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# Bearing capacity of gravity based foundations, rostock 2012, jakob hausgaard lyngs

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Presentation at the 12th Baltic Sea Geotechnical Conference, 1 June 2012

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### Bearing capacity of gravity based foundations, rostock 2012, jakob hausgaard lyngs

1. 1. 1 12th Baltic Sea Geotechnical Conference, 1 June 2012 Bearing capacity of gravity based foundations for offshore wind turbines under combined loading Jakob Hausgaard Lyngs (presenting) Jørgen S. Steenfelt › COWI A/S › Denmark
2. 2. Agenda 2 Motivation and definition Code based approach for spread foundations Finite element study Yield surface in literature Case: the Rødsand 2 offshore wind farm
3. 3. 3 › Gravity based foundations for offshore wind turbines are: › numerous › expensive › Subjected to combined loading › Typically designed using bearing capacity formulas › Analysis of bearing capacity for › circular foundation › on level surface › combined loading (VHM) › Tresca soil (cohesive, undrained, c = cu,  = 0) Motivation and definition D H V M
4. 4. R / A' = cu Nc sc ic › Prandtl, 1921 › Effective area - Meyerhof, 1953 › Shape factor - Skempton, 1951 › Inclination factor - Green, 1954 Code based approach 4 V M H
5. 5. Finite element study 5 › Abaqus › Probe tests › Swipe tests
6. 6. 6 Finite element study › Good match in VH- and VM-plane › Bearing capacity formulas conservative in combined loading
7. 7. 7 › Rectangular footings › Proposed yield surface, formulated in normalised forces v = V / Vult h = H / Hult m = M / Mult › Much better fit for combined loading Gourvenec, 2007 ℎ ℎ∗ 2 + 𝑚 𝑚∗ 2 = 1 ℎ∗ = 0.25 − 𝑣 − 0.5 2 0.25 for 𝑣 < 0.5 1 for 𝑣 ≥ 0.5 𝑚∗ = 4(𝑣 − 𝑣2 )
8. 8. Significant increase in bearing capacity in principle possible 8 › Code vs. Gourvenec › Difference
9. 9. 9 Rødsand 2 › 90 nos. offshore 2.3 MW wind turbines › Baltic Sea (south of Lolland, Denmark) › Completed 2010 › Gravity foundations › octagonal › width 17 m › height up to 16 m › concrete mass up to 1300 tonnes › Clay till, cu > 250 kPa › 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
10. 10. › cu-reduction carried out to bring foundation into yield › Located at point near yield surface with little difference › Difference may be express in terms of › cu (excess material strength) 38% of code value › V (potential ballast saving) 4% of code value Rødsand 2 – typical ULS load level 10
11. 11. Conclusion › Circular, level surface, Tresca, VHM › For VH loading and VM loading, the standard bearing capacity formulas are accurate › For combined VHM loading, the standard bearing capacity formulas are conservative › The yield surface by Gourvenec (2007) provides closer fit to finite element data › The location in load space determines if any optimisation from code-based approach yields significant changes. 11