Using the example of a wind-platform supply ship that has recently been in operation with a variable speed unit, we can give a glimpse of the advantages.
On a typical platform supply ship, four units with a capacity of approximately 2,100 kW each are used to power the vessel. An arrangement of the units with their electrical components is shown in Figure 3.
The high performance that this arrangement can offer is necessary due to redundancy requirements and comes in handy in cases of bad weather – but is not needed for a majority of a ship’s operation time. Having this arrangement in place during typical use for diesel engines results in high proportions of low load phases that result in an average genset utilization of only about 35 percent. This is due to the ship being in a waiting position for extended periods while on-site and having to hold its position.
Figure 1 illustrates this. The time proportion is represented by the size of the circles. These load shares are further clarified by Figure 2.
To see the fuel savings that are possible with a variable speed unit, the following evaluation shows the results of the supply ship operating in the North Sea, powered by four mtu engines of type 16V 4000 M63L (variable speed) from which loads of <25 percent were driven approximately 80 percent of the time.
These operating points are associated with high specific fuel consumption for constant speed-operated units. This is where the decisive advantage of a variable speed unit comes into play: namely the fuel economy. In the present case, this amounts to about 15 percent, which roughly equals € 250,000 per year or, with a term of 20 years, a total savings of about € 4 million.
