The comprehensive shipbuilding solution

With its innovative propulsion plant, the Nils Holgersson has proven a great success after two years in service.

The 36,468gt RoRo ferry Nils Holgersson, in service since July 2001 on the Baltic between Travemuende (Germany) and Trelleborg (Sweden), has proved to be a great green success for its German owner TT Line.

After more than two years in service, both the Siemens division ?Industrial Solutions and Services? (I&S) and the Hamburg shipowner expressed satisfaction with the concept and the technical solutions incorporated in the ship?s design. The most innovative aspect of the design is the propulsion plant with a pure diesel-electric drive using two Siemens-Schottel 11 MW pods ? the Nils Holgersson being the first RoRo ferry with this particular propulsion plant.

Although the capital cost of this propulsion system is much higher than that of a conventional propulsion plant with the same output, Arndt-Heinrich von Oertzen, managing director of TT Line, pointed out that this was compensated by the larger carrying capacity. The podded propulsors enable naval architects to design ships with larger cargo spaces due to the arrangement of the generating sets in the sidewall structure. Another advantage is that the total fuel consumption of this system is in the order of 15 to 20% lower than comparable vessels with conventional diesel mechanical plants because the five generating sets are always running at optimal power output.

These claims, made by von Oertzen, are based on in-service experience of the company?s ferry fleet. TT Line operate seven similar sized vessels with an average age of only six years, of which four are so-called ?green ships?, two with conventional diesel-electric drives and two with pod drives. Their sulphur-dioxide emissions are 85 % and their NOx emissions 50 % lower than conventional drives while their lower fuel consumption has resulted in lower CO and CO2 emissions. Von Oertzen also stated that the TT Line board of management is very satisfied with the pod propulsion system supplied by Siemens I&S.

The Nils Holgersson is in fact a good example of how comprehensive shipbuilding solutions can strengthen the competitiveness of the shipowner. Part of this approach is the co-operation with Schottel concerning development and marketing of pod drives. For Nils Holgersson and its sister ship, Peter Pan, the scope of supply included, besides the 11 MW type SSP10 pods, the medium voltage system with the 6.6 kV generators, the switchboards, two bow-thrusters, the propulsion control system, alarm and monitoring systems, the low voltage distribution system as well as various auxiliary and ship safety systems.

Axel Rothe, director sales of Siemens Marine Solutions, did not conceal the fact that both these vessels had been delivered late because of problems with the pods. In the first case, water penetrated into the pods during sea trials resulting in essential modifications to the design having to be carried out. This had a knock-on effect on the Peter Pan whose pods also had to be modified.

Part of the propulsor contract was the availability of a fifth pod to avoid long delays in case of a breakdown. This spare pod was in fact used after a bearing breakdown in one of the pods on Nils Holgersson. Within only three days, the replacement pod was fitted at the yard in Bremerhaven and the vessel was back in service after five days. The reason for this breakdown was both simple and potentially dangerous because, during assembly, two sensor cables were allegedly swapped over. This mistake could not be monitored and therefore the crew was not able to detect the lack of oil. Although this was an isolated case, Schottel has modified the design while all pods in service have been checked. Both Siemens and TT Line reported that there have been no other failures of the pods.

Besides the excellent manoeuvrability of a vessel fitted with two pods, there is a safety feature which should not be overlooked. This is the ability for such vessels to make a crash stop in emergencies by rotating the pods at full power output in less than one third of the time needed with conventional propellers resulting in a similar reduction in the stopping distance.

Despite a number of teething problems and high-profile in-service breakdowns for pods in general, these propulsors are gradually becoming more reliable while their efficiency is recognised by both shipowners and crew.