Instability during Installation of Foundations for Offshore Structures

Transcription

1 Instability during Installation of Foundations for Offshore Structures S.Madsen, L. V. Andersen & L. B. Ibsen Department of Civil Engineering, Aalborg University Division of Structures, Materials and Geotechnics Research Group for Offshore Foundations Outline of presentation

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4 & 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%

5 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

6 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.

7 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

8 & 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)

9 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.

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11 & 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)

12 Wilhelmshaven installation failure

13 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

14 Fabrication Fabricated by Bladt Industries A/S, Denmark Steel structure Plate girder lid

15 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.

16 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.

17 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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21 MBD Offshore Power A/S -> Universal Foundation A/S Universal Foundations - Concept IP Holder Universal Foundations Solution Provider Fred.Olsen Dong Energi Novasion Aalborg University

22 Wind turbine Cabling 22 The Supply Chain Project development Fabrication Transport Installation Operation Universal Foundation Solution Supplier Concept IP s / Configuration / Contract Management / R&D Buckets a wide-ranging foundation solution Client Contract Fred. Olsen United AS Fred.Olsen & Co / Support by way of Legal Accounting HR Finance Rave May Bremerhaven

23 Dogger Bank: Two Metmast installations - August m of water. Firth of Forth: Metmast installation August m of water.

24 Conclusions and 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.

25 References Aldridge, 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 Barbour, R.J. & Erbrich C. (1994). Analysis of Insitu Reformation of Flattened Large Diameter Foundation Piles Using ABAQUS. UK ABAQUS Users Conference, Oxford, September 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 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, 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, Pinna, R. & Ronalds, B.F. (2000). Hydrostatic buckling of shells with various boundary conditions. Journal of Constructional Steel Research 56 (1), 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, Schmidt, H. (2000). Stability of steel shell structures: General Report. Journal of Construtional Steel Research 55, Teng, J.G. (1996). Buckling of thin shells: Recent advances and trends. Applied Mechanics Reviews 49 (4), Edlund, B.L.O. (2007). Buckling of metallic shells: Buckling and postbuckling behaviour of isotropic shells, especially cylinders. Structural Control and Health Monitoring 14,

26 Thank you for listening Questions? Lars Bo Ibsen