Author(s): Christian Lonoy; Morten Bjerkas; Arne Gurtner
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Keywords: No Keywords
Abstract: A single-degree-of-freedom (SDOF) model of the Norstr"omsgrund lighthouse has been established and the structural response caused by dynamic ice actions has been modelled. Predefined forcing functions to represent dynamic ice actions have been outlined in accordance with two recent design codes; ISO/DIS 19906 and Recommendations for Design of Offshore Wind Turbines (IEC). The present work shows that IEC predicts significantly larger dynamic responses than ISO. In the case of 1 m ice thickness and an assumed structural damping coefficient of 0.02, the harmonic forcing function proposed by IEC yields 430% larger waterline displacement of the Norstr"omsgrund lighthouse than the sawtooth shaped forcing function suggested by ISO/DIS 19906 design code. Based on full-scale data from the Norstr"omsgrund lighthouse, it has been found that ISO/DIS 19906 may overestimate the stability of the structure and suggests that it is not very susceptible to have frequency locked ice induced vibrations. Fullscale data shows however that ice induced vibrations are present for the structure at specific conditions. Sensitivity analyses of parameters defined in design codes have been performed and new values are presented to indicate that ice induced vibrations occur at given conditions. One key parameter is the stability coefficient which determines whether the structure is susceptible to have frequency locked vibrations. The present work shows that, depending on the choice of structural damping coefficient, this stability coefficient should be 45-190% larger than proposed by ISO.
Year: 2010