Verification of vessel resistance in old brash ice channels through model scale tests

Winter navigation is vital for the societies and communities living close to the polar region to ensure the year-round security of supply. Winter navigation in the Baltic Sea is based on intergovernmental cooperation on icebreaker assistance and the Finnish-Swedish Ice Class Rules. The icebreaker fleet assists merchant ships with ice-classification by providing escort on request. The ship's eligibility for icebreaker assistance depends on the current traffic restrictions, which define the lowest allowed ice class and gross tonnage to acquire assistance. The ice class of the ship sets requirements for the minimum engine power for a ship, aiming to verify sufficient ice performance for the ship with specific ice class in an old brash ice channel, which is a typical operational condition for assisted ships. Model scale tests are an essential way to verify the ship's ice performance. Ship's performance in ice is one of the key factors in the winter navigation system, however, recent environmental standards have generated substantial changes in the merchant fleet bow shapes, meaning verifying model scale practices suitability in the new circumstances is crucial. This thesis investigates ship's resistance in frequently operated, old brash ice channels using experiments on brash ice and ship correlation tests in full scale and in model scale. Through analysing the processes forming the resistance and determining the forces contributing to the processes, it scrutinises the ice properties contributing to the forces. As the thesis shows, the current model scale test practices do not sufficiently simulate all substantial processes which contribute to ship's resistance in brash ice. The results indicate that the current practices are suboptimal for realistically and uniformly simulating ship's resistance: based on evidence from correlation tests, the model tests performed according to the current practices might overestimate the brash ice resistance of certain new bow types. To secure uniform and accurate ice performance predictions for all ice-classed ships, this thesis proposes a new approach for performing a model scale test in an unconsolidated old brash ice channel. The main discovery of the thesis is to show that realistic simulation of ship resistance in an old brash ice channel in physical model scale tests necessitates realistic simulation of brash ice fragment interaction. It does this through introducing a novel methodology for model scale simulations that improve the current problems of the modelling of ice-ice interaction. This leads to improved simulation of processes, which are related to displacing the brash ice mass sideways. The new scaling approach abandons the current practice of scaling down the ice flexural strength and proves that solid ice fragments more realistically model the interaction between the ice fragments when compared to traditional model ice with scaled-down strength. The understanding acquired within this thesis improves the performance predictions in brash ice channels.