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Write your master’s thesis with us and face a wide range of complex engineering and scientific challenges. You’ll get the opportunity to develop and deploy some of the world’s largest renewable infrastructure assets, solving complex challenges that require innovative solutions in close cooperation with other leading specialists, leveraging each other’s know-how and expertise.

Overall project description
Analysis of how larger diameters of monopiles affect the drivability of piles, primarily focusing on energy losses.

Detailed project description
As wind turbine generators (WTGs) are getting larger and hub heights are increasing, higher demands are put on the support structure. In the case of offshore WTGs, the support structure is typically comprised of a tower, a transition piece (TP) and a monopile foundation (MP). The increased size of the WTG increases the demands on especially MP dimensions to ensure adequate structural capacity (loads) and stiffness (natural frequency). The most feasible solution is typically to increase the MP diameter.

The MP is typically driven into the soil by a large hammer (the kinetic energy of the hammer before impact is typically ~4000kJ). After a hammer impact, an axial stress wave travels from the top of the MP towards the bottom of the MP where some of the energy goes into driving the MP into the soil and some goes into reflecting the stress wave. There will also be energy dissipation along the length of the MP due to skin friction and through local bending at transitions in cross-section.

Because of Poisson’s ratio, the axial stress wave will induce a radial deformation of the cylinder, whereby additional interaction with the soil becomes present. For large-diameter piles, the natural frequency of a radial deformation mode is close to the frequency at which the hammer transmits energy to the MP [1,2]. Hence, energy may not only be dissipated due to skin friction, but some of the axial stress wave energy may be kinetic (and elastic) energy in the form of radial vibrations. Consequently, less energy will be available for penetration into the soil.

We would like to get a better understanding of how much energy is dissipated from the axial stress wave, and how important the phenomenon is for large-diameter MPs. We’re also interested in the interaction with the surrounding soil and would like to be able to model the out-of-plane displacement of the cylinder wall. It’s believed that an axisymmetric model of an MP where the cylinder wall is modelled by beams on elastic springs [3] subjected to impulse-like loadings will be a representative model. The axial stress wave typically has a fairly short wavelength and therefore special consideration will have to be given to time integration methodology and spatial discretisation.

References:

  1. Meijers, P.C., and Tsouvalas, A. and Metrikine, A.V., The effect of stress wave dispersion on the drivability analysis of large-diameter monopiles, Procedia Engineering, 199:2390-2395, X International Conference on Structural Dynamics, EURODYN 2017, 2017
  2. Meijers, P.C., Pile driving of larger diameter monopiles: Current practice and challenges, Presentation at Delft University of Technology, Faculty of Civil Engineering and Geosciences, Section of Offshore Engineering, October 2017
  3. Krenk, S., Mechanics and Analysis of Beams, Columns and Cables, 2nd Edition, Springer, Berlin, DE, 2001.

Project overview
The scope of the project is to

  • get a better understanding and an initial assessment of the effect of out-of-plane vibrations for large-diameter MPs
  • develop a program that can model impulse-like loads on an axisymmetric MP. This includes the dynamics of beams on elastic foundations, a suitable time integration approach, etc.
     

During the project, you’ll

  • gain insight into a critical part of a key problem in the offshore wind industry
  • gain experience from working with the world’s largest offshore wind developer.
     

Technical content

  • Beams on elastic foundations
  • Finite element modelling
  • Structural dynamics.
     
  • Your competences include that you
  • preferably have knowledge about and experience with beam theory, finite element modelling and structural dynamics
  • have a structured and enthusiastic approach to the challenge of doing a master’s thesis in an industry setting
  • speak and write English fluently.
     

Functional area: Foundations

Number of students: One or two

Academic level: MSc

Working hours: 30-35 ECTS

Project supervisor(s)/department(s)

Lasse Tidemann, PhD, Structural Engineer, Engineering/Foundations, by email: LATID@orsted.dk
 

Would you like to help shape the renewable technologies of the future?

Send your application to us as soon as possible and no later than 15 November 2019, as we’ll be conducting interviews on a continuous basis.

Desired start date: Spring semester 2020

Please don’t hesitate to contact windthesis@orsted.dk if you’d like to know more about the project.

Please note that for your application to be taken into account, you must submit your application via our online career pages.

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