Trans RINA, Vol 156, Part C1, Intl J Marine Design, Jan - Dec 2014


which requires a stronger structure in that zone. Once the pressures are determined, the minimum thickness, inertia and section modulus required are calculated using the formulas given in the Lloyd’s Register design rules. These formulas take into account different coefficients depending on the structural assumptions of the model. The scantlings of the three block sections shown in Figure 4 are determined using this procedure. Including the stiffening members dimensions.


Figure 3: New CLF trimaran concept 4.1


SCANTLING PROCEDURE


For the preliminary structural design, the hull has been split in three blocks. The first one covers the engine room section, the second one is the greatest part of cargo decks and the last one considers the slamming loads at the bow. The use of aluminium alloy required to spacing of the ordinary stiffeners and the transversal primary stiffeners to be reduced to 400mm and 1600mm respectively. In order to use Lloyd’s Register rules to compute the minimum scantling dimensions required, geometrical assumptions were made. The bulkhead positions were arranged according to different criteria, such as stability and


fire protection. For this reason the original


arrangement of the steel trimaran preliminary structural design [5] were adopted for the calculations.


In order to increase the longitudinal strength, the cross deck, which links the lateral hulls and the central hull together was modified. It is the most critical zone subjected to stresses, due to the reduce strength of aluminium alloy compared to steel. It was lengthened fore of the lateral hulls and integrated into the wet deck structure along the whole length of the hull, as shown in Figure 3, to distribute stress. The geometric shape of the sections at 24m, 60m, 90m from the Aft Perpendicular are shown in Figure 4. Each of these sections are used as input geometry for each block in LR SSP software. Where constant


sections of the stiffeners have been


assumed for the length of each block. For the load estimations, the design regulations prescribe different components for the pressure acting on the hull. These are used for the computation of local design criteria. The components are determined from the environmental conditions for the specific area of operation of the craft, such as wind, wave and currents from which design data are derived.


Each structural element, such as plating, framing, double bottom structures, bulkheads, etc., have relative design pressure. Estimating these values the greatest ones are used from a parametric combination of the geometric input data. The evaluated design pressures include the effects of combined static and dynamic load components as well as the effects of impact or slamming loads. The variation of the different components of vessel design pressure with


longitudinal distance forward of Aft


Perpendicular are shown in Figure 5. The increase in pressure in the bow of the hull is due to slamming,


© 2014: The Royal Institution of Naval Architects


Figure 4: Hull Sections at 24m, 60m, 90m from the AP


Figure 4: Hull Sections at 24m, 60m, 90m from the AP


Figure length


5: Design pressure


distribution over the ship


At this preliminary design stage a main section checking is required. Due to the particular hull


shape it is C-141


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