Double bottom 1 General

6.1.1 Double bottom extend Ref. SOLAS Ch. II-1, Part B, Reg. 12-1

A double bottom is to be fitted extending from the collision bulkhead to the afterpeak bulkhead.

6.1.2 Framing system

For ships greater than 120 m in length, the bottom, the double bottom and the sloped bulkheads of hopper tanks are to be of longitudinal system of frame arrangement at least within the cargo hold area. The spacing of the floors and bottom girders is not only governed by frame spaces but requirement in absolute value, in metres, is also indicated in [6.3.3] and [6.4.1].

6.1.3 Height of double bottom

Where a double bottom is required to be fitted the inner bottom shall be continued transversely in such a manner as to protect the bottom to the turn of the bilge.

Such protection will be deemed satisfactory if the inner bottom is not lower at any part than a plane parallel with the keel line and which is located not less than a vertical distance h measured from the keel line, as calculated by the formula:

h = B/20

However, in no case is the value of h to be less than 760 mm, and need not be taken as more than 2,000 mm.

Where the height of the double bottom varies, the variation is generally 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 this is impossible, suitable longitudinal structures such as partial girders, longitudinal brackets etc., fitted across the knuckle are to be arranged.

RCN 1 to July 2010 version (effective from 1July 2012) 6.1.4 Dimensions of double bottom

The breadth of double bottom is taken as shown in Fig 17.

Figure 17: Breadth of double bottom

6.1.5 Docking

The bottom is to have sufficient strength to withstand the loads resulting from the dry-docking of the ship.

Where docking brackets are provided between solid floors and connecting the centreline girder to the bottom shell plating, the docking brackets are to be connected to the adjacent bottom longitudinals.

6.1.6 Continuity of strength

Where the framing system changes from longitudinal to transverse, special attention is to be paid to the continuity of strength by means of additional girders or floors. Where this variation occurs within 0.6L amidships, the inner bottom is generally to be maintained continuous by means of inclined plating.

Bottom and inner bottom longitudinal ordinary stiffeners are generally to be continuous through the floors.

The actual net thickness and the yield stress of the lower strake of the sloped bulkhead of hopper tanks, if any, are not to be less than these ones of the inner bottom with which the connection is made.

6.1.7 Reinforcement

The bottom is to be locally stiffened where concentrated loads are envisaged such as under the main engine and thrust seat.

Girders and floors are to be fitted under each line of pillars, toes of end brackets of bulkhead stiffeners and slant plate of lower stool of bulkhead. In case girders and floors are not fitted, suitable reinforcement is to be provided by means of additional primary supporting members or supporting brackets.

When solid ballast is fitted, it is to be securely positioned. If necessary, intermediate floors may be required for this purpose.

6.1.8 Manholes and lightening holes

Manholes and lightening holes are to be provided in floors and girders to ensure accessibility and ventilation as a rule.

The number of manholes in tank tops is to be kept to the minimum compatible with securing free ventilation and ready access to all parts of the double bottom.

Manholes may not be cut in the girders and floors below the heels of pillars.

6.1.9 Air holes and drain holes

Air and drain holes are to be provided in floors and girders.

Air holes are to be cut as near to the inner bottom and draining holes as near to the bottom shell as practicable.

Air holes and drain holes are to be designed to aid full ballast water and sediment removal to allow for effective ballast water exchange.

6.1.10 Drainage of tank top

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 depth of the height of double bottom

6.1.11 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 bottom plating.

6.1.12 Duct keel

Where a duct keel is arranged, the centre girder may be replaced by two girders generally spaced, no more than 3 m apart.

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

6.2 Keel

6.2.1

The width of the keel is to be not less than the value obtained, in m, from the following formula:

200 / 8 .

0 L

b= +

6.3 Girders

6.3.1 Centre girder

The centre girder is to extend within the cargo hold area and is to extend forward and aft as far as practicable, and structural continuity thereof to be continuous 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 0.25B from the centreline, etc.

6.3.2 Side girders

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

6.3.3 Spacing

The spacing of adjacent girders is generally to be not greater than 4.6 m or 5 times the spacing of bottom or inner bottom ordinary stiffeners, whichever is the smaller. Greater spacing may be accepted depending on the result of the analysis carried out according to Ch 7.

6.4 Floors

6.4.1 Spacing

The spacing of floors is generally to be not greater than 3.5 m or 4 frame spaces as specified by the designer, whichever is the smaller. Greater spacing may be accepted depending on the result of the analysis carried out according to Ch 7.

6.4.2 Floors in way of transverse bulkheads

Where transverse bulkhead is provided with lower stool, solid floors are to be fitted in line with both sides of lower stool. Where transverse bulkhead is not provided with lower stool, solid floors are to be fitted in line with both flanges of the vertically corrugated transverse bulkhead or in line of plane transverse bulkhead.

RCN 1 to July 2008 version (effective from 1 July 2009)

6.4.3 Web stiffeners

Floors are to be provided with web stiffeners in way of longitudinal ordinary stiffeners. Where the web stiffeners are not provided, fatigue strength assessment for the cut out and connection of longitudinal stiffener is to be carried out.

6.5 Bilge strake and bilge keel

6.5.1 Bilge strake

Where some of the longitudinal stiffeners at the bilge part are omitted, longitudinal stiffeners are to be provided as near to the turns of bilge as practicable.

6.5.2 Bilge keel

Bilge keels are not be welded directly to the shell plating. An intermediate flat is required on the shell plating.

The ends of the bilge keel are to be sniped as shown in Fig.18 or rounded with large radius. The ends are to be located in way of transverse bilge stiffeners inside the shell plating and the ends of intermediate flat are not to be located at the block joints.

The bilge keel and the intermediate flat are to be made of steel with the same yield stress as the one of the bilge strake. The bilge keel with a length greater than 0.15L is to be made with the same grade of steel as the one of bilge strake.

The net thickness of the intermediate flat is to be equal to that of the bilge strake. However, this thickness may generally not be greater than 15 mm.

Scallops in the bilge keels are to be avoided.

about 3h min. 100

ht

about 1.5b

t

r 2≥ b

Figure 18: Example of bilge keel arrangement