Page 284 - Ship Construction.DJ Eyres 6Ed

Ch23-H8070.fm Page 273 Wednesday, October 18, 2006 7:38 AM
Liquefied Gas Carriers 273
similar to that fitted to the Independent Type A tanks described earlier is
fitted (see Figure 23.4).

SEMI-MEMBRANE TYPE B TANKS The Japanese ship builder IHI
has designed a semi-membrane, Type B tank to carry LNG cargoes
which has been used in a number of LPG carriers. The rectangular tank
consists of plane unstiffened walls with moderately sloped roof and
system. --- ใช้เพื่อการศึกษาเท่านั้น---
rounded edges and corners which are not supported so that expansion
and contraction is accommodated. The tank is of 15 to 25 mm thick alu-
minium alloy supported on a layer of PVC insulation and the partial sec-
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
ondary barrier is made of plywood 25 mm thick integral with a PVC
foam insulation.

General Arrangement of Gas Carriers

Gas carriers have a similar overall arrangement to tankers in that their
machinery and accommodation are aft and the cargo containment is spread
over the rest of the ship to forward where the forecastle is fitted.
Specific gravity of LPG cargoes can vary from 0.58 to 0.97 whilst LNG
ึ
ships are often designed for a cargo specific gravity of 0.5 so that a charac-
teristic of LNG ships in particular and most LPG ships is their low draft
and high freeboards. Water ballast cannot be carried in the cargo tanks so
adequate provision is made for it within the double hull spaces, double
bottom, bilge tank, and upper wing tank spaces.
The double hull feature of LNG carriers and many LPG ships is a
required safety feature and the tanks of LPG ships which do not have this
feature are required to be a minimum distance inboard of the shell.
Fore end and aft end structure is similar to that for other ships. The
cargo section is transversely or longitudinally framed depending primarily
on size in the same manner as other cargo ships, the inner hull receiving
special consideration where it is required to support the containment

All gas ships have spaces around the tanks which are monitored for gas
leaks and in many ships these spaces are also inerted, an inert gas system
being fitted aboard the ship. Liquid gas cargoes are carried under positive
pressure at all times so that no air can enter the tanks and create a flammable
mixture.
Liquefaction equipment is provided aboard LPG ships, ‘boil off’ vapour
from the tanks due to any heat ingress is drawn into the liquefaction plant
and returned to the tank. Boil off vapour from LNG ship tanks can be utilized
as a boiler fuel in steam ships, otherwise it is vented to atmosphere, although
this is not permitted in many ports, and several other solutions have been
developed to overcome this problem.