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Dynamic stiffness of monopile supporting offshore wind turbine generators

Bhattacharya, S and Shadlou, M (2016) Dynamic stiffness of monopile supporting offshore wind turbine generators Soil Dynamics and Earthquake Engineering, 88. pp. 15-32.

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Very large diameter steel tubular piles (up to 10 m in diameter, termed as XL or XXL monopiles) and caissons are currently used as foundations to support offshore Wind Turbine Generators (WTG) despite limited guidance in codes of practice. The current codes of practice such as API/DnV suggest methods to analysis long flexible piles which are being used (often without any modification) to analyse large diameter monopiles giving unsatisfactory results. As a result, there is an interest in the analysis of deep foundation for a wide range of length to diameter (L/D) ratio embedded in different types of soil. This paper carries out a theoretical study utilising Hamiltonian principle to analyse deep foundations ( L/ 2 D≥ ) embedded in three types of ground profiles (homogeneous, inhomogeneous and layered continua) that are of interest to offshore wind turbine industry. Impedance functions (static and dynamic) have been proposed for piles exhibiting rigid and flexible behaviour in all the 3 ground profiles. Through the analysis, it is concluded that the conventional Winkler-based approach (such as p–y curves or Beanon-Dynamic Winkler Foundations) may not be applicable for piles or caissons having aspect ratio less than about 10 to 15. The results also show that, for the same dimensionless frequency, damping ratio of large diameter rigid piles is higher than long flexible piles and is approximately 1.2–1.5 times the material damping. It is also shown that Winkler-based approach developed for flexible piles will under predict stiffness of rigid piles, thereby also under predicting natural frequency of the WTG system. Four wind turbine foundations from four different European wind farms have been considered to gain further useful insights.

Item Type: Article
Subjects : Civil & Environmental Engineering
Divisions : Faculty of Engineering and Physical Sciences > Civil and Environmental Engineering
Authors :
Bhattacharya, S
Shadlou, M
Date : 27 May 2016
DOI : 10.1016/j.soildyn.2016.04.002
Copyright Disclaimer : © 2016. This manuscript version is made available under the CC-BY-NC-ND 4.0 license
Uncontrolled Keywords : Wind Turbine, Foundation Stiffness
Depositing User : Symplectic Elements
Date Deposited : 01 Nov 2016 12:01
Last Modified : 02 Nov 2016 18:32

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