Environmental regulations imposed on
operators means that they must adopt new technology to comply with the new
rules. In many cases the choice is LNG or low sulphor HFO or MDO.
by Earl of Cruise, based on THE NAVAL ARCHITECT, October 2016
Cruise vessel design of KNUD E. HANSEN - courtesy KNUD E. HANSEN
Danish naval architects, KNUD E.
HANSEN takes an informed look at the merits and demerits of both fuels,
LNG and HFO, on a medium sized cruise liner. The naval architects have
designed a medium sized cruiseship, ostensibly for a Greek operator to
deploy in the Mediterranean. But KNUD E. HANSEN have made a comparison
between the HFO fuelled version and its LNG variant.
According to KNUD E. HANSEN´s comparison,
the operating expenses for the LNG version are lower, including the fuel
consumption when the scrubber fuel (HFO) consumption is included in
the calculation.
Scrubbers take out SOx from the
exhaust when vessels are operating in Environmental Control Areas (ECA).
These are f.e. 200sm off coast North America, the British Chanel, the North Sea,
Baltic Sea and as well in Polar Regions. The Mediterranean is soon to follow,
as well the Caribbean.
IMO regulation say the step will be
in 2020, years ahead of the once planed schedule. Fuel has to be with at best 0,1%
to a maximum of 0,5% sulphor.
LNG–fuelled ships will not
need scrubbers to meet the SOx rules. On the other side all conventional
fuelled ships need scrubbers to pass the new regulations. The LNG–fuelling is
cutting back on both capital expenditue and operating costs. Further
space is safed within the ship, or for the funnel design (dimensions). In
addition, the LNG ships will not need a selective catalytic reduction
(SCR) to meet NOx rules, while HFO heating and purifying equipment
would be unnecessary. HFO needs to be heated to around 60° to become
"a fluid".
Furthermore, there would be no need
for settling and daily tanks for the fuel and no waste
chemicals.
Gas carriers often take advantage
of boil-off gas. LNG is stored in liquid form at -163°C and will emit
small amounts of gas as the liquid warms. This boil-off gas is used
for powering the main engine. But a cruise vessel can take advantage of this boil-off
rate to cover the higher demands in electric load in the hotel section
of the vessel.
In these days where safety is an
issue as well as the availability of LNG in some parts of the world,
the ability to switch fuels offers the cruise operator an `extra degree of redundancy´,
explains KNUD E. HANSEN. The design company also explained that
the LNG version prompted them to explore more innovative solutions in
terms of engine room and consequently funnel location.
Such solutions, however, are
no longer unique to the LNG fueled vessel.
Each new construction has to consider
the new IMO machinery system redundancy requirements of the INO - SAFE RETURN to
PORT RULE, e.g. two sperated machine rooms. Similar arrangements are now
feasible for any diesel-electric configuration with only limited
impact on construction costs.
And the final advantage
according to KNUD E. HANSEN is that the bunkering process can take place
with passengers onboard saving time in the operation of the ship.
The list of disadvantages is
shorter, but could ultimately prove more costly, at least in the
short term.
Higher capital expenditure will
be needed to build the vessel with all the safety features necessary
for using a low flashpoint fuel. Safety will necessarily be a
significant issue and the crew will need special training in the use
of equipment and the handling of LNG.
In addition, the LNG
tank arrangement uses up a lot of technical space. Membrane tanks
negate this effect compared to other types of tanks such as Type C
tanks, while the LNG tank structure adds up a considerable amount of
weight.
LNG fuelled cruise vessel design of KNUD E. HANSEN - courtesy KNUD E. HANSEN
Depicting the new configuration for LNG fuelled cruise ships
Strict bunkering procedures
require the isolation of the main fire zone (MFZ) although there are `no
official regulations regarding the bunkering procedure, only
universally accepted guidance´, explains the design company.
Bunkering LNG itself could also
prove a difficulty in some parts of the world.
