Image analysis methods for the flow of ice pieces on a hull surface in ice tank tests

When sailing in level ice fields, an icebreaker moves forward with breaking ice and pushing down/away the broken ice pieces along the hull surface. The broken ice pieces make hull resistance and cause propeller-ice interaction. Therefore, evaluating the flow of ice pieces around a hull surface is important to hull form development. When taking ice tank tests, we observe the flow of ice pieces around a model ship using observation windows on the floor of the ice tank. However, visual observation is limited to qualitative analysis. Therefore, we developed quantitative analysis method for the flow of ice pieces from bottom-view movie using the image analysis techniques. In this paper, ice concentration and the motion of ice pieces on a hull surface were analyzed for the flow of ice pieces. Ice concentration is analyzed by extracting ice pieces from the original movie using “Otsu's method”. The distribution of ice concentration on a hull surface is analyzed by projecting the coordinate points of hull surface onto the movie, considering the positional relationship between the hull and the camera. The motion of ice pieces is analyzed using CSR-DCF tracking method. The three-dimensional position of ice pieces are estimated by projecting tracking results onto the hull surface based on the assumption that ice pieces move on the hull surface. The accuracy of these analysis methods was verified using a simplified model test in the ice tank. As a result, the distribution of ice concentration was almost the same as the true distribution although it tended to be overestimated. Moreover, the error in the position of an ice piece on the simplified model was less than 10mm at the model scale. We analyzed the flow of ice pieces of the new “Arctic Research Vessel” of Japan. As the result, it was confirmed that the ice pieces moved sideways along with they moved from the fore to aft area, and that the motion of the ice pieces became linear as the ship’s speed increased. By using these analysis methods, the flow of ice pieces on a hull surface can be analyzed quantitatively. Moreover, the analysis results of the flow of ice pieces can be used for validation to develop more accurately simulation methods for icebreakers.