Section 8 Double bottom structure

8.1.1 The symbols used in this Section are defined as follows:

L, and as defined in Pt 4, Ch 1, 5 Definitions

B as defined in Pt 4, Ch 1, 5 Definitions but need not exceed

=

maximum distance between longitudinal bulkheads, in metres

=

Rule depth of centre girder, in mm

=

actual depth of centre girder, in mm

=

head from top of inner bottom to top of overflow pipe, in metres

=

load head as defined in Table 6.7.1 Watertight and deep tank bulkhead scantlings

8.2.1 In general, double bottoms need not be fitted in non-propelled units and column-stabilised units, except where required by a National Administration.

8.2.3 The requirements in this Section are, in general, applicable to double bottoms with stiffeners fitted parallel to the hull bending compressive stress. When other stiffening arrangements are proposed the scantlings will be specially considered, but the minimum requirements of this Section are to be complied with.

8.2.4 The arrangements of drainage wells, recesses and dump valves in the double bottom will be specially considered.

8.2.5 If it is intended to dry-dock the unit, girders and the side walls of duct keels are to be continuous and the structure is to have adequate strength to withstand the forces imposed by dry-docking the unit.

8.2.6 Adequate access is to be provided to all parts of the double bottom. The edges of all holes are to be smooth. The size of the opening should not, in general, exceed 50 per cent of the double bottom depth, unless edge reinforcement is provided. In way of ends of floors and fore and aft girders at transverse bulkheads, the number and size of holes are to be kept to a minimum, and the openings are to be circular or elliptical. Edge stiffening may be required in these positions.

8.2.7 Provision is to be made for the free passage of air and water from all parts of tank spaces to the air pipes and suctions, account being taken of the pumping rates required. To ensure this, sufficient air holes and drain holes are to be provided in all longitudinal and transverse non-watertight primary and secondary members. The drain holes are to be located as close to the bottom as is practicable, and air holes are to be located as close to the inner bottom as is practicable, see also Pt 3, Ch 8 Process Plant Facility.

8.3.1 When a double bottom is fitted to a self-elevating unit, the scantlings of the double bottom will be specially considered in accordance with Pt 4, Ch 4, 3 Self-elevating units but the general requirements of this Section are to be complied with.

8.3.2 The longitudinal extent of the double bottom will be specially considered in respect of the design and safety of the unit but it should extend as far forward and aft as is practicable. A double bottom need not be fitted in pre-load deep tanks or other wing deep tanks.

8.3.3 The depth of the double bottom at the centreline, , is to be in accordance with Pt 4, Ch 6, 8.3 Self-elevating units 8.3.4 and the inner bottom is, in general, to be continued out to the unit’s side in such a manner as to protect the bottom to the turn of bilge. When pre-load wing deep tanks are fitted port and starboard, the inner bottom may be terminated at the deep tank longitudinal bulkheads.

8.3.4  The centre girder is to have a depth of not less than that given by:

=

28B + 205 mm

nor is t + t_c to be less than 760 mm. The centre girder thickness is to be not less than:

t

=

(0,008 + 4) mm

nor less than 6,0 mm. The thickness may be determined using the value for without applying the minimum depth of 760 mm.

8.3.5  Side girders are to be fitted below longitudinal bulkheads. In general, one side girder is to be fitted where the breadth, B, exceeds 14 m and two side girders are to be fitted on each side of the centreline where B exceeds 21 m. Equivalent arrangements are to be provided where longitudinal bulkheads are fitted. The side girders are to extend as far forward and aft as practicable and are to have a thickness not less than:

nor is t + t_c to be less than 6,0mm. In general, a vertical stiffener, having a depth not less than 100 mm and a thickness equal to the girder thickness, is to be arranged midway between floors.

8.3.7 If the depth of the watertight side girders exceeds 915 mm but does not exceed 2000 mm, the girders are to be fitted with vertical stiffeners spaced not more than 915 mm apart and having a section modulus not less than:

Z

=

5,41 s k x 10–9 cm3

The ends of the stiffeners are to be sniped. Where the double bottom tanks are interconnected with side tanks or cofferdams, or where the depth of the girder exceeds 2000 mm, the scantlings of watertight girders are to be not less than those required for deep tanks, see Pt 4, Ch 6, 7.3 Watertight and deep tank bulkheads, and the ends of the stiffeners are to be bracketed top and bottom.

8.3.9 The sides of the duct keels are, in general, to be spaced not more than 2,0 m apart. Where the sides of the ducts keels are arranged on either side of the centreline or side girder, each side is, in general, to be spaced not more than 2,0 m from the centreline or side girder. The inner bottom and bottom shell within the duct keel are to be suitably stiffened. The primary stiffening in the transverse direction is to be suitably aligned with the floors in the adjacent double bottom tanks. Where the duct keels are adjacent to double bottom tanks which are interconnected with side tanks or cofferdams, the stiffening is to be in accordance with the requirements for deep tanks, see Pt 4, Ch 6, 7.3 Watertight and deep tank bulkheads. Access to the duct keel is to be by watertight manholes or trunks.

