7 Double Bottom Structure

7.1 General

7.1.1 Framing system

For ships greater than 120 m in length, the bottom shell, the inner bottom and the sloped bulkheads of hopper tanks, if any, are to be longitudinally framed within the cargo hold region. Where it is not practicable to apply the longitudinal framing system to fore and aft parts of the cargo hold region due to the hull form, transverse framing may be accepted on a case-by-case basis subject to appropriate brackets and other arrangements being incorporated to provide structural continuity in way of changes to the framing system.

7.1.2 Variation in height of double bottom

Any variation in the height of the double bottom is to be made gradually and over an adequate length; the knuckles of inner bottom plating are to be located in way of plate floors. Where such arrangement is not possible, suitable longitudinal structures such as partial girders, longitudinal brackets, fitted across the knuckle are to be arranged.

7.1.3 Breadth of inner bottom

Breadth of inner bottom, in m, is to be measured at mid-length of the cargo hold as shown in Figure 17.

Figure 17 : Breadth of inner bottom

7.1.4 Drainage of tank top

For ships designed to carry solid cargoes, effective arrangements are to be provided for draining water from the tank top. Where wells are provided for the drainage, such wells are not to extend for more than one-half height of the double bottom.

7.1.5 Striking plate

Striking plates of adequate thickness or other equivalent arrangements are to be provided under sounding pipes to prevent the sounding rod from damaging the plating.

7.1.6 Duct keel

Where a duct keel is arranged, the centre girder may be replaced by two girders spaced, no more than 3 m apart. Otherwise, for a spacing wider than 3 m, the two girders are to be provided with support of adjacent structure and subject to the Society’s approval.

The structures in way of the floors are to provide sufficient continuity of the latter.

7.2 Keel plate

7.2.1 Keel plating is to extend over the flat of bottom for the full length of the ship.

The width of the keel, in m, is not to be less than 0.8 + L/200, without being taken greater than 2.3 m.

7.3 Girders

7.3.1 Centre girder

When fitted, the centre girder is to extend within the cargo hold region and is to extend forward and aft as far as practicable. Structural continuity of the centre girder is to be maintained within the full length of the ship.

Where double bottom compartments are used for the carriage of fuel oil, fresh water or ballast water, the centre girder is to be watertight, except for the case such as narrow tanks at the end parts or when other watertight girders are provided within 0.25 B from the centreline.

7.3.2 Side girders

The side girders are to extend within the parallel part of the cargo hold region and are to extend forward and aft of the cargo hold region as far as practicable.

7.4 Floors

7.4.1 Web stiffeners

Floors are to be provided with web stiffeners in way of longitudinal ordinary stiffeners. Where the web stiffeners are not welded to the longitudinal stiffeners, design standard as given in Ch 9, Sec 6, [2] applies unless fatigue strength assessment for the cut out and connection of longitudinal stiffener is carried out.

7.5 Bilge keel

7.5.1 Material

The material of the bilge keel and ground bar is to be of the same yield stress as the material to which they are attached.

In addition, when the bilge keel extends over a length more than 0.15 L, the material of the bilge keel and ground bar is to be of the same grade as the material to which they are attached.

Figure 18 : Bilge keel construction

Figure 19 : Bilge keel end design

Figure 20 : Bilge keel end design (continued)

7.5.2 Design

The design of single web bilge keels is to be such that failure to the web occurs before failure of the ground bar. This may be achieved by ensuring the web thickness of the bilge keel does not exceed that of the ground bar.

Bilge keels of a different design, from that shown in Figure 18, are to be specially considered by the Society.

7.5.3 Ground bars

Bilge keels are not to be welded directly to the shell plating. A ground bar, or doubler, is to be fitted on the shell plating as shown in Figure 18 and Figure 19. In general, the ground bar is to be continuous.

The gross thickness of the ground bar is not to be less than the gross thickness of the bilge strake or 14 mm, whichever is the lesser.

7.5.4 End details

The ground bar and bilge keel ends are to be tapered or rounded. Tapering is to be gradual with a minimum ratio of 3:1, see items (a), (b), (d) and (e) in Figure 19/Figure 20. Rounded ends are to be as shown in item (c) of Figure 19. Cut-outs on the bilge keel web, within zone ’A’ (see items (b) and (e) of Figure 19/Figure 20) are not permitted.

The end of the bilge keel web is to be not less than 50 mm and not greater than 100 mm from the end of the ground bar, see items (a) and (d) of Figure 19/Figure 20.

Ends of the bilge keel and ground bar are to be supported by either transverse or longitudinal members inside the hull, as indicated as follows:

• Transverse support member is to be fitted at mid length between the end of the bilge keel web and the end of the ground bar, see items (a), (b) and (c) of Figure 19.
• Longitudinal stiffener is to be fitted in line with the bilge keel web, it is to extend to at least the nearest transverse member forward and aft of zone ’A’ (see items (b) and (e) of Figure 19/Figure 20).

Alternative end arrangements may be accepted, provided that they are considered equivalent.

7.6 Docking

7.6.1 General

The drydocking arrangement itself is not covered in these Rules.

The bottom structure is to withstand the forces imposed by drydocking the ship.

7.6.2 Docking brackets

Docking brackets connecting the centreline girder to the bottom plating, are to be connected to the adjacent bottom longitudinals.