The concept of electric propulsion is not new, the idea originated more than 100 years ago. However, with the possibility to control electrical motors with variable speed in a large power range with compact, reliable and costcompetitive solutions, the use of electrical propulsion has emerged in new application areas during the 80’s and 90’s.
Electric propulsion with gas turbine or diesel engine driven power generation is used in hundreds of ships of various types and in a large variety of configurations.
Installed electric propulsion power in merchant marine vessels was in 2002 in the range of 6-7 GW (Gigawatt), in addition to a substantial installation in both submarine and surface war ship applications.
By introduction of azimuthing thrusters and podded thrust units, propulsion configurations for transit, maneuvering and station keeping have in several types of vessels merged in order to utilize installed thrust units optimally for transit, maneuvering and dynamically positioning (dynamic positioning – DP).
At present, electric propulsion is applied mainly in following type of ships: Cruise vessels, ferries, DP drilling vessels, thruster assisted moored floating production facilities, shuttle tankers, cable layers, pipe layers, icebreakers and other ice going vessels, supply vessels, and war ships.
There is also a significant on-going research and evaluation of using electric propulsion in new vessel designs for existing and new application areas.
Advantages of ship electric propulsion
The following characteristics summarize the main advantages of electric propulsion in these types of vessels:
- Improved life cycle cost by reduced fuel consumption and maintenance, especially where there is a large variation in load demand. E.g. for many DP vessels a typically operational profile is equally divided between
transit and station keeping/maneuvering operations.
- Reduced vulnerability to single failure in the system and possibility to optimize loading of prime movers (diesel engine or gas turbine).
- Light high/medium speed diesel engines.
- Less space consuming and more flexible utilization of the on-board space increase the payload of the vessel
- Flexibility in location of thruster devices because the thruster is supplied with electric power through cables, and can be located very independent on the location of the prime mover.
- Improved maneuverability by utilizing azimuthing thrusters or podded propulsion.
- Less propulsion noise and vibrations since rotating shaft lines are shorter, prime movers are running on fixed speed, and using pulling type propellers gives less cavitation due to more uniform water flow.
These advantages should be weighted up against the present penalties, such as:
- Increased investment costs. However, this is continuously subject for revisions, as the cost tends to decrease with increasing number of units manufactured.
- Additional components (electrical equipment – generators, transformers, drives and motors/machines) between prime mover and propeller increase the transmission losses at full load.
- For newcomers a higher number and new type of equipment requires different operation, manning, and maintenance strategy.
High availability of power, propulsion and thruster installations, as well as safety and automation systems, are the key factors in obtaining maximum operation time for the vessel. The safety and automation system required to monitor, protect, and control the power plant, propulsion and thruster system, becomes of increasing importance for a reliable and optimum use of the installation.