Trans RINA, Vol 153, Part A4, Intl J Maritime Eng, Oct-Dec 2011 EFFECT OF DOUBLE BOTTOM HEIGHT ON THE STRUCTURAL BEHAVIOUR OF


BULK CARRIERS (DOI No: 10.3940/rina.ijme.2011.a4.217)


P D Contraros, PDC Maritime & Theta Marine, UK S P Phokas, Theta Marine, Greece SUMMARY


This is the first of a series of companion papers that the authors propose to present on the effect that the new CSR Rules will have on the design of bulk carriers. Our initial focus will be on the new design framework established for the inner bottom height of such vessels, a parameter critical to their structural integrity. It examines the effect that double bottom height reduction has on the reliability of the bulk carrier structure, by applying a finite element 3D - 3 hold analysis of varying double bottom heights to a typical current Panamax bulk carrier design. The results are compared to pre and post IACS CSR[2] requirements. The conclusion reached is that the establishing of the double bottom height should not be left to direct calculations. A minimum acceptable height should be established in order to maintain a minimum level of structural reliability and safety.


NOMENCLATURE CSR


UR Common Structural Rules


IACS International Association of Classification Societies Unified Requirement


SH ABS Safe Hull V10 FE


Version 10.0 Finite Element


FEA 3D CH


DWT L


lDB B


D BDB


Finite Element Analysis 3 Dimensional Cargo Hold


Deadweight (tonnes) Scantling Length of the ship (m)


Length of the double bottom between Lower Stool’s footings (m)


Moulded Breadth of the vessel (m) Moulded Depth of the vessel (m)


Breadth of the double bottom between bilge hoppers (m)


dDB (or H) Height of double bottom (mm) d


BHTW


Molded draught to the summer load line of the vessel (m)


Width of Bilge Hopper Tank (m)


TSTW Width of Top Side Tank (m) DB Double Bottom HTS 32 or 36 Higher Tensile Strength Steel (of Yield Point 3200 or 3600 kg/mm2) Von Misses Stress (kg/mm2) Plate Thickness (mm)


VM t


DLA Dynamic Loading Approach 1.


INTRODUCTION


Bulk carriers are the workhorses of the merchant fleet, carrying a wide variety of cargoes. Cargoes that are not always stowed in the exact same locations as on container vessels, which do not have the same behaviour as the liquids of the tank vessels and with a wide range of stowage factors. Cargoes with specific gravities declared by the shippers (with a high degree of uncertainty), and


which are loaded at exceptionally high rates controlled not by the vessel’s master but by the terminal operators. Designing a robust bulk carrier is a demanding exercise. Due to the uncertainties previously stated, their designers should place considerable reliance on the experience gained from the operation of these vessels through the years. For the past 100 years this experience was traditionally reflected in the Class Rules. In the last 35 years it was in the form of explicit upper or lower limits for the scantlings and arrangements of


their primary


supporting members. In the development of the CSR Rules the IACS group on bulk carriers chose to rely heavily on FEA. Most of the previously set limits on the design and arrangement of primary supporting members of bulk carriers were lifted. Voluminous and valuable written contributions in support of these limits by bulk carrier operators, some of which have followed for years the everyday operations and problems of


fleets larger


than those of individual Societies, were set aside. The fact that operators’ experience is based on every day follow up of this enormous fleet, while Class Surveyors board the vessels a limited number of times each year was not considered. Rising demand for increased cargo volume and deadweight of bulk carriers has led designers to increase the depth and draught of current vessels. Shipyards followed by increasing use of higher tensile strength steel to almost 100%. The introduction of computer aided design allowed designers to eliminate margins inherent in the traditional Class Rules, and the new design optimization philosophy of “carry cargo and not steel” led to an unacceptable number of casualties and ship losses in the 80s and early 90s. IMO’s intervention resulted in a number of retroactive IACS UR’s, which were applied at the expense of the owners. The introduction of the CSR by IACS has created a new design philosophy that permits greater flexibility to designers, and supports the modern trend of increasing the vessel’s carrying capacity without increasing its breadth or length. This has resulted in ship designs with reduced double bottom heights and cross sections of the lower and upper side tanks (see Figure 1).


©2011: The Royal Institution of Naval Architects


A-247


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