Development of velocity dependent ice flexural failure model and application to safe speed methodology for polar ships

Sazidy, Mahmud Sharif (2015) Development of velocity dependent ice flexural failure model and application to safe speed methodology for polar ships. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The main focus of this research work is to develop a velocity dependent ice flexural failure model through numerical investigation of ship icebreaking process. In addition, the present work involves development of Excel-VBA software using this flexural failure model to determine ice impact load, minimum plate thickness, frame dimensions and safe speed methodology for Polar ships. First of all, individual material models of ice crushing, ice flexure and water foundation are developed using the FEM software package LS DYNA. Two different material models of ice are used to represent the ice crushing and ice flexure. The input parameters of these ice material models are selected from numerically conducted ice crushing test and four point bending test. The water foundation effect is modeled using a simple linear elastic material. The material models are incorporated into the numerical models of ship icebreaking. Two collision scenarios are considered for the ship icebreaking models; a head-on collision with a flat inclined ship face and a shoulder collision with an R-Class ship. In these models, the rigid ship impacts a cantilever ice wedge. The ice wedge rests on the water surface. Both collision scenarios are investigated with and without considering radial cracks in the level ice. The ice impact force and wedge breaking length are extracted from these numerical models of ship ice wedge breaking. Results indicate that the ship velocity, normal ship frame angle, ice wedge angle, ice thickness and radial crack significantly affect the breaking process. At higher ship velocities, the bending crack location shifts toward the ice crushing zone and results in a higher impact force. Higher impact force is produced for thicker ice, higher wedge angle and lower ship normal frame angle at a particular ship velocity. The existence of radial cracks reduces the magnitude of impact force and influences the breaking patterns. A methodology is presented to estimate the dynamic ice failure load using existing static failure models and dynamic amplification factors. The comparative study with these dynamic failure loads indicates that the developed numerical model results are in good agreement. A flexural failure model is developed based on validated numerical model results. The model provides velocity dependent force required to break an ice wedge in flexure. The developed model is validated with full scale test data and with non-linear finite element based dynamic bending model results. Application of this model is demonstrated to estimate the limit bow impact load and design ice load parameters. Finally, the Excel-VBA software “Safe Speed Check for Polar Ships” is developed using the velocity dependent flexural failure model and Polar Rules based limit state equations. This software and the velocity dependent flexural failure model are believed to help in establishing a rational basis for safe speed methodology as well as in improving ship structural standards and assessing ice management capability.

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