Marine Installation Manual
2021-08
4-38
4 Ancillary Systems
4.4 Fuel gas system
X72DF
4.4.4
Fuel gas supply system
Fuel gas can typically be stored as LNG at atmospheric pressure, or be pressur
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ised. The design of the external Fuel Gas Supply System (FGSS) may vary, how
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ever it should provide natural gas with the correct temperature and pressure to
the engine. The gas piping can be of either single- or double-wall type according
to its installation position in compliance with the relevant rules. Any gas pipe in
the engine room must be of the double-wall type.
Master fuel gas engine shut-off valve
For dual-fuel engines, the IMO IGC and IGF Codes require a master fuel gas en
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gine shut-off valve to be installed in the fuel gas feed system, outside of the en
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gine room (see the ‘Gas Fuel System’ in the MIDS,
). In addition, a
manual shut-off valve must be placed upstream of the master fuel gas engine
shut-off valve. Alternatively, the master fuel gas engine shut-off valve can be de
-
signed with manual override function.
To enable independent operation of different fuel gas consumers, it is recom
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mended that each fuel gas consumer’s supply line is equipped with an inde
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pendent fuel gas shut-off valve. At least one of these shut-off valves should close
in suitable time to prevent unnecessary venting of fuel gas or at least to restrict
the amount of vented fuel gas.
Tank type
The best-suited tank type will depend on system requirements such as the neces
-
sary size, design pressure, the expected quantity of Boil-Off Gas (BOG), and the
expected duration for the tank to hold the LNG (and resulting BOG).
As defined by the IMO, LNG tanks are classified as either ‘Integrated’ or
‘Free-standing’. Free-standing tanks are then further classified as sub-types A, B
and C. A description of the main tank types is provided below.
Integrated –
Membrane tank
Integrated type tanks, such as a membrane tank, are built into the hull as part of
the vessel structure to ensure an efficient utilisation of space. This is the defining
feature compared to a free-standing tank (also known as an independent or
self-supporting tank) which is not built into the hull. Integrated tanks are
low-pressure tanks, designed for pressure less than 0.7bar(g). They are a
common tank type option for LNGC cargo tanks and are appropriate for large
LNG-fuelled vessels, such as container vessels, bulk carriers and oil tankers.
