Brash Ice and Level Ice Growth, Effects of Snow

Sammanfattning: Brash ice occurs due to frequent navigation in ice-infested waters, typically along established navigation tracks that are regularly maintained by icebreakers, and harbours. The accumulation and consolidation of brash ice between two ship passages are influenced by meteorological factors including the cumulative freezing air temperatures, and the mechanical processes such as ice-breaking due to ship passages. Brash ice's physical and thermodynamic properties also have a marked influence on the accumulation process. Brash ice formation and growth can occur rapidly, and the accumulations pose hazards to shipping operations. Vessels lacking ice-breaking capabilities often need assistance from icebreakers, resulting in heightened operational costs, greater fuel consumption, and ultimately, increased greenhouse gas emissions. Navigation in icy waters relies on operational strategies that can be enhanced and guided by reliable forecast models. Accurate prediction of brash ice growth in Subarctic and Arctic regions is crucial for port and ship design. However, our understanding of brash ice development remains incomplete, and existing brash ice models can be improved and validated through the integration of full-scale data. Moreover, there has been limited research into the physical and mechanical properties of brash ice. Full-scale results are invaluable for both model-scale studies and developers seeking to estimate ship performance in brash ice conditions.This thesis presents the study of various parameters affecting brash ice formation and is validated by data from three full-scale brash ice channels and two harbours situated in the Bay of Bothnia, Luleå, Sweden. The primary objective was to assess the impact of snow on brash ice formation and growth. To begin with, laboratory-scale experiments were conducted to study the simultaneous growth of water and slush under identical meteorological conditions. Concurrently with the laboratory-scale experiments, the growth of brash ice in three full-scale channels and the growth of level ice adjacent to the ship channels were examined. Parameters under investigation included thickness profiles along the channel cross-section, macroporosity, distribution of brash ice piece sizes, and lateral movement of brash ice. Additionally, the study explored snow ice content, microstructure, microporosity, density, and uniaxial compressive strength.The laboratory scale experiments revealed that slush freezing occurred more rapidly than water freezing, primarily due to the porous nature of the slush, where freezing took place within the water-filled pores. Also, the freezing front of the slush remained insulated from direct contact with the warmer water from the slush layer underneath, whereas the freezing front of water was in direct contact with the water column. Based on the laboratory-scale experiments and full-scale observations, two key effects of snow on brash ice growth were identified: snow insulation between two ship passages and the transformation of snow into slush and then into snow ice after navigation. These effects were integrated into a brash ice growth model, and the model results were validated using full-scale data. The findings of the modelling supported by the measurement data, showed that in ship channels with low frequency of navigation and where snow remained on the brash ice for extended periods, snow insulation had a more substantial impact on brash ice growth compared to the snow-slush-snow ice transformation. Conversely, in frequently navigated channels where incoming snow was more often submerged, the snow-slush-snow ice transformation played a more prominent role in brash ice growth. It was also found that brash ice had the lowest snow ice content, while the level ice had the highest.In addition, the channel's initial macroporosity was estimated through the correlation between measured porosities and cumulative freezing air temperatures. This estimated initial porosity was integrated into the brash ice growth model along with the sideways motion of brash ice at each ship passage. It was found that in the less frequently navigated and narrower channels, the amount of brash ice expelled at each vessel passage was significantly higher than in the wider and more frequently navigated channels. The piece sizes and piece distribution were investigated for two full-scale ship channels and two different harbours, revealing that the brash ice pieces were larger in ship channels compared to harbours. The piece size distributions could be well described by a three-parameter lognormal probability density function.