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Conference programme 

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Poster session

Lead Session Chair:
Stephan Barth, Managing Director, ForWind - Center for Wind Energy Research, Germany
Bernd Kuhnle ForWind - University of Oldenburg, Germany
Bernd Kuhnle (1) F P Martin Kühn (1)
(1) ForWind - University of Oldenburg, Oldenburg, Germany

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Presenter's biography

Biographies are supplied directly by presenters at OFFSHORE 2015 and are published here unedited

Research assistant at University of Oldenburg, MSc Aerospace Engineering at University of Stuttgart and student assistant at the Endowed Chair of Wind Energy, 5 years of experience in wind energy. His responsibilities are within the INNWIND.EU project in Task 4.1. focusing on load mitigation strategies for large OWCs.


Unfavourable trends of rotor speed and systems dynamics for very large offshore wind turbines - analysis of the 10mw reference turbine


In the scope of the INNWIND.EU project innovative design concepts for very large 10-20 MW turbines are developed. The design of the reference jacket is mainly governed by the turbulent wind excitation and the resonances between multiples of the rotor speed and natural frequencies of the system. Whereas the rotor speed decreases due to the increased rotor diameter at almost constant maximum tip speed, the support structure fundamental natural frequency hardly changes even for a taller and heavier structure. Analysing a Campbell diagram of the three-bladed INNWIND.EU reference turbine reveals a resonance in the lower rotor speed range.


The influence of the natural frequency and the blade passing frequency on support structure and wind turbine loading is assessed. Therefore, a parameter study is carried with aero-elastic simulations of the 10 MW INNWIND.EU reference turbine (10 MW, 179 m diameter, rotor speed range from 6 rpm to 9.6 rpm). Apparently this turbine a typical example for resonance issues occurring for large offshore wind turbines recently.

Main body of abstract

The study is carried out by variation of material parameters, instead of varying the geometry of the support structure. For the tower base sideways moment damage equivalent load increasing from less than 10 MNm to over 30 MNm are found, which is an increase of about 300%. A 10% deviation of the natural to the blade passing frequency already leads to reductions of fatigue loads of 50%. Furthermore, load signals at different nodes in the support structure and blade root moments will be presented. However, increasing sizes might lead to the necessity of compliant operation of offshore wind turbines, which means that a design solution with resonances of the multiples of the rotor speed and the natural frequencies is accepted in order to achieve economic turbine designs. Therefore, measures to improve operation in resonance from a structural but also from a control point of view shall be presented.


Resonant operation can become a design problem for large offshore wind turbines. It results in uneconomic exploitation of wind energy and countermeasures have to be taken, e.g. operational and/or structural control.

Learning objectives
The presentation shall provide the audience with a deeper insight into the relevance of blade passing and natural frequency resonances for very large offshore wind turbines. Although in state-of-the-art design mostly prevented, compliant design might be necessary for economic designs. However, possible measures to realise such operational statuses shall be presented to the audience and offer suggestions for further innovations.