Sea ice features made of ice rubble, such as rubble fields and ice ridges, are the most difficult ice condition encountered by offshore structures or vessels operating in first-year ice. Thus, scientific understanding of the mechanical behaviour of ice rubble is crucial to correctly estimate ice loads on offshore structures. Ice rubble is a granular material and its mechanical properties are often studied assuming that the ice blocks constituting the rubble do not break. It has, however, been observed in laboratory experiments that the ice blocks within deforming ice rubble may break due to compressive ice-to-ice contact forces. This occurs as the load is transmitted within the rubble through force chains, sequences of contacting ice blocks under high compressive stress. The present thesis studies ice block breakage using three approaches: 1) an experimental study on ice block breakage under compressive ice-to-ice contacts, 2) high resolution bonded particle modeling (BPM) of the aforementioned breakage experiments, and 3) the development of a simplified ice block breakage model for DEM simulations. In the experiments, three ice blocks were set to form two ice-to-ice contacts and compressed until the failure of the three block system. The experiment results revealed that shear failure was the primary failure mode of the ice blocks in compressive ice-to-ice contacts and that the force transmitted by the contact was limited by the bulk strength of the ice. BPM simulations further confirmed the hypothesis that the failure of ice blocks occur due to shearing governed by the bulk strength of the material. Moreover, the stress distribution within the ice blocks, obtained from the simulations, showed that the Mohr-Coulomb failure criterion can be used to model the observed shear failure in a simple yet reliable manner. Following that, a simplified block breakage model based on shear failure and the Mohr-Coulomb failure criterion could be developed. The breakage model is for DEM simulations of ice rubble. The breakage model was integrated into an existing DEM tool and direct shear box experiments on ice rubble were simulated. The simulation results revealed that block breakage is one of the modes of force chain failure within deforming ice rubble. Simulations further showed that block breakage reduces the shear resistance and the shear strength of ice rubble. The findings of the present thesis indicate that the common approach of modeling contact failure of ice blocks in DEM simulations by using a contact force limit based on local crushing of ice is not always accurate. Instead, this thesis suggests that the shear failure of ice blocks has to be accounted as well, since block breakage limits the load transmitted by force chains. Moreover, the reduction of the shear strength of the ice rubble due to block breakage implies that the ice load estimates obtained from DEM simulations of ice-structure interaction processes without a block breakage model may be overly conservative.
|Julkaisun otsikon käännös||Combined experimental and numerical study on ice block breakage|
|Tila||Julkaistu - 2023|