86
LNG
INDUSTRY
MARCH
2016
Norwegian passenger ferry
Glutra
became the first non-LNG
carrier to use LNG as fuel. LNG is a clean burning fuel, which
allows the vessels to comply with upcoming, stricter marine
emissions legislation. Regular marine diesel engines operating
on residual marine fuels would require exhaust gas after
treatment in order to comply with the new emission standards.
Dual-fuel diesel engines emit hardly any SO
X
, as there is no
sulfur in LNG. Furthermore, they have lower NO
X
and CO
2
emissions than regular diesel engines.
However, LNG fuel is much harder to store onboard the ship
than traditional marine fuels, such as residual and distillate fuel,
as it requires tanks that are able to withstand the cryogenic
temperature of LNG. Furthermore, the energy density of LNG is
only half that of traditional marine fuels, making it necessary to
store twice as much fuel for the same operational range.
In addition, marine diesel fuels are stored in hull conformant
bunker tanks, whereas all of the LNG-fuelled commercial
vessels have Type C pressurised cylindrical tanks. The geometry
of these tanks adds to the space losses, as a cylindrical tank in a
relatively square fuel tank space under deck results in poor hull
space utilisation. As a result, it can generally be said that LNG,
when stored in Type C tanks under deck, needs three to four
times as much space as the same amount of energy in the form
of marine diesel fuel. When the Type C LNG tanks are located
on deck, the revenue making space losses are not as dramatic
as mentioned above.
The question is, would there be any benefit for LNG-fuelled
vessels if they moved away from the cylindrical Type C
pressurised LNG tank to the membrane, Moss or SPB systems?
To find the answer, one has to evaluate the alternative tank
system using a number of criteria, such as volumetric efficiency
(howmuch space is required for a certain amount of LNG fuel to
be stored onboard and ability to shape the tanks to make the
most of the available space within the hull), operational
implications (tank pressure management and BOG handling,
bunkering, sloshing damage risk), installation issues, availability
and cost.
When it comes to fitting the LNG fuel tanks into the hull, the
prismatic tanks are better than spherical or cylindrical tanks,
giving the membrane systems and the SPB tank an advantage
over the Moss system and the Type C tank in terms of space
utilisation. Both membrane systems and the SPB system can be
shaped to make the most out of the available space within the
hull, whereas the Moss systemmust retain its (almost) spherical
shape and the Type C pressure vessel is always cylindrical.
Because of the obvious geometrical advantages, many new
LNG containment system designs are prismatic systems, such
as: the Lattice Pressure Vessel (LPV) developed by Lattice
Technology in South Korea; the Norwegian-designed LNT A-box
tank from LNG New Technologies; the Aker Solutions ADBT
tank from Norway; the General Dynamics NASSCO FSP tank
developed in the US; and the NLI Type B tank, also from
Norway.
Operationally, the membrane tanks need careful tank
pressure management in order to avoid damage to the primary
membrane and the insulation boxes. BOG from the membrane
tanks needs to be directed to either the dual-fuel engines or to
the BOG reliquefaction plant, or, if neither are available, to the
gas combustion unit (GCU). The membrane tanks can only be
bunkered from LNG tanks at atmospheric pressure, because of
their low tolerance to tank pressure rise. The Moss and the SPB
systems are less critical in terms of tank pressure management,
because of their more robust structural design. Both systems
can accept a 0.7 bar(g) tank pressure increase – further tank
pressure rise has to be handled by gas consumers or
reliquefaction.
Bunkering of the Moss and SPB tank system can be carried
out with atmospheric tanks or with pressurised tanks, provided
that the tank pressure has been brought down below the design
parameters of the Moss and SPB tanks. The Type C tank allows
for the most flexibility; the tank pressure can rise above 2 bar(g)
and even up to 10 bar(g), depending on the working pressure and
the tank safety valve settings. Four-stroke gas and four-stroke
dual-fuel engines can be directly fed from pressurised Type C
tanks without the need for cryogenic pumps. Type C tanks can
also be bunkered from a wide range of cryogenic tanks, thus
allowing for maximum flexibility in refuelling.
Contrary to LNG shipping, in which the cargo tanks are either
full or almost empty, the LNG fuel tanks onboard LNG-fuelled
vessels operate anywhere between the two levels. For the
membrane tank systems, operations with slack tanks in adverse
weather could lead to sloshing issues, with its inherent risk of
damage to the containment system. Careful attention to the tank
geometry and the tank location in the vessel could be a way to
mitigate some of this risk. Tanks in or on the bow or stern of the
vessel or tanks high up on deck are subjected to more
acceleration than tanks close to the centre of gravity of the vessel.
However, to avoid sloshing damage and its associated risks, it is
advisable to use LNG fuel tanks that are not susceptible to
sloshing damage, such as the Moss tanks, the SPB tanks and the
Type C cylindrical tanks.
Installation
There are considerable differences in the installation of LNG
tank systems. Membrane tanks are built into the vessel, which
means that the vessel’s structure has to be completed first,
before installation of the tanks can commence. Ensuring a
temperature controlled, clean environment for the installation of
the membrane systems in a shipyard is not easy.
The Moss tanks, the SPB tanks and the Type C tanks are
self-supporting tanks that are built off-hull and inserted or
lowered into the hull whenever convenient. For these LNG tanks,
construction of the ship and the LNG fuel tanks is partly in
parallel, which could have an advantage in completion time of
the vessel. Building these tanks in specialised facilities has a
positive effect on quality. However, transportation of the finished
tanks from the factory to the shipyard, especially for larger tanks,
is not without cost and risk.
Figure 2.
The prismatic FSP LNG tank system currently under
development by FSP LNG (source: FSP LNG).
