Trans RINA, Vol 153, Part A4, Intl J Maritime Eng, Oct-Dec 2011
2. SELECTION OF THE TYPE OF BULK CARRIER TO BE INVESTIGATED
Bulk carriers are defined in IACS UR Z11.2.2 [1] as self propelled ships which are constructed generally with single deck, double bottom, hopper side tanks, top side tanks, with single or double side skin construction in the cargo length area, and intended to carry dry cargoes in bulk. Commonly carried cargoes today are bulk ore, coal, light cargoes, (commodities such as grain, wheat, soya beans, sugar etc.), steel products, log, chip and lumber and a number of other not so bulk type cargoes. Given that bulk carrier design is market driven, increased demands from developing countries in the last few years had a considerable effect on the design of bulk carriers of all types, and their sizes / capacities.
In general current bulk carriers fall into the following categories:
Handy bulk carriers that are less than 40,000 DWT having 5 or less cargo holds with B less than 32.2 m
Handymax and Supramax bulk carriers with 5 Cargo Holds, between 40,000–60,000 DWT, having L between 170.0 m–190.0 m and B at 32.2 m
Panamax bulk carriers with DWT between 60,000 to 80,000 DWT, and Kamsharmax bulk carriers between 80,000 and 90,000 DWT, usually with 7 cargo holds and B ≥ 32.2 m
Cape size bulk carriers between 100,000–180,000 tons with 9 cargo holds, and B well over 32.2 m
For this investigation a Panamax bulk carrier was chosen, due to design failures suffered by a number of newly built vessels of this size. It also represents the current middle range of the fleet and is the size on which the Baltic Dry Freight rate is based.
3. EVOLUTION OF PRE AND POST CSR OF IACS CLASS SOCIETIES’ REQUIREMENTS ON DOUBLE BOTTOM HEIGHT
IACS Class Rules for bulk carriers provide parametric equations for the calculation of the major ship parameters such as the double bottom height, and spacing of DB girders and floors. These requirements formed the lower limit for the design of any bulk carrier. In the 80s and early 90s these were followed by a FEA in order to verify and refine the results of the parametric equations, and locate specific areas in need of additional reinforcement. These limits were first introduced to provide adequate safety margins, and compensate for the uncertainties involved in
the rule loadings. The minimalistic /
fragmented FEA covered only the cargo holds located at 0.4L amidships1, the computer power dependent FE modeling and other general
input such as boundary A-248
This practice could not of course handle and prevent failures such as the casualty of similar open type
1 structure “MSC Napoli” [16]
Figure 1: Typical schematic view of the evolution of the Handymax and Panamax bulk carriers built in 1970’s until today (2000’s)
For example, ABS Rules [3] prior to 1991, in paragraphs 23.1.3 & 7.3.2 required that the minimum double bottom height for vessels carrying heavy cargoes is to be as per the following equation.
dDB = 32B + 190√d
conditions, unsymmetrical loading that could not be applied accurately on the model, size and type of elements, etc. It was clearly understood at that time that the FEA was indeed a strong mathematical tool but one which possessed neither convergence nor uniqueness of solutions. As such, it should be used with extreme care by entities that possess a good understanding of the structure analyzed and its expected behavior. It safe to consider that FEA reflects the absolute truth.
is not
Table 1 shows the evolution of the bulk carrier double bottom design from early 70s until today as compared with pre and post CSR philosophies, together with the results of
the new formula proposed by the authors.
Evidently the new designs were developed to produce bulk carriers with reduced double bottom height, reduced number of double bottom girders (widely spaced), and increased double bottom width due to the reduced width of the bilge hopper box girder tank [see Figure 1]. Given that the cargo hold’s length has remained almost constant, this practice alters the width to length aspect ratio of the double bottom
resulting in an appreciably reduced stiffness due to the reduced height of the double bottom.
©2011: The Royal Institution of Naval Architects
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