Interest to sailing in arctic zone is increasing, as due to the climatic change, the seasons when northeast and northwest passages are open enough for see transportation, are getting every year longer and longer. Some other activities like oil and gas exploration and drilling at Barents Sea require also regular sea traffic connections to be opened. Sea operations at arctic zone are challenging, because thick ice generates a high magnitude dynamic load against the hull and the propulsion units. Turning and backward sailing in thick ice field are the most critical operations, in which the steerable propulsion units are in totally different service as in the regular open sea cruising. In such operations the ice field, when guided downwards along the slope of the hull, is broken to large plates, which then are fed against the propulsion unit. The steering propulsion unit itself is a vertically mounted inverse mast column, at the top of which the horizontally spinning propeller(s) can be vertically turned to follow the steering commands. Such cantilever structure is now under random collision process when the column is breaking the underwater ice plates to smaller blocks. For hydrodynamic reasons the column has a limited cross-sectional area as compared to the propeller area making it sensitive to bending vibrations. Another dynamic interaction with ice is coming from the periodical blade-ice contact when the ice blocks pushed down to the propulsion depth are completely milled by the units. These two parallel dynamic processes have been the reason for several serious damages and losses of propulsion units leading to expensive service operations by means of support vessels. The purpose of this study has been to model the underwater propulsion system with all essential structures, parts and interactions with the surrounding fluid field and floating ice blocks. This brings a complex analysis, in which random collisions and periodical machining forces are loading the elastic hull-mounted inverse mast column with high end mass. The response behavior led to predictions for the reasons of the observed damages especially in case of collapsed bolt connections in the units.

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