Influence of the ice shape on ice-structure impact loads

Abstract

Ships that are navigating in ice covered waters are at risk of colliding with icebergs or ice floes, which can lead to severe damage to the ship‘s hull in certain conditions. Predicting these loads is of great importance for the design and safety of ships. Ice loads are driven by a range of parameters, including ice properties such as the compressive strength, the geometry of the ship's hull and values related to the interaction (e.g. impact velocity). Classification rules and requirements for polar vessels account for these parameters to a certain extent. However, sea ice exhibits not only variability of its properties but also the geometry of the ice shows a high degree of diversity: icebergs and ice floes exist in multiple different shapes. Therefore, this paper addresses the ice-structure interaction process, focusing on the influence of the initial ice contact shape and its impact on the local load. Additionally, the paper explores the feasibility of pinpointing geometric parameters of the ice responsible for influencing these loads. Ultimately, understanding which ice shape in contact poses the greatest risk would be valuable for improving numerical simulations for the worst case scenario and, consequently, enhancing the safety of ships in ice-prone environments.To achieve this, a comprehensive drop tower test campaign on a small-scale has been conducted at the test facilities of the Hamburg University of Technology. In these experiments, single impacts of differently shaped ice specimens against a rigid 50 mm steel plate in normal direction were investigated. In the experiments, a drop hammer was released from a predefined drop height to realize a brittle failure of the ice at an interaction velocity of about 2000 mm/s. This falls within the lower range of ship-ice interaction speeds. The drop hammer fell onto the ice specimen which was positioned centered underneath the drop hammer. The investigated specimen geometries were cylindrical shapes of 200 mm diameter with varying caps of cone, dome, wedge, ellipse and other shapes. Overall, 27 different shaped ice specimens were tested. The analysis includes the failure behavior of the ice specimens and the load introduced by the specific contact shapes. Two main failure modes were observed depending on the ice geometry, which showed significant differences in the load magnitude. Further relations between the initial contact area, the cone angle, and the specimen length to the peak force, maximum pressure, and energy were identified.