8.3.10  Inner bottom plating is, in general, to have a thickness not less than:

t

=

0,00136 (s + 660) ( L ) mm

nor is t + t_c to be less than 6,5 mm.

8.3.11 The thickness, t, of the inner bottom plating as determined in Pt 4, Ch 6, 8.3 Self-elevating units 8.3.10 is to be increased by 10 per cent in machinery spaces but in no case is the gross thickness (t + t_c) to be less than 7,0 mm.

8.3.12 A margin plate, if fitted, is to have a thickness throughout 20 per cent greater than that required for inner bottom plating.

8.3.13 Where the double bottom tanks are common with side tanks or cofferdams, the thickness of the inner bottom plating is to be not less than that required for deep tanks, see Pt 4, Ch 6, 7.3 Watertight and deep tank bulkheads, and the load head is to be measured to the highest point of the side tank or cofferdam.

8.3.14 Inner bottom stiffeners are in general to have a section modulus not less than 85 per cent of the Rule value for bottom shell stiffeners, see Pt 4, Ch 4, 3.3 Structural design 3.3.1. When the inner bottom design loading is considerably less than 9,82T_T kN/m² (T_T tonne-f/m²) the scantlings of the inner bottom stiffeners will be specially considered. Where the double bottom tanks are interconnected with side tanks or cofferdams, the scantlings are to be not less than those required for deep tanks, see Pt 4, Ch 6, 7.3 Watertight and deep tank bulkheads.

8.3.15  Plate floors are to be fitted under heavy machinery items and under bulkheads and elsewhere at a spacing not exceeding 3,8 m. The thickness of non-watertight plate floors is to be not less than:

t

=

(0,009 + 1) mm

nor is t + t_c to be less than 6,0 mm. The gross thickness need not be greater than 15 mm, but the ratio between the depth of the double bottom and the thickness of the floor is not to exceed 130. This ratio may, however, be exceeded if suitable additional stiffening is fitted. Vertical stiffeners are to be fitted at each bottom shell stiffener, having a depth not less than 150 mm and a thickness equal to the thickness of the floors. For units of length, L, less than 90 m, the depth is to be not less than 1,65L mm, with a minimum of 50 mm.

whichever is the greater, but t + t_c is not to exceed 15 mm on floors of normal depth. The thickness is also to satisfy the requirements for deep tanks, see Pt 4, Ch 6, 7.3 Watertight and deep tank bulkheads, with the load head measured to the highest point of the side tank or cofferdam if the double bottom tank is interconnected with these tanks. The scantlings of the stiffeners are to be in accordance with the requirements of Pt 4, Ch 6, 7.3 Watertight and deep tank bulkheads for deep tanks, but in no case is the modulus to be less than:

Z

=

5,41 s k x 10–9 cm3

Vertical stiffeners are to be connected to the inner bottom and shell stiffeners.

8.3.17 Between plate floors, transverse brackets having a thickness not less than 0,009 mm are to be fitted, extending from the centre girder and margin plate to the adjacent longitudinal. The brackets, which are to be suitably stiffened at the edge, are to be fitted at every frame at the margin plate, and those at the centre girder are to be spaced not more than 1,25 m.

8.3.18 Where floors form the boundary of a sea inlet box, the thickness of the plating is to be the same as the adjacent shell, but t + t_c is not to be less than 12,5 mm. The scantlings of stiffeners, where required are, in general, to comply with Pt 4, Ch 6, 7.3 Watertight and deep tank bulkheads for deep tanks. Sniped ends for stiffeners on the boundaries of these spaces are to be avoided wherever practicable. The stiffeners should be bracketed or the free end suitably supported to provide alignment with backing structure.

8.4.1 Where a double bottom is fitted in the lower hull of column-stabilised, tension-leg, deep draught caisson, buoy or sea bed-stabilised units, the scantlings of the double bottom structure will be specially considered but the general requirements of Pt 4, Ch 6, 8.3 Self-elevating units are to be complied with where applicable. The minimum scantlings of the double bottom structure are to be in accordance with Pt 4, Ch 6, 8.4 Column-stabilised, tension-leg, deep draught caisson, buoy, and sea bed-stabilised units 8.4.2.

8.4.2 The scantlings of tank boundaries are to comply with the requirements for tank bulkheads in Pt 4, Ch 6, 7 Bulkheads but the load head is not to be taken less than the distance measured to . When the internal double bottom compartment is a void space the scantlings of watertight boundaries are to comply with Pt 4, Ch 6, 7.5 Watertight void compartments 7.5.1 and Table 6.7.1 Watertight and deep tank bulkhead scantlings.

8.4.3 The boundaries of a sea inlet box are to comply with the requirements of Pt 4, Ch 6, 8.3 Self-elevating units 8.3.18.

8.4.4 The strength of the bottom structures in sea bed-stabilised units is also to comply with Pt 4, Ch 4, 2.1 General 2.1.6 .