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An overall comparison between natural gas spark ignition and compression
ignition engines for a ro-pax propulsion plant: Proceedings of the 3rd
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Chapter · June 2016
DOI: 10.1201/b21890-95
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An overall comparison between natural gas spark ignition and
compression ignition engines for a ro-pax propulsion plant
U. Campora
Department of Mechanical, Energy, Management and Transportation Engineering, University of Genoa,
Polytechnic School, Genoa, Italy
M. Laviola & R. Zaccone
Department of Electrical, Electronic, Telecommunications Engineering and Naval Architecture University of
Genoa, Polytechnic School, Genoa, Italy
ABSTRACT: The paper presents a study concerning the repowering of an existing ro-pax operating across the
Strait of Messina (Italy). Considering the short ferry routes (about 4 km) and the ship operating profile (24
hours/day of continuative operation), two different solutions are considered and compared for the substitution
of the original four stroke diesel engines: traditional diesel engines (DE) and natural gas (NG) fueled marine
engines. This second alternative is considered due to the better performance in terms of pollutant emissions
and efficiency, if compared to DE of the same power and rotational speed, as verified by the authors in a recent paper. The different engines fuel types require different solutions for the fuel storage and handling that
involve significant differences in the engine room layout, vessel structures, general arrangements, load capacity and ship stability. In the paper, all these subjects are analyzed for the considered propulsion power solutions, including also other important aspects as fuel, machinery and maintenance costs.
1 INTRODUCTION
The here presented study is motivated by the intention of the Caronte & Tourist S.p.a. shipping line to
evaluate the repowering of an existing bidirectional
ro-pax vessel, named Enotria (length 96 meters),
built in the 2002 and employed in the Strait of Messina (Italy), in order to reduce the pollutant emissions and the fuel costs, increase the ship velocity
and, if possible, increase the vessel load capacity.
Starting from an authors’ recent paper (Benvenuto et
al. 2016), regarding a detailed comparison between
two recently marketed Rolls-Royce (RR) diesel and
natural gas four-stroke marine engines, in the present
study both solutions have been considered to replace
the originally adopted engines (four four-stoke diesel
ones delivering 735 kW at 2100 rpm each. The increasing use of four-stroke dual-fuel or NG engines
mainly in alternative to the conventional DE for ropax vessels applications (Smart 1991, ATENA
2014,2015, Di Natale 2014, Livanos et al. 2012) is
due to the limitations in fuels sulphur content and
engines nitrogen oxide emissions brought by the
Marine Environment Protection Committee (MEPC)
meeting (MEPC 2008). Sulphur oxide (SOx) and nitrogen oxides (NOx) emissions are subject to more
strict limits in the Emission Control Ar-eas (ECA),
actually located in the European North Sea, Baltic
Sea and English Channel. ECA limits are however
suitable to a future extension to others water areas,
as visualized in Figure 1 (Pedersen 2015).
Figure 1. Existing and possible future world ECA areas.
This fact has involved an increase of the Liquefied
Natural Gas (LNG) fueled fleet in the 2010-2014 period, and a further increase is expected as visualized
in Figure 2 (WPCI). Possible applications are starting to look at, in addition to large commercial and
passenger ships, also smaller working boats as tugs
and fishing vessels (Altosole et al. 2014a). Moreover, the International Maritime Organization (IMO)
recently introduced the Energy Efficiency Design Index (EEDI) to estimate the merchant ships carbon
dioxide emission efficiency (MEPC 2009, 2013). An
increase of LNG market is as well expected in the
Mediterranean sea (as reported in Fig. 1) (Pedersen
2015) and in Italy in particular (Cazzulo 2015), as
nine ports are going to be equipped by LNG storage
and fueling ships devices in the next years. The use
of DE or NG engines changes the conception of on
board fuel storage and handling, including installations’ security, particularly in the case of NG engines, in comparison with the ferry original DE engines. In the paper, a comparison between the two
considered solutions is carried out, taking into account the arrangement of the fuel tanks, the engine
efficiency and room layout, the emissions of carbon
dioxide and other pollutant substances, the ship displacement and load capacity. Also the main economic aspects (fuel, machinery and maintenance costs)
are taken in consideration in the comparison. The
original propulsion scheme and propeller typology is
unchanged. Results are reported in tables, graphs and
ship general plans.
