Preliminary design of offshore wind turbine support structures

The importance of proper mode shape estimation

Conference paper (2013)

Authors

Department
Hydraulic Engineering (CEG) (TU Delft)
Copyright: © 2013 Van der Male, P.
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Published Date

04-02-2013

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Source:

EWEA 2013: Europe's Premier Wind Energy Event, Vienna, Austria, 4-7 Febraury 2013

Faculty

Civil Engineering and Geosciences

Department

Hydraulic Engineering

Abstract

Offshore wind turbines are highly exposed to timevarying loads. For support structures, estimation of the fatigue damage during the lifetime of the structure is an essential design aspect. This already applies for the preliminary design stage. In determining the dynamic amplification in the frequency domain, modal analysis is a common tool. This paper describes a main drawback of the application of modal analysis in preliminary support structure design. Exact mode shapes are not available, due to the concentrated inertia of the rotor nacelle assembly (RNA). Generally the mode shapes of a cantilever beam are applied, in which the RNA mass is neglected. In analyzing three turbine types (V90-3.0MW, SWT-3.6-107 and NREL 5-MW), an overestimation of dynamic amplification due to sea level loading is observed. Estimation of the structural response at the second natural frequency turns out to be poor. Within the range of considered RNA masses, the actual magnitude of the tower top mass does affect the relative error of the modal analysis much. Besides, by increasing the tower and support structure stiffness, the relative error diminishes.

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