Renk studies optimum AWD propulsion system

Selection of the propulsion system for the Air Warfare Destroyer will be amongst the most critical decisions for the project design team, and will need to take place early in the evolution of the design. The considerations of this decision revolve around the role of the ship, and the speed and range it will be required to have.

There are always design trade-offs to be considered, not the least of which is cost. But in the formative stages of deciding on the best answer, the designers will have to deal with the issue of maximising the volume of the ship for its combat system and weapons payload, while enabling the critical issues of speed and range to be met.

Complicating this further is the need to be able to arrange a propulsion system in such a manner as to minimise blast and shock damage that may be sustained during operations.

Commodore Andrew Cawley in the December 2005/January 2006 edition of ADM has already indicated his understanding of those operational performance issues to be confronted by the RAN when he said '...we know it's going to have a combination of diesel-gas turbine propulsion system'. This understanding mirrors those of several countries now building modern warships which also have this interrelated problem of maximising operational payload, as well as the endurance of the ship.

Whereas the USN DDG-51 has an operational range in the order of 4,000nm, the RAN requirement is for 5,000nm, and no doubt more would be better, provided the weapons and combat system payload is not affected.

Using performance envelope information available in the public domain and its own experience with highly sophisticated Naval propulsion systems, Renk AG has applied its considerable expertise in estimating what could be the solution the project is seeking.

The company has been able to show how, in the Australian context, the in-service speed profile of the USN DDG-51, estimated during design and found in real operation as shown in Figure 1 could be satisfactorily met with a combined diesel and gas propulsion arrangement (CODAG) driving two shafts through a very modern gear system.

Figure 1 also shows that, based on the "one diesel engine" and "two diesel engine mode" approximately 85% of the total time underway could be operated with one or two diesel engines, which gives an overall advantage with regard to fuel oil consumption and the fuel oil bunker capacity aboard the vessel.

The range of 5000nm is comfortably achieved while giving an excellent ability to configure the propulsion system to maximise the life of the engines and their associated systems.

The power required to meet the speed requirements of a ty
Speed is not the only consideration; in the Australian context there is the need for a very healthy maximum range by minimising fuel consumption. Renk's calculations show that there is a significant fuel penalty of having only a gas turbine configuration COGAG (orange line in Figure 3), and that for the AWD a Combined Diesel And Gas (CODAG) configuration with an automatic cross connecting gear system as shown by the green line in Figure 3 would deliver the required megawatts at the lowest possible fuel consumption.

The RAN has experience using CODOG (Combined Diesel Or Gas turbine) with its Anzac Frigates, although those ships have a manual clutch system of transitioning from "one diesel mode" to a two diesel mode and a separate gas turbine mode and back again.

Renk's PLC controlled clutch arrangements enable a completely flexible selection of propulsion mode and also avoids the risk of mis-managing the system in a combined gas turbine and diesel engine mode.

Figure 4 shows the Factory Acceptance Test (FAT) test run of the CODAG arrangement at the Renk test bed in Augsburg. This is the second of a series of eight ship sets which Renk is delivering to the US Coast Guard for the National Security Cutter vessel as part of the Deep Water Program. This gear box plant has been designed in close cooperation with the US Coast Guard and the US building yard based on Renk's experience with the CODAG arrangement for the German F124 Frigate, which has now more than 5,000 hours of experience in service.

Deriving information from the DDG 51 design, Renk estimates that a single gas turbine and two diesel engines in a CODAG arrangement will deliver the optimum performance and fuel consumption for the AWD, and particularly enable maximum service life of the propulsion system to be realised through sharing of engine time between the two diesels, which can each drive one or both propeller shafts as shown in Figure 5.

Being able to arrange the propulsion system in a manner that minimises risk from shock and battle damage is of great importance in a warship and Figure 6 shows the proposed AWD CODAG arrangement suggesting the position of the water tight bulkheads between the compartments.

This gear system is able to be designed for emergency operation even when submerged by sea water up to 6 metres above the propeller shafts. The wide range of casualty modes of operation of this CODAG system is shown in the following table.

Type of Damage
Remaining Operating Modes
one diesel inoperable
other diesel engine operating with or without gas turbine on both propellers
two diesels inoperable
gas turbine driving one or two propeller shafts
gas turbine inoperable
either or both diesels on both propellers
cross connect gear inoperable
each diesel drives its own propeller shaft
one main gear and cross connect gear inoperable
diesel on other main gear and referring shaft
gear compartment flooded
all modes possible
diesel engine compartment flooded
gas turbine driving one or two propeller shafts
gas turbine compartment flooded
either or both diesels on both propellers

In summary, a critical evaluation of the AWD requirement by Renk confirms that a combined diesel engine and gas turbine driven system is the optimum solution for the AWD's requirements for range and speed. A CODAG power plant optimally matched with Renk's cross connect gearing system would give the RAN the most survivable and operationally flexible propulsion system possible.

By Gregor Ferguson, Adelaide