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Design and Assessment Methods for Ships in Ice

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SIPW03 - Ice-structure interaction

Design and assessment of ships in ice is a topic that concerns a wide range of people, from owners, to designers, builders, regulators and the insurance industry, with the public, the environment and the economy being impacted by arctic shipping risks. The methods that are being used to design and evaluate ice class ships have evolved over many decades and are currently continuing to change. In this discussion, the focus is on the steel structure of ice-going ships, including structurally important ice loads and the nature of the structural response. 

As we seek to better understand the nature of the interaction between ships and ice, we are increasingly focusing on complex multi-body interactions and non-linear behaviours. In such situations, general solutions are not only not currently available, but may not be achievable at all. While we can assemble specific solutions, we must ask whether a general understanding of complex nonlinear systems is possible, and if so, what mathematics can we use to develop them?

Concern for structural ice loads has, until now, mainly focused on a single situation; the “design condition”. For that condition, the structure behaves linearly or pseudo-linearly. The “design” loads have been largely developed empirically, from a combination of measurement data and loads inferred from past successful practice.

While much of the current industrial and regulatory practice still employs relatively simple models of load and response, new sets of tools and approaches are taking shape and are being applied by the most sophisticated ship owners, builders and operators. These new approaches seek to directly model a growing set of complex issues and scenarios, with consequent improvements in the fidelity of ice loads and structural response. The growing computational power and improving simulation tools are enabling these developments. To illustrate these approaches, some of the author’s own efforts will be described.

One recent and developing technique is called safe speed analysis. This method involves modeling a wide variety of discrete ship-ice interaction cases, modelling the load and overload capacity of the vessel and then combining these results to produce a multi-parameter map of acceptable operations (speed, ice size, etc). The method makes use of available tools and solutions, including “Popov” type collision models (algebraic solutions of two-body collisions), and explicit dynamic finite element models (LS-Dyna) to capture interaction kinematics, contact and a full range of structural responses (ductility, dynamics, stability).

A second, but related technique is a model called GEM , which uses simple event solutions and simple equations of motion to model large scale operations of vessels in ice. GEM , while seeking a reasonable level of accuracy, focuses on high simulation speeds and practical decision support rather than on fidelity and universality.

After presenting these two methods, the presentation raises three mathematical challenges. The first challenge relates to the problem of interacting chaotic systems and wonders whether a calculus for such systems is possible. The second challenge questions the application of probabilistic design to ice class ships. The third challenge relates to the simulation of multi-body systems. Such systems are highly nonlinear and computationally costly to model. Could an asynchronous or quasi-synchronous timestep algorithm yield improvements in overall speed?

This talk is part of the Isaac Newton Institute Seminar Series series.

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