Gravity based foundations for the Rødsand 2 offshore wind farm, Denmark

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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


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Rødsand 2 foundation geometry

39 nov 2010


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


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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


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


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


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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


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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


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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, Denmark

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