Scaling principles for model testing in old brash ice channel

Abstract

Climate warming sets higher requirements for sustainability in ship industry, which fuels the introduction of stricter power restrictions and new types of ship hull design. The new designs invite us to reconsider the principles in model scale testing in ice, which is the state-of-the-art method to predict vessel's ice performance prior to its construction. Ice model testing principles were originally developed for the evaluation of performance in level ice at slow speed, but currently those principles are widely applied for ice model testing in other ice conditions as well. The forces forming the vessel resistance in a brash ice channel are different compared to forces forming the vessel resistance in level ice. In consequence, the current model scale testing principles are suboptimal when predicting vessel's performance in an old, frequently operated brash ice channel, as our earlier research indicates that the prediction is conservative especially for modern merchant ship bow shapes with high stem angles. An old brash ice channel is especially interesting, as it is the determining ice condition in Finnish-Swedish Ice Class Rules ice class granting process. This paper addresses the problem by introducing a new similitude number – a Channel number – to be applied alongside the currently applied Froude and Cauchy similitudes. The Channel number aims to realistically scale the dominating forces that form the resistance of the vessel in an old brash ice channel by developing further the modelling of the resistance component induced by displacing the ice fragments. This necessitates considering the brash ice fragment interaction and inertia forces. The new scaling principle, which supplements the currently applied Froude-Cauchy scaling principle by the Channel number, is considered more accurate for brash ice conditions. It is designed to predict fairly and uniformly the ship resistance for all bow shapes avoiding potential overpowering of the modern bow types with a high stem angle. This would contribute to providing a good harmonization of ships' expected ice performance with ice classes, and, further, improve the fleet schedule predictability that the logistic chains demand from the transportation.