Page 268 - Ship Construction.DJ Eyres 6Ed

Ch22-H8070.fm Page 257 Wednesday, October 18, 2006 6:59 AM
Tanker Construction 257
is permitted by the classification society (see Figure 22.3). Higher tensile
steel longitudinals are to be continuous irrespective of ship length.
The longitudinals can be offset bulb plates which may be built up to give
the required scantling on large ships. It is not however uncommon to find on
many tankers longitudinals having a web and symmetrical flat plate flange.

FLOORS AND TRANSVERSES To support the bottom and inner bot-
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tom longitudinals solid plate floors are fitted. Stiffened transverse plate webs
support the side shell and inner shell longitudinals, deck longitudinals and
longitudinal bulkhead stiffening. In way of the centre tank a bracketed deck
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transverse supports the deck longitudinals and large plate brackets are fitted
between the longitudinal bulkheads and inner bottom (see Figure 22.2).
Transverses are as a rule built of a plate web, and heavier flat face bar the
depth being adequate to allow sufficient material abreast the slots through
which the longitudinals pass. Within the double hull side space they form a
solid web and are supported by horizontal stringers (see Figure 22.2). In the
wing tank space they support the deck longitudinals and fore and aft bulk-
head longitudinals and in turn are supported by a mid-height longitudinal
plate stringer (see Figure 22.2).
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BOTTOM AND DECK GIRDERS A centre line girder is required in
the double bottom space, which with the heavy keel plate constitutes the
immediate structure through which docking loads are transmitted when the
vessel is placed on the keel blocks.

Bulkheads

Bulkhead spacing throughout the cargo tank space is determined by the
permissible length of cargo tanks. MARPOL requires that the length of each
cargo tank shall not exceed the greater of 10 metres or a length expressed as
a percentage of the ship’s length which is dependent on the number of
longitudinal bulkheads fitted and the minimum distance from the ship’s side
of the outer longitudinal bulkhead. Tankers with two or more longitudinal
bulkheads may have wing and centre tank lengths up to 20 per cent of the
ship’s length. Lloyd’s Register require the disposition of transverse bulk-
heads to comply with Table 18.1, as applicable to ships with machinery aft.
Cofferdams, which may be formed with two adjacent oiltight transverse
bulkheads at least 760mm apart, are required at the ends of the cargo space.
However in many cases a pump room is fitted at the after end of the cargo
space (also forward on some products carriers), and a ballast tank is fitted
at the forward end, each of these compartments being accepted in lieu of a
cofferdam. A cofferdam is also provided between any accommodation and
oil cargo tanks.