Arctic technology: Winterisation of FPSO 38 Cruise ... - Ship & Offshore

SHIPBUILDING & EQUIPMENT | GREEN SHIP TECHNOLOGY
Bunkering, infrastructure,
storage, and processing of LNG
EMISSIONS REDUCTION To comply with future environmental regulations, improvements of
ships’ emissions are urgently required. LNG as a ship’s fuel will reduce NOx to clearly below
Tier III level, SOx and particulate matters to about zero and CO 2 by 20 to 30% without any
treatment of combustion gases. Main challenges of the usage of LNG, however, include the safe
storage and processing of the liquefi ed gas as well as the bunker infrastructure,
procedure and equipment.
Jürgen Harperscheidt
Using LNG as fuel has been a common
technology for decades on LNG carriers.
The safety record for loading/unloading
of such vessels as well as for the operation
of propulsion systems based on burning
boil-off gas is very good. During the last
10 years, operational experience has been
gained in Norway where small ships have
been equipped with LNG propulsion, e. g.
ferries and offshore supply vessels.
The diffi culty when providing LNG as fuel
to a wider scale of ships and shipping areas
is the bunker infrastructure to make
LNG available wherever ship’s operators
may need it. It is therefore crucial for the
introduction of LNG as a fuel to have an
infrastructure in place that secures safe, fast
and reliable accessibility to LNG for the
operators - a major task for those involved
in small scale LNG.
Containment systems
In respect to storage, one basic disadvantage
of LNG is its low density: LNG takes
up roughly twice the volume of fuel oil for
the same energy content. There are several
types of containment systems for LNG available
but some of them are not feasible for
the given conditions on ships using LNG
as fuel. The current regulatory approach is
based on self supporting tanks as defi ned in
the IMO IGC code: Type A (designed as ship
structures) and type B (prismatic or spherical)
tanks are generally feasible for fuel gas
tanks but their requirement for pressure
maintenance and secondary barrier raise
problems which have not yet been solved
in a technically and commercially sound
way. This may be a future solution for ships
carrying large amounts of LNG as fuel.
Hence IMO type C tanks (pressure vessels)
turn out to be the preferred solution
for current designs. Firstly, the tanks are
very safe and reliable, secondly, their high
design pressures allow high loading rates
and pressure increase due to boil-off and
fi nally, they are easy to fabricate and install.
12 Ship & Offshore | 2011 | N o 1
Figure 1: Principal view of IMO type C
bilobe and cylinder tank Image: TGE
Figure 2: One possible tank layout for a
project study on a container feeder
Image: TGE, GL, MAN, Neptun SK
The major disadvantage of this type of tank
is the space consumption due to restriction
to cylindrical, conical and bilobe shape. In
addition to the unfavorable volume/energy
effi ciency, these design restrictions lead to
a factor of 3 to 4 times required volume to
carry the same amount of energy in comparison
to oil based tankers.
LNG tanks have to be insulated for two reasons:
One is to reduce boil-off vapour generation
by heat ingress and the other is to
protect adjacent ship structures from very
cold temperatures. For ships with more or
less continuous consumption of LNG and
only short periods of low or no demand,
the conventional foam insulation will be
the most economical type of insulation.
LNG consumption of the engines will keep
tank pressure low. For ships with longer periods
of low consumption, for example operational
patterns based on LNG use only in
ECA and conventional fuel outside ECAs, it
might be essential to improve tank insulation
in order to reduce pressure rise in tanks.
In regards to small tanks this can be done by
using vacuum insulation as seen in Norway.
These vacuum insulated tanks are limited
to cylindrical shape and do not allow for
in-tank inspections or mounting of in-tank
equipment as they usually have no manhole.
For tanks clearly exceeding 500 cbm or
requiring bilobe or conical shape, the use of
special insulation panels is proposed to improve
insulation performance.
Process systems
Depending on the design parameters such
as storage size, number of tanks, engine
consumption and layout, required bunkering
rate and arrangement of the system
components, tailor-made solutions for
LNG fuel gas systems are available. One
general rule is that for larger storage volumes
the cost impact of high tank operating
pressures becomes signifi cant; these
systems should be preferably equipped
with mechanical pressure rising (pumps,