Mitsubishi Heavy Industries Technical Review Vol. 46 No. 1 (Mar. 2009)1

Japan's First Dual-Fuel Diesel-Electric Propulsion
Liquefied Natural Gas (LNG) Carrier

HAJIME NUMAGUCHITOSHIFUMI SATOHTOSHINORI ISHIDASHOICHI MATSUMOTO

KAZUHIRO HINO

TAKESHI IWASAKI|1. Introduction
A steam-turbine propulsion plant using dual-fuel boilers has long been used as the propulsion
system for liquefied natural gas (LNG) carriers. In recent years, however, increasing numbers ofLNG shipowners have required the use of diesel engines, traditionally used in ships other thanLNG carriers, for the propulsion plant due to the lack of crews skilled in handling steam plants
containing high-pressure boilers and the potential energy savings that can be realized because of
the soaring price of crude oil. Mitsubishi Heavy Industries, Ltd. (MHI) has designed andconstructed an electric propulsion carrier for MISC Berhad that makes use of dual-fuel dieselengines. This paper describes our dual-fuel diesel–electric propulsion LNG carrier, which is thefirst ship of its kind constructed in Japan.

|2. Dual-fuel LNG carrier overview and propulsion plant
The dual-fuel ship is a 157,000 m3 membrane LNG carrier built for MISC Berhad. Its main
specifications are listed in Table 1. Based on the shipowner’s requirements at the time ofconstruction, MHI adopted a propulsion system using dual-fuel diesel engines, which consume lessfuel than steam-turbine propulsion.
Table 1 Main specifications of the dual-fuel LNG carrier
Ship classOverall length
Length between perpendiculars
Width
Depth
Calculated draft
Gross tonnage

(m)
(m)
(m)
(m)
(m)
(t)

BV
294.60281.6046.50
25.80
11.15
107 633

Cargo tank volume
Propulsion motor power
Generator power
Maximum power
Sea speed

(m3)
(kW)
(kW)

157 720
12 900 × 2 sets
11 400 × 3 sets
5 700 × 1 set(kW × min-1)
24 750 × 78
(kt)
19.5

The propulsion plant consists of four generators, two propulsion electric motors, a reduction
gear, and a screw propeller for propulsion. The redundancy of the propulsion plant was enhancedby adopting several generator engines, propulsion electric motors, and independent power supplysystems (Figure 1).
The generator engine consists of four medium-speed dual-fuel diesel engines (12V50DF  3,6L50DF  1) manufactured by Wärtsilä. This generator engine can be operated by switching
between MDO-mode, which uses only diesel oil as fuel, and GAS-mode, which uses gas as a main
fuel and diesel oil as a pilot fuel. Also, this generator engine can use the boil-off gas generated in
the LNG tank as fuel.
The propulsion electric motor (PEM) consists of two high-speed motors (rated at 640 min-1)
manufactured by ABB. The output shafts of the PEM are connected to a reduction gear and activate
the screw propeller for propulsion. Each PEM has an independent power supply system and can be
individually operated.
The arrangement of the engine room is shown in Figure 2. The generator engines are placed
separately into left and right engine rooms, and a bridge wall separates the rooms for safety
reasons.

Mitsubishi Heavy Industries Technical Review Vol. 46 No. 1 (Mar. 2009)
2Figure 1 Generation-power supply system

Figure 2 Engine-room layout plan

|3. Gas supply system
3.1. System outline
The gas supply system of this carrier is shown in Figure 3.
The fuel gases for the generator engines consist of natural boil-off (NBO) gas spontaneously
generated in the LNG tank and forced boil-off (FBO) gas obtained by vaporizing the LNG. The
fuel gases are pressurized by gas compressors and supplied to the generator engines. The required
gas pressure for the generator engines is 0.55 MPa. MHI adopted two-stage pressurization for thegas compressors. To obtain a sufficient compression ratio, the gas compressors are required tomaintain the suction temperature of less than –100C. A precooler is installed on the NBO side tolower the temperature by spraying LNG. Non-gasification ingredients in the LNG sprayed by theprecooler are separated by mist separators, collected in a drain tank, and returned to the LNG tank.
FBO is the gas obtained when LNG in the tank is transferred to a steam-heated forcing
vaporizer by a fuel gas pump. LNG sprinkling sprayers are installed on the exit side of thesteam-heated forcing vaporizer to maintain the FBO gas temperature at less than –100C. Twostages are used for the sprayer to ensure good cooling performance. The remaining non-gasification
ingredients in the gas after spraying the LNG are separated by the mist separator dedicated for theFBO gas and returned to the LNG tank.

