Tue 22 Sep 2015 by David Foxwell
Diesel-electric power is almost considered the norm for many types of offshore support vessels (OSVs), with most ships having four or six diesel gensets and a pair of main propulsion motors, sometimes driving through a gearbox but often directly linked to the main propulsors, especially where the thrust is provided by azimuth thrusters rather than conventional propellers.
Multiple gensets allow total power output to be matched to demand with the engines running at optimum speed. When less power is needed, there is no need for all the engines to be running, thus allowing for fuel savings. The number of engines also confers a high degree of redundancy on a diesel-electric ship where even one engine will permit the vessel to maintain some controlled motion in all but the heaviest of seas.
Edda Ferd was the first application of Siemens BlueDrive Plus C concept
The flexibility that diesel-electric propulsion can bring does come at a price, with open seas propulsion efficiency being adversely affected by energy losses in the additional equipment that is necessary. However, for an offshore vessel, where open seas propulsion accounts for perhaps just one-third of the use of the engines, this is much less of a problem because of the optimum running of engines that can be managed at other times. A further benefit comes in the flexibility of layout, as there need not be a clear direct line between engine and propellers. Thus, engines can be located in line of one and another or offset in a layout that can take account of other needs of the vessel.
As diesel-electric has become more popular, so system makers have striven to improve the efficiency and reduce costs simultaneously. The result is a proliferation of concepts from enginemakers and others. Among these are Wärtsilä’s Low Loss Concept (LLC), Siemen’s BlueDrive PlusC, MAN’s EPROX (electric propulsion excellence) and ABB’s Onboard DC Grid.