Optimization of Bow Plating for Icebreakers

Abstract

The objective of the work reported here is to ensure that ice-capable ships are both safe and economical. It is largely based on research conducted over the past two decades. The present methodology allows the designer to calculate a bow plating thickness which will resist local ice loads and be cost-effective. Full-scale data for the MV Canmar Kigoriak and USCGC Polar Sea were ranked, curves were fitted through the tail of each data set, and a Type I extreme probability distribution was derived for each data set. The Canmar Kigoriak data were then subdivided based on contact area and a simulation was performed to derive the load distributions on subregions of the instrumented panel, termed subpanels. On the basis of this analysis, a local ice load model was validated and is used in the subsequent analysis. To evaluate the strength of the bow plating, three limit stages (three-hinge collapse, permanent set, and membrane collapse) are considered. Statistical distributions for each of the input parameters are established. The probability of failure is calculated for each limit state corresponding to a range of plate thicknesses, frame spacings, and annual numbers of impacts, using First Order Reliability Method software. Plate thickness is optimized for minimum cost. Minimum safety levels for permanent set and membrane collapse are also specified. The objective function considers costs due to construction, aesthetics, repair, and replacement. An empirical expression for the expected annual cost of damage is developed. Optimum plating thicknesses are specified. Costs associated with lost use of the vessel or increased steel weight can also be specified.