Mitsubishi Heavy Industries Technical Review Vol. 46 No. 1 (Mar. 2009)
3The temperature at the gas compressor exits rises to nearly 100C, and could cause thetemperature of the gas at the inlet to the generator engines to exceed the required range of 0 to60C. To lower the temperature, after-coolers are installed at the sides of gas compressor exits tospray the LNG. The after-coolers also control the quantity of LNG sprayed to balance the required
gas quantity in the field, since the quantity of fuel gas runs short when only NBO gas is used whilean overabundance of fuel gas exists if the forcing vaporizer is used. To prepare for the case when
the NBO gas yield surpasses the required fuel quantity, a gas combustion unit (GCU) is installed todispose of the extra NBO gas.
To ensure that the gas temperature is maintained at a value greater than 0C (e.g., when
supplying NBO gas to the GCU only using the internal pressure of the LNG tank without driving
the gas compressors), gas heaters are installed downstream of the after-coolers and used to heat theLNG tank temperature to a normal temperature.

Figure 3 Gas supply system

3.2. Gas supply
The two-speed gas compressors are started at low speed. After the inlet temperature of thegas compressors falls below –100C due to the precooler, the gas compressors are shifted to highspeed, increasing the discharge pressure up to 0.55 MPa. The dual-fuel engines cannot dispose of
the discharged gas from the gas compressors in this process due to insufficient pressure. The gas
discharged during this process is usually burned by the GCU, but in our new carrier, piping isinstalled to return the gas to the LNG tank to avoid useless gas consumption. This piping isequipped with a control valve. When the gas consumption rapidly decreases due to the fuel-mode
switching of the dual-fuel diesel engines, this control valve returns the gas from the gas compressor
exits to the LNG tank. Thus, the control valve also controls the gas so as not to affect the gas
supply system, including the gas compressors.

3.3. Fuel tank pressure control
The gas supply system not only transfers the fuel gas to the diesel engines, but also plays animportant role controlling the pressure in the LNG tank. Three functions are available to control thepressure: an automatic start/stop of the GCU, flow control of the forcing vaporizer, and spray
quantity control of the after-coolers.

|4. Trial results
4.1. Sea trial
The measured fuel oil consumption rate (FOCR) of this carrier during its sea trial and FOCRof an identical horsepower steam-turbine propulsion plant are shown in Figure 4. The propeller
shaft horsepower FOCR indicated approximately 20% energy savings compared to thesteam-turbine propulsion plant.

Mitsubishi Heavy Industries Technical Review Vol. 46 No. 1 (Mar. 2009)4

4.2. Gas test
The measured stability of the gas supply system is shown in Figure 5. When three dual-fueldiesel engines were in operation in GAS-mode, the variation in the gas supply pressure wascontrolled to approximately 8%, even if the loading on the engines was drastically lowered byapproximately 40%. This rapid loading change on the engines did not affect the GAS-mode
operation of the diesel engines, and consequently, the stability of the gas supply system wasdemonstrated to be adequate.

Figure 4 Comparison of fuel oil
consumption rate

Figure 5 Stability of the gas supply system|5. Conclusions
The first dual-fuel diesel–electric propulsion LNG carrier in Japan finished its gas tests in
November 2008, meeting its design objectives. Its delivery procedure was completed in January
2009. The advantages of the diesel–electric propulsion plant over a steam-turbine power plant are
mixed with regard to maintenance, such as parts replacement during periodic inspections andflexibility in the pressure control of the LNG tank. However, since MHI is now able to deliver anelectric propulsion plant that makes use of dual-fuel diesel engines, in addition to a conventional
steam-turbine plant for LNG carriers, the company is in a better position to respond tobroad-ranging customer requirements.

Authors
Hajime Numaguchi
Manager,
Ship Designing Department,
Nagasaki Shipyard & MachineryWorks

Toshifumi SatohManager,
Ship Designing Department,
Nagasaki Shipyard & MachineryWorks

Toshinori Ishida
Manager,
Ship Designing Department,
Nagasaki Shipyard & Machinery
Works

Shoichi MatsumotoManager,
Ship Designing Department,
Nagasaki Shipyard & Machinery
Works

Kazuhiro Hino
Koyagi Construction Department,
Nagasaki Shipyard & Machinery
Works

Takeshi Iwasaki
Ship Designing Department,
Nagasaki Shipyard & Machinery
Works