Page 264 - Ship Construction.DJ Eyres 6Ed

Ch22-H8070.fm Page 253 Wednesday, October 18, 2006 6:59 AM
Tanker Construction 253
of these tanks had the effect of providing full or partial double bottoms
and/or side tanks in way of the cargo tank space. Subsequent amendments
to MARPOL required every tanker of 5000 tonnes deadweight or more
that commenced construction after 1993 to have the entire cargo tank
length protected by ballast tanks or spaces other than cargo and fuel oil
tanks, or other provisions offering equivalent protection against oil pollu-
tion. As we have seen in Chapter 3 double hulls will be required for Cate-
--- ใช้เพื่อการศึกษาเท่านั้น---
gory 1 oil tankers from 2005 and for Category 2 and 3 oil tankers from 2010.
(there is however provision for the flag State to permit some oil tankers of
Category 2 and 3 to operate beyond these dates to 2015 or until their 25th
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
anniversary but port States may deny such ships entry to their ports).

Materials for Tanker Construction

Mild steel is used throughout the structure, but higher tensile steels may
also be introduced in the more highly stressed regions of the larger vessels.

MILD STEEL As with dry cargo ships it is a requirement that Grade B, D
ึ
and E steels be used for the heavier plating of the main hull strength mem-
bers where the greatest stresses arise in tankers. These requirements are
the same as those indicated in Table 17.1. Grade E plates which we have
equated with the ‘crack arrester strake’ concept (see Chapter 8) will be seen
to be required over the midship region in ships exceeding 250 metres in
length and are also required as shown in Table 22.1.

HIGHER TENSILE STEEL Higher tensile steels are often used for the
deck and bottom regions of the larger tankers, Grades DH and EH being
used for the heavier plating. As indicated in Chapter 5 the use of higher
tensile steel leads to a reduction in the scantlings of these structural items
with advantages both for the shipbuilder and owner. The extent of this plat-
ing and section material is indicated in Figure 22.2.

TABLE 22.1
Use of grade E steel in tankers

Location Thickness
Stringer plate, sheerstrake, rounded gunwale. Greater than 15 mm
Bilge strake, deck strake in way of longitudinal bulkhead. Greater than 25 mm
Main deck plating, bottom plating, keel, upper strake of Greater than 40 mm
longitudinal bulkhead.