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Ewec 2007 Scour Protection around Offshore Wind Turbine Foundations……Full-scale Measurements

Ewec 2007 Scour Protection around Offshore Wind Turbine Foundations……Full-scale Measurements. Erik Asp Hansen DHI (Present company DNV) Hans Jacob Simonsen DHI Anders Wedel Nielsen DHI Jan Pedersen DONG Energy Michael Høgedal Vestas Wind Systems.

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Ewec 2007 Scour Protection around Offshore Wind Turbine Foundations……Full-scale Measurements

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  1. Ewec 2007 Scour Protection around Offshore Wind Turbine Foundations……Full-scale Measurements • Erik Asp Hansen DHI (Present company DNV) • Hans Jacob Simonsen DHI • Anders Wedel Nielsen DHI • Jan Pedersen DONG Energy • Michael Høgedal Vestas Wind Systems

  2. 80 Wind Turbines at Horns Rev • Water depth 6.5 m - 13 m • Extreme High water level +2.5m • Extreme Low water level -1.8m • Extreme Current 1.2 m/s • Extreme Significant Wave Heights 4-6m • Seabed sand 0.1 mm - 1.0 mm • Tower Diameter 4.2m • Towers installed in 2002, • (original purpose of the present study, to compare stability Full scale and model scale measurements of armour and filter layers)

  3. Why Scour Protection? • Fix the support at a certain level (typically the present seabed level) • Local Scour Unprotected mono-towers on sand in pure Current • (scour depths up-to 2.0 times the tower diameter) • General Scour (Associated with overall sediment transport) • (estimated to be max 0.5m at Horns Rev) max6-8 m depth

  4. Overview How to Design of Scour Protection, 1 • Input • Environment conditions • Seabed properties • Tower dimension • Design Philosophy • No Maintenance • Some damage allowed • Scour protection installed before tower installation • Scour protection installed after tower installation

  5. Overview, How to Design of Scour Protection, 2 • Type of scour protection • Armor Stones above one or two filter-layers • Artificial sea-grass • Mattresses • Armor Stones on geo-textile, Mattresses on geo-textile • ……. • Engineering • number of filter layers, Grain sizes, Densities • Thickness, Horizontal Extension • Cost • Time of Installation…. • If required Physical experiments (Froude Scaling) • Stability of Armor layer , Stability of Filer Layer • Note….Sand can not be studied in a Froude scaled model

  6. The Horns Rev scour protectionradius =9.5 m (from tower center) armour stones

  7. Over View Surveys

  8. Windturbine no. 35 Difference 2005 and 2002 Surveys Net transport: app..-270m3 away from the turbine

  9. EW 2002 2005 NS Windturbine no 35

  10. Windturbine no 12 Red 2002, Blue 2005

  11. bed development in m3 from 2002-2005

  12. Average lowering around the turbines

  13. Horns Rev Wave Hindcast wavemodelling Numerical Grid

  14. Horns Rev Wave Hindcast

  15. 1 hour maximum significant wave height at each of the wind turbines ( 1 January 2004 - 1 January 2006. N E No unique relation between max waves and lowering have been found

  16. What has happened? • Armor stones transported away from the tower? • Is it possible? • Where is the armor stones now? • Filter layer has transported away from the tower? • Is it possible? • Where is the filter stones now? • Seabed material transported away from the tower? • Is it possible?

  17. What has happened? 2 • Armor stones transported away from the tower? • Is it possible to move stones close to the tower in the upwards direction ?

  18. Traditional Filter Criteria fulfilled?

  19. Traditional Filter criteria P- P+ SAND Filter Stone FlowDirection Established under stationary flow conditions Only a function of the grain size distributions Not dependent on • Pressure gradient • Grain densities • Period of oscillation • …..

  20. Sumer, Cokgor og Fredsøe 2001 : ’Suction Removal of Sediments Between Armour Blocks’. One of the main objectives Why/When/How are stones moving downwards?

  21. The Result from one layer tests, can they be used • When several layers above each other are present? • When vertical velocities are present? • When vertical pressure gradients are present?

  22. Physical model test of a Scour Protection (no filter) Erosion in waves and current (seen from downstream side)

  23. Erosion in waves and current (seen from downstream side) 0.15mm sand have moved up to though a 2mm holeat similar pressure gradients as at Horns Rev !

  24. UB ?? F ?? Transport of Sand and Filter Stones upwards through Armour Layer

  25. Transport of Sand and Filter Stones upwards through the Armour Layer ( A Simple Model ) • Step 1, determine near bed environment • Step 2, evaluate the effect from tower and protection • Step 3, determine the flow inside the scour protection • Step 4, determine stability of the interfaces • Step 5, evaluate sand movement during one wave cycle

  26. pressure gradient from the flow outside the scour protectionDarcy's formula(modified for turbulence and inertia forces) Step 3, flow inside the scour protection V is the velocity I is the pressure gradient a,b and c empirical constants dependent the porosity, reynolds number,… , (to be used with caution)

  27. ShearForce Gravity Step 4, stability of the interfaces Possible movement of the sand/filter material at the inter-phase is determined by the Shields Parameter: Shields Parameter: Relation between the shear stresses and the gravity acting on the acting on a grain

  28. Filter layer :Shields parameterArmour layer D50 = 0.4 m, Hs = 3.5 m, Tp = 10 s, and h = 10 m movement No movement

  29. Sand Layer Shields parameter Filter layer D50 = 0.1 m, Hs = 3.5 m, Tp = 10 s, and h = 10 m movement No movement

  30. Step 5, sand movement during one wave cycle sediment fall velocity Upwards Directed flow Downwards Directed flow 0

  31. Sand: Vertical movement during one wave cycle.Upper layer D50 = 0.1 m, Hs = 3.5 m, Tp = 10 s, and h = 10m α ratio between vertical and horizontal velocities

  32. The simple model shows/indicates • Why the most severe lowering has taken close to the tower • Sand finer than 0.45 mm close to the tower can move 0.5 m upwards (all the way through the filter layer) during one wave cycle for wave heights equal 3.5 m.

  33. Conclusions • The surveys indicate that the armour layers have been stable • Only a small part of the filter layer has moved up through • the armour layer • The sand has moved up through the filter layer and further • through the armour layer • In order to avoid the lowering of the armour layer two filter layers or much thicker filter layer with a wider gradation or a geo-textile could have been used. • Do not underestimate the value of a good filter design!

  34. The financial support of the Danish Public Service Program (PSO) under Contract PSO: 6508 (FU5102) is gratefully acknowledged. Thank You for Your Attention Erik Asp Hansen DHI

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