Figure 3. Actual Enotria ferry longitudinal view (a) and deck
load plant view (b) with 22 trailers.
Table 1. Actual Enotria ferry main dimensions and equipment
features.
______________________________________________
ACTUAL MAIN SHIP
FEATURES
______________________________________________
Overall length [m]
97
Maximum beam [m]
18
Design draught [m]
4
Dead Weight [tons]
2000
Main engines power [kW]
4x735
Main electric generator [kW]
3x160
Emergency electric gen. [kW]
1x80
Maximum speed [knots]
13.5
Accommodation: persons
300
Accommodation: cars
108
Accommodation:
trailers
22
______________________________________________
As shown in Figure 3, the actual ferry configuration
is characterized by a single garage/open deck. As the
central area of the main deck/garage, between frames
-20.5 and +20.5, is covered by a superstructure
houses, loading operations take place through two
doors ramps hinged at the ship’s extremities, next to
which two small superstructures (manoeuvring
decks), are used to open and close the ramps and for
docking and mooring operations. The ship has a full
time (h-24) continuative operation. The crew working time is arranged into tree turns, each shift lasts 8
hours. Trips last about 35 minutes, with about 15
minutes stop between, for roll on/roll off operations.
Figure 2. Increase world LNG-fueled fleet type in 2010-2014
and foreseeable increase in 2018.
2 ORIGINAL RO-PAX CHARACTERISTICS
The considered Enotria ferry is a bidirectional ro-pax
vessel, able to the transport of passengers, cars, vehicles and trailers, operating on the Strait of Messina
(Italy) and owned by Caronte & Tourist S.p.a. The
Figure 3 shows a longitudinal view of the ship, while
the Table 1 reports the ferry main dimensions and
features.
features.
3 ACTUAL AND NEW PROPULSION
ENGINE COMPARISON
As mentioned in the introduction, Enotria ferry originally adopted four four-stroke diesel engines (MAN
D2042LE408) have been replaced by the more efficient four-stroke marine RR “Bergen” C series engines (Rolls-Royce 2012), available both in the
compression-ignition (DE) and spark-ignition (NG)
version in different powers: 2000÷3000 kW the DE,
1620÷2430 the NG, both at maximum continuous
rating (MCR) load conditions. The C25:33L6P DE
and C26:33L8PG NG RR “Bergen” engines considered in (Benvenuto et al. 2016) are characterized respectively by a delivered power of 2000 kW and
2160 kW, both at 1000 rpm, therefore both are suitable to replace the original ferry engines. Tab. 2
shows the original Enotria ferry MAN DE characteristics compared to the RR DE and NG possible alternative engines which are both more efficient (particularly the NG one), with respect to the actually
adopted MAN engines. Figure 4a shows the RR DE
constant efficiency map, provided by the manufacturer (Rolls-Royce Marine. 2015), referred to the entire engine load field (area bounded from the dashed
red lines in figure, normalized by referring to the RR
NG engine MCR data).
