GLOBAL STRENGTH ANALYSIS USING FEM

The finite element method (FEM) is a numerical method for solving complex engineering problems. A structural 3D model built up from 2D shell and beam elements can be loaded and the resulting deformations and stresses can be analyzed.


Why a global strength analysis?
Because ships become more complex due to discontinuities in the structure, like large openings in the shell or deck structure and complex loading conditions, ship owners and class societies demand proof of strength of the vessel and local stability (buckling) structural items. A global finite element calculations can provide this proof to a high degree of accuracy and therefore higher allowable stresses can be used. Furthermore, these calculations provide a lot of information to optimize the structure and/or perform more detailed calculations.

How to perform a global strength analysis?
Structural finite element calculations are ideal to simulate these complex structures and/or loading conditions. To perform such an analysis a good geometrical model is crucial. At DEKC Maritime we use Rhinoceros to create our models. In-house developed scripts help us to create a more accurate model within less time. Examples of these scripts are automatic splitting tools for crossing areas and an automatic edge splitting tool to provide the necessary common edges. After completion, the model can be exported to Ansys. Our export script creates an identical model in Ansys, including all thicknesses, used materials and identification. This model is then meshed, for a global model a coarse mesh is normally used. Interesting areas can already be meshed using a finer mesh.

Global loading usually includes a light weight and buoyancy distribution together with a wave load distribution. For a sailing yacht the rigging load and for a cargo vessel the cargo distribution is added. This cargo distribution can be exported from our ship stability program NAPA and applied to the global model.
 
What results can be obtained from the analysis?
From the calculated results valuable information can be extracted. Of course, the global stress results can be evaluated in an envelope of the stress results. These envelopes (including corresponding load case) give a direct overview of the occurring stresses. Together with the global deflection and the reactions, they also provide a good way to check the results. Based on the results, areas in the model can be identified where reinforcement may be necessary, or where structural material may actually be removed.  Further deflection results can be extracted to calculate cargo hold deflections or determine the relative deformations of window frames.

Are there more options?
The global results can be the base for further sub model analysis. In this case the deflections from the global model are used as prescribed translations/rotations at the sub model boundaries to include the global deflection effects.
These sub models are normally provided with a more detailed geometry and a finer mesh to calculate the detailed stress results. In case of stresses above the yield limit an analysis, using nonlinear material properties, could prove the structural integrity of the structure and allow local yielding. To calculate the stability of an unstiffened plate panel with unusual shapes and/or loading patterns an “in house” developed method can be used. This method isolates the unstiffened panel and load pattern obtained from the larger FE model. After applying an imperfection, a nonlinear analysis determines the load bearing capacity of the panel.

Very usefully
Recapitulating: Finite element analysis is a powerful tool that can provide very useful detailed information to analyze and optimize new built structures or existing structures. At DEKC we can perform these finite element analyses and more.
At DEKC Maritime we use Ansys APDL to perform our finite element calculations. Our licenses provide us with all the necessary simulation options: structural non linearities (stability analysis), nonlinear material behavior (calculate local yielding), applying strain (simulate weld shrink) and contact analysis (friction analysis). We use APDL because this offers the possibility to use scripting. These scripts can be used for multiple projects and automate time consuming operations: applying loads, making envelopes etc.