-202 Streamlines for 0.7m/s current, X+ View ............................................................................................. 300
Figure 10-203 Streamlines for 0.7m/s current, X- View.............................................................................................. 301
Figure 10-204 Streamlines for 0.7m/s current, Y+ View ............................................................................................. 301
Figure 10-205 Streamlines for 0.7m/s current, Y- View.............................................................................................. 302
Figure 10-206 Streamlines for 0.7m/s current, Z+ View ............................................................................................. 302
Figure 10-207 Streamlines for 0.7m/s current, Z- View .............................................................................................. 303
Figure 10-208 Streamlines for 1.42m/s current, Isometric View ................................................................................ 303
Figure 10-209 Streamlines for 1.42m/s current, X+ View ........................................................................................... 304
Figure 10-210 Streamlines for 1.42m/s current, X- View............................................................................................ 304
Figure 10-211 Streamlines for 1.42m/s current, Y+ View ........................................................................................... 305
Figure 10-212 Streamlines for 1.42m/s current, Y- View............................................................................................ 305
Figure 10-213 Streamlines for 1.42m/s current, Z+ View ........................................................................................... 306
Figure 10-214 Streamlines for 1.42m/s current, Z- View ............................................................................................ 306
Figure 10-215 Streamlines for Basic Model, 0.5m/s current, Isometric View............................................................. 309
Figure 10-216 Streamlines for Basic Model, 0.5m/s current, X+ View........................................................................ 310

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Figure 10-217 Streamlines for Basic Model, 0.5m/s current, X- View .........................................................................310
Figure 10-218 Streamlines for Basic Model, 0.5m/s current, Y+ View.........................................................................311
Figure 10-219 Streamlines for Basic Model, 0.5m/s current, Y- View .........................................................................311
Figure 10-220 Streamlines for Basic Model, 0.5m/s current, Z+ View.........................................................................312
Figure 10-221 Streamlines for Basic Model, 0.5m/s current, Z- View .........................................................................312
Figure 10-222 Streamlines for Rectangular Collar Model, 0.5m/s current, Isometric View ........................................313
Figure 10-223 Streamlines for Rectangular Collar Model, 0.5m/s current, X+ View ...................................................313
Figure 10-224 Streamlines for Rectangular Collar Model, 0.5m/s current, X- View....................................................314
Figure 10-225 Streamlines for Rectangular Collar Model, 0.5m/s current, Y+ View ...................................................314
Figure 10-226 Streamlines for Rectangular Collar Model, 0.5m/s current, Y- View....................................................315
Figure 10-227 Streamlines for Rectangular Collar Model, 0.5m/s current, Z+ View ...................................................315
Figure 10-228 Streamlines for Rectangular Collar Model, 0.5m/s current, Z- View ....................................................316
Figure 10-229 Streamlines for Triangular Collar Model, 0.5m/s current, Isometric View...........................................316
Figure 10-230 Streamlines for Triangular Collar Model, 0.5m/s current, X+ View ......................................................317
Figure 10-231 Streamlines for Triangular Collar Model, 0.5m/s current, X- View.......................................................317
Figure 10-232 Streamlines for Triangular Collar Model, 0.5m/s current, Y+ View ......................................................318
Figure 10-233 Streamlines for Triangular Collar Model, 0.5m/s current, Y- View.......................................................318
Figure 10-234 Streamlines for Triangular Collar Model, 0.5m/s current, Z+ View ......................................................319
Figure 10-235 Streamlines for Triangular Collar Model, 0.5m/s current, Z- View.......................................................319
Figure 10-236 Streamlines for Rounded Collar Model, 0.5m/s current, Isometric View.............................................320
Figure 10-237 Streamlines for Rounded Collar Model, 0.5m/s current, X+ View........................................................320
Figure 10-238 Streamlines for Rounded Collar Model, 0.5m/s current, X- View.........................................................321
Figure 10-239 Streamlines for Rounded Collar Model, 0.5m/s current, Y+ View........................................................321
Figure 10-240 Streamlines for Rounded Collar Model, 0.5m/s current, Y- View.........................................................322
Figure 10-241 Streamlines for Rounded Collar Model, 0.5m/s current, Z+ View ........................................................322
Figure 10-242 Streamlines for Rounded Collar Model, 0.5m/s current, Z- View.........................................................323
Figure 10-243 Streamlines for Helical Wire (Half Wire) Model, 0.5m/s current, Isometric View................................323
Figure 10-244 Streamlines for Helical Wire (Half Wire) Model, 0.5m/s current, X+ View...........................................324
Figure 10-245 Streamlines for Helical Wire (Half Wire) Model, 0.5m/s current, X- View ...........................................324
Figure 10-246 Streamlines for Helical Wire (Half Wire) Model, 0.5m/s current, Y+ View...........................................325
Figure 10-247 Streamlines for Helical Wire (Half Wire) Model, 0.5m/s current, Y- View............................................325
Figure 10-248 Streamlines for Helical Wire (Half Wire) Model, 0.5m/s current, Z+ View...........................................326
Figure 10-249 Streamlines for Helical Wire (Half Wire) Model, 0.5m/s current, Z- View............................................326
Figure 10-250 Streamlines for Helical Wire (Full Wire) Model, 0.5m/s current, Isometric View ................................327
Figure 10-251 Streamlines for Helical Wire (Full Wire) Model, 0.5m/s current, X+ View ...........................................327
Figure 10-252 Streamlines for Helical Wire (Full Wire) Model, 0.5m/s current, X- View ............................................328
Figure 10-253 Streamlines for Helical Wire (Full Wire) Model, 0.5m/s current, Y+ View............................................328
Figure 10-254 Streamlines for Helical Wire (Full Wire) Model, 0.5m/s current, Y- View ............................................329
Figure 10-255 Streamlines for Helical Wire (Full Wire) Model, 0.5m/s current, Z+ View............................................329
Figure 10-256 Streamlines for Helical Wire (Full Wire) Model, 0.5m/s current, Z- View ............................................330
Figure 10-257 Finite Length Pile Model, Geometry, Reverse Isometric ......................................................................331
Figure 10-258 Finite Length Pile Model, Geometry, Side View....................................................................................331
Figure 10-259 Finite Length Pile Model, Meshing, Reverse Isometric.........................................................................332
Figure 10-260 Finite Length Pile Model, Meshing, Side View......................................................................................332
Figure 10-261 Streamlines for Finite Length Pile Model, 0.5m/s current, Isometric View ..........................................333
Figure 10-262 Streamlines for Finite Length Pile Model, 0.5m/s current, X+ View.....................................................334
Figure 10-263 Streamlines for Finite Length Pile Model, 0.5m/s current, X- View......................................................334
Figure 10-264 Streamlines for Finite Length Pile Model, 0.5m/s current, Y+ View .....................................................335
Figure 10-265 Streamlines for Finite Length Pile Model, 0.5m/s current, Y- View......................................................335
Figure 10-266 Streamlines for Finite Length Pile Model, 0.5m/s current, Z+ View .....................................................336
Figure 10-267 Streamlines for Finite Length Pile Model, 0.5m/s current, Z- View......................................................336