Engine parameters ____________
Actual engine
Renewed
engines
_____________
MAN D2842 C25:33L6P C26:33L8PG
______________________________________________
Length [mm]
1491
4036
4796
Height [mm]
1105
3195
3195
Width [mm]
1230
1775
1748
Dry weight [kg]
1790x4
19650x2
20700x2
Cylinders number 12V
6L
8L
Bore [mm]
128
250
260
Stroke [mm]
142
330
330
Fuel type
MDO
HFO/MDO NG
Brake power [kW] 735
2000
2160
B.m.e.p. [bar]
19.1
24.7
18.5
Speed [rpm]
2100
1000
1000
Efficiency
[%]
42.4
45.5
47.4
_____________________________________________
120
normalized power [%]
110
DE engine manufacturer constant eff. lines
cpp propeller curve
100
1.01 1
effD = 1.02
90
80
13.5 knots
power limt
power limt
70
60
50
14.0 knots
1
0.99
0.98
0.94
0.92
40
30
simulation code (including in cylinder calculations
and the supercharger group), developed by the authors (Benvenuto et al. 2016) and validated, with
good agreement, with reference data provided by the
engine manufacturer, for different engine running
load conditions and speeds (the validation errors are
generally less to 2%, with a maximum value lower
to 4.3% (Benvenuto et al. 2016). The numerical
model of the engine is developed in Simulink® environment, by using the simulation approach successfully adopted for other recent marine applications
(Altosole et al. 2012a; Altosole et al. 2013; Altosole
et al. 2014b). This kind of simulation analysis allows
to predict engine performance for emissions evaluation (Altosole et al. 2012b) and control design purposes (Altosole et al. 2014c).
normalized power [%]
Table 2. MAN and selected Rolls-Royce “Bergen” series engines main dimension and design (MCR) performance.
______________________________________________
_____________________________________________
0.83
20
0.71
10
130
NG vs DE differential efficiency
120
effM > effD ;
effM = effD ;
effM < effD)
(
110
NG power limit line
14.4 knots
14.0 knots
100
DE power limit line
90
13.5 knots
NG cpp curve
1.5
80
DE cpp curve Diffeff = 1.7
1
70
2
60
0.5
50
0.2
40
30
0.0
20
-0.2
-0.4
10
-0.8
0
45 50 55 60 65 70 75 80 85 90 95 100 105 110
normalized speed [%]
0
45 50 55 60 65 70 75 80 85 90 95 100 105 110
(a)
normalized speed [%]
normalized power [%]
100
90
NG engine constant efficiency lines
cpp propeller curve 14.4 knots
eff. = 1
0.99
14.0 knots
80
13.5 knots
0.97
70
0.95
60
0.93
50
40
0.89
power limt
0.84
0.76
30
0.67
0.55
20
10
0
45 50 55 60 65 70 75 80 85 90 95 100 105 110
normalized speed [%]
(b)
Figure 4. Experimental diesel engine (a) and calculated natural
gas (b) “Bergen” efficiency maps with Enotria ferry propeller
curve and working points for 14.4 (NG only), 14 and 13.5
knots vessel speeds
The Figure 4b shows the RR NG engine efficiency
map, determined by a fully thermodynamic engine
normalized power [%]
(a)
130
NG vs DE differential CO2
120
NG power limit line
110
14.4 knots
DE power limit line
14.0 knots
100
NG cpp curve
13.5 knots
90
DE cpp curve
-25.5
80
70
DiffCO2 = -25.5
60
50
40
-24
30
-22
20
-20
-17
10
-10
0
45 50 55 60 65 70 75 80 85 90 95 100 105 110
normalized speed [%]
(b)
Figure 5. Natural gas vs diesel engine percent map efficiency
(a) and carbon dioxide emission comparison (b) with Enotria
ferry propeller curve and working points for 14.4 (NG only), 14
and 13.5 knots vessel speeds
A comparison between the here considered “Bergen”
C25:33L6P DE and C26:33L8PG NG engines, regarding the global efficiency and the dioxide carbon
emissions, in the respective working fields, is shown
in Figure 5. In Figure 5a the two considered engines
global efficiency difference (Diffeff) is shown, expressed in percentage and normalized by equation:
Diff eff =
η NG − η D
100
ηD
(1)
The power reported in the figure is normalized with
respect to NG engine, while the efficiency difference
is normalized by referring to diesel engine, and the
engine speed is the same for both the engines. Finally, the efficiency parameters ηNG and ηD have been
respectively obtained by simulation and from manufacturer data, and are both referred to the same engines load and speed. Figure 5b shows the comparison between the two examined “Bergen” engines in
terms of the carbon dioxide (CO2) emissions. In the
figure, the data regarding the carbon dioxide mass
produced from the two engines is determined, for
each working point, as function of the respective fuel
mass consumption and fuel typology (NG and HFO),
by the burned fuel mass to produced CO2 mass table
reported in (MEPC 2012). The CO2 emissions comparison results, reported in Figure 5b, are presented
in percentage normalized form with respect to DE
CO2 emission, obtained, for each engines working
conditions, by equation:
Diff CO 2 =
M CO 2 NG − M CO 2 D
M CO 2 D
100
(2)
In accordance with literature data (Livanos et al.
