Improved method for non-linear FE analysis of polar class ship primary structures

In the IACS Polar Class Rules, primary structures are required to be assessed with direct calculation methods, which in most cases is either linear or non-linear finite element analysis. For linear analysis a clear set of requirements and acceptance criteria are provided in the rules. This approach is currently the most widely used assessment method although non-linear analysis would provide better insight into the behavior of the structure, giving designers tools to reduce weight and/or improve safety. The current rule requirements for non-linear analysis are high-level in nature and not easily applicable to practical design work. In order to implement non-linear analysis practically, a simple-to-use acceptance criteria is be needed. In this paper, a robust and simple-to-use assessment methodology and acceptance criteria is proposed. The proposed method provides means to ensure proper structural hierarchy. Appropriate safety margins are described through the plastic response of the local frame and the primary structural member. Allowable plastic deformation caused by the design ice load is determined by relating the permanent set of the structure to newbuild vessel production tolerances. An example analysis comparing the proposed method to existing linear and non-linear methods is presented. The proposed method ensures that the structure remains serviceable throughout the lifetime of the vessel.