Challenges in the design of foundation piles for offshore wind parks
RWE Innogy GmbH is planning the construction of the first phase of the offshore wind farm Innogy Nordsee in Germany’s North Sea. The sub area IN1 extends over an area of 41 km², in which 54 wind turbines with a total output of 332,1 MW will be constructed. 6,15 MW REpower turbines will be installed on pre piled jackets. The Foundation piles of the jacket structure will be driven with a template at an approximate water depth of -28 mLAT to a maximum pile toe level of -45m below mudline.
The site IN 1 is characterized predominantly by dense to very dense sand combined with small silty clay layers. Usually there is no great challenge by this type of soil to design a piled foundation.
The guidelines of the BSH Standard changed in 2012 during the development phase of this project. Therefore the cyclic and dynamic design of the foundation piles had to be adjusted according to the recommendations of the national Working Group "Piles" (EA Pfähle) as described in chapter 13. The chosen analytical design methods reflect the latest state of science. The soil parameters used are based partially on non-standardised laboratory testing and therefore evaluation of the parameters in itself is complex. The complexity of this operation led inevitably to a consultation and discussion between the independent geotechnical expert (Baugrundsachverständier – GeoEngineering Bremen) and the foundation designer and author of the method (tkb Bremerhaven and GuD Consult Berlin) concerning this calculation procedure, execution and post processing of the cyclic laboratory tests and their results. Both - the novel design approach, and the necessary laboratory tests, represented a major challenge to the designer.
In the meantime, the design of the piles was completed and certifiable approved by the Germanischer Lloyd. The additional pile lengths determined by the cyclic and dynamic calculation method are from 4.3% up to 11.8 % (mean value of 6.4 %) above the design length calculated for static loads according to the API method.
The project is currently expecting to secure the unconditional grid connection consent by the end of 2012.
Daniel Bartminn CEng, ICE Member in Germany, worked on this project with Dr Volker Herwig, the Senior Geotechnical Engineer for Wind Energy Offshore, RWE Innogy GmbH in Hamburg, Germany.