2012, Aymelek et al. 2015), the comparison results
reported in the Figure 5b, shows about 25% less carbon dioxide emission for the NG engine, in comparison to the DE one, at MCR and in the great part of
the engines load areas; for engines power less than
about 30%, the NG carbon dioxide emission advantage progressively reduces. This fact is easily explained by the NG efficiency reduction as compared
with the diesel one, as the delivered power decreases, as shown in Figure 5a. The data reported in Tab.
3, referred to engines running at normal condition
rating, puts in evidence the greater advantage of the
NG, vs the conventional liquid marine fuels (HFO
and MGO), as regard of pollutant emissions without
considering the methane slip phenomena. The NG
emissions advantage will increase even more with
future development of methane slip risk prevention
(Aymelek et al. 2015).
Table 3. Typical naval four-stroke natural gas engines emissions reduction vs two-stroke low speed DE (HFO) (Livanos et
al. 2012) and vs four-stroke (MDO) (Aymelek et al. 2015),
both at normal condition rating load conditions.
______________________________________________
Marine engine
Pollutant type
NG FS engine
NG FS engine
difference [%]
difference [%]
______________________________________________
Carbon dioxide (CO2)
-25÷30
-25
Carbon monoxide (CO)
-25
-Nitrogen oxide (NOx)
-85
-85
Sulphur oxide (SO2)
-100
- more 98
Particulate
matter
-99
more 90
_____________________________________________
4 OLD AND RENEWED ENGINES
ARRANGEMENT COMPARISON
The NG ro-pax conversion project has been performed taking into account the main regulations in
the marine industry related to this kind of ship:
- Stability calculations are in compliance with the
European directive 2009/45 / EC;
- The structural calculation is in line with the RINA
class requirements;
- The estimate of the electric load for essential services board and for emergency conditions has been
carried out with the support of RINA (PtC, Ch2,
Sec3);
- All the specific requirements for ship propelled by
methane gas are in line with the IGF DRAFT code.
The cryogenic conditions of the liquid natural gas
stored on board require storage systems highly resistant to low temperatures and equipped with high
degree of insulation technologies. The LNG is stored
inside prefabricated vacuum-isolated type C cryogenic tanks, in redundant quantity as required by IGF
draft code (2 tanks), placed at a distance of B/5 from
the shipside and equipped by a vent mast positioned
at 6 m height as required by IGF code.
Figure 6. RR NG powered Enotria ferry longitudinal view and
LNG tanks layout (above), (below) additional cargo inner deck
plant view.
The choice to place the NG tanks on the deck 4 (as
shown in Fig. 6) has been supported by safety reasons. The possibility of arranging the tanks in a naturally ventilated open space and low exposed to
shocks and collisions aims to increase the degree of
safety. Furthermore, the released space where the
HFO fuel tank were placed (in the inner bottom at
the center area of the vessel, as shown in Fig. 7) it
can now use be used to increase the payload. The
new fuel tank arrangement is estimated to increase
the original payload by 31 cars (Di Natale 2014)
which find place in the inner deck, equipped by two
ramps for roll on/roll off operations (Fig. 6). The
bunkering station and the nitrogen cylinders used for
inerting system refueling operations are also present
on the deck 4.
