instability during installation of foundations for …...unsolved structural problems, not turnin g...

NGM 2012 ▪ Copenhagen ▪ 9-12 May 2012 ▪ S. Madsen, L. V. Andersen & L. B. Ibsen

Department of Civil Engineering | Offshore Foundations Group

Outline of presentation Introduction General Instability Buckling of Piles Buckling of Bucket Foundations Future work

S.Madsen, L. V. Andersen & L. B. IbsenDepartment of Civil Engineering, Aalborg UniversityDivision of Structures, Materials and GeotechnicsResearch Group for Offshore Foundations

Instability during Installation of Foundations for Offshore Structures



IntroductionGeneral InstabilityBuckling of PilesBuckling of Bucket FoundationsFuture work



Introduction & Motivation Offshore wind energy is an upcoming source of renewable energy

• Forecast: 16 GW installed by the end of 2014, and global total of 75 GW by 2020 (Cumulative capacity in 2010: 3554 MW)

Foundations constitute approx. 1/3 of total cost of offshore wind farm Deeper water (> 25 m): up to 50%




General Instability Optimization of cross section (against global buckling) leads to a hollow cylinder (prone to local

buckling). Literature on buckling behaviour of shell structures from the past decades, is vast

• The abovementioned optimization leads to a high imperfection sensitivity.

Review by others• Teng (1996)– Research within buckling of shells is very active and will be for a long time

• Schmidt (2000): – Much more research is needed. – Future research should focus on applying the high theoretical knowledge and numerical solutions to

unsolved structural problems, not turning the basic shell cases round and round.• Edlund (2007): – Buckling of shells is a very challenging field of research




Mono piles 75% of all wind parks today Simple fabrication with welded steel pile No preparations of the seabed are necessary. Requires heavy duty piling/drilling equipment Not suitable for locations with many large boulders in

the seabed.




Jackets and Tripod Suitable for larger water depths. Minimum of preparations are required at the site before

installation Complex welded main structure Known technology from oil & gas industry




Introduction & Motivation

Instability also an issue for piles

Local buckling (Extrusion of initially deformed pile / Propagation of damaged pile tip)• Goodwyn A platform (1992): 15 of 20 piles severely crushed

Global buckling (Euler buckling)• Pile stick-up during installation• Loss of lateral support caused by scour or liquefaction in the

surrounding soil during an earthquake

(Barbour and Erbrich, 1994 as cited by Erbrich et al., 2011)




Buckling of Piles

Local buckling• Aldridge et al. (2005): Very simplified analytical expressions• Earl (2002): Finite element model to analyse extrusion buckling (Too simple)• Erbrich et al. (2011): More sophisticated FE model with pile-soil interaction by p-y springs

Global buckling• Bhattacharya et al. (2005): Pile stick-up during installation• Dash et al. (2010): Bending-buckling interaction as a failure mechanism caused by lateral load

during an earthquake combined with a high axial load.




Introduction & Motivation

The bucket foundation is a thin shell structure At deeper waters, the diameter of the bucket increases Large aspect ratio between caisson diameter and wall thickness Instability (buckling) critical issue during installation

Crucial case of buckling in Wilhelmshaven, Germany in 2005• Bucket diameter 16 m, height 15 m, and skirt thickness 25 mm • 6 MW offshore wind turbine• Buckling at a penetration of 6.8 m (vessel impact)




Wilhelmshaven installation failure




The prototype for Wilhelmshaven E-112

Our conclusion: The installation was jeopar-dized by a impact of the auxiliary vessel during operation outside the predicted wind

window.

Learning: Plans and procedures to be followed




Fabrication Fabricated by Bladt Industries

A/S, Denmark

Steel structure

Plate girder lid




Fabrication The geometric skirt imperfections was measured by a

3D point cloud laser scanner

The maximum out-of-roundness was 50mm

The largest imperfections were along the vertical welding's.




Buckling of Bucket Foundations Analytical and semi-emperical expressions for the structural buckling pressure of circular cylindrical

shells require assumptions of idealized boundary conditions such as pinned, fixed or free. Pinna and Ronalds (2000) modelled the lateral restraint of the soil by elastic Winkler springs

• Top end of cylinder pinned or clamped. Imperfections not included Pinna et al. (2001) modelled the soil both by an elastic model and an elasto-plastic Tresca model.

