Trans RINA, Vol 153, Part A4, Intl J Maritime Eng, Oct-Dec 2011
Additionally ABS Rules in paragraphs 23.15.1, 23.15.2 and 6.3.5 (c) dictate that both the spacing of the double bottom side girders and floors should be not less than 3 meters apart, thus essentially defining the bottom grillage properties.
DNV 1973 Rules [4] Chapter II Sec. 10/B301 required that the height of the center girder/double bottom is to be not less than:
H = (600 + 9 B√d) x 1.05 in mm
which as per Ch III sec. 5/C304, it was to be increased by 5% for vessels carrying heavy cargoes.
Additionally, in line with the ABS philosophy, for the strength of the double bottom structure DNV Rules Chapter II section 5/C200 and C300 stipulated that the floors and side girders should be fitted with spacing not greater than:
Spacing ≤ 4.5 (L + 240)
IACS CSR 3/6.1.3 dictates that the height of double bottom be not less than:
DB Height = B/20 or 2 m whichever is less CSR 3-6/6.3.3 states that the spacing of the bottom
girders should generally be not greater than 4.6 m or 5 times the spacing of the bottom or inner bottom ordinary stiffeners, whichever is
the smaller. CSR’s 3-6/6.4.1
states that the spacing of the floor should not be greater than 3.5 m or 4 frame spaces whichever is the smaller. However, for both DB girders and floors greater spacing may be accepted, depending on the FEA results.
The CSR formula for the calculation of the DB Height is
independent of the vessel’s draught. This is quite strange given that this formula is applicable to all sizes of bulk carriers above 90 m in length, which have a wide range of draughts. It is quite unreasonable to size the depth of a grillage structure on a unidirectional span (B), ignoring the load that it is required to carry. The load and the grillage
f d B DB DB ( , ,
of the double bottom is a function of )
.
The formulae for the requirements of double bottom height should include variables such as the draught of the new design which is the ship’s principal dimension, to which the load applied on the double bottom is directly proportional. The length of the vessel controls the primary stress of the double bottom structure and the breadth of
strength and stiffness (most
the double bottom structure controls of
the likely) have cargo the
transverse direction. In addition, Handymax & Supramax bulk carriers
double bottom in a 5
holds,
Panamaxes have 7 cargo holds and Cape Sizes have 9 cargo holds, thus the length per hold is almost standard for most types of bulk carriers over 150 m in length. The
A-250
following parametric equation was developed to fit the data indicated in Table 1 that compares well with the formulations given by other pre-CSR IACS Class Societies Rules:
dDB = 45BDB +80√d + (L+240) Where dDB = height of double bottom (mm) BDB = breadth of the double bottom between bilge hoppers (m) [as shown in above mentioned Figure 1]
d = molded draught to the summer load line of the vessel (m)
L In combination
requirements:
= Scantling length of the vessel in m (as per Rule) with the following
The spacing of the adjacent girders is
greater than about 3.0m or 4 times the spacing of the bottom or inner bottom ordinary whichever is the smaller.
proposed to be not stiffeners,
The spacing of the floors is to be not greater than 3.0m or three (3) frame smaller.
spaces, whichever is
The above proposed formulae and requirements were used to calculate the double bottom height (dDB) and the spacing of floors and girders for the
included in Table 1. 4.
VESSEL’S STRUCTURAL ARRANGEMENTS
Given that the focus of this paper is the double bottom height, and since our decision was to use a Panamax bulk carrier for the model ship, we have selected a single sided vessel with a double
bottom structure which reflects the current trend in the design of such vessels.
The midship section of the vessel is shown in Figure 2 (b) below. The vessel is longitudinally framed with transverse hold frames between the top side and hopper tanks. The transverse hold frames are spaced at 860 mm apart, with the inner bottom and outer bottom longitudinals spaced at 810 mm. The spacing of the deck longitudinals is 880 mm and the spacing of the side shell and top side slopped bulkhead longitudinals is 850 mm.
The materials are predominantly HTS 36 for the main deck and longitudinal members within the upper 3.0 m (measured from the main deck) and HTS 32 for the remaining longitudinal material, its inner bottom plating which is mild steel. Material used for the double bottom girders is HTS 36 at their end sections (under the stools and one bay between floors, located aft and fore of the stools). The remaining longitudinal materials of the bottom girders and floors are HTS 32. In summary, the whole double bottom structure was built with HTS 32 material apart from the inner bottom plating that was
bulk carriers
©2011: The Royal Institution of Naval Architects
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