Page 282 - Ship Construction.DJ Eyres 6Ed

Ch23-H8070.fm Page 271 Wednesday, October 18, 2006 7:38 AM
Liquefied Gas Carriers 271
the underside of the dome projecting through the deck which is used for
access and piping connections, etc. The tanks sit on insulated bearing blocks
so that surfaces are accessible for inspection and are located by anti-roll
and pitch keys in such a manner that expansion and contraction can take
place relative to the ships structure. Anti-flotation chocks are provided to
prevent the tank floating off the bearings if the hold were flooded. Tanks
are constructed of a notch ductile steel for the normal minimum operating
--- ใช้เพื่อการศึกษาเท่านั้น---
temperature of − 43 °C the boiling point of propane.
The ship has a double hull extending over the bottom and bilge area, the
secondary barrier being provided by low temperature (notch ductile) steel
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
at the inner bottom, sloping bilge tank, part side shell, and sloping bottom
of topside tank. Transverse bulkheads may be single or double plate (cof-
ferdam) type between cargo holds. Insulation can be either on the tank or
the secondary barrier for this type of ship.

Liquefied Natural Gas Ships

There are over twenty approved patent designs of containment vessel for
LNG ships, the majority of which fall into the membrane or independent
ึ
tank categories. Those types which have been or are more commonly found
in service are described below. A feature of LNG ships is their double hull
construction within which are fitted the cargo tanks and the secondary
barrier system.
At the beginning of 2002 of the 130 LNG ships then trading, more than
half had independent Type B tanks of the Kvaerner-Moss design with most
of the others being of the membrane type. The GAZ Transport membrane
system being twice as prevalent as other membrane systems.

INDEPENDENT TYPE A TANKS Early LNG ships such as the ‘Methane
Princess’ and ‘Methane Progress’ were fitted with self-supporting tanks of
aluminium alloy having centreline bulkheads (see Figure 23.2). The balsa
wood insulation system was attached to the inner hull (secondary barrier)
and each insulated hold contained three tanks. Later vessels built with
tanks of this category have adopted a prismatic tank design.

INDEPENDENT TYPE B TANKS The Kvaerner-Moss group have
designed an independent Type B tank containment system which has been
well accepted and is installed in a good number of LNG ships. Tanks con-
sist of either an aluminium alloy or 9 per cent nickel steel sphere welded to
a vertical cylindrical skirt of the same material which is its only connection
to the hull (see Figure 23.3). The sphere expands and contracts freely all
movements being compensated for in the top half of the skirt. The outer
surface of the sphere and part of the skirt is covered with a polyurethane