Trans RINA, Vol 153, Part A4, Intl J Maritime Eng, Oct-Dec 2011
Figure 8: Resultant stresses in the same double bottom girder modeled with and without manholes – Redistribution of stresses around the manholes show the multiplicity of critical areas as defined/required by SOLAS Ch. II-1/Reg 3-6.4.2 versus the FE model without the manholes
9. CONCLUSIONS
In conclusion, if the stress reduction is to be obtained solely by increasing the double bottom height of the vessel investigated, this height should be drastically increased to well above 2000 mm instead of the basic ship’s value of 1680 mm. This height could be reduced to about 1900 mm if additional double bottom girders are introduced, as will be shown in the companion paper. This coincides with the views of other investigators such as A. Kawamura, D Sakai et al., 1974 paper entitled “Full scale measurements and strength analysis of
60,000 DWT bulk carriers” [14].
The current IACS CSR formulation (dDB = B/20 or 2 m which ever is less) requires urgent revision. The formula that controls the double bottom height should include parameters related to the draught of the vessel, the aspect ratio of the double bottom (i.e. width of the double bottom between the hopper tanks, over the length of the cargo hold, in relation to the vessel length). In addition more realistic spacing of the double bottom floors and girders should be adopted support and accurate
to ensure double bottom transmission of more balanced shear forces to the transverse bulkheads. The IACS CSR A-258 ©2011: The Royal Institution of Naval Architects
requirement concerning the calculation of the minimum double
bottom height should not include vague
statements like “
..in general..” or “… both DB girders and floors greater spacing maybe accepted depending on the FEA results” for the reasons highlighted in paragraph 3 of this paper.
The omission of the manhole and pipe openings from the FE model may well contribute to structural failures that may require redesigning in part or in whole, of the double bottom structure (say by adding floors/girders, or increasing the height of the double bottom). This may contribute to the failure of establishing the position of all the critical areas within the cargo block area as required by SOLAS Ch. II-1/Reg 3-6/4.2.
Table 5 below provides a comparison between the current and old rule formulations, as well as values for a proposed interim formula for establishing a minimum acceptable double bottom height, (based on the proposed spacing of the double bottom floors and girders), which will be further refined in the companion paper, already mentioned.
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