LNG supply and infrastructure
remains underdeveloped. Meanwhile, a lack of experience in the design
and operation of LNG powered cruise vessels means that investors
are reluctant to risk being the first to make the investment. Tank
sizing was based on three operational scenarios. In all three of
them there is the assumption that the energy requirements of the
vessel can be covered relying solely on either LNG or HFO.
Technical Particulars - Reference
vessel
Loa
|
250m
|
Breadth
|
034m
|
Depth
|
009.5m
|
Design draught
|
007m
|
Service Speed
|
18 Kn
|
Persons onboard
|
2650 (passenger and crew)
|
Propulsion
|
podded electric thrusters
|
LNG Version
|
6 medium speed dual fuel engines.
LNG as main fuel, in combination
with MDO for pilot fuel and compliance with SRtP rules.
Membrane type of tank for LNG
storage HFO Version 6 medium speed diesel generators HFO in all normal
operating conditions Scrubbers and possible SCR/EGR
|
KNUD E. HANSEN research results are
as follows:
Scenario 1
|
includes five day
cruises, with four trips per month and bunkering intervals at 14
days
|
Scenario 2
|
has 2.5 trips a month with
12-day cruises and bunkering intervals set at six-12 days
|
Scenario 3
|
includes a
21-day transatlantic cruise, 1.4 trips per month, and bunkering
intervals of every 10 days.
|
The outcome of this research
was that an LNG tank of 2,000m³, would be sufficient to cover the
vessel’s needs for the first 2 scenarios, while for the third one,
a future retrofit and potential addition of a tank could be an
option.
HFO fuelled cruise vessel design of KNUD E. HANSEN - courtesy KNUD E. HANSEN
Depicting the "classic" configuration for modern day cruise vessels
The pilot fuel has been calculated
as the equivalent to 5% of the LNG thermal capacity that is onboard.
Additional MDO to comply with the safe return to port rules and for
redundancy purposes would also need to be taken into account.
For the two versions of the
cruiseship design there are potentially small changes in the
dimensions of the vessels midship section. The vessel will include two engine
rooms located fore and aft of the LNG tanks. A reduction on the amount
of equipment that will be required in the LNG version offers the
possibility of rearranging the machinery area to gain space.
This will also necessitate
changes to the crew cabins location, the crew public spaces and the provisions
area. A change in the vessel’s profile due to double funnels, located
on the centreline of the machine room will also mean changes to the
external open deck arrangement and facilities that would be combined with
the two funnels.
On the LNG version the membrane tanks
are preferred as these types of tank use the available space more
efficiently.
According to the naval architect: "80% of the vessel’s inner hull volume in the certain compartment that will accommodate the membrane, will constitute the membrane. There is a difference of 30-40% in the usable volume of the area where the tank is located between Type C and membrane tanks."
In addition, there is "no minimum filling limit in membrane tanks (no thermal stresses restrictions as in type C tanks)".
According to the naval architect: "80% of the vessel’s inner hull volume in the certain compartment that will accommodate the membrane, will constitute the membrane. There is a difference of 30-40% in the usable volume of the area where the tank is located between Type C and membrane tanks."
In addition, there is "no minimum filling limit in membrane tanks (no thermal stresses restrictions as in type C tanks)".
Irrespective of the type of
tanks, cruiseships can turn the potential problem of boil off gas to
an advantage through the powering of the hotel areas. "Managing the boil-off gas on a
cruise vessel is not considered a major issue, due to the extended
hotel load," says KNUD E. HANSEN. "Often, boil-off gas is produced on purpose to cover the
needs in electrical load."
When this is not the case, the `excess´ boil-off
gas can be used in various ways, including the innovative combination
of an LNG fuelled vessel with significant battery storage onboard.
The boil-off gas can be used for battery charging via DC electric
storage, increasing the LNG efficiency onboard while offering a
further more eco-friendly solution.
The excess gas can also be used to
fuel the auxiliary boiler and produce steam for various purposes or to
fuel the incinerator. Other solutions, including gas
combustion units, which just waste the energy, and reliquefaction units,
are costly and apply mostly to other, more specialised types
of vessel such as gas carriers. These are not neccessary for cruise
vessels.
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