• Still idealized assumptions of the end boundary conditions. Only the lowest Eigen-mode considered as imperfection.

We have included the “real” structure of the bucket lid.• A large imperfection based solely on mode 1 can increase the buckling load.

IntroductionGeneral InstabilityBuckling of Bucket FoundationsBuckling of PilesFuture work



Buckling of Bucket Foundations

The first 21 modes are included as imperfect geometries• Mode 1 is not the worst mode• For large imperfections higher modes are

more critical

Mode 1 Mode 7 Mode 14




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Project development Fabrication InstallationTransport Operation

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Dogger Bank: Two Metmast installations - August 2012. 24m of water.

Firth of Forth: Metmast installation – August 2012. 38m of water.




Conclusions and Future work Instability issues related to both pile foundations and bucket foundations have been addressed.

• For piles, two overall buckling groups: global buckling (Euler buckling) and local buckling (propagation of initially deformed pile).

• Extensive work has been carried out for basic shell cases. • Some research has been carried out for the buckling issue of bucket foundations. • The authors have initiated their own analyses of the buckling of bucket foundations, and presented

some initial results.

The authors propose increased focus on non-linear finite element analysis of the local buckling issues of piles where the soil is modelled by a continuum model; and not by p-y springs.

Further, more sophisticated methods of predicting the “real” imperfection geometry of the bucket foundation should be implemented in the buckling investigations by the authors.

Also, more advanced analysis of the soil-structure interaction and a nonlinear soil material model should be considered.




ReferencesAldridge, T.R., Carrington, T.M. & Kee, N.R. (2005). Propagation of pile tip damage during installation. Proceedings of the International Symposium on Frontiers in Offshore Geotechnics (ISFOG 2005) (Cassidy, M. & Gourvenec, S. (eds)), Taylor & Francis Group, London. 823-827.Barbour, R.J. & Erbrich C. (1994). Analysis of Insitu Reformation of Flattened Large Diameter Foundation Piles Using ABAQUS. UK ABAQUS Users Conference, Oxford, September 1994.Bhattacharya, S., Carrington, T.M. & Aldridge, T.R. (2005). Buckling considerations in pile design. Proceedings of the International Symposium on Frontiers in Offshore Geotechnics (IS-FOG 2005) (Cassidy, M. & Gourvenec, S. (eds)), Taylor & Francis Group, London. 815-821.Dash, S.R, Bhattacharya, S. & Blakeborough, A. (2010). Bending-buckling interaction as a failure mechanism of piles in liquefiable soils. Soil Dynamics and Earthquake Engineering 30, 32-39.Earl, R.J. (2002). Growth of Imperfections in Piles During Installation. PhD Thesis, University of Western Australia.Erbrich, C.T, Barbosa-Cruz, E. & Barbour, R. (2011). Soil-pile interaction during extrusion of an initially deformed pile. Frontiers in Offshore Geotechnics II (Gourvenec & White (eds)), Taylor & Francis Group, London, 489-494.Pinna, R. & Ronalds, B.F. (2000). Hydrostatic buckling of shells with various boundary conditions. Journal of ConstructionalSteel Research 56 (1), 1-16.Pinna, R., Marin, C.M. & Ronalds, B.F. (2001). Guidance for Design of Suction Caissons Against Buckling During Installation in Clay Soils. Proceedings of the Eleventh International Offshore and Polar Engineering Conference, vol. 2, 662-668.Schmidt, H. (2000). Stability of steel shell structures: General Report. Journal of Construtional Steel Research 55, 159-181.Teng, J.G. (1996). Buckling of thin shells: Recent advances and trends. Applied Mechanics Reviews 49 (4), 263-274.Edlund, B.L.O. (2007). Buckling of metallic shells: Buckling and postbuckling behaviour of isotropic shells, especially cylinders. Structural Control and Health Monitoring 14, 693-713.



Thank you for listening

Lars Bo Ibsen

[email protected]

www.civil.aau.dk

Questions ?

instability during installation of foundations for …...unsolved structural problems, not turnin g...

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