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Gravity based foundations for the Rødsand 2 offshore wind farm, Denmark
19 nov 2010
Gravity based foundations for the Rødsand 2 offshore wind farm, DenmarkISFOG 2010, Perth
Presentation by:Jakob Hausgaard Lyngs, COWIDenmark
Co-authors:Lone KroghJørgen S. SteenfeltCOWI, Denmark
Owner: E.ON Wind Sweden with Grontmij Carl Bro as consultant
Contractor: Aarsleff-Bilfinger Berger JVwith COWI as designer
Certifying body: Det Norske Veritas (DNV)
Wind turbine supplier: Siemens Wind Power
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Location
90 2.3 MW wind turbines complementing the neighbouring Nysted offshore wind farm (2002)
InauguratedOctober 2010
CO2-reduction700.000 t/year
Total cost~AUD 700 million
29 nov 2010
Gravity based foundations for the Rødsand 2 offshore wind farm, DenmarkISFOG 2010, Perth
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Rødsand 2 foundation geometry
39 nov 2010
Gravity based foundations for the Rødsand 2 offshore wind farm, DenmarkISFOG 2010, Perth
Gravity based foundation most favourable
1300 t pre-fabricated RC caissons with:
• Octagonal base, width 17 m
• Cylindrical shaft (length variable)
• Conical shape to reduce ice loads
• Hyperite stones in ballast chambers (up to 800 t)
• Sand in shaft to protect J-tubes
Same geometry for all 90 foundations
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Design procedure and special challenges
Traditional limit state design:
Bearing capacity
Sliding capacity
Differential settlements and tilt
Special challenges:
• Soil-foundation-wind turbine dynamic with loads and system natural frequencies being mutually dependent
• Non-linear soil stiffness –load iterations a must
• Dominating horizontal loads
• Sliding capacity: remoulded soil
• High ballast dead weight – minimum structural dead weight
• Extent and quality of the geotechnical investigations
• Tight schedule due to UN COP15 meeting on climate change
49 nov 2010
Gravity based foundations for the Rødsand 2 offshore wind farm, DenmarkISFOG 2010, Perth
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Ground conditions and geotechnical investigations
Owner's Design Basis: Danish Clay till, su=200 kPa
Original ground investigation (2007):
• 183 CPTs to a depth of 2.9 ± 2.4 m; 86 nos. outside footprints
• 80 geotechnical boreholes;majority to a depth of 7 m
• 200 UU tests; in lieu of field vane testing
• 16 valid triaxial CAU tests
Supplementary ground investigations by Contractor (2009):
• 31 geotechnical boreholes with SPTto a depth of 15-20 m
• 6 triaxial CAU tests and oedometer tests
Need for confirmation of strength at depths!
59 nov 2010
Gravity based foundations for the Rødsand 2 offshore wind farm, DenmarkISFOG 2010, Perth
Row 2
-12
-10
-8
-6
0 500 1000 1500
cu [kPa]
Level
[m]
Su [kPa]
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Re-evaluation of soil strength for design
Effective parameters: Large variation in results of individual tests:
φtr'=24.4º - 39.5º, c'=23 - 116 kPa
69 nov 2010
Gravity based foundations for the Rødsand 2 offshore wind farm, DenmarkISFOG 2010, Perth
Undrained shear strength:Correlation between lab and field testing
Re-evaluation of Design Basis: su=250 kPa
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Foundation sliding capacity
Nysted OWF unsolved issue:Sliding governing for design due to an upper, weak layer of remoulded soil
Rødsand 2 OWF solved issue:Careful planning of pit excavation and cleaning processes proved the non-existence of an upper remoulded layer
Verification by:
• Multibeam survey
• Diver's knife tests and video
• CPTs within footprint
79 nov 2010
Gravity based foundations for the Rødsand 2 offshore wind farm, DenmarkISFOG 2010, Perth
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Bearing capacity: 2D Plane strain analyses
Foundations on clay till and sand till soils: Purpose-made bearing capacity formula design schemes to handle the multitude of load combinations.
Foundations on non-standard ground conditions (~10%):Plaxis 2D plane strain analysis (Mohr-Coulomb soil)
Plaxis outcome:
• Verification of sufficient capacity of all foundations on non-standard soil profiles(except M14)
• An accurate match of the results ofthe spreadsheet and Plaxis analysis of foundations on clay till soils
• Indication of rupture figure
89 nov 2010
Gravity based foundations for the Rødsand 2 offshore wind farm, DenmarkISFOG 2010, Perth
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Bearing capacity: 3D Abaqus modelling
3D Abaqus analysis of foundation M14 on low-strength clay till with interbedded melt water deposits
Abaqus outcome:A large increase of capacity compared to 2D analyses (20-35%) due to:
• Correct modelling of the base plate geometry (no assumption of an effective area with fully developed plastic stress distribution)
• Hence, up to 50% larger contact area in 3D compared to the 2D effective area
• Direct application of torsion (no assumption of an equivalent horizontal loading model)
• Development of real rupture figure in 3D
99 nov 2010
Gravity based foundations for the Rødsand 2 offshore wind farm, DenmarkISFOG 2010, Perth
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Summary
• Design of 90 foundations performed with a very tight schedule
• Practical and economical design usingbearing capacity formula schemes & numerical modelling
• Constructability integrated in optimised design for each location (numerical studies allowed unchanged generic foundation geometry)
• Careful planning between project parties
optimized design prior to site activitiese.g. resolved sliding issue
• Purpose-planned supplementary investigations allowed increase of design properties
considerable ballast saving
109 nov 2010
Gravity based foundations for the Rødsand 2 offshore wind farm, DenmarkISFOG 2010, Perth