Warships face a unique set of operating conditions and therefore developing gas turbines for them presents several challenges. For instance, surface combatants are subjected to the harsh salt environment and have varying operational load demands. Engines also can be subjected to major shock events while the warship is in harm’s way. It is therefore of vital importance to have the gas turbine always available and operating under these very challenging conditions.
There are more than 1200 LM2500 gas turbine units operating in 35 navies around the world, including with the US, Canadian, Spanish, Australian, Canadian, German, and South Korea navies, among others.
The LM2500 gas turbine is derived from the CF6 aero engine. However, George Awiszus, Director of Military Marketing for GE Marine, explained that there is a key difference between aero engines and industrial and maritime gas turbines.
Aero engines are designed to generate thrust, and as a result, involve a different set of loadings to industrial and maritime turbines. When developing aero engines for use in maritime applications, GE first adapts the engine for use in industrial applications, and when the unit is proven here, it is then developed for use in maritime settings.
Industrial usage gives experience in the modifications, although the changes are often small.
Reliability is the critical issue for gas turbines used in naval ships. A thirdparty track of reliability and availability data has been made through the Operational Reliability Analysis Program (ORAP). This demonstrated that the LM2500 achieved greater than 99 per cent reliability and greater than 98 per cent availability.
The biggest difference between the industrial and the maritime versions is in the package. In a naval environment, space and weight are at a premium. Power density of a gas turbine is a major requirement. Reducing size is important in space-constrained frigates, which need to balance propulsion, weapons and crew accommodations. Awiszus said that the LM2500 has the best in class in power density, traditionally 20 per cent better than the nearest equivalent sized turbine and increasing to 30 per cent.
The reason for the 10 per cent power density improvement has been through using composite materials. These were developed as a collaboration between the US Navy, General Dynamics Bath Iron Works and GE. They developed a new one-piece composite carbon fibre enclosure for the LM2500. It was designed and performance verified in accordance to a complete set of US Navy military and shock specifications.
Replacing steel with this composite in the enclosure has enabled a weight reduction of 2500 kg. Using composite rather than steel walls give better sound attenuation, and the outside of the wall is not as hot, with a surface temperature heat reduction of 15-35°C, making it safer to touch. This is an important consideration when working in the confined space of a ship’s engine room, particularly when manoeuvrability of the ship is likely to cause significant changes in speed and direction, and hence stability of the platform for operators.
Doors into the package made with composites are both larger and lighter, making access significantly easier. In addition, composites do not corrode in salt water, while steel does.
Another important factor required for gas turbines in a naval ship is the ability to withstand shock loading. To test that their gas turbines meet US Navy requirements, GE carries out shock testing in accordance with MIL-S-901. This involves mounting the gas turbine on a barge, which is then floated in a test pond. Then depth charges are detonated at set distances in the water, and the effect on the turbine measured. The turbine continued to operate after the test. This test has been carried out on GE gas turbine equipment four times, and it provides a clear demonstration of the robustness of the turbine under such conditions.
A third issue facing gas turbines in naval applications is that of Foreign Object Damage (FOD).
The LM2500 and LM6000 gas turbine employ a horizontal split casing. This enables it to be opened in-situ, and the blades can often be repaired or replaced in the ship. If the unit did not have this horizontal split casing, the turbine would have to be removed from the ship and sent off for repair, a costly process that could lay-up the ship for an extended period, especially if there is only one gas turbine.
The Royal Canadian Navy (RCN) has 24 LM2500 gas turbines in 12 Halifax-class frigates. Since 2001, GE has provided customised maintenance and logistic support services for this gas turbine fleet under a contract with Public Services and Procurement Canada. The contract has provided the RCN with many benefits, including an availability of the LM2500 fleet of 99.9 per cent over the 17 years of the contract.
The agreement with the Royal Canadian Navy includes:
- Repair, overhaul, and engineering support
- Parts warehousing and inventory management
- Field service representative support (home port and deployed)
- Support of RCN engineering school training curriculum for on-engine and equipment maintenance
- Operational level maintenance
- Configuration management
- Supply and distribution of technical manuals.
