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Presenter

Presenter's biography

Biographies are supplied directly by presenters at WindEurope Summit 2016 and are published here unedited

Gesa Ziemer has joined the Hamburg Ship Model Basin (HSVA) in 2012 after receiving a Diploma in Naval Architecture. She works as Project Manager at the Arctic Technology Department. Next to physical model tests of ships in ice, her main area of interest is ice loads and ice-induced vibrations of fixed offshore structures. She has been working on the BRICE research project dealing with ice loads on offshore wind turbine foundations, and has initiated the international research project IVOS (ice-induced vibrations of offshore structures) which aims at development of physical and numerical models to predict dynamic ice-structure interaction.

Abstract

Ice actions of offshore wind turbine foundations – mitigating ice loads and ice-induced vibrations

Introduction
Offshore wind turbines located in moderate to cold climate have to be designed robust against ice action. Ice loads significantly increase the design loads, as they are often considerably higher than loads induced by winds, waves and current. However, at most offshore wind farm sites ice is only present in very severe or extremely severe winters. Overdesigned turbine foundations to withstand the extreme ice loads are therefore not cost-efficient. A proper knowledge on ice loads to be expected on the foundation is inevitable for profitable design. Another issue is the occurrence of ice-induced vibrations at the foundation, which are transferred to the blades and can pose a serious threat to the integrity of the offshore wind turbine. As the excitation mechanism of these vibrations has not been entirely deciphered yet, rules and guidelines follow a conservative approach to prevent structural failure.
Approach
Numerical methods are not yet capable of reliably predicting ice load level and characteristic ice breaking frequency on vertical and conical structures. Therefore, physical model tests are the most suitable approach to study the influence of foundation geometry on these values. Afterwards, results can be used to calibrate a numerical model. Model tests in ice have been performed in several test campaigns during the BRICE (Breaking the ice) Project at the Hamburg Ship Model Basin in the Large Ice Basin. The slope angle of a conical foundation was varied between 50° and 90° to find an optimum regarding loads and vibrations for a large range of ice drift velocities.

Main body of abstract
The highest ice loads arise when the ice fails in crushing. This is the case on vertical structures (90°). 80° cones exhibit a mixture of bending- and crushing failure, while the ice fails in pure bending on smaller slopes. Consequently, the ice loads increase with increasing slope. At the same time, cost and construction effort are inverse proportional.
The breaking length decreases almost linearly over increasing slope up to angles between 70° and 80°. When crushing load components become dominant, the breaking frequency changes and can adapt to the natural frequency of the structure, resulting in severe resonant vibrations. Whether such resonance case can occur is highly dependent on the ice conditions, i.e. ice thickness and drift speed. Hence, the optimum slope angle depends on the dynamic properties of the structure, but also its location and site-specific ice parameters.

Conclusion
The study provides basic aids for the choice of proper foundation geometry, and highlights issues that should be taken into account during design. In addition, results can be used to calibrate and validate numerical models, which are needed for cost-efficient design.

Learning objectives
- basic considerations regarding ice loads on offshore wind turbine foundations
- optimal slope angle to mitigate ice loads and vibrations