Section
6 Double bottom structure
6.1 General
6.1.1 The requirements
given in this Section provide for double bottom construction of steel
mono-hull craft in association with either transverse or longitudinal
framing.
6.1.2 Double
bottoms are generally to be fitted in accordance with Pt 3, Ch 2, 6.6 Double and single bottom structure and where fitted are to
extend from the collision bulkhead to the after peak bulkhead, as
far as this is practicable within the design and proper working of
the craft. In addition, the inner bottom is to be continued to the
craft's side in such a manner as to protect the bottom to the turn
of bilge or chine.
6.1.3 The double
bottom structure in way of girders and duct keels is to be sufficient
to withstand the forces imposed by dry-docking the craft.
6.1.4 The centreline
girder and side girders are to extend as far forward and aft as practicable
and care is to be taken to avoid any abrupt discontinuity. Where girders
are cut at bulkheads, their longitudinal strength is to be maintained.
6.2 Keel
6.2.4 The duct
keel width is in general to be 15 per cent of the beam or 2 metres,
whichever is the lesser, but in no case is it to be taken as less
than 630 mm. The inner bottom and bottom shell within the duct keel
are to be suitably stiffened with primary stiffening in the transverse
direction, whilst the continuity of the floors is maintained. Access
to the duct keel is to be by means of watertight manholes or trunks.
6.3 Centre girder
6.3.3 The overall
depth of the centre girder, d
DB, is to be
taken as not less than 630 mm and is to be sufficient to give adequate
access to all parts of the double bottom.
6.4 Side girders
6.4.1 Where the
floor breadth does not exceed 6,0 m, side girders are not required.
Vertical stiffeners are to be fitted to the floors on each side, the
number and positions of these stiffeners being dependent on the arrangement
of the double bottom structure.
6.4.2 Where the
breadth of floor is greater than 6,0 m, additional side girders having
the same thickness as the floors are to be fitted. The number of side
girders is to be such that the distance between the side girders and
centre girder and margin plate, or between the side girders themselves,
does not exceed 3,0 m.
6.4.3 Side girders
where fitted are to extend as far forward and aft as practicable and
are in general to terminate in way of bulkheads, deep floors or other
primary transverse structure.
6.4.4 Where additional
side girders are fitted in way of main machinery seatings, they are
to be integrated into the structure of the craft and extended forward
and aft as far as practicable.
6.4.5 Under the
main engine, girders extending from the bottom shell to the top plate
of the engine seating are to be fitted. The height of the girders
is to be not less than the height of the floor. Engine holding-down
bolts are to be arranged as near as practicable to the girders and
floors. Where this cannot be achieved, bracket floors and/or hanging
brackets are to be fitted.
6.5 Plate floors
6.5.3 Plate floors
are, in general, to be continuous between the centre girder and the
margin plate.
6.5.4 In longitudinally
framed craft, plate floors or equivalent structure are, in general,
to be fitted in the following positions:
-
At every half frame
in way of the main engines, thrust bearings, and bottom of the craft
forward.
-
Outboard of the engine
seatings, at every frame within the engine room.
-
Underneath pillars
and bulkheads.
-
Outside of the engine
room at a spacing not exceeding 2,0 m.
6.5.5 Vertical
flat bar stiffeners are to be fitted to all plate floors at each longitudinal.
Each stiffener is to have a depth of not less than 10t
w and a thickness of not less than t
w,
where t
w is the thickness of the plate floor
as calculated in Pt 6, Ch 3, 6.5 Plate floors 6.5.1.
6.5.6 In transversely
framed craft, plate floors are to be fitted at every frame in the
engine room, under bulkheads, in way of change in depth of double
bottom and elsewhere at a spacing not exceeding 2,0 m.
6.6 Bracket floors
6.6.1 Between
plate floors, the shell and inner bottom plating is to be supported
by bracket floors. The brackets are to have the same thickness as
plate floors and are to be stiffened on the unsupported edge.
6.6.2 In longitudinally
framed craft, the brackets are to extend from the centre girder and
margin plate to the adjacent longitudinal, but in no case is the breadth
of the bracket to be taken as not less than 75 per cent of the depth
of the centre girder. They are to be fitted at every web frame at
the margin plate, and those at the centre girder are to be spaced
not more than 1,0 m apart.
6.6.3 In transversely
framed craft, the breadth of the brackets, attaching the bottom and
inner bottom frames to the centre girder and margin plate, is to be
not less than 75 per cent of the depth of the centre girder.