In addition, the Royal Australian Navy (RAN) has 28 LM2500 units on its Adelaide and Anzac-class frigates, and its two Landing Helicopter Dock ships HMAS Canberra and HMAS Adelaide. GE provides customised shipboard and depot-level maintenance for these turbines. In addition, the RAN’s new Hobart-class Air Warfare Destroyers (AWD) will also be powered by GE LM2500 gas turbines.
The depot-level maintenance is provided by Air New Zealand, an authorised LM2500 service centre and longtime RAN service provider.
LM2500 marine gas turbine
The LM2500 marine gas turbine is a simple cycle, two-shaft engine. It was derived from GE’s TF39 and CF6-6 aircraft engines.
The LM2500 has four major components: a 16-stage, 18:1 pressure ratio compressor with seven stages of variable stators and inlet guide vanes; a fully annular combustor with externally mounted fuel nozzles; a two-stage, aircooled high-pressure turbine which drives the compressor and the accessorydrive gearbox; and a six-stage, aerodynamically coupled, low-pressure turbine which is driven by the gas generator’s high-energy exhaust gas flow.
Range of turbines
GE has six propulsion gas turbines available, with power outputs from 4.6 MW to 52.7 MW (see Table 1).
This range enables provision of power to ships ranging in size from fast patrol boats, at around 200 tons displacement, through to amphibious ships, of around 45,000 tons displacement.
The Royal Canadian Navy is planning to replace the Iroquois-class destroyer and eventually replace the Halifax-class frigate. The Halifax-class frigates use the LM2500, which has proven to be a reliable and effective unit for the RCN. The fact that the RCN is already familiar with the system simplifies operation of these units, and training of the sailors on the units.
Currently there are two battle-ready designs being offered for the new Canadian surface combatant, built by the world’s leading shipyards, that are powered by GE LM2500 gas turbines:
- The Spanish Navy’s F105 frigate Cristobal Colon, built by Navantia.
- Alion is offering the Dutch De Zeven Provincien-class frigate.
The Navantia and Alion ships each employ two gas turbines, as does the Halifax class frigate. The employment of two gas turbines offers significant benefits, as this enables the ship to achieve greater speed and possess inherent redundancy. Should one gas turbine be lost, for whatever reason, the ship will still be able to operate at mission speeds of 70-80 per cent of maximum speed.
In addition, the United States Navy plans to purchase up to 20 new FFG(X) frigates as successors to the current Littoral Combat Ships (LCS). GE’s LM2500 family of gas turbines meets the requirements of the US Navy. GE currently provides 97 per cent of the commissioned propulsion gas turbines in the US Navy fleet, demonstrating experience at sea.
The US Navy’s request for information for the FFG(X) frigate outlined tiered ranking of key frigate attributes. GE analysed market data to better understand how its engines compare to the needs of past frigates. Amongst the findings were the following:
- Gas turbines make up 80 per cent of the prime mover market, and are used in a variety of configurations, often in combination with diesel engines.
- Gas turbines are preferred when lower weight and volume, higher availability, lower maintenance requirements, and lower noise are needed.
The average top frigate speed is 28- 30 knots, and this can be achieved with a gas turbine of 35 MW or smaller.
The US Navy ranked different attributes into three tiers, reflecting its view of the comparative importance of the attributes:
- First tier (the most important): availability, reliability, service life, and survivability
- Second tier (next in importance): manning accommodation, range, and thermal loads
- Third tier: power density and sustained speed.
The US Navy survivability requirements states the propulsion system are to be Grade A shock tested. The LM2500 gas turbine family are the only gas turbines that have met these shock tests.
Elsewhere, there is also a great deal of interest in the larger power GE LM6000 gas turbine, which received naval certification in 2015 after successfully completing engine and module testing. There are over 1300 industrial engines in service, 17 in marine applications, with over 33 million operating hours. The LM6000 is being proposed for the South Korean FFX Batch 3 frigate. It is being also being considered in a generator set configuration for India’s IAC-2 aircraft carrier, which will leverage GE’s naval generator set packaging expertise.