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| Fast ro-pax vessel. |
LNG fuels ro-pax vessel
Wärtsilä has developed a new environmentally
friendly machinery concept for ro-ro and ro-pax vessels designed to operate
on natural gas. The introduction of gas onboard commercial ships introduces
a lot of questions regarding the new technology involved and the impact
on the ship design.
The shipping industry is facing ever-increasing demands to be more environmentally
friendly. More efficient machinery concepts and emission reduction technology,
such as direct water injection and selective catalytic reduction units,
has already made it possible to reduce some emissions.
 However,
more drastic means are required to cut all relevant exhaust emissions,
including CO2, from ships. One way to do this is by switching from traditional
oil based fuels to natural gas (NG). Operation on natural gas gives very
low emissions owing to the clean burning properties of the gas and the
low content of pollutants in the fuel. Natural gas consists mainly of
methane, which is the most efficient hydrocarbon fuel with regard to energy
content per amount of carbon. Natural gas operation therefore reduces
the CO2 emissions by over 20 per cent compared to a diesel operation.
A new machinery concept
A new ro-pax vessel and machinery concept has been developed in co-operation
between Wärtsilä Corporation and Kvaerner Masa-Yards Technology
as part of the Finnish research project SeaTech 2000+.
The
machinery is based on the new Wärtsilä Dual-Fuel (DF) gas engines
and is intended to operate on liquefied natural gas (LNG).
The
same engines and machinery technology is already being applied to a LNG
carrier, which is under construction in France and will be the first gas-engine
electric LNG carrier in the world. A new high speed ro-pax vessel was
designed for the DF-electric machinery. The hull of the proposed ro-pax
vessel is of full-displacement type and features a very long and slender
form with a single centreline skeg to offer low resistance.
The
superstructure is located in the forward part of the ship leaving the
aft end of the upper car deck open. The cargo is transported on the two
large ro-ro decks. The hull sides are flared out to increase the width
of the ro-ro decks, while keeping the breadth at the waterline level narrow.
This makes it possible to increase the number of lanes and thereby the
lane meters without impacting on the resistance.
The
enlarged car decks also makes it possible to carry all cargo on two decks
and no lower hold is needed. This allows for faster loading and unloading
since no internal ramps are used.
The loading and unloading of vehicles is arranged with double level stern
ramps directly to both the main and upper deck.
The
simple and fast cargo-handling concept is in line with the high-speed
philosophy of this ship. The vessel can also be equipped with a bow ramp
for drive through loading.
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| Relative space demand and weight for the storage of LNG and
compressed natural gas (CNG) compared to that of diesel fuels. |
Machinery concept
The machinery is based on the electric power plant principle with Wärtsilä
DF engines as the power source. The DF engines are mainly operated on
natural gas with marine diesel oil (MDO) as a pilot fuel but they can
as well be run purely on MDO, which acts as a back-up fuel.
Electric propulsion is selected to get the benefits of constant speed
operation, which is better suited for gas operation.
The
power plant is dimensioned to meet the total power demand of about 43
MW at the intended service speed of 28 knots, including ship services.
It consists of four Wärtsilä 12V50DF and two Wärtsilä
9L32DF gensets with a total installed engine power of 51,9 MW, giving
an appropriate engine margin.
The
smaller engines are mainly intended for harbour use when the load is low,
but they can also be used when maximum propulsion power is needed.
CRP propulsion
The
propulsion is fully electric featuring a novel arrangement with one shaft
driven propeller located straight in front of a contra rotating propeller
(CRP) powered by an electric pod.
The podded CRP configuration offers better hydrodynamical efficiency compared
to a conventional vessel with twin screws on long open shafts, because:
The
aft propeller takes advantage of the rotative energy left in the slipstream
of the forward propeller when it turns in the opposite direction. This
improves the rotative efficiency of the propulsion.
The
single skeg hull form offers a more favourable wake than an open shaft
line, resulting in better hull efficiency.
The
resistance of the single skeg hull form with a single pod is lower than
that of a twin screw hull with two open shaft lines. Especially the lack
of appendages, such as rudders, shaft brackets, bossings and stern thrusters
contributes to the lower resistance of the single skeg hull.
The podded CRP drive also offers other beneficial characteristics such
as excellent manoeuvring and steering capabilities owing to the pods ability
to turn 360°.
This
concept still requires more development before it can be commercially
introduced in full scale. Wärtsilä is currently conducting research
into this concept.
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| Relative exhaust emissions for the entire machinery (engines
+ OFB). |
Safety arrangement
The use of gas as fuel requires some special machinery arrangements to
comply with the classification rules and to offer a functional layout.
The
machinery is designed according to an emergency shutdown (ESD) safety
philosophy, which has the following features:
Automatic
switch to MDO use or shutdown of one of the engine rooms in case of a
gas leak.
Gas
detectors located in strategic locations to quickly detect any gas leak.
Engines
located in two separate engine rooms with a cofferdam in between to minimise
the consequences of gas leakage in either engine room.