6.7 Watertight floors
6.8 Tankside brackets
6.9 Inner bottom plating
6.10 Inner bottom longitudinals
6.10.1 Inner
bottom longitudinals are to be supported by inner bottom transverse
web frames, floors, bulkheads or other primary structure, generally
spaced not more than 2 m apart.
6.10.2 The inner
bottom longitudinals are to be continuous through the supporting structure
and are to be satisfactorily stiffened against buckling.
6.10.3 Where
it is impracticable to comply with the requirements of Pt 6, Ch 3, 6.10 Inner bottom longitudinals 6.10.2, or where it is desired to
terminate the inner bottom longitudinals in way of bulkheads or integral
tank boundaries, they are to be bracketed in way of their end connections
to maintain the continuity of structural strength. Particular care
is to be taken to ensure accurate alignment of the brackets.
6.10.4 The requirements
for section modulus, inertia and web area are to be determined from
the general equations given in Pt 6, Ch 3, 1.17 Stiffening general,
using the design pressures from Pt 5, Ch 3, 3.1 Hull structures or Pt 5, Ch 4, 3.1 Hull structures for
non-displacement or displacement craft as appropriate, and the coefficients ΦZ, ΦI, and ΦA as detailed in Table 3.1.1 Section modulus, inertia and web
area coefficients for the load model (b).
6.11 Inner bottom transverse web framing
6.11.1 Inner
bottom transverse web frames are defined as primary stiffening members
which support inner bottom longitudinals. They are to be continuous
and to be substantially bracketed at their end connections to bottom
web frames, bottom floors and tankside brackets.
6.11.2 Where
it is impracticable to comply with the requirements of Pt 6, Ch 3, 6.11 Inner bottom transverse web framing 6.11.1, or where it is desired to
terminate the inner bottom transverse web frames in way of centre
or side girders, bulkheads or integral tank boundaries, etc. they
are to be bracketed in way of their end connections, to maintain the
continuity of structural strength. Particular care is to be taken
to ensure accurate alignment of the brackets.
6.11.3 The requirements
for section modulus, inertia and web area are to be determined from
the general equations given in Pt 6, Ch 3, 1.17 Stiffening general,
using the design pressures from Pt 5, Ch 3, 3.1 Hull structures or Pt 5, Ch 4, 3.1 Hull structures for
non-displacement or displacement craft as appropriate, and the coefficients ΦZ, ΦI, and ΦA as detailed in Table 3.1.1 Section modulus, inertia and web
area coefficients for the load model (a).
6.12 Margin plates
6.12.1 A margin
plate, if fitted, is to have a thickness as required for inner bottom
plating.
6.13 Wells
6.13.1 Small
wells constructed in the double bottom structure are not to extend
in depth more than necessary. A well extending to the outer bottom
may, however, be permitted at the after end of the shaft tunnel of
the craft. Other well arrangements (e.g. for lubricating oil under
main engines) may be considered provided they give protection equivalent
to that afforded by the double bottom.
6.14 Transmission of pillar loads
6.14.1 In double
bottoms under widely spaced pillars, the connections of the floors
to the girders, and of the floors and girders to the inner bottom,
are to be suitably increased. Where pillars are not directly above
the intersection of plate floors and girders, partial floors and intercostals
are to be fitted as necessary to support the pillars. Manholes are
not to be cut in the floors and girders below the heels of pillars.
Where longitudinal framing is adopted in the double bottom, equivalent
stiffening under the heels of pillars is to be provided, and where
the heels of pillars are carried on a tunnel, suitable arrangements
are to be made to support the load.
6.15 Manholes
6.15.1 Sufficient
manholes are to be cut in the inner bottom, floors and side girders
to provide adequate access to, and ventilation of, all parts of the
double bottom. The size of the manhole openings is not, in general,
to exceed 50 per cent of the double bottom depth unless edge reinforcement
is provided. Holes are not to be cut in the centre girder, except
in tanks at the forward and after ends of the craft, and elsewhere
where tank widths are reduced unless additional stiffening and/or
compensation is fitted to maintain the structural integrity.
6.16 Pressure testing
6.16.1 Double
bottoms are to be tested upon completion with a head of water representing
the maximum internal pressure which could be experienced in service,
but not less than a head of water equivalent to the level of the upper
deck.
6.17 Drainholes in bottom structure
6.17.1 Sufficient
limber holes are to be cut in the internal bottom structure to allow
for the drainage of water from all parts of the bilge to the pump
suctions.
6.17.2 Particular
care is to be given to the positioning of limber holes to ensure adequate
drainage and to avoid stress concentrations.
6.17.3 Suitable
arrangements are to be made to provide free passage of air from all
parts of tanks to the air pipes.
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