Page 262 - Ship Construction.DJ Eyres 6Ed

Ch22-H8070.fm Page 251 Wednesday, October 18, 2006 6:59 AM

22
Tanker Construction

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Ships designed specifically to carry bulk liquid cargoes are generally
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referred as tankers. Tankers are commonly associated with the carriage of
oil, but a wide variety of liquids are carried in smaller tank vessels and there
are a considerable number of larger tank vessels dedicated to carrying
chemicals in bulk.

Oil Tankers

Small tankers not exceeding 75 metres in length, involved principally in the
coastal trade have a single longitudinal bulkhead on the centre line providing
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two athwartships tanks. The machinery is aft, and an expansion trunk, if fit-
ted, is on the centre line in way of the tank spaces (see Figure 22.1).
This chapter is concerned with the construction of the larger ocean-going
type, which may be considered in two classes. There are those ships which
carry refined oil products, and perhaps some other cargoes like molasses,
which tend to be in the smaller 12 000 tonnes to 50 000 tonnes deadweight
range. Then there are the crude oil carriers which extend to the 500 000
tonnes deadweight range. The former vessels, the smaller of which may
have a single centre line longitudinal bulkhead (see Figure 22.4), have a
greater number of tanks, and more complicated pumping arrangements
which permit the carriage of a number of different products on a single
voyage.
Both types of ship have traditionally been single flush deck ships with
longitudinal bulkheads and a structure within the tank spaces consisting of
a grillage of longitudinal and transverse members. The structural arrange-
ment over the cargo tank length is dictated by the requirements of the
MARPOL convention (see Chapter 29). Since 1980 new crude tankers of
20 000 tonnes deadweight or more and new products carriers of 30 000
tonnes deadweight or more were required to be provided with segregated
ballast tanks (SBT’s). The capacity of the SBT’s being so determined that
the ship could operate safely on ballast voyages without recourse to the use
of cargo tanks for ballast water. These SBT’s were to be located within the
cargo tank length and arranged to provide a measure of protection against
oil outflow in the event of grounding and or collision. The protective location