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

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. 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
› 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. 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. Finite element study
5
› Abaqus
› Probe tests
› Swipe tests

6. 6
Finite element study
› Good match in VH- and VM-plane
› Bearing capacity formulas conservative in combined loading

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. Significant increase in bearing capacity in principle
possible
8
› Code vs. Gourvenec › Difference

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. › 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. 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.
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