| Summary: | Vertically-sided offshore structures occasionally experience sustained vibration due to drifting ice sheets crushing against them. These vibrations may lead to problems associated with structural integrity and safety. Traditionally, three regimes of interaction are distinguished: intermittent crushing, frequency lock-in and continuous brittle crushing. These regimes correspond to the dynamic ice-structure interaction at low-, intermediate- and high ice sheet velocities respectively. In this paper the effect of friction at the ice-structure interface on the frequency lock-in regime is studied. This investigation is a follow-up of a comparison between prediction models and full-scale data for ice induced vibrations which has been carried out in the framework of an Ice Induced Vibrations JIP (IIV JIP). This comparison showed that no simultaneous match could be found for both the structural acceleration of and ice force on cylindrical structures. This raised the question of whether or not the neglected in the models frictional interaction at the ice-structure interface could be a reason for such an inconsistency. In order to answer this question the interaction at the ice-structure interface is implemented, in a simplified manner, according to the Coulomb friction law in one of the models tested in the framework of the IIV JIP. In this model it is assumed that the three regimes of the dynamic ice-structure interaction can be described based on the distribution of ice strength in the ice sheet. It is concluded that friction alone cannot explain the large inconsistency in case of cylindrical shaped structures. It is suggested that the way the current models translate the available measurement data to input might be worth further investigation. Effects of friction on the range of velocities for which frequency lock-in might occur are expected to be minimal when a fully confined scenario is considered, although an overall increase in loads is predicted. The influence of friction for scenarios with marginal confinement ...
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