This paper describes the hull form optimization method of an icebreaker. The objective function is resistance in ice which is calculated by ICHM (Ice-Covered Hull Model) we developed. A genetic algorithm is adopted as the numerical optimizer. We optimize a hull form which has the same hull form parameters as the preliminary designed hull form as an application example. As the optimization result, the hull form which has smaller ice resistance than the preliminary designed hull form is obtained.
In the development of an icebreaker, we design a hull form which meets the performance requirements at its navigating sea area in both ice-covered and open water conditions. For evaluation of icebreaker’s performance in ice, ice tank tests are needed. These tests take longer time than general open water tests, because they need to make model ice for each test. Therefore, to develop an icebreaker’s hull form with better performance within a limited development period, it is important to preliminarily design a hull form whose performance is close to the requirement before ice tank tests. For a conventional hull form, many optimization methods using CFD, Rankine source methods or other resistance estimation methods have been studied for minimizing resistance in open water. However, when an icebreaker navigates in ice fields, its dominant resistance components come from ice breaking phenomena and hull-ice interaction. Thus, for the first step to the development of hull form optimization system for icebreakers, an optimization method for minimizing resistance in level ice is devised. Moreover, optimization performed only around the bow because bow form is dominant in ice resistance.
The optimization problem of an icebreaker was investigated by Edwards, Major, Kim, German, Lewis and Miller (1976). In that paper, the principal particulars of an icebreaker were optimized with the focus on ice resistance, costs and maneuverability in ice. However, since ice resistance and maneuverability in ice were calculated based on the results of parametric ice tank tests, it could not optimize a hull form in detail. On the other hand, the other calculation methods commonly used for estimating resistance in ice (Lindqvist, 1989; Ionov, 1988) employ hull form parameters around the planned water line. Hence, these methods can hardly consider underwater hull geometry. In the present study, to optimize a hull form in detail, we use ICHM (Ice-Covered Hull Model) (Anzai, Yamauch and Mizuno, 2019) in which, ice resistance is decomposed into ice breaking resistance, resistance due to ice buoyancy and clearing ice pieces and each resistance component is calculated by estimating ice pieces distribution on hull surface.