Navies not only want more engine power, there are also coming under increasing pressure to become environmentally conscious.
The power and propulsion (P&P) systems for naval vessels have advanced considerably in recent decades. This feature aims to highlight the latest developments in the design and operation of the prime movers in those P&P systems. The prime movers are the gas turbines, diesel engine and steam turbines that convert the energy in fuel into mechanical energy that is used for propulsion or electrical systems. For modern naval vessels the steam turbine has largely been replaced by arrangements of gas turbines (GTs) and diesel engines. Depending on the operating profile of the warship it will need different types of prime movers to generate the necessary power and propulsion to perform its mission sets. Because these systems are so important to a ship’s capability and are in essence its defining characteristic, designs of new ships have to consider the power and propulsion system first and foremost before anything else.
Most naval vessels will use diesel engines to provide standard levels of power for propulsion, sensors, weapons and the ‘hotel load’ to sustain the living conditions of the sailors such as heating/ cooling, lighting, charging, cooking etc. Diesel engines can provide ships with speeds up to a maximum of about 28 knots although they are generally used to provide much lower patrolling or transit propulsion speeds. But frontline warships, especially surface combatants like frigates and destroyers have a specific requirement for high-speed manoeuvres and this requires the ability for a sudden boost in power to achieve this. Adding GTs into a ship’s power and propulsion arrangement offers a capability to provide a surge in power to attain top speeds from 28kts to in excess of 40kts. Typically marine GTs would provide propulsion power from 18-30kts in most frigates and destroyers with less than 18kts powered by direct-drive diesels or diesel gensets via electric motors. Marine GTs have developed considerably over the past 20 years and have become an extremely specialised niche industrial capability. Today the two main providers of GTs for navies are Rolls-Royce and General Electric, which produce the most modern high-powered GTs, the MT30 and LM2500 respectively. Ukraine’s Zorya-Mashproekt also builds marine GTs and since it stopped delivering its products to Russia following the Crimean invasion, Moscow has been developing a new marine GT capability at its aero-GT company NPO Saturn.
Marine GTs have been developed from aircraft gas turbines and the MT30 is one of the most modern on the market. Derived from the Trent 800 aero engine, with 80 percent commonality between them, the MT30 is classed as a fourth generation GT that can produce 36MW of power going to 43MW if required. The reason the MT30 is able to provide such large quantities of power is because the roots of its development lay in the 1980s aviation GT contract arrangement and 1990s computer power. During this period, Rolls-Royce entered into more leasing arrangements for its GT engines with commercial airlines whereby the company retained ownership of the engines while the airlines were charged for the power usage by the hour. This is one of the reasons why GT providers have been so hard hit by the COVID-19 crisis, because their engines are not being used. As this type of contractual arrangement became more common, it meant that the risk of engine reliability was transferred to the owner giving them a huge incentive to conduct analysis of health and usage monitoring of the engine’s components and improve performance through a total care package.
At the same time, by the early to mid-1990s engines were becoming more powerful as aerospace technology advanced through improvements in computer processing and the use of computational fluid dynamics and finite element analysis. The Trent 800 followed from this. Being a large diameter GT engine with a larger air mass flow it was far more powerful with enhancements to the compressor, combustion systems and the turbines, as well as giving higher margins on temperature. The result was that from this period onwards the Boeing 777 aircraft was able to operate with just two engines instead of the conventional four.
According to Richard Partridge, chief of Naval Systems at Rolls-Royce, the MT30 was developed because of the power requirements in the naval market for a much more power dense 36MW GT with higher levels of reliability. The Trent 800 was therefore selected for marine conversion rather than the Trent 700 or 500 because it was able to offer better outcomes. “For the Japan Maritime Self Defense Force’s new Mogami-class (FFM) frigate programme we have actually been able to offer 43MW of power,” Partridge said.
The fact that it is so power dense means that ships can move to a one-GT power and propulsion arrangement alongside the diesel engines instead of two GTs allowing more space and flexibility in design. This was also evident in Korea where the MT30 was adapted to provide power to smaller ships and was selected for the new Republic of Korea Navy Daegu-class (FFX-II) guided missile frigates. Furthermore, the reliability of the MT30 gives it an additional advantage in maintenance as it will never need to taken out of the ship for overhaul offering lower cost of ownership. “The overhaul life is so long, based on typical naval utilisations of 300-600 hours per engine per year. It is literally a sort of fit and forget boost capability and that’s fantastic for the operator in terms of reliability but also through-life costs,” Partridge explained.
Predictions are that the MT30 will have to undertake in excess of 25,000 hours of operations before needing a full overhaul and the lead engine produced has not yet reached that level of use. The MT30’s large diameter core increases the air mass flow, which is then compressed creating a high temperature before fuel is injected and ignited in the combustor at the right air-gas ratio to produce the right amount of highpressure exhaust gas that will turn the turbines. “You can imagine just the centrifugal force trying to pull the engine rotatives apart exacerbated by the heat, but that allows much more efficiency from the turbine than hitherto available,” Partridge said. But it is not just the metrology (the measurements and calibration) that are a factor, there are additional niche technologies implemented to help the system withstand these pressures. While the turbine blades have a thermal barrier coating in order to withstand the heat generated, one of the big differences that gives the GT the ability to function within high temperature margins is the cooling system. “The High Pressure Turbine, which is immediately downstream of the combustor, has a very intricate series of cooling passageways inside each of the rotating blades. It needs those to remove heat from the metal and allow the blades to actually survive in those conditions and to rotate at a very high rpm,” Partridge said.
The level of design detail and the technologies required to build a properly functioning, reliable and efficient gas turbine are considerable. The level of knowledge in GT manufacturing companies has been built up over 70 years making it a high barrier for the entry of new companies into this market. Because of the costs of design and development, it is not often that new GTs can be produced for the naval market unless there is significant demand and a gap or weakness in existing offerings. It is possible to create an even more powerful GT but this would require an increase in complexity and cost. Therefore the products have to match the market requirements whilst remaining competitive.
Meanwhile, although diesel engines are not able to match the power density offered by the GT, there have been improvements that can give warships more power and propulsion capability to sustain higher speeds and larger electrical loads as well as meet tougher environmental regulations. It means that diesel engines are becoming more attractive as an option if a navy is questioning the value of buying GTs. The attractiveness of diesel engines has been helped by the move towards electric power and propulsion over the past two decades that started in the UK in the 1990s. With the inclusion of an electric motor, the diesel engines can be removed from the shaft line allowing more flexibility to meet some mission requirements.
Simon Riddle, general manager, Naval & Research Vessels at Wärtsilä Marine Solutions explains that another benefit is redundancy: “You can have maybe four gensets, and still operate the ship on three, but you’d still be able to maintain the fourth engine in reserve. It also means that you can improve the engine loading when the power demand from the vessel is lower, because you can shuffle the load around from one genset to another,” he explained, “And you have a power management system that makes the loading on the engine more controlled than having a diesel engine running into a conventional gearbox.”
Although naval forces do not prioritise fuel efficiency as much as the commercial market, this is still an important attribute in diesel engine performance. Riddle said that it has been the “biggest driver” behind the development of the new Wärtsilä 31 marine engine that was launched in 2018, which achieved a reduction in throughlife costs.
“The first thing that we do with all engines is ask how can we design them with a reduced number of components. Reduced numbers of components means fewer actual chances of failures. Then we design it to mitigate a lot of service operations on the engