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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
Niels Verkaik Keppel Verolme, The Netherlands
Co-authors:
Niels Verkaik (1) F P Yilmaz Salman (1) Sebastian Schafhirt (2) Torbjørn Hagen (3) Michael Muskulus (2)
(1) Keppel Verolme, Rotterdam-Botlek, The Netherlands (2) Norwegian University of Science and Technology, Trondheim, Norway (3) OWEC Tower AS, Oslo, Norway

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Poster
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Presenter's biography

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

Niels Verkaik obtained the degree of Master of Science in Offshore Engineering at Delft University of Technology and in Technology-Wind Energy at Norwegian University of Science and Tchnology. He is currently a research engineer at Keppel Verolme.

Abstract

Impulse based substructuring applied to offshore wind turbine jackets

Introduction

Jacket structures are fit for large multi-megawatt offshore wind turbines installed in deeper waters. However, the structural complexity of jackets carries also complex structural dynamics that will impact its long-term structural integrity. Current mainstream time-domain analysis tools make structural simplifications that enable cost-effective assessments but that also significantly limit the quality and quantity of structural dynamics captured in the models.
This paper looks at the Impulse Based Substructuring method, a modelling approach that notably increases the dynamics captured while helping turbine and structure intellectual property protection and featuring lower computational costs than currently used methods.


Approach

The Impulse Based Substructuring method uses Impulse Response Functions, i.e. the structural response to given impulses, to characterise the dynamic response of the structure under loading. This technique is applied to an offshore wind turbine jacket, where an innovative low computational cost analytic and parametric approach is tested.

Main body of abstract

The Impulse Based Substructuring (IBS) method has been proposed as an effective approach to model the dynamic response of offshore wind turbine support structures since it captures in detail the internal dynamics of the structure, while saving computational resources and allowing an intellectual property-friendly working framework for both structure and turbine designers. This paper shows how the IBS method can be used to compute the dynamic behaviour of the multibody system composed of turbine, tower and support structure by using the linear Impulse Response Functions (IRFs) of the latter, a finite element model of the tower and an aero-elastic model of the turbine, achieving the required accuracy at low computational cost. Additionally, an IRF analytical solution is shown to provide further savings and flexibility, using a summation of certain eigenmodes of the support structure and a flexibility compensation for the truncated modes. It is also shown that a parametric approximation can deliver satisfactory IRFs at even lower costs. These techniques are showcased for a jacket structure, comparing the IBS results with those of a fully-coupled analysis where the support structure is FE-modelled.

Conclusion

Impulse Based Substructuring is shown to be a suitable approach for the offshore wind industry. It enables higher accuracy and lower cost in the computation of the coupled dynamic responses of the multibody system “rotor nacelle assembly, tower and offshore support structure”. Additionally, an analytical instead of numerical solution of the Impulse Response Functions provides even lower computational costs while allowing the parametrisation of the Impulse Response Functions, which in turn enables multibody structural optimisation by adjusting only the support structure parameters.


Learning objectives
The paper will explain the concept of substructuring methods, in particular the IBS method. This includes how structural dynamics are captured in the model, how it can be integrated in current mainstream calculation methods, why it is computationally cost-effective and how analytic Impulse Response Functions can improve the method even further.