design considerations for aluminum hull structures - Ship Structure ...

-122-redistribution of the still water buoyancy to reflect the wave. Therefore,such a calculation maybe used to evaluate the levels of totalbending moments (static plus dynamic) to be expected. Accordingly, standardbending moment calculations for several waveheights and for still watermay be made to produce an Mj vs Q log (M>Mj) plot by plotti~ ~he obtained( hebending moments at the probabilities corresponding to the X ~ values at)which the bending moments have been calculated. The sense (hog or sag) ofthis curve of expected moments will be the same as that of the still waterbending moment. By subtracting 2 times the still water bending from itsordinates a curve of expected moments of the opposite sense may be obtained.The foregoing procedure should be based upon the load condition of thedesign yielding the highest still water bendi~ moment, since the effectof load distribution is more significant to the calculated wave bendingmoment than is the draft at which it occurs. Finally, if the base scaleof probability is multiplied by the expected number of oscillations in theintended life of the ship, the resultant log 10 base scale will indicatethe numbers of oscillations during the life of the ship in which it shouldbe expected the Mj lev-elsof bending moments are reached.4. In applyi~ the foregoing proceduz-es to the subject designit was considered that probabilities based on average North Atlanticweather would be realistic yet not overly conservative, in view of contemplatedtramp operations in the various seas of the world. Accordingly,the values given by Figure 18 of Refereme (Al) for a 600 foot ship areconsidered valid for this application, subject to minor correction forFroude number. Based on Figure 12 of Reference (Al), increasing theFreud rnunberfrom 0.1 to about 0.2hwould increase the ~ value atLQ = lo-lofrom about .0485 shown on Figure 18 of Reference (Al) to .O~.5. Bending moment estimates for the various loadings of theMV CHALLENGER were reviewed. Noti,ngthat the subject design will have adeeper load line than the loadings reviewed, and further that light shipweight is expected to be about 2900 tons less than that used in the estimates,Mandelli methods were used to correct for the effects of these.Accordingly, the design bending moment was estimated to vary from 1S0,000foot tons (sagging) in still water to 67~,ooo foot tons for anhe— = .O.!J wave. The resulting Mj vs Q log 10 (M>Mj) plot is shown onLFigure Al. Based upon an expectancy of 1.26 x 108 cycles in as computedthereon for the designis lifetime, it may be expected that once in itslifetime the bending moment will exceed S7S,2~0 foot tons. Also in itslifetime, -thereis a 99 per cent probability that the bending moment willnot exceed 680,2s0 foot tons. This is obtained by a statistical methodgiven in Appendix A of Reference (A2) and is recommended as the bendingmoment to be used in yield stress considerations. However, for fatigueconsiderations the values in the probability range from 1 to 1.26 x 10-8are considered appropriate.