04_scour protection around monopiles

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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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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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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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The Sinking of the Scour Protection

Upper panel: As installedsurvey (2002).

Lower panel: Scour protection3 years later (2005).


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Flow in the Scour Protection


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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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Flow in the Scour Protection


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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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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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Mobility of the Sediment Underneath theScour Protection


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Bed Shear Stresses under the ScourProtection


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Sinking of Scour Protection with FilterLayer

Significant reduced sinking if themobility number is below thecritical (


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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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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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Selected Streamlines in the ScourProtection


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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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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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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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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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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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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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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.

04_scour protection around monopiles

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