FEA Modeling Guidelines: Some Critical Problems in Analyzing Offshore Structures

Structural finite element analysis (FEA) is routinely used in the design of modern ship and offshore structures. The benefits of FEA are apparent when used proficiently, but it can yield incorrect and misleading results when misused. There are certain types of errors in FEA that are unfortunately not that rare, but many of these errors can be avoided if clear and direct advice is provided to the analyst. SNAME Hull Structure Panel (HS3) aims to help analysts recognize the limitations of FEA tools and provide guidance on how to overcome them. A generic semisubmersible model is studied in order to demonstrate the simplification of the real structure in an FEA model, studying beam idealization in ship/offshore structure FEA, along with how to deal with the global and local loading. The effect of modeling geometry discrepancy is identified with numerical examples to illustrate the benefits of the guidelines. When a local model is built and boundary conditions are to be applied, displacement from the global model is often used to transfer the global loads. The difference between a detailed shell local model and the simplified coarse beam/shell global model requires the use of so-called rigid elements, such as RBE2/RBE3 in the commercially-available program ANSYS, which was used in developing these guidelines. Other than displacement, the local model may be analyzed using force boundary conditions obtained directly from the global model beam element internal forces. While the two methods are acceptable in the applications, there are issues with each of the methods. Theoretical formulation is used to show that either of the choices of boundary conditions could be established in order to solve the basic equation. However, a judicious process to deal with the boundary conditions will help analysts determine the best approach. An investigation of the effects of different methods is shown to illustrate the consequences. A summary of guidelines to these particular problems is presented. Future work is also discussed