Ships in ice : the interaction process and principles of design

Zou, Bin (1996) Ships in ice : the interaction process and principles of design. Doctoral (PhD) thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (18MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

For ships operating in arctic and sub-arctic waters, ice load is a major threat. Due to the uncertainties in ice conditions and varying operating situations, an accurate estimation on design ice load is difficult. The objective of the present research is to investigate the ice loads and the associated structural strength from aspects of mechanics, statistics and design principles. -- First, the ice-structure interaction process is investigated from the view point of mechanics. The interaction is characterized by ice fracture and damage. The ice load is highly localized within high pressure regions termed critical zones. A numerical analysis was carried out to investigate how a crack may propagate in an ice sheet and how the ice material is damaged during an ice-structure interaction. The analysis showed that small shear cracks, with mixed modes, are more likely the candidates for the fracture spalls and the formation of critical zones. -- Critical zones vary in space and time. These critical zones are characterized using parameters such as spatial density, zonal area, and the zonal force. These parameters in the model were calibrated using ship trial data of CCGS Louis St. Laurent. The ice loads on a design area were modelled as a random number of critical zones, each with a random force. Based on this model and extreme value theory, a design curve was proposed for the estimation of extreme ice loads. -- Third, the strength of the structure was investigated. A long plate, loaded by uniform pressure was proposed as the design model for the plating. Due to the randomness of ice load, there are uncertainties associated with the design model. To understand this uncertainty, various load scenarios were investigated using the finite element method. The results show that the plate fails at a dominant section, which fails in a way similar to an "equivalent long plate". Factors affecting the failure of the panel are lateral support and interaction between critical zones. These factors were investigated and empirical formula were derived based on finite element modelling. -- A simplified model was proposed to investigate the failure of the "equivalent long plate". This model was used, together with factors of lateral support, location and interactions between critical zones from empirical formula, in Monte Carlo simulation scheme to model the uncertainty of the design model of the structure. The simulated results of the uncertainty factor were approximated by a lognormal distribution. -- Finally, the results from the analysis on the ice loads and the structural resistance were used in discussion of the design principles. Two design methods, i.e., reliability design and code design methods, were discussed. Principles in selecting design load and resistance were discussed. These principles were applied in an example design of an offshore oil tanker. Reliability of the plates from different design strategies were evaluated. It was found that, for ultimate rupture, a yearly maximum with a probability of exceedance of 10⁻⁴ is appropriate as the design load.

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