The signature and occurrence of frequency lock-in (FLI) vibrations of full-scale offshore structures are not well understood. Although several structures have experienced FLI, limited amounts of time histories of the responses alongside measured met-ocean data are available in the literature. This paper presents an analysis of 61 measured events of resonant vibrations of the Norströmsgrund lighthouse from 2001 until 2003. The vibrations of most of these events did not reach a steady state; thus, they violate an often-quoted criterion for frequency lock-in vibrations and remain outside any modes of ice-induced vibrations suggested in standards. Met-ocean data from both in situ measurements and from the Copernicus marine service information database are further used to better understand the occurrence of resonant ice-induced vibrations. All events between 2001 and 2003 occurred during days with ice concentrations of 8–10/10, closely packed consolidated drift ice. The locally measured ice velocity and thickness ranged from 0.023 to 0.075 m s−1 and from 0.26 to 1.9 m, respectively. These measurements included level ice, rafted ice and ridged ice. The events of resonant vibrations are further compared with measurements from the same structure between 1979 and 1988. Most events of resonant vibrations were recorded in the winter of 1988, followed by the winters of 2003 and 1980. The winter of 1988 had fewer freezing degree days (FDD) than the 65-year average, whereas the winters of 2003 and 1980 had more FDD than the 65-year average.@en

The sea ice interaction with a structure may cause system changes and affect the feasibility of common approaches to track modal parameters. In this paper, a covariance-driven stochastic subspace identification method is used to identify the natural frequencies from 190 time series of ice actions against a lighthouse structure. The results are sorted into groups defined by the observed ice conditions and governing ice failure mechanisms during the ice-structure interaction. The identified natural frequencies vary substantially within each individual group and between the groups. Recordings with flexural failures and a north-south ice-drift direction consistently rendered the same identified frequencies, whereas crushing seemed to create large amounts of variability in the identified frequencies. The results show the need for more high-fidelity data to assess whether modern system identification methods can be used to monitor system changes and support decision-making for operations of structures in ice-infested waters, such as offshore wind structures.@en

In the FATICE project we have addressed the fatigue damage on fixed offshore structures exposed to drifting ice. This is an important challenge in the development of energy production from offshore wind in the Baltic and involves at least five element: a) define ice statistics, b) predict the structural response (ice-structure interaction simulations), c) estimate the fatigue damage and d) carry out scale-model tests. We have used the Copernicus database and simple analytical equations to define the large-scale ice statistics and studied down-scaling to structural scale by comparing with ice load data on the Norströmsgrund lighthouse (LOLEIF and STRICE data). The VANILLA model allows for ice-structure interaction simulations and has been validated against the full-scale LOLEIF and STRICE data and against the model-scale ice in HSVA. The fully coupled and the traditional methods are compared. In the fatigue estimations studies the assumption of linear damage accumulation is challenged and load combinations from wave, wind and ice studied by assessing simulated time-series of the different loads. The main results is that sea ice cause the higher loads than wind and waves do , but the cumulative frequency of ice loads is much smaller than for wind and waves. The traditional model-scale ice tends to be too soft and/or too viscous so that a realistic breaking pattern combined with realistic force-time series is not been obtained for large aspect ratios. HVA has developed a crushing model ice (ICMI) in which the ice crystals are larger and the texture more uniform.@en