(b)
Figure 8. Repowered Enotria ferry with RR DE (a) and NG (b)
right engine rooms plants view (see Fig. 3), the respective left
ones are speculars.
It has not been possible to find a similar solution for
the production of electrical energy in case of emergency state, so the emergency electrical load is provided by a traditional emergency diesel 120 kW
fueled by MDO. The start of the gas engine is made
possible by using compressed gas stored in pressurized cylinders.
5 PROPELLER, UPDATED MATCHING
Figure 7. RR DE powered Enotria ferry longitudinal section
with diesel tanks layout.
The TCS (the set of the devices required for the safe
storage of liquid gas and to re-gasification for feeding the engine) is directly integrated in each tank.
The gas flows from the tanks to the engine via double walled steel pipes. The capacity of two NG tanks
of 50 m3 each is estimated to guarantee one week of
sailing as required by the shipowner. The ventilation
of the engine is 30 times the volume per hour and
the gas regulation valve is placed within 10 m from
the engine. The estimated electrical load in normal
navigation is 300 kW (Di Natale, 2014), however,
no NG engines of this size are available on the market: for this reason, two shaft generators for each
shaft line (as shown in Fig. 8) have been used to
supply the electrical load, each one providing 150
kW; this solution allows the propulsion and electrical load to be fully produced by gas technology.
A performance prediction of the propulsion system
has been carried out in in order to compare the considered solutions. The hull hydrodynamic resistance
Rt has been computed using Holtrop & Mennen
method (Holtrop & Mennen 1978): results are
shown in Figure 9. The original Enotria propulsors
have been maintained: the two Voith Schneider Propellers (VSP), which principal data is summarized in
Table 4, are arranged symmetrically with respect to
amidships in accordance with the bidirectional layout of the ship.
Table 4. Principal Voith Schneider Propeller data
______________________________________________
Propeller features
______________________________________________
Number of blade
5
Blade orbit diameter [m]
2.6
Blade length [m]
1.965
Blade
density
0.242
_____________________________________________
The thrust (T) required to each propulsor is given by
the following:
T=
(a)
Rt
2(1 - t)
(3)
Where t is the thrust deduction factor. The power
prediction can be solved by several methods (Figari
& Altosole 2007), however, proper attention has to
be paid to the definition of the main VSP performance coefficients, slightly different in comparison
with traditional propellers. The VSP open water diagrams provide the non-dimensional thrust (Ks) and
torque (KD) coefficients as well as the consequent
open water efficiency, respectively defined as follows:
KS =
T
0.5 ρDLu 2
(4)
KD =
4Q
ρDLu 2
(5)
Figure 10 shows the propeller power absorption
curves in function of the fast reduction gear shaft
speed, at different pitches, represented on the engine
load diagram, as well as the pitch – revolution speed
combined law, which has been chosen in order to
ensure a reasonable margin from the limit line at low
engine loads, and to reach the maximum speed at
95% of MCR and nominal speed, as suggested by
the engine manufacturers.
Bergen C26:33L8PG (METHANE) EFFICIENCY
C26:33L8PG engine constant eff. lines
100
eff. = 1
0.99
90
(6)
The above defined coefficients are provided in function of the advance coefficient (λ):
λ=
VA
nD π
(7)
NORM. POWER [%]
80
K
ηO = S λ
KD
0.97
70
0.95
60
0.93
50
0.89
40
0.76
20
0.67
0.55
10
0
45
In order to eliminate the dependency from the propeller revolution speed (which is unknown at design
stage), the following auxiliary variable is used:
K=
KS
λ2
=
T
0.5 ρDLV A
(8)
2
This allows, for the design condition, to choose the
speed and pitch that maximize the efficiency of the
propeller, and thus to compute the proper reduction
factor i = 11.56. Once this last is fixed, the performance prediction is carried out at different pitch values (60% to 100% of design pitch).