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Table of Tables
Table 2-1 Characteristics of Offshore Wind Farm Sites [24] ......................................................................................... 11
Table 3-1 Average Sediment Distribution based on the three test boreholes, refer to Figure 5 [25]........................... 16
Table 3-2 Dependence of Local Scour Depth at bridge piers affected by the Relative Depth of Flow [63] .................. 38
Table 3-3 Equations for Maximum Scour Depth ........................................................................................................... 38
Table 3-4 Granular materials used in the studies of threshold of motion, as seen in Figure 3-49above. [92] ............. 56
Table 4-1 Inflation Layer Options .................................................................................................................................. 70
Table 4-2 Element Sizes in Volume Meshing Blocks...................................................................................................... 75
Table 4-3 Face Sizing on Volume Meshing Blocks ......................................................................................................... 75
Table 4-4 Fluid Properties ............................................................................................................................................. 77
Table 4-5 Solution Methods for SST Transition Model.................................................................................................. 78
Table 4-6 Solutions Methods for k-ε Model.................................................................................................................. 78
Table 4-7 Element Size in Volume Meshing Blocks, Meshing Independence Test........................................................ 83
Table 4-8 Face Size in Volume Meshing Blocks, Meshing Independence Test.............................................................. 83
Table 4-9 Element and Node Count, Meshing Independence Test............................................................................... 83
Table 4-10 Monitoring Points X, Y, Z Coordinates......................................................................................................... 86
Table 4-11 Monitoring Points Values, Meshing Independence Test 1.......................................................................... 87
Table 4-12 Element Sizes in Volume Meshing Blocks.................................................................................................... 92
Table 4-13 Face Sizing on Volume Meshing Blocks ....................................................................................................... 92
Table 4-14 Coefficient of Pressure Parameter Values................................................................................................... 97
Table 4-15 Element Sizing of Blocks in the Volume Meshing...................................................................................... 115
Table 4-16 Inflation Options, Helical Wire Models...................................................................................................... 116
Table 4-17 Parameters for Shields Equations.............................................................................................................. 121
Table 4-18 Interception Points and Rearranged Equations......................................................................................... 123
Table 4-19 Critical Seabed Shear Stress required for Sediment Movement of various Sediment Sizes ..................... 123
Table 6-1 Threshold Velocity for live-bed initialisation for different sediment sizes.................................................. 160
Table 10-1 Textural Names of Classification seen in Figure 84 [28]............................................................................ 192
Table 10-2 Shear Stress Calculation Parameters......................................................................................................... 196
Table 10-3 Boundary Reynolds Number and Critical Shields Stress Calculations, 0.0002m........................................ 197
Table 10-4 Interception Point for 0.0002m................................................................................................................. 199
Table 10-5 Sediment Movement Threshold Values, 0.0002m .................................................................................... 199
Table 10-6 Critical Seabed Shear Stress required for Sediment Movement of various Sediment Sizes, with a
Sediment Density of 2082kg/m3
.................................................................................................................................. 200
.................................................................................................................................. 200
.................................................................................................................................. 200
Table 10-9 Monitoring Points Values, Basic Model..................................................................................................... 307
Table 10-10 Monitoring Points Values, Meshing Independence Test 2...................................................................... 307
Table 10-11 Monitoring Points Values, Meshing Independence Test 3...................................................................... 308

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Nomenclature
Abbreviations
CFD – Computational Fluid Dynamics
CO2 – Carbon Dioxide
DES – Detached Eddy Simulation
DNS – Direct Numerical Simulation
EC – European Commission
EU – European Union
GWh – Gigawatts Hour
LES – Large Eddy Simulation
MW – Megawatts
MWe – Megawatt Electrical
RANS – Reynolds-Averaged Navier Stokes
SST – Shear Stress Transport
UDF – User Defined Function
Units
b – Pier Width (m)
D – Diameter (m)
D – Sediment Particle Diameter (m)
°C – Degrees Celsius
Fr – Froude Number
g – Gravity (
𝑚
𝑠2)
h – Water Depth (m)
hr – Hour
L – Characteristic length of the object (m)
PSU – Practical Salinity Unit (
𝑔
𝑘𝑔
)
Re – Reynolds Number
Re*
- Shear Reynolds Number, Boundary Reynolds Number
S – Separation Line (Figure 21)
S – Scour Depth (m)
Suc – Ultimate Scour Depth (m)
U – Flow Velocity (
𝑚
𝑠
)
ū – Velocity Gradient (
𝑚
𝑠
)

Mark Donnelly-Orr
xx
u – Velocity at height y (
𝑚
𝑠
)
U – Reference Velocity at Height δ (
𝑚
𝑠
)
U*
- Shear Velocity = √
𝜏 𝑜
𝜌 𝑤
(
𝑚
𝑠
)
Uc – Depth Averaged Critical Velocity (
𝑚
𝑠
)
V – Mean Inlet Velocity (
𝑚
𝑠
)
Ws – Average Settling Velocity (
𝑚
𝑠
)
xS – Distance in front of pile (m) (Figure 21)
y – Depth of flow (m)
y – Height of interest in velocity profile (m)
y+
- Y Plus Value
δ – Boundary Layer Thickness (m)
Θc – Critical Shields Stress (Pa)
μ – Dynamic Viscosity (
𝑃𝑎
𝑠
)
ν – Kinematic Viscosity (
𝑚2
𝑠
)
ξ – Normalised Distance =
𝑦+
𝑅𝑒∗
ρ – Density (
𝑘𝑔
𝑚3)
ρs – Density of Seabed Sediment (
𝑘𝑔
𝑚3)
ρw – Density of Fluids (
𝑘𝑔
𝑚3)
τs – Shear Stress (Pa)
τo – Dimensional Shear Stress (Pa)
𝜏 𝑚𝑎𝑥
𝜏∞
– Maximum Shear Stress Amplification
Φ, Phi – Sediment Size Measurement

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