Trans RINA, Vol 153, Part A4, Intl J Maritime Eng, Oct-Dec 2011


It was noticed that Panamaxes designed in 1974 with DWT 62,600 tons, have much heavier scantlings than the Panamax in this study (latest generation of bulk carriers) with DWT of 75,000 tons (almost 20% more DWT). See Figure 2 (a) compared to 2 (b) and also Table 1. A large number of bulk carriers of the late 90s and 2000s were designed with reduced double bottom height (i.e. from 1800 mm to 1680 mm) and a reduced number of their double bottom girders.


As shown in Table 3, the resultant stresses in floors and girders can decrease with the corresponding increase of the height of the double bottom to levels that are more reasonable, but yet not satisfactory. The problem with the reduced number of bottom girders will be the subject of a companion paper.


8. ASSESSMENT OF NOMINAL AND HOT SPOT STRESSES IN DOUBLE BOTTOM FLOOR AND LONGITUDINAL GIRDER


Resorting to solutions such as the “postage stamp” inserts as a local reinforcement is not strictly covered by the published IACS Class Societies past rules or the present CSR. The current linear FEA for thin shell analysis cannot handle adequately the behavior of such abnormal changes of thickness. The currently applied S- N curves for the calculation of fatigue are not designed / developed for such abnormalities without any correction. Designers have taken advantage of the increase of the mesh size of the FE elements, and by using solid floors and girders (without including in the FE model manhole and pipe openings – see DNV FE model in [16]), which disguise the resultant high stresses. Figure 8 below shows the resultant stresses on a double bottom girder modeled with and without the manhole and pipe openings. CSR for bulk carriers, Chapter 7, allows modeling the floors and double bottom girders without the openings. It is also shown that the critical areas as defined in SOLAS Ch. II- 1 Reg. 3-6/4.2 may well be missed if the FE model does not depict accurately the manhole and pipe openings. In addition, for the new type of bulk carriers and oil tankers contracted


SOLAS Ch.II-1/Reg. 3-6/5 requires the vertical manhole openings


to


for construction after 1st January be


800x600mm (from the customary


600x400mm). Evidently the same dimensions are to be maintained for clearance


between obstructions (i.e.


piping) and internals for the safe access of a person on a stretcher, a port state control inspector, crew member or class surveyor wearing a self-contained air-breathing apparatus. The omission (as per current version of CSR) of now enlarged openings from the FE model will exacerbate the resultant stress distribution around and adjacent to these openings located in critical areas (see Fig.5). This may force the designer to redesign partially the double bottom structure (by providing additional floors and/or girders), or as a whole (by increasing the double bottom height), to avoid the introduction of the very thick “postage stamp” (inserts) plating that will induce additional hot spot


stresses due to increased


The stress concentration factor is the hot spot stress divided by the nominal stress which, in this case, is about 2.0. The nominal stress has been calculated over 3t and corresponds to about


1/10 of the spacing of the


longitudinal stiffeners. The allowable stress is as per ABS SH-DLA. It therefore follows that in order to satisfy the VM stresses criterion shown above, the height of the double bottom should have been raised well over the as per “basic ship” 1680 mm. The stresses shown above are well beyond the yielding and ultimate strength of the material. That means that a plastic hinge will form on the joints under consideration, and redistribution of the load will take place. Additional stresses due to lateral load and deflection have not been taken into account either in CSR’s


ultimate strength check [13] as 2005,


stiffness locally. SOLAS Ch. II-1/Reg. 3-6/4.2 requires the establishment of the critical areas by calculation (and as per IACS UI SC 191/4.2 - by advance analysis techniques), applicable to the whole cargo block area (within 0.4L and outside 0.4L). CSR provide FEA procedures for the primary supporting members only of the cargo block area within 0.4L, and ignore the cargo block areas outside 0.4L.


The current S-N curves included in IACS CSR and ABS SafeHull 5-3-A1 Fig. 1 [1] were extracted from UK HSE Guidance Notes [8] for Offshore Structures (previously known as DEn) Section 21. In UK HSE Guidance Notes paragraph 21.2.12 c) states the following:


“For welded joints the fatigue performance is dependent on member thickness,


performance decreasing with


increasing thickness for the same stress range…..The basic design S-N curves are applicable to thickness less than the basic thickness tB which for both classes P and T is 16 mm.“ The intent of this statement has not been considered in the IACS CSR.


Evidently the designers of bulk carriers in the late 90s and 2000s have taken advantage of the IACS Rules’ omissions with regard to the variation of plate thickness and the method of calculating the hot spot stresses. This variation of thickness of heavily loaded plating creates a stress concentration in the immediate connections as well as in the transition zone between thick to thin plate.


In order to examine the structural behavior of the heavy insert, the mesh of the FE model in these areas was refined. The connections of the longitudinal


girders,


floors, lower stool diaphragms, lower stool side plating and inner bottom plating were modeled with fine mesh elements. The sizes of the elements next to the joints were equal to the thickness of the plate, the adjacent element 2 times the thickness of the plate, the next 3 times the plate thickness, and so forth as per ABS SH 5- 3-A1/Figure 17 and [7]. Then the hot spot stress and stress concentration at the joint was calculated as shown in Figure 6. Hot spot stresses have been calculated as shown on Table 4.


A-256


©2011: The Royal Institution of Naval Architects


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