Feature 2 | CAD/CAM UPDATE
Figure 6 UC on buckling envelope for the upper deck.
processing phase, plots are generated showing Unity Checks (UC) on yield and (plate) buckling, enveloping all load cases. Figure 6 shows a UC on buckling envelope for the upper deck. Obviously, buckling problems can be
expected aſt of the hopper opening, giving rise to detailed examination of this region. Also, for each element the governing
load case number is retrieved. Based on this information, the governing case for particular areas can be identified and the stress response for that wave can be reviewed. figure 7 shows the load case, inducing maximum shear stresses in the ‘tween deck aſt of the hopper. Te high stresses aſt of the hopper are due
to shear leg effects related to the transition of the (open) cargo part to the closed aſt ship part. Tis result was used to optimise the lay out of decks and bulkheads and fine tuning the plate thickness. Tis is a typical example where FEA can make a substantial contribution to the design of both safe and efficient hull structures. Finally, all results are analysed and in
• Modify and / or refine geometry • Refine mesh
cooperation with the design team one or more of the following actions are taken:
• Change plate thickness. Where necessary, sub-modelling is
Figure 7 The load case, inducing maximum shear stresses in the ‘tween deck aft of the hopper.
indicates the position of the wave crest with regard to the ship. Sometimes a regular wave which generates
the design value of one load effect severely over-estimates the value of another at the same time. Tis illustrates the large generality of the rule design values which apparently may lead to conflicting load effects in particular cases. On such an occasion, the highest load effect in the wave should be downscaled, based on common engineering sense. Tis downscaling is supported by the rules (1).
42 Table 1 presents the design waves
with the sectional loads for this ship. Te figures in bold indicate the dominant load effect of each wave, which equals the rule reference value. It can be seen that the wave reproducing the rule torsion simultaneously induces a horizontal bending of almost twice the rule value.
Strength analysis Load cases are assembled combining the still water and wave load sets. In a post
performed on parts of the model in order to retrieve accurate stress results in structural details, serving either a yield check or a fatigue analysis. Aſter a final run over all load cases is made the basic construction plan is fine tuned and ready for class approval.
Outside the rules Some ships of particular design fall outside the range of application of ship rules. A good example is Pieter Schelte, designed by Allseas Engineering, Delſt, Te Netherlands, and currently in the basic design phase. Vuyk Engineering is assisting Allseas with the overall hull girder analysis, using an FE model as shown in figure 8. Due to the vessel’s proposed exceptional
shape, common rule design values cannot be used. Terefore, 3D diffraction analysis
The Naval Architect July/August 2009
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