For offshore wind farms which are planned in sub-arctic regions like the Baltic Sea and Bohai Bay, support structure design has to account for load effects from dynamic ice-structure interaction. There is relatively high uncertainty related to dynamic ice loads as little to no load- and response data of offshore wind turbines exposed to drifting ice exists. In the present study the potential for the development of ice-induced vibrations for an offshore wind turbine on monopile foundation is experimentally investigated. The experiments aimed to reproduce at scale the interaction of an idling and operational 14 MW turbine with ice representative of 50-year return period Southern Baltic Sea conditions. A real-time hybrid test setup was used to allow the incorporation of the specific modal properties of an offshore wind turbine at the ice action point, as well as virtual wind loading. The experiments showed that all known regimes of ice-induced vibrations develop depending on the magnitude of the ice drift speed. At low speed this is intermittent crushing and at intermediate speeds is ‘frequency lock-in’ in the second global bending mode of the turbine. For high ice speeds continuous brittle crushing was found. A new finding is the development of an interaction regime with a strongly amplified non-harmonic first-mode response of the structure, combined with higher modes after moments of global ice failure. The regime develops between speeds where intermittent crushing and frequency lock-in in the second global bending mode develop. The development of this regime can be related to the specific modal properties of the wind turbine, for which the second and third global bending mode can be easily excited at the ice action point. Preliminary numerical simulations with a phenomenological ice model coupled to a full wind turbine model show that intermittent crushing and the new regime result in the largest bending moments for a large part of the support structure. Frequency lock-in and continuous brittle crushing result in significantly smaller bending moments throughout the structure.

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Fixed offshore wind turbines are increasingly developed for high latitude areas where not only wind and wave loads need to be considered, but also moving sea ice. Current structural design rules do not adequately consider the effect of ice loading on fatigue life, due to missing studies on fatigue strength of welded joints under combined wind, wave, and ice action. Thus, a methodology to determine combined variable-amplitude loading (VAL) spectra was developed in a previous study. The stress state time-history at an exemplarily selected point in the support structure of an offshore wind energy monopile was translated into a VAL sequence. This sequence is used as an input for fatigue tests of butt-welded joints in the current study. The current study presents the VAL spectrum and the corresponding VAL time series, the results of the fatigue tests and compares them to typical fatigue damage sums for other stress spectra.@en

With the recent surge in development of offshore wind in the Baltic Sea, Bohai Sea and other ice-prone regions, a need has arisen for new basin tests to qualify the interaction between offshore wind turbines and sea ice. To this end, a series of model tests was performed at the Aalto ice basin as part of the SHIVER project. The tests were aimed at modeling the dynamic interaction between flexible, vertically-sided structures and ice failing in crushing. A real-time hybrid test setup was used which combines numerical and physical components to model the structure. This novel test setup enabled the testing of a wide range of structure types, including existing full-scale structures for which ice-induced vibrations have been documented, and a series of single-degree-of-freedom oscillators to obtain a better understanding of the fundamental processes during dynamic ice-structure interaction. The tests were primarily focused on the dynamic behavior of support structures for offshore wind turbines under ice crushing loads. First results of the campaign show that the combination of the use of cold model ice and not scaling time and deflection of the structure can yield representative ice-structure interaction in the basin. This is demonstrated with experiments during which a scaled model of the Norströmsgrund lighthouse and Molikpaq caisson were used. The offshore wind turbine tests resulted in multi-modal interaction which can be shown to be relevant for the design of the support structure. The dataset has been made publicly available for further analysis.@en

Fixed offshore wind turbines continue to be developed for high latitude areas where not only wind and wave loads need to be considered but also moving sea ice. Current rules and regulations for the design of fixed offshore structures in ice-covered waters do not adequately consider the effects of ice loading and its stochastic nature on the fatigue life of the structure. Ice crushing on such structures results in ice-induced vibrations, which can be represented by loading the structure using a variable-amplitude loading (VAL) sequence. Typical offshore load spectra are developed for wave and wind loading. Thus, a combined VAL spectrum is developed for wind, wave, and ice action. To this goal, numerical models are used to simulate the dynamic ice-, wind-, and wave-structure interaction. The stress time-history at an exemplarily selected critical point in an offshore wind energy monopile support structure is extracted from the model and translated into a VAL sequence, which can then be used as a loading sequence for the fatigue assessment or fatigue testing of welded joints of offshore wind turbine support structures. This study presents the approach to determine combined load spectra and standardized time series for wind, wave, and ice action.

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Due to increasing trend of building offshore wind turbines (OWTs) in seas at high latitudes where seasonal sea ice occurs, novel methods for design of such structures are needed. Specifically, the effect of ice-induced vibrations (IIVs) on fatigue life of the structures is currently poorly understood. Therefore, the goal of this paper is to analyze the current state-of-the-art approach for estimating the ice loads contribution to the fatigue life of OWTs and identify the current knowledge gaps. Moreover, the paper proposes a methodology for developing a combined load spectrum of wind, waves and ice using numerical simulations, with an ultimate goal to develop applicable Gaßner curves characterizing the variable-amplitude nature of the loading pattern. Finally, the use of small-scale fatigue tests under sub-zero temperatures in order to develop the appropriate S-N and Gaßner curves is discussed.@en

Offshore wind turbines at locations where sea or lake ice is present need to be designed to withstand ice-induced loading. For vertical-sided support structures, such as monopiles, the effects of ice-induced vibrations need to be considered in the design. Current practice is either to use approaches provided in design standards, or for example to apply pre-generated ice load time series in the wind turbine aeroelastic model. These approaches have the drawback that the coupling between ice failure behavior and structural motion is not included. The effect of omitting this coupling on predictions for fatigue and ultimate limit states is currently not known. To enable fully coupled simulations in the design of offshore wind turbines, an existing simulation model for ice crushing has been recently coupled (“VANILLA”) to the in-house aeroelastic software package BHawC. In this paper this fully coupled model is applied to simulate ultimate limit state design load cases (DLCs) for a recent design of an offshore wind turbine on a monopile foundation. The project that is chosen for this case study is situated in the Southern Baltic Sea. The loads obtained for ice- and wind loading with the VANILLA model are compared to wind- and wave-induced loading. It is found that intermittent crushing is the governing ice interaction mode for offshore wind turbine support structures and rotor-nacelle-assembly components.
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