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Dual fuel diesel electric propulsion systems LNG carriers-Electric transmission losses & fuel cost

One of the strongest arguments against electric drive of LNG carriers has always the losses in the electric transmission between the prime mover and the propeller.
Indeed there are electric transmission losses of less than 8%, but even with these losses a dual fuel electric propulsion system is far more efficient than a steam turbine drive system. The thermal efficiency of the dual fuel diesel engine in gas mode is approximately 47%. Then the thermal efficiency at the electric motor flange to the gearbox would be over 43.5%.

Currently, all electric propulsion (EP) systems for LNGC’s have medium speed electric motors driving the propeller via a reduction gearbox, because medium speed electric motors are cheaper and smaller than low speed electric motors. Factoring in the losses in the gearbox (1.5%) and the shafting losses (1%), the total thermal efficiency of the DFDE propulsion system is about 42.5%. This compares very favorably with the steam turbine drive system’s thermal efficiency of less than 30%. Reducing fuel consumption by 30 – 40% has a significant impact on the total cost price of the LNG that is being transported to the consumer market.

Electric transmission losses by LNG ships
Fig :Electric transmission losses by LNG ships


Medium speed diesel engines, which include the dual fuel diesel engines used in the DFDE LNGCs, need more maintenance and more manpower to carry out the maintenance than steam turbine propulsion systems. Steam turbine propulsion systems are much lower in maintenance requirements.

Apart from the maintenance on the dual fuel diesel engines, there is also maintenance required on the electric drive system. Initially, there was a great deal of uncertainty about these costs, but careful examination of the major cost drivers has shown that these cost are a mere fraction of the maintenance cost of the dual fuel diesel engines.

Duel fuel electrical propulsion system
Fig: Duel fuel electrical propulsion system

Lube Oil

Medium speed diesel engines have higher lube oil consumption than steam turbines. The main reason is that the lube oil in the diesel engine is exposed to the combustion process in the cylinders. The specific lube oil consumption for the dual fuel diesel engines is quoted at 0.5 g/kWh when running in gas mode. On an annual basis this means up to 100 tons of lube oil.

When running in HFO mode the lube oil consumption might be higher as it has to protect the engine against the corrosive attack of the sulfur contained in the residual fuel. The difference in properties between gas fuel and HFO might make it necessary to change between lube oils with higher and lower TBN number to adequately protect the engines. Most recently a preliminary advice came to use a TBN 30 lube oil when alternating between HFO and gas fuels. At this stage experience with HFO on dual fuel engines is too limited to draw any firm conclusions.

Gas combustion unit (GCU)

Electric driven vessels need additional equipment in the form of a gas combustion unit (GCU) to handle excess boil-off gas (BOG), while steam turbine vessels burn the excess BOG in the boilers and dump the steam in the condenser. The additional CAPEX for procurement and installation of the GCU are included in the 2 – 4% higher initial price for a DFDE LNGC.

Related article:

The Steam driven LNGCs & fuel option

Duel fuel electrical propulsion system for LNG ships

Benifits of compressed gas technology

Transporting economically viable compressed gas liquids from remote fields

Advantages of the dual fuel diesel electric propulsion & crew training

Defining various gas carrier types

Fuel flexibility of LNG ships

LNG ship spillage risk

LNG shipment

Initial Cool Down of cargo tanks

Leaks on the Cargo System, Continuous Flow - how to prevent

LNG tank leaks and immediate action by gas carriers

Leaks from a Loading Arm due to Tidal or Current Effects

Minor or major leaks from LNG tanks

Procedures for LNG cargo loading

Procedures for LNG cargo discharging

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