0.84
30
50
55
60
65
70
75
80
85
90
95
100 105 110
NORM. SPEED [%]
Figure 10. Propeller power absorption curves at different pitch
values and combined law.
In Table 4 is reported a comparison, regarding the
main parameters, between the actual and the two
versions of the repowered Enotria ferry. In the repowered versions, the ship overall length, maximum
beam and the design draught parameters are the
same of the actual Enotria configuration, also reported in Table 1.
Table 5. Actual and repowered Enotria ferry main parameters
comparison.
__________________________________________
Main ship features
Actual engine _____________
Proposal engines
____________
MAN D2842 C25:33L6P C26:33L8PG
______________________________________________
Hull Resistance
250
Dead weight [tons]
Main engine power [kW]
Main electric generator [kW]
Shaft electric generator [kW]
Emergency generator [kW]
Ship maximum speed [knots]
Ship design speed [knots]
Accomodation: persons
Accomodation: cars
Accomodation: cars + trailers
Resistance [kN]
200
150
100
2207
2 x 2000
-2 x 150
1 x 120
14
13.5
300
103
22 + 0
2233
2 x 2160
-2 x 150
1 x 120
14.4
14
300
134
22+34
__________________________________________
50
0
2200
4 x 735
3 x 160
-1 x 80
13.5
13
300
108
22 + 0
0
5
10
15
Speed [kn]
Figure 9. Predicted hydrodynamic resistance vs, speed curve
(Holtrop & Mennen 1978).
Table 5 shows a comparison, between the two considered alternative propulsion plants (RR DE or NG
engines), in terms of main components efficiency,
fuel consumption and carbon dioxide emission, for
two different ship speeds (13.5 and 14 knots). The
slight deadweight difference between the two engines version of the repowered ferry (see Tab. 5) is
Table 6. Repowered Enotria ferry main parameters comparison
at 14 knots.
______________________________________________
Main ship features
C25:33L6P
C26:33L8PG
______________________________________________
Hull resistance [kN]
229
229
Engines efficiency [%]
45.6
47.0
Propellers efficiency [%]
63.0
63.0
Normal seagoing fuel cons. [kg/h] 351.5
298
Carbon
dioxide
emission
[kg/h]
1099
819.5
_____________________________________________
Table 7. Repowered Enotria ferry main parameters comparison
at 13.5 knots.
______________________________________________
_____________________________________________
Main ship features
C25:33L6P
C26:33L8PG
______________________________________________
Hull resistance [kN]
211
211
Engines efficiency [%]
46
46.8
Propellers efficiency [%]
62.0
62.0
Normal seagoing fuel cons. [kg/h] 312
247
Carbon
dioxide
emission
[kg/h]
983
679
_____________________________________________
The same engines working points considered in Tables 6 and 7 are represented, in the Figures 4-5.The
data reported in Tables 6 and 7 shows unequivocally
the smaller NG engine fuel consumption and CO2
emissions as compared to the DE one. This fact is
due to the NG engine better efficiency and, mainly,
to the NG greater lower heating values and less molecular carbon percentage in comparison with to DE
fuels.
6 FINANCIAL EVALUATION OF LNG
RETROFITTING SOLUTION
The ferry annual machinery, fuel and maintenance
costs, for each proposed engines typologies, are determined considering that the ship has a full time (h24) continuative operation; crew working time is arranged into tree turns, each shift lasts 8 hours. Each
trip between the Messina strait lasts about 35
minutes, at whose end the vessel stops for about 15
minutes, for an annual total of 8000 hours/year
(these data are provided by the shipowner’s company). For the fuel, a MDO price of 676 €/tons and
LNG price of 477 €/tons (+ 10% to consider the
LNG price of transport from port to port), are considered in accordance with (Bunker Index 2016), referred to December 15, 2015. The machinery costs
(i.e.: engines, propellers, fuel tanks and LNG gasification system, electric generators, maintenance) are
provided by the respective manufacturers. The total
ferry annual machinery costs are presented in Figure
11 (ten years of amortization are considered for the
machinery costs), both for NG and DE engines and
for two different ferry speed (13.5 and 14 knots).
percent
neglected as the same hull resistance is considered.
