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 CR for the Baltic Sea. It is found that the velocity effect may produce global pressures equivalent to a CR 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.

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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

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