ABSTRACT
FSI modelling and Geotechnical design of Offshore Wind Turbines A thesis submitted for the degree of Master Science
Carmine Donatelli
Scuola di Ingegneria, Università di Pisa
Offshore wind parks promise to become an important source of energy in the next future and many parks will be installed in the European Seas.
Onshore wind energy has already grown enormously over the last years, but its further expansion is limited for many reasons, such as noise, nuisance and aesthetic issues, so the problem in the development of wind energy can be solved by the installation of wind turbines in offshore regions. The work described in this dissertation was developed within the Technical University of Civil Engineering of Bucharest. It is focused on the description of the approach, the techniques and the tools used to perform a fluid-mechanical simulation of an offshore wind turbine, located in the North Sea.
Modelling and Simulation is a branch of engineering, that is continuously developing across the industry, and following the evolution of computer technology and simulation tools.
By means of simulation, it is possible to explore and test several design solutions in a virtual environment, and to obtain performance predictions before the physical devices are actually produced.
The design problems are several and complex because different engineering fields met; hence specific technical competences are necessary, some of them even coming from the field of design of platforms for oil extraction. In order to solve the issues that we met in performing such an operation, Structural, Geotechnical, and Fluid-dynamics competences are necessary.
In this case the particular shape of the structure requires a more comprehensive study including both wind and current sea dynamic effects.
In order to achieve this result we decided to choose a very innovative analysis for this kind of structure: the Co-simulation, a modelling widely used in other engineering fields (aerospace, mechanical and biomedical) where the fluid-structure interaction in the design has to be considered with particular attention.
The software used to get our goal is Abaqus 6.14, that gives us the possibility to couple a Computation Fluid Dynamics (CFD) analysis with a structural one, using Abaqus/Standard as the structural solver and Abaqus/CFD as the incompressible flow solver.
In the structural model we modelled also the soil-structure interaction, using an elasto-plastic constitutive law, showing how the natural frequencies of the structure change in this case.
The result is an iterative procedure which can evaluate the mutual interaction between fluid dynamic field and shape of structure and it can also predict the final shape of the body (deformed).
To describe the fluid acting on the skew surface of generator wings different pre processing software programs can be used to prepare the meshed geometry of the model or with a trick it is possible to make it directly in Abaqus working on the input file as we will show in this work.
In the first part of the work the on shore wind turbine’s case is analysed. With the results obtained from the Co-simulation, we focused on the study of drag and lift displacements that we used to elaborate a spectral analysis aims to read more clearly the results. The latter has been obtained thanks to MathCad 14.
In the second part a comparison in terms of displacements of the blades in the flow direction is carried out, considering two cases: in the first one the wind speed is 25 m/s (standard case), while in the second one the wind speed changes from 0 m/s to 60 m/s (210km/h) in a very short time. The latter model is made using the Euler Lagrange Coupling (CEL).
At the end we faced the problem of finding the foundation pile’s displacements in the working situation, to see if they are acceptable for the proper functioning of the turbine.
Report set up
This thesis consists of five chapters:
1. Formulation of the problem description and the objective of the thesis;
Chapter 1 discusses the research motivation, research questions and scope of the thesis. At the end of the chapter the thesis outline is presented.
2. State of Art;
A review of support structures of offshore wind turbines are presented, focusing our attention on the foundation piles usually used and the installation steps
3. Actions on Offshore Wind Turbine;
We made a description of the all actions with particular consideration of the enviromental ones (wind, waves and currents).
4. CFD and Fluid-structure interaction (FSI) problems formulation;
Chapter 4 illustrates the CFD analysis and about the two main approaches exist for the simulation of FSI-problems: monolithic and partitioned approach.
5. FSI modelling and Geotechnical design of Offshore Wind Turbines;
Chapter 5 defines a fluid-dynamic and structural problem in Abaqus 6.14 in which we analysed both the onshore and offshore wind turbine. Furthermore, in the same chapter, the Euler-Lagrange coupling modelling is presented and compared with the Co-simulation technique.
This section concludes the approch and results of this research in a comprehensive way. This discussion is followed by highlighting the contribution of the research to the state of the art knowledge.
