Microstructure of sea spray ice. Prediction of icing on marine structures.

Sea spray icing implies a considerable risk and safety problem for the shipping and offshore industries. In cold , stormy weather sea spray ice can form extremely rapidly on ships and offshore structures. This process is, in contrast to normal ice formation on the surface of the sea or a lake, difficult to predict. This is mostly due to three complex interacting processes. First, the sea spray flux depends on spray generation created by breaking waves the transport of droplets with the wind. Second, sea spray ice accumulation is also dependent on cold air and water temperatures, the geometry and surface properties of a structure on which the ice is accumulating. Third but not least, accumulation rates depend on the amount of unfrozen seawater entrapped in the porous spray ice: this water can contribute to more than half of the sea spray ice loads. It is the latter contribution that currently make predictions of sea spray ice formation uncertain. The present project focuses on the microstructure of sea spray ice and crystal growth processes during sea spray ice accumulation. 3D X-ray tomographic imaging will provide important novel knowledge about the microstructure and properties of sea spray ice. This will enable us to improve models and predictions of its accumulation, e.g. through better knowledge of its liquid fraction. We will also obtain important information about ice-surface contacts for different materials, providing fundamental data and insight for the development of anti-icing materials and coatings. Improving our understanding of icing processes in the sea, the project will contribute to improved safety and risk evaluation in ice-covered waters. During the first phase of MICROSPRAY we have developed a laboratory setup that allows us to grow spray ice under different growth conditions (wind, air and water temperature, water salinity) and on different surfaces. We have constructed a coolng stage for imaging sea spray ice samples at the NTNU hub of the Norwegian Centre for Nano-scale X-ray tomography (https://www.next.uio.no/hubs-and-capacities/) and successfully performed the first X-ray tomographic scans of saline spray ice. We found that the microstructure of laboratory grown spray ice looks very similar to ice formed from sea spray in the field (collected at Longyearbyen harbour in spring 2021), showing pronounced brine drainage channels that so far had onlz been detected in sea ice. The comparison of field and laboraorz ice gives us credence to continue dtailed studies with our setup. In the next phase we will integrate in situ measurements of ice adhesion strength, and perform systemtic tests of spray ice growth for a wide range of growth conditions, in order to collect a large combined dataset on sea spray adhesion and microstructure.

Ice formation from sea spray is an important process regarding offshore operations in polar regions. Due to its safety risk for vessels and structures it has been become part of operational weather forecasting for half a century. However, while models of different complexity have been formulated to predict icing, their performance is still rather limited. There is a general consensus that one of the causes for this lack in performance is due to the freezing process of sea spray and the largely unknown microstructure of the forming ice. In recent years it has been proposed to include microstructure models to predict sea spray ice growth. However, so far no observations of dendritic structure of spray ice exist. The present project aims to close this knowledge gapin microstructure observations through imaging by nondestructive 3-d X-ray micro-tomography (XRT). Another challenge of marine icing is to understand the influence of different surfaces on the microstructureand properties of sea spray ice. We will perform ice adhesion experiments on different substrates, and also here observe the 3-d microstructure in dependence on growth conditions. Of particular interest in the freezing process of saline ice, its microstrcture and adhesion is the fate of saline brine that is expelled during freezing. Also this process will be studied for the first time by XRT imaging, and making uyse of high spationtemporal resolution at synchrotron radiation facilities, as well as high performance radiography . Our approach will gain new fundemental knowledge of microstructure and adhesion properties of sea spray ice. This is of particular importance for the predictability of ice loads on vessels and offshore structures. The project will also lay the ground for development of feasible coatings/materials of low ice adhesion, of high usefulness in cold regions operations. The project is of particular importance in view of a changing climate and increasing logistics in cold waters.