The comparison does not include the actual Enotria
propulsion plant configuration due to information
lacks.
110
100
90
80
70
60
50
40
30
20
10
0
LNG 13.5 kn Diesel 13.5 LNG 14 kn Diesel 14 kn
kn
Capital cost
Fuel costs
Maintenance costs
Figure 11. Repowered ferry (NG and DE engines) annual machinery, fuel and maintenance costs comparison for two different ship speed (CAPEX and OPEX (Livanos et al. 2012)).
The data reported in figure, presented in percent, are
normalized by referring to 14 knots speed with diesel engines. It is important to observe that these two
different vessel speeds do not permit an increase of
the trip/day, but only an increase of the ferry stop
time in port in the case of the 14 knots speed. The
annual costs data reported in Figure 11 show clearly
the less costs of the NG engines solution: this is due
to the less NG cost referred to MDO, despite an
about 70% greater NG engines cost referred to DE
ones. The propulsion plants maintenance costs are
substantially the same for both the engines types. Finally, Figure 11 shows the economic convenience of
a ship speed of 13.5 knots in comparison to 14
knots, mainly because of the less hull resistance, as
reported in Tables 6-7.
7 CONCLUSIONS
A study concerning the repowering of an existing bidirectional ro-pax ferry is presented in the paper.
The substitution of the original engines with two
new engine options is considered: a four-stroke natural gas fueled engine and a traditional four-stroke
diesel one, both commercialized by the same manufacturer (Roll-Royce Marine) and belonging to the
same engine series (named “Bergen”). Both engines,
characterized by very similar design power, are also
object of a detailed comparison, as reported in (Altosole et al. unpubl.). The actual ferry propellers and
main electric generators are as well substituted by
new different typology ones in the here proposed
new vessel propulsion plant configurations. The here
proposed ferry repowering and propulsion plant reconfiguration presents the following advantages, by
referring to the actual ship propulsion plant layout:
• A potentially increased ship cruise speed, due the
greater power of the new engines;
• An about 30% ferry load capacity increase (cars
only), in the case of adopting NG engines;
• A fuel consumption reduction for each trip (at
equal cruise speed), due the greater efficiency of the
new engines, in particular the NG ones;
• A minor reduction of the pollutant emissions with
the new DE engines, as well as a significant reduction in the case of NG;
• An annual costs reduction, due to the less fuel
consumption of the new DE engine, and a relevant
annual costs reduction in the case of NG one, due to
further fuel consumption reduction and natural gas
lower price, if compared to the DE.
This considerations lead to the conclusion that, in
authors’ opinion, the repowering of the considered
ferry is convenient especially if the currently adopted
engines are substitute with the NG ones. This solution, despite the greater machinery cost as referring
to the new DE engines, implies a load capacity increase, a greater reduction of the pollutant emissions
and a consistent annual fuel costs reduction (due to
the high amount of annual working hours, that allow
to recover in a short time the major NG plant cost).
At last, the expected increase (in the next future) of
the LNG infrastructures in the Italian and in the
Mediterranean Sea ports makes the use of the NG a
particularly interesting alternative to the conventional DE fuels also in this sea area.
AKNOWLEDGMENTS
A particular grateful is expressed to Mister Andrea
Cerutti and engineer Irene Zanin of Rolls-Royce Italy, for you precious collaboration. The authors wish
also to thank for the support given by Caronte &
Tourist S.p.a. shipping line.
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