OPTIMIZATION OF BOW PLATING FOR ICEBREAKERS

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 co nducted 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 Kig oriak and USCGC Polar Sea were ranked, curves were fitted through the tail of each data set, and a Type I extreme probability distribution w as derived for each data set. The Canmar Kigoriak data were then subdi vided based on contact area and a simulation was performed to derive t he load distributions on subregions of the instrumented panel, termed subpanels. On the basis of this analysis, a local ice load model was v alidated and is used in the subsequent analysis. To evaluate the stren gth of the bow plating, three limit stages (three-hinge collapse, perm anent set, and membrane collapse) are considered. Statistical distribu tions for each of the input parameters are established. The probabilit y of failure is calculated for each limit state corresponding to a ran ge of plate thicknesses, frame spacings, and annual numbers of impacts , using First Order Reliability Method software. Plate thickness is op timized 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 em pirical expression for the expected annual cost of damage is developed . Optimum plating thicknesses are specified. Costs associated with los t use of the vessel or increased steel weight can also be specified.