Liquefied Gas Carrier

Home page||| LNG handling ||| LPG handling||| Other Gas products||| Fire & Safety||| Emergency response |||

Materials of cargo tank construction and insulation of gas carriers

The choice of cargo tank materials is dictated by the minimum service temperature and, to a lesser degree, by compatibility with the cargoes carried. The most important property to consider in the selection of cargo tank materials is the low-temperature toughness. This consideration is vital as most metals and alloys (except aluminium) become brittle below a certain temperature.

Treatment of structural carbon steels can be used to achieve low-temperature characteristics and the Gas Codes specify low-temperature limits for varying grades of steel down to -55°C. Reference should be made to the Gas Codes and classification society rules for details on the various grades of steel.

LNG carrier moss tanks
Fig:LNG carrier moss tanks

According to the Gas Codes, tankers carrying fully refrigerated LPG cargoes may have tanks capable of withstanding temperatures down to -55°C. Usually, the final temperature is chosen by the shipowner, depending on the cargoes expected to be carried. This is often determined by the boiling point of liquid propane at atmospheric pressure and, hence, cargo tank temperature limitations are frequently set at about -46°C. To achieve this service temperature, steels such as fully killed, fine-grain, carbon-manganese steel, sometimes alloyed with 0.5 per cent nickel, are used.

Where a tanker has been designed specifically to carry fully refrigerated ethylene (with a boiling point at atmospheric pressure of -104°C) or LNG (atmospheric boiling point -162°C), nickel-alloyed steels, stainless steels (such as Invar) or aluminium must be used for the material of tank construction.

LNG carrier moss tanks cross section
Fig:LNG carrier moss tanks cross section

Tank Insulation

Thermal insulation must be fitted to refrigerated cargo tanks for the following reasons:

• To minimise heat flow into cargo tanks, thus reducing boil-off.

• To protect the tanker structure around the cargo tanks from the effects of low temperature.

Insulation materials for use on gas carriers should possess the following main characteristics:

• Low thermal conductivity.

• Ability to bear loads.

• Ability to withstand mechanical damage.

• Light weight.

• Unaffected by cargo liquid or vapour.

The vapour-sealing property of the insulation system, to prevent ingress of water or water vapour, is important. Not only can ingress of moisture result in loss of insulation efficiency but progressive condensation and freezing can cause extensive damage to the insulation. Humidity conditions must, therefore, be kept as low as possible in hold spaces. One method to protect the insulation is to provide a foil skin acting as a vapour barrier to surround the system.

Thermal insulation may be applied to various surfaces, depending on the design of the containment system. For Type 'B' and 'C' containment systems, insulation is applied directly to the cargo tank's outer surfaces. For Type 'A' cargo tanks insulation can be applied either directly to the cargo tank or to the inner hull (if fitted) although its application to the cargo tank is more common.

As most insulation materials are flammable, great care is required at times of construction or refit to ensure that fires are avoided.

Related Information:

  1. Gas cargo containment systems - primary barrier (the cargo tank),secondary barrier, thermal insulation and more

  2. Preparatory operations of drydocking for LNG carriers

We have extracted gas carrier images from the publication ‘LNG Shipping Knowledge’ by Witherby Seamanship

Gas cargo containment systems - primary barrier (the cargo tank),secondary barrier, thermal insulation and more

Preparatory operations of drydocking for LNG carriers

Procedure for transporting remote gas

Development and potential of todays emerging gas technologies

Transporting economically viable compressed gas liquids from remote fields

The risk of laden voyage - a brief guide to liquefied gas carriers

Connection and disconnection of cargo hoses and hard arms

The risk of ballast voyage - a brief guide to liquefied gas carriers

The risk of laden voyage - a brief guide to liquefied gas carriers

External links :

  1. International maritime organization

// Home page/// LNG handling /// LPG handling/// Sea transport /// Gas products///

Cargo work ///Fire precautions ///Health hazards ///Safety Precautions

///Emergency response ///

Copyright © Liquefied Gas All rights reserved.

The content published in this website are for general reference only. We have endeavoured to make the information as accurate as possible but cannot take responsibility for any errors. For latest information please visit . Any suggestions, please Contact us !

///Links &Resources // Terms of use/// Privacy policy///Home page///