Redundant
power generation, so that half the power is available even after one of
the engine rooms has been shutdown.
Low
gas pressure (under ten bar).
Single-wall
gas piping within the engine room.
Gas
pipes enclosed in ducts outside the engine room.
The Wärtsilä DF engines are well suited for the ESD philosophy,
owing to their ability to operate both on gas and MDO. This adds redundancy
and makes it possible to use only a single gas tank. Also the low gas
pressure of about five bar is a requirement for this safety concept.
Gas storage
The gas fuel for the proposed ro-pax is stored in liquid form (LNG), due
to its lower weight and space demand compared to compressed natural gas
(CNG). The LNG tanks are located in a designated tank room with the following
features to comply with the classification rules:
vThe tank room is thermally isolated from the hull to avoid the cold LNG
coming into contact with the steel structure in case of leak.
The
space is vent upwards to a safe location.
The
tanks are located inside the B/5 line.
There
are buffer zones to any A class machinery spaces .
Machinery impact on the ship design
The extra space demand for the LNG tanks and the electrical propulsion
equipment in combination with the divided engine room arrangement with
buffer zones requires more machinery related space than for a conventional
vessel.
However,
the proposed arrangement with only two wide car decks and no lower cargo
hold offers a lot of free space under the main car deck, which is well
suited for this machinery.
The
engine rooms are located inside the B/5 bulkhead enabling both the engines
and their generators to fit into common spaces. This way the need for
expensive gas tight bulkhead seals is avoided. The engine rooms are made
compact with most auxiliaries outside to reduce the air volume and make
gas detection quicker. The steel structure is also placed on the outside
of all the engine room bulkheads, to give a smooth surface and reduce
the risk of local gas pockets.
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| DF-electric machinery configuration for the 28 knot ro-pax |
Environmental impact
The proposed DF-electric machinery concept has been compared with a conventional
ro-pax machinery concept to quantify the environmental benefits of the
new solutions. The reference vessel is a typical diesel-mechanical ro-pax
machinery with four Wärtsilä 12V46 propulsion engines and three
6L26 gensets all operated on HFO. A fully diesel-electric alternative
with podded CRP propulsion and SCR units for all engines is also included.
It
can be seen that the DF-electric machinery using LNG as fuel offers by
far the lowest emission levels. The second best result is achieved with
the diesel-electric option equipped with SCR units.
The
great benefit of the LNG option is the clear reduction in CO2 emissions,
which cannot be achieved when burning oil based fuels. Also the lack of
sulphur in the LNG fuel eliminates all SOx emissions, while the clean
burning properties of a lean burn gas engines reduce the NOx emissions
to a fraction of those from a conventional diesel engine.
 |
| A contra rotating pod behind a propeller connected to a conventional
shaft line. |
Marine use of gas as fuel
There are some issues that need to be addressed before NG becomes a major
fuel option for marine applications. The availability of LNG is limited
due to the lack of infrastructure.
Currently,
there are only a few ports close to LNG terminals with bunkering capability.
Other ports must either rely on LNG brought in with trucks or build small
liquefaction plants for pipeline gas.
One
solution is to introduce small LNG carriers, with a cargo capacity of
a few thousand cubic meters, to distribute LNG from the terminals directly
to the ships or local storage tanks in the ports. A Norwegian operator
has already ordered the first vessel of this type for LNG distribution
along the Norwegian cost.
Another
issue is the need for rules and regulations for gas fuelled ships. The
first classification rules have recently been released by DNV, but most
national authorities have not released any rules yet.
The
price of natural gas operation is also an issue that needs to be evaluated
before the introduction of new ships. According to calculations for the
proposed ro-pax vessel, the first cost of the vessel will be higher than
that of a conventional ro-pax owing to the complexity of the special gas
arrangements and the high cost of a fully electric propulsion system.
The
operating cost will also be higher than a standard ship operating on HFO
due to the slightly higher price of LNG. However, when comparing the DF-electric
ro-pax against an environmentally sound conventional vessel, which is
equipped with SCR units and is burning high quality fuel, such as MDO,
the cost advantage is in favour for the LNG ship.
Since
the LNG ship offers the lowest emissions, the choice of fuel for the environmentally
conscious operator is leaning towards LNG. However, the cost of LNG can
vary a lot from one area to another. The feasibility of gas use could
be further boosted if emission trading will be introduced in marine transport.
National or company specific emission reduction goals could be achieved
by introducing gas-fuelled ships instead of doing more costly measures
to reduce emissions from land based operations.
Summary
The new Wärtsilä DF engines operating on LNG offers an interesting
alternative for future ro-pax vessels when environmental issues are of
high importance.
The
technology required for a gas fuel ship is available since the introduction
of the Wärtsilä DF engines.
However,
the LNG distribution infrastructure needs to be expanded and international
rules for gas fuelled applications needs to be made available before gas
becomes a common fuel on the seven seas.
//Oskar Levander, Wärtsilä Corporation, Marine Division
Back to SSG 8, Spetember 20
Latest update 18-10-2006 8:49
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