The design of flexible vertical offshore structures exposed to crushing ice, such as offshore wind turbines, can become governed by ice loads and the structural response associated with low relative speeds between ice and structure. Low ice speeds can cause significant loads due to pressure synchronization and/or increase in contact, potentially larger than those observed at high ice speeds, which is often referred to as the velocity effect. In this study, the dataset from the full-scale measurement campaign at the Norströmsgrund lighthouse is reanalyzed. Several instances of ice load amplification are identified and presented, to confirm that synchronization and the velocity effect developed. The increase in ice load is quantified and discussed in the context of a theoretical framework, and model-and full-scale observations of the velocity effect on other structures. Then several events of high-speed crushing are investigated and the potential global pressures at low speeds for those events are estimated based on the theoretical framework. These estimates are compared to typical high-speed global crushing pressures used to define the ice strength coefficient R for the Baltic Sea. It is found that the velocity effect may produce global pressures equivalent to a R factor above 0.9 MPa. The results provide a theoretical substantiation for inclusion of the velocity effect and a possible physical interpretation of the recommended value of 1.8 MPa in the ISO 19906 design standard.@en

EU urgently needs to increase the development of secure and green energy, and this includes renewables such as Offshore wind energy. An expansion of Offshore wind will include the Baltic where sea ice is one of the major uncertainties. To ensure that the w ind turbines are safe for people and the environment, while keeping them economically competitive better guidelines and regulations should b e developed collaboratively by European industry and academia. There are unsolved challenge s with respect to ice action on structures for offshore wind. However, in the current draft for Horizon Europe Work Programme 2023-2024 on Climate, Energy and Mobility1, the challenges related to sea ice with regards to Offshore wind energy are not mentioned. In order to meet the crucial green energy goals, it is our statement that it is imperative to include sea ice i n the final version.@en

EU urgently needs to increase the development of secure and green energy, and this includes renewables such as Offshore wind energy. An expansion of Offshore wind will include the Baltic where sea ice is one of the major uncertainties. To ensure that the wind turbines are safe for people and the environment, while keeping them economically competitive betterguidelines and regulations should be developedcollaboratively by European industry and academia. There are unsolved challenges with respect to ice action on structures for offshore wind. However, in the current draft for Horizon Europe WorkProgramme 2023-2024 on Climate, Energy and Mobility1, the challenges related to sea ice with regards toOffshore wind energy are not mentioned. In order to meet the crucial green energy goals, it is our statement that it is imperative to include sea ice in the final version.

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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

In bodies of water where ice is not an annual occurrence, such as in the Southern Baltic Sea, the design of offshore wind turbines is complicated by the difficulty involved in estimating the relevant ice parameters (thickness, velocity, and strength) and their corresponding probabilities of occurrence (return periods). In this paper, the use of a Copernicus reanalysis product is evaluated for its applicability in preparing drift ice thickness distributions in the design phase of offshore wind turbines. An area surrounding the Kriegers Flak wind farm site in the Southern Baltic Sea is used as a case study. The drift ice thickness statistics of ice within the region which could potentially drift into the site were weighted according to drift directions, based on the wind direction frequency in the area. We found that between 1993-2017, drift ice at Kriegers Flak mainly occurred in 1996 with 0.1 m maximum ice thickness, in good agreement with estimations reported in the literature. Ice thickness probabilities have been created from the 1996 winter data and used as input for a fatigue damage analysis of an offshore wind turbine. The additional steps required to improve the suitability of Copernicus reanalysis data for use as input into design calculations are discussed.@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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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

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.

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