Dual fuel diesel electric propulsion systems LNG carriers

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Dual fuel diesel electric propulsion systems LNG carriers

Dual-fuel engine is the diesel engine that can run on both gaseous and liquid fuels. When running in gas mode, the engine works according to the Otto process where the lean airfuel mixture is fed to cylinders during the suction stroke. Efficiencies exceeding 47% have been routinely recorded. When running in diesel mode, the engine works according to the Diesel process where the diesel fuel is fed to cylinders at the end of compression stroke. The engine is optimised for running on gaseous fuels and diesel fuel is used for back-up fuel operation.

The DF technology enables the engine to be operated on either natural gas, light fuel oil or HFO. Switching between fuels can take place seamlessly during operation, without loss of power or speed. The engine is designed to have the same output regardless of the fuel used.

LNG shipping : Since the beginning of the 21st century, LNG has been very much in the picture. Oil majors are expecting that LNG will be taking a much larger share in the energy mix in the next 25 years. The LNG value chain necessarily involves LNG shipping. It’s in this part of the value chain that we’ve seen some major technology changes recently. .

Fig:LNG ship at sea

The introduction of the dual fuel diesel electric (DFDE) LNG carrier (LNGC) and the slow speed diesel driven LNGC with onboard reliquefaction (DRL) are radical departures from the currently dominant steam turbine drive technology. The introduction of the dual fuel diesel electric drive systems could also open the door for dual fuel gas turbine electric (DFGTE) propulsion systems, as they are based on the same electric drive concept.

Electric propulsion systems for LNGC’s include the following components:

•Prime movers, i.e. dual fuel diesel engines;
• Prime mover driven generators;
• Main switchboard to distribute the generated power to the various consumers;
• Propulsion transformers;
• Frequency converters;
• Electric propulsion motors;
• Propeller(s);

Fig: Duel fuel electrical propulsion system

The introduction of electric drive in LNGC propulsion might seem like a radical step, but in fact the move toward electric propulsion was an evolutionary process. With the LNGC growing in size, the onboard power requirements increased so much that 440 V installations couldn’t handle the load safely. This led to the introduction of medium voltage switchboards and pumps motors. This familiarization with medium voltage systems was the first step towards electric propulsion. In cruise vessels, medium voltage propulsion systems were introduced more than a decade earlier and have since gained an excellent track record.

Fig:Dual Fuel Diesel Electric Propulsion Systems for LNG carriers

The advantages of the dual fuel electric propulsion system over the conventional steam turbine drive system are:

• Improved thermal efficiency of the propulsion plant, both at sea and in port;
• Lower installed power, as the power plant serves both the propulsion load as well as the harbor load;
• Increased redundancy with 4 prime movers and two (almost) independent drive lines;
• Increased cargo capacity within the same overall dimensions of the vessel;
• Crews with diesel electric experience are more readily available the crews with steam certification;
• Electric propulsion system is easier to operate and faster to start up and to shut down;

However, there are also a few disadvantages:

• Steam turbines have proven to be very reliable over time, more reliable than diesel engines;
• Steam turbine plants need less maintenance than diesels;
• Diesel engines have higher lube oil consumption than steam turbines;
• Electric driven vessels need additional equipment to handle excess boil-off gas;
• Steam turbines are very flexible in terms of fuel types and fuel mixing ratios, whereas dual fuel diesel engines operate either in gas mode or in diesel mode;

LNG as an alternative/duel fuel for maritime operations offers

Lower costs per measurement energy of fuel
Adheres to new emissions regulations
Technological and operational improvements in vessels and terminals

LNG as a fuel needs to follow a carefully planned implementation process that is fully integrated (economic, technologically, public awareness, political, protect the environment, safety, etc.)

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