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Advantages of dual fuel diesel electric propulsion of LNG carriers
Increased Cargo Capacity for LNG ships
A number of studies have focused on the increase in cargo capacity that could be obtained from
changing to dual fuel diesel electric (DFDE) propulsion. When comparing the engine room size of the conventional steam
LNGC with the DFDE LNGC, it would be possible to move the engine room bulkhead further aft when
using DFDE propulsion.
Electric propulsion systems allow the prime movers to be located away from
the propeller shaft into areas where there is more space. By having only the propulsion motors and the
reduction gearbox on the tank top, the engine room can be shortened substantially. While additional
cargo capacity estimates fluctuate, the consensus is that it should be possible to gain some extra
cargo space. The space saving argument is reinforced by the lower fuel consumption of the DFDE
LNGC. Because the fuel consumption is lower, less bunkers need to be carried, which saves
additional space and weight.
Fig:Gas carrier sea passage
However, this additional cargo space is mostly located in way of cargo tank number 4 just in front of
the engine room bulkhead, where the hull starts to taper in towards the propeller. Lengthening tank 4
too much would result in sloshing issues.
Additional cargo capacity also results in higher cost for
building the containment system and tanks with complicated geometry are more expensive to build
than parallel mid ship tanks. Without having access to shipyard engineering data it is very difficult to
exactly predict the cost of the additional cargo capacity. Consequently, it becomes difficult to precisely
predict the economic benefits of the additional cargo space.
Much has been written about the shortage of steam qualified crew for LNG carriers. By moving away
from steam to DFDE propulsion this issue is not entirely solved, as there are still many steam driven
LNGC on order and the current fleet of more than 200 steam driven LNGCs will need steam qualified
crews for decades to come.
However, it might be a bit easier for DFDE LNGS operators to find crews.
The dual fuel diesel engines basically work on the same principles as regular medium speed engines.
The dual fuel system is not very complicated to understand and has been in marine operations for a
few years now. With a bit of equipment specific training most engineers should be able to operate and
maintain the engines properly.
The electric drive system is mostly made up of components with which most engineers onboard are
very familiar; switchboards, generators, electric motors and transformers. Everything is just a bit bigger
and the voltage is higher. High voltage safety training should be a prerequisite for operation and
maintenance of these systems.
The only “new” technology for the crews to be introduced onboard the
DFDE LNGCs is the frequency converters that control the speed of the electric motors. These
frequency converters are actually not new technology as they have been used in many demanding onshore
applications for many years. There are equipment specific training courses available from the
OEM to train the crews in proper operation and maintenance of these frequency converters.
Actually, the main question might not be whether the crews can handle the propulsion plant, but can
they perform all cargo operations adequately and do these engineers have sufficient experience
onboard gas vessels
Fuel cost for dual fuel diesel electric propulsion of LNG carriers
- Benifits of compressed gas technology
- Compressed gas liquid carriers (CGLC)
- Transporting economically viable compressed gas liquids from remote fields
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