Floe Ice – Sloping Structure Interactions

Abstract

Several interaction processes can be identified during floe ice - sloping structure interactions. However, fierce processes around the waterline are the most conspicuous phenomena. These violent processes feature ice fracturing and the potential rotation of subsequent ice blocks. This thesis focuses primarily on the fracture of ice floes. The exact failure pattern of an ice floe is complicated by the actual ice conditions in which the sloping structure is positioned. Regarding the ice condition, we do not predefine it as a ‘level ice’ or a ‘broken ice’ field. Instead, considering an ice field composed of ice floes of varying sizes is a more realistic approach. Studying the interaction between an ice floe and a sloping structure (i.e., floe ice - sloping structure interactions) can yield more general results because an ice floe has a spatial scale that typically ranges from 10 m to 10 km. The ice floe can be large enough to be regarded as level ice or can be small enough to be treated as a member of a broken ice field. Currently, Arctic exploration and exploitation are expanding into a relatively ‘open’ ice condition, within which, a knowledge gap exists regarding the fracture of a finite size ice floe in the context of ice-structure interactions. To advance our understanding, this thesis studied the following observed failure modes of a finite size ice floe within such an ‘open’ ice condition: 1) In-plane splitting failure; 2) Out-of-plane flexural failure; and 3) Competition between different failure modes in the context of ice - sloping structure interactions. This thesis addresses influential factors, such as an ice floe’s size, geometry, confinement, and ice-structure contact properties. In a decoupled manner, these different failure modes were studied primarily using the concept of fracture mechanics. The major contributions of this study are the following: 1) Analytical solutions for each failure mode were proposed for ice floes of varying sizes, which range over a large spatial scale (i.e., from approximately 1 m to 10 km); and 2) Each analytical solution within the established analytical framework was rigorously verified, either by numerical simulations, existing idealised analytical results, experimental measurements, or all of the above. Theoretically, through derivations, implementations and validations of all these analytical solutions, many interesting results were obtained, such as the following: 1) The validity of using Linear Elastic Fracture Mechanics (LEFM) to study an ice floe’s splitting failure on an engineering scale was confirmed; 2) An ice floe’s confinement has a much more profound effect on increasing the force required to split an ice floe in comparison with the influence of floe geometry; 3) Three out-of-plane flexural failure scenarios were further conservatively identified and analytically studied; and 4) A floe size requirement (i.e., a square floe’s size L <27(ice thickness)3/4 ) was suggested for the radial-crack-initiation-controlled fracture of an ice floe. From a practical point of view, with all these analytical solutions of failure modes at our disposal, we are thus able to construct a failure map that relates an ice floe’s dominant failure mode to its geometry, relevant contact and material properties. In addition, these analytical formulae can be effectively incorporated with multi-body dynamic simulators to assess the performance of Arctic offshore structures in ice over large temporal and spatial scales. In addition, as opposed to relatively ‘open’ ice conditions, we have also studied ice - sloping structure interaction in ‘tight’ ice conditions. One extreme interaction scenario within these ice conditions is when a large amount of ice rubble accumulates in front of a sloping structure. This scenario has long been recognised as one of the controlling design conditions and has previously been under thorough investigation. In this thesis, a novel approach that combines both physical model tests (i.e., measured by tactile sensors and load cells) and a theoretical model was employed to study ice load’s spatial and temporal variations on a sloping structure under the influence of rubble accumulation. The following results were found: 1) The presence of rubble accumulation increases the global ice resistance; 2) The maximum value of ice resistance occurs in a location below the waterline, which signifies the importance of the ice rotating process and rubble accumulation effect; and 3) The developed theoretical model, which was validated by both physical model tests and existing theoretical models, can serve as a preliminary tool to study ice load’s temporal and spatial distribution under the influence of rubble accumulation. As an extension, we also explored a seemingly promising numerical method’s (i.e., the Cohesive Element Method (CEM)) applicability in studying ice - sloping structure interactions under the influence of rubble accumulation. Preliminary results demonstrate that there is still a substantial knowledge/computational gap in using this numerical method to simulate ice and sloping structure interactions in a three-dimensional setting. The primary deliverable contributions of this thesis to the scientific and engineering community are the proposed analytical solutions and the framework for different failure modes under two different ice conditions. Such analytical treatment prepared us to simulate loads related to floe ice - sloping structure interactions, which are important for Arctic exploration and exploitation on large temporal and spatial scales.