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Scour Protection of offshoreWind Farms
Anders Wedel Nielsen
Supervisors:B. Mutlu SumerJrgen FredseErik Damgaard Christensen
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2 DTU Mechanical Engineering, Technical University of Denmark
Outline
Background for the project
The sinking mechanism
Flow in the scour protectionaround a mono pile in current
Sinking of scour protection
around a mono pile in waves Comparison with Horns Rev I
Offshore Wind Farm
Conclusion
Copyright: DONG Energy A/S
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3 DTU Mechanical Engineering, Technical University of Denmark
Background for the Project
0.5 to 2 m sinking adjacent to thepiles within 3 years afterinstallation.
Mechanisms of scour protectionsare not good enough understood.
Better understanding of the
mechanismes of the scourprotection is required due to thehigh costs of scour protections.
Map by Havala, via Wikimedia Commons.
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4 DTU Mechanical Engineering, Technical University of Denmark
Bathymetry of the Horns Rev 1 Wind Farmarea (2001)
Pile diameter: 4.2 m.
Water depth: 6.5 to 13 m.
Tidal current: 0.5 m/s.
Extreme current: 0.8 m/s
Breaking and non-
breaking waves.
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5 DTU Mechanical Engineering, Technical University of Denmark
The Sinking of the Scour Protection
Upper panel: As installedsurvey (2002).
Lower panel: Scour protection3 years later (2005).
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6 DTU Mechanical Engineering, Technical University of Denmark
Flow in the Scour Protection
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7 DTU Mechanical Engineering, Technical University of Denmark
Scour around a Protected Pile in Current-Overall flow pattern
Small vortecies at front of scourprotection causing edge scour.
Large horseshoe vortex at the pilecauses sinking of the stones.
A stagnation line where the inflowbetween the stones (streamwise
direction) and the large horseshoevortex meets.
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8 DTU Mechanical Engineering, Technical University of Denmark
Flow in the Scour Protection
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9 DTU Mechanical Engineering, Technical University of Denmark
Sediment Bed Experiments
Several tests were conducted withsediment bed. The followingparameters were varied:
Pile size.
Cover stone size.
Number of cover stone layers.
Diameter of the scour protection.
Flow velocity.
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10 DTU Mechanical Engineering, Technical University of Denmark
Sinking of the Scour Protection
Sinking of the scourprotection for one, two andthree layers of cover stones.
The larger the value of themobility parameter, the
more agitation the basesediment will undergo.
The larger the diameter ofthe pile, the largerhorseshoe vortex, thelarger sinking.
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11 DTU Mechanical Engineering, Technical University of Denmark
Mobility of the Sediment Underneath theScour Protection
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12 DTU Mechanical Engineering, Technical University of Denmark
Bed Shear Stresses under the ScourProtection
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13 DTU Mechanical Engineering, Technical University of Denmark
Sinking of Scour Protection with FilterLayer
Significant reduced sinking if themobility number is below thecritical (
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14 DTU Mechanical Engineering, Technical University of Denmark
Computational Calculation of the Flow andBed Shear Stresses in the Scour Protection
Flow and bed shear stresses inthe scour protection is calculatedusing FLOW-3D.
The scour protection is modelledas: 1) spheres, 2) porous mediaand 3) hybrid model a
combination of spheres andporous media.
The models were tested insteady current.
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15 DTU Mechanical Engineering, Technical University of Denmark
Velocities in the scour protection
The top panel shows thevelocities measured in thephysical model
The lower panel shows thevelocities calculated with thescour protection represented as
spheres.
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16 DTU Mechanical Engineering, Technical University of Denmark
Selected Streamlines in the ScourProtection
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17 DTU Mechanical Engineering, Technical University of Denmark
Bed Shear Stresses Measured andCalculated
How is the bed shear stressunder the porous mediumcalculated?It is calculated using apipe analogy: The poresbetween the stones is seenas a pipe with a diameter
of the stone size. The bedshear stress can then becalculated as the wall shearstress in the pipe multipliedby a factor: 4.5.
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18 DTU Mechanical Engineering, Technical University of Denmark
Comparison with Horns Rev I Wind Farm-Mobility of the Sediment
Diameter of fundation: 4.2 m.
Size of cover stones: 40 cm.
Size of filter stones: d50=100mm (d15=20 mm and d85=200mm)
2 layers of cover stones and
50 cm filter layer.
Sand size: d15=0.2 mm
Water depth: 10 m.
Typical extreme current: 0.8 m/s
Typical sinking near the pile: 1.5m.
Installation of cover layer. Copyright: DONG Energy A/S
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19 DTU Mechanical Engineering, Technical University of Denmark
Bed Shear Stress under the Horns Rev 1Scour Protection
-FLOW3D porous media model of the Horns Rev 1 Offshore Wind Farm(turbine 44).
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20 DTU Mechanical Engineering, Technical University of Denmark
Bed Shear Stress under the Horns Rev 1Scour Protection
Calculated (FLOW3D)Shields number under ascour protection around aHorns Rev 1 foundation(turbine 44).
The bed is unstable forShields numbers above
approximately 0.05 (redand yellow colours).
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21 DTU Mechanical Engineering, Technical University of Denmark
Sinking of the Scour Protection at Horns Rev1 Offshore Wind Farm
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Conclusion
The horseshoe vortex canpenetrate into the scourprotection.
The horseshoe vortex can movesediment under a thick scourprotection with filter layer.
Sediment under the scourprotection at Horns Rev I OffshoreWind Farm was mobile underextreme current events.
The problem can be modelledusing CFD.
Copyright: DONG Energy A/S
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23 DTU Mechanical Engineering, Technical University of Denmark
Acknowledgements
The study was funded by DTU, Statkraftthrough the Ocean Energy ResearchProgramme, DHI and Danish Council forStrategic Research through the researchprogram Seabed and Wind FarmInteraction.
The study was conducted undersupervision of B.M Sumer, J. Fredse,E.D Christensen and X. Liu (UTSA).
Parts of the physical model tests wereconducted by A. Hudecz and F.
Roignant.
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24 DTU Mechanical Engineering, Technical University of Denmark
References
Anders W. Nielsen; B. Mutlu Sumer; Jrgen Fredse; Erik D. Christensen(2011): Sinking of armour layer around a cylinder exposed to acurrent. Institution of Civil Engineers Maritime Engineering, vol. 164issue MA4, pp. 159-172.
Anders W. Nielsen; Xiaofeng Liu; B. Mutlu Sumer; Jrgen Fredse: Flow
and bed shear stresses in scour protections around a pile in a current.Submitted to Coastal Engineering.