Page 117 - Marine Structural Design

Chapter 5 Ship Hull Scantling Design by Analysis 93

Stress Concentration Area:
In the areas where high stress concentrations may take place, local fine mesh models are to be
applied by using forces or forced deformations as boundary conditions based on the results
obtained in the global analysis. Alternatively, sub-modeling, super-element techniques or
direct mesh refinement may be introduced.
Attention should be paid particularly to the following areas:
Areas around large openings,
Longitudinal stiffeners between transverse bulkheads and the first frame at each side of
the bulkhead,
Vertical stiffeners at transverse bulkheads with horizontal stringers in the way of the inner
bottom and deck connections,
Horizontal stiffeners at transverse bulkheads with vertical stringers in the way of the inner
side and longitudinal bulkhead connections (Figure 5.4),
Corrugated bulkhead connections.
Fatigue Model
If fatigue is of concern, analysis of critical structural details should be performed. Fine mesh
models shall be completed for critical structural details in the areas such as the following:
Hopper knuckles in way of web frames,
Topside support stools,
Details in way of the moonpool,
Other large penetrations in longitudinal load bearing elements,
Longitudinal bulkhead terminations,
Stiffener terminations,
Pontoon to column or column to deck connections,
Other transition areas when large changes in stiffness occur.
The size of the model should be such that the calculated hot spot stresses are not affected
significantly by the assumptions made for the boundary conditions. Element sizes for stress
concentration analysis should be of the same order of magnitude as the plate thickness.
Normally, shell elements may be used for the analysis. Only dynamic loads are to be applied
on the model, because only these affect the fatigue life of the structure. The correlation
between different loads such as global bending, external and internal pressure, and
acceleration of the topside should be considered in the fatigue assessment.

5.3.2 Boundary Conditions
Defining boundary conditions is one of the most important steps in FEM analysis. For local
analysis models, the boundary conditions imposed by the surrounding structures should be
based on the deformation or forces calculated from the global model.
The boundary conditions, for a global model, have no othm purpose than to restrict the rigid
body motion. Fixing 6 degrees of freedom @OF) at both ends (and comers) of the model