Unconsolidated ice rubble modelling with continuum approach

Abstract

In ice-infested waters and in the absence of icebergs, ice ridges are responsible for the highest quasi-static loads on offshore structures. The main body of an ice ridge is located below the waterline. This submerged part (keel) consists of an upper consolidated layer and a lower unconsolidated layer, ice rubble. This thesis focuses on the description of the mechanical behavior of ice rubble through the development of a continuum model of ice rubble. Ice rubble consists of separate ice fragments. Therefore, the continuum modelling will result in random errors due to material inhomogeneity when an insufficient number of ice fragments is considered. To estimate these errors, a 2D discrete element model of ice rubble, developed at Aalto University, was used. The average stress was calculated using the contact forces inside a virtual ice-rubble sample. The discrepancy between the average stress tensor and the boundary forces was investigated for uniaxial and biaxial boundary conditions. The material model used in this work was based on experimental data available in the literature, the results obtained at the Hamburg Ship Model Basin (HSVA) and the results from the cold laboratory at NTNU. Based on experimental data, the model was developed using plasticity theory. In particular, the critical state theory was shown to provide a more consistent description of ice rubble shear strength than that provided by the Mohr-Coulomb criterion. In addition, continuum breakage mechanics was used to provide the link between the icefragment properties and the overall ice-rubble characteristics. The material model was implemented in the Abaqus finite element software which was used to model the ice rubble - structure interaction process. The model predicted the load level measurements at the HSVA. In addition, varying model parameters suggested that ice rubble accumulation controls the load, while the breakage of ice fragments has only a minor effect on the load level.