Direct calculations methods for ice strengthened hulls in the Finnish-Swedish Ice Class Rules

The goal of the study was to lay groundwork for implementing guidelines in the Finnish-Swedish Ice Class Rules (FSICR) for using direct calculation methods (e.g. Finite Element Method, FEM) for assessing hull structural strength in the case of ice loads. Major part of the work was to study in practice with FE analysis how the proposed concepts on modelling and criteria would work on different sized vessels with different ice class. It was decided to focus on general / bulk cargo vessels as these are most frequent ship type operating in ice in the Baltic Sea. Three different sizes (3000 DWT, 10 000 DWT and 58 000 DWT) of vessels, corresponding to typical sizes on the Baltic Sea, were designed and modelled. From 10 000 DWT ship three different ice class (IA, IASuper and IC) versions were modelled. The Baseline 10 000 DWT IA model worked as test model for testing how mesh size, model size, boundary conditions and different material models affect the results. From these studies, results were gathered for suitable proposals for the future guideline text. For determining the plastic capacity of the structure, criteria were chosen to be to limit the permanent deformation of the structure to remain below limits of IACS newbuilding quality standard [1]. All chosen vessels were loaded with various load patches to understand the capacity of the structure against the chosen criteria, and to find the most onerous load patch locations and sizes. The load patches followed the FSICR. In general, the plastic capacity of the shell and framing was found to be about 300 to 400 % of elastic rule design load. For stringers and web frames, the typical double side construction resulted in structure where capacity is not limited by bending capacity, but rather by buckling, and clear conclusions of capacity of structures according to current rules could not be drawn. For this, further research with single skin type vessels is recommended. Finally, some recommendations for future rule formulation are given. However, further research is required before the final rule formulation can be given.