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

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Presentation held at the ISFOG 2010, Perth, Australia, 9 November 2010

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

    1. 1. # Gravity based foundations for the Rødsand 2 offshore wind farm, Denmark 1 9 nov 2010 Gravity based foundations for the Rødsand 2 offshore wind farm, Denmark ISFOG 2010, Perth Presentation by: Jakob Hausgaard Lyngs, COWI Denmark Co-authors: Lone Krogh Jørgen S. Steenfelt COWI, Denmark Owner: E.ON Wind Sweden with Grontmij Carl Bro as consultant Contractor: Aarsleff-Bilfinger Berger JV with COWI as designer Certifying body: Det Norske Veritas (DNV) Wind turbine supplier: Siemens Wind Power
    2. 2. # Location 90 2.3 MW wind turbines complementing the neighbouring Nysted offshore wind farm (2002) Inaugurated October 2010 CO2-reduction 700.000 t/year Total cost ~AUD 700 million 2 9 nov 2010 Gravity based foundations for the Rødsand 2 offshore wind farm, Denmark ISFOG 2010, Perth
    3. 3. # Rødsand 2 foundation geometry 3 9 nov 2010 Gravity based foundations for the Rødsand 2 offshore wind farm, Denmark ISFOG 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
    4. 4. # 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 4 9 nov 2010 Gravity based foundations for the Rødsand 2 offshore wind farm, Denmark ISFOG 2010, Perth
    5. 5. # 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 SPT to a depth of 15-20 m • 6 triaxial CAU tests and oedometer tests Need for confirmation of strength at depths! 5 9 nov 2010 Gravity based foundations for the Rødsand 2 offshore wind farm, Denmark ISFOG 2010, Perth Row 2 -12 -10 -8 -6 0 500 1000 1500 cu [kPa] Level [m] Su [kPa]
    6. 6. # 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 6 9 nov 2010 Gravity based foundations for the Rødsand 2 offshore wind farm, Denmark ISFOG 2010, Perth Undrained shear strength: Correlation between lab and field testing  Re-evaluation of Design Basis: su=250 kPa
    7. 7. # 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 7 9 nov 2010 Gravity based foundations for the Rødsand 2 offshore wind farm, Denmark ISFOG 2010, Perth
    8. 8. # 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 of the spreadsheet and Plaxis analysis of foundations on clay till soils • Indication of rupture figure 8 9 nov 2010 Gravity based foundations for the Rødsand 2 offshore wind farm, Denmark ISFOG 2010, Perth
    9. 9. # 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 9 9 nov 2010 Gravity based foundations for the Rødsand 2 offshore wind farm, Denmark ISFOG 2010, Perth
    10. 10. # Summary • Design of 90 foundations performed with a very tight schedule • Practical and economical design using bearing 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 activities e.g. resolved sliding issue • Purpose-planned supplementary investigations allowed increase of design properties  considerable ballast saving 10 9 nov 2010 Gravity based foundations for the Rødsand 2 offshore wind farm, Denmark ISFOG 2010, Perth

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