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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

Agenda

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Motivation and definition

Code based approach for spread foundations

Finite element study

Yield surface in literature

Case: the Rødsand 2 offshore wind farm

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› 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

R / A' = cu Nc sc ic

› Prandtl, 1921

› Effective area - Meyerhof, 1953

› Shape factor - Skempton, 1951

› Inclination factor - Green, 1954

Code based approach

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V M H

Finite element study

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› Abaqus

› Probe tests

› Swipe tests

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Finite element study

› Good match in VH- and VM-plane

› Bearing capacity formulas conservative in combined loading

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› Rectangular footings

› Proposed yield surface, formulated in normalised forcesv = 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)

Significant increase in bearing capacity in principle possible

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› Code vs. Gourvenec › Difference

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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 JVwith COWI as designer

› Certifying body: Det Norske Veritas (DNV)

› Wind turbine supplier: Siemens Wind Power

› 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

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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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