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Tank types

Tank types

The Interim Guidelines on Safety for Natural Gas Fuelled Engine Installation in Ships (Resolution MSC.285(86)) mandate that LNG fuel tanks must be selected from “Independent Types A, B, or C” of the IGC Code, chapter 4.

 

The International Code Of Safety For Ships Using Gases or Other Low-Flashpoints Fuels (IGF Code) (Resolution MSC.370(93)) that will take effect on 1 January 2017, in addition thereto includes the use of membrane tanks for such use.

 

For LNG-fuelled merchant ships there are several options for installing an LNG tank. For smaller LNG-fuelled vessels, prefabricated vacuum-isolated cryogenic type C tanks are available in a wide range of sizes (up to 500 m3), with a maximum allowable working pressure of 20 bar. Available tank sizes are expected to increase significantly over the next few years (1,000-10,000 m3). Type C tanks are already in operation on ferries and offshore supply vessels. There are several designs for larger LNG-fuelled ships that propose using type B tanks because they require less space, but the industry is not unanimous on this issue. Membrane tanks are now (2016) offered for use as fuel tanks and several have been contracted.

 

LNG carriers are typically equipped with membrane-type tanks or type B tanks, but smaller ships may be equipped with type C tanks (e.g. Coral Methane).

 

Space requirements for LNG storage are greater than for conventional fuel. This may reduce cargo capacity, depending on the type of vessel, type of fuel tank and onboard potential for adequate location of the LNG tanks. Type C tanks require the most space: about 2-4 times more than an HFO tank.

 

Type A and B tanks require a full or partial secondary barrier respectively to prevent potential release of the liquefied gas in the event of a tank failure. With a type C tank this risk is lower and no secondary barrier is therefore needed.

 

Although an LNG tank will be properly insulated, there will be some transmission of heat from outside into the liquid, which will cause evaporation and pressure increase. Vacuum insulation or a 20-30 cm layer of polyurethane foam covered by protection sheets is applied for fuel tank insulation.

 

Type A and B tanks will require handling of the boil-off gas by (propulsion) engines, boilers or reliquefaction. A 200 m3-type C tank can hold the boil-off for about 25 days before reaching the maximum allowed tank pressure.

 

Main characteristics of the different tank types

Tank type Description Pressure Pros Cons
A Prismatic tank, adjustable to hull shape; full secondary barrier <0.7 bar g Space-efficient

Boil-off gas handling. More complex fuel system required

 

High costs

B Prismatic tank, adjustable to hull shape; partial secondary barrier <0.7 bar g Space-efficient

Boil-off gas handling. More complex fuel system required

High costs

Spherical tank; partial secondary barrier Reliably proven in LNG carriers Boil-off gas handling. More complex fuel system required
C Pressure vessel, cylindrical with dished ends >2 bar g

Allows pressure increase

Simple fuel system

Little maintenance

Easy installation

Lower costs

On board space requirements

 

Further reading

LNG tanks

All LNG fuelled ships in the current fleet (2015) are fitted with a type C tank, a cylindrical pressure vessel requiring 2-4 times more space than a conventional fuel storage tank.