Alewijnse Celebrates Delivery of MV Abis Dover with Third-Generation DC Diesel-Electric Propulsion System
Innovative drive demonstrates unparalleled operational versatility
Built by Shipkits in a joint project that included Vuyk Engineering Groningen, Alewijnse Marine Systems and Vacon, MV Abis Dover is the second of an exciting new class of special-purpose cargo vessel designed to offer exceptional capacity and stowage flexibility for their size. Together with her sister ship MV Jaguar, MV Abis Dover represents a major advance in diesel electric propulsion systems as one of the first operational commercial vessels to use DC current for enhanced versatility and efficiency. Alewijnse Marine Systems was responsible for the design, installation and commissioning of the complete electrical installations on board both vessels, including the third-generation DC diesel-electric propulsion systems.
MV Abis Dover is owned by Abis Shipping and operated for Amasus Shipping BV. She has a DWT of 6,000 tonnes and an LOA of 107 metres yet the length of her upper hold is a full 80 metres, a remarkable achievement for a vessel of her size. That, along with her dynamic position (DP) system, makes her ideal for a wide range of roles including offshore support as well as wind farm operations.
Her ability to use so much of her internal volume is a direct result of her diesel electric propulsion system. This does away with the need for one or two bulky diesel engines and replaces them with six, much smaller, Scania diesel generators that are positioned at the aft end of the hull in convenient locations and have the potential for variable alignments. Instead of propeller shafts the propulsive power comes from steerable Z-drive thrusters.
However, up until now all maritime diesel electric systems have used AC power which has a number of shortcomings for certain operational requirements. One of these is that standard AC configurations require all the generators to run synchronously so as to avoid damaging each other. This requires complex control gear and has the effect of making the system less fuel efficient than it might be, and it also takes valuable time to synchronise the generators with each other. Another feature of AC systems is that all the motors run at a fixed speed dependent upon the frequency (Hz) supplied to them.
To achieve the variations in speed necessary for propulsion the supply frequency must itself be variable. Traditionally this is done through the use of a variable speed drive, but these can cause harmonics which can be disastrous for an electric distribution system, particularly where large motors are involved. To address all these issues Alewijnse Marine Systems worked with Vacon to develop a ‘third generation diesel electric system’ that uses DC current to overcome these harmonic distortion problems.
DC generators offered a number of immediate advantages. To start with they can run at different speeds simultaneously, giving greater flexibility in selecting overall power outputs and as a consequence the possibility of greater fuel economy. Also the fact that the generators do not require synchronisation means that power is available just a few seconds after one of more additional units are brought on line. This is important for vessels like MV Abis Dover where the operational requirements demand both low-energy station-holding using a DP system and high-energy open sea steaming. It also removes the requirement for complex control apparatus. To address the issue of harmonics Vacon designed into the system an AFE (Active Front End) AC/DC rectifier that feeds into a 750 volt DC bus. This in turn feeds variable frequency AC/DC inverters to power the Z-drive thrusters and bow thrusters, all of which are AC motors.
On MV Abis Dover only one or two of the six generators will be needed in DP mode whereas all are required for full propulsion at maximum tonnage. The remaining motors for pumps, winches and other onboard systems are fed by either fixed or variable speed inverters depending on their design. Fixed frequency inverters feed all the other systems as they do not need to change speed. A power management system designed and installed by Alewijnse, along with the rest of the onboard electrics, ensures that the optimum number of generators is delivering the correct amount of current at any one time.
MV Abis Dover has just completed a series of sea trials and is now on her maiden commercial voyage.
The success of the first two vessels has been confirmed by further orders for additional ships, and DC diesel electric systems now look set to become widespread for commercial applications that require high degrees of design and operational versatility along with minimal emissions and fuel consumption.