Foundations

A practical design and monitoring procedure for foundations of offshore wind turbines under cyclic loads

Project leader:
BAM - Federal Institute for Materials Research and Testing - Division VII.2 (www.bam.de)

Up to now, there are many open questions concerning the construction of offshore wind turbines with a reliable grounding on the seabed. The foundation plays a key role on the safety of the whole turbine, since for instance excessive deformations of the bedding or progressive changes in the soil’s stiffness can make the turbine unfit for service or even lead to the collapse of the whole construction.

For the given pile dimensions of the different foundation types and soil characteristics in the North and Baltic Seas, the effect of the cyclic dynamical loads from wind, waves and service on the behaviour of the foundation is still unclear.

This project shall produce analysis criteria for the selection of appropriate foundation systems (monopile, tripod, jacket, etc.) for the offshore wind turbines. The intended investigations aim to develop calculation methods and models for the design of the different foundation alternatives. The design procedure shall contemplate the safety level and limits for both the load-bearing capacity (failure of the foundation) and the service ability (excessive deformations or inclination of the tower).

A set of reduced-scale (1:32) and small-scale (1:100) model tests will provide information about the evolution of significant parameters of the embedment (soil stiffness, pore water pressure, axial load transfer, etc) during the cyclic load. Additionally, a numerical model including a high quality constitutive model calibrated with laboratory tests shall be developed to investigate the influence of the pore water pressure in the system’s behaviour. The results from the model tests, the numerical model and the present state of the art shall lead to practical design proposals for the engineer.

The validation of the models and procedures will be done with measurements from a real tripod foundation in the offshore test site alpha ventus. Strain gauges and accelerometers will inform indirectly about the lateral bedding and axial load transfer (skin friction and tip resistance) of the quasi-static and dynamic systems under the effects of the permanent service loads and the occasional extreme events. The stress condition of the surrounding soil will be registered through measurements of the pore water pressure and total stress near the pile.

The results from this project will improve the safety and availability of offshore wind turbines while reducing at the same time the financial costs for inspection and maintenance tasks.

Main research questions

  • Can we predict the long-term behaviour of the turbine’s foundation?
  • What’s the influence of the pore water pressure on the pile’s resistance?
  • What is the effect of the combined lateral and axial load on the pile’s resistance?

Literature

  • Rücker, W. (2007): Offshore wind energy plants: Problems and possible solutions. Proc. EVACES 2007 conference in Porto, Portugal.
  • Rohrmann, R. G.; Rücker, W.; Thöns, S. (2007): Integrated Monitoring Systems for Offshore Wind Turbines. Proc. 6th International Workshop on Structural Health Monitoring, 11-13 September, 2007, Stanford, USA
  • Tasan, H.E.; Rackwitz, F.; Savidis, S.A. (2007): Modellversuche in der Geotechnischen Versuchsgrube zur Untersuchung des Tragverhaltens von Offshore-Monopilegründungen. Veröffentlichungen des Grundbauinstitutes der Technischen Universität Berlin, Heft 41, S.197-213.

Contacts

  • Prof. Dr. Werner Rücker (werner.ruecker(at)bam.de)
  • Matthias Baeßler (matthias.baessler(at)bam.de)
  • BAM - Federal Institute for Materials Research and Testing - Division VII.2, Buildings and Structures, Unter den Eichen 87, D-12205 Berlin (www.bam.de)
  • TUB, Technische Universität Berlin (http://www.grundbau.tu-berlin.de/)
  • Adwen GmbH (www.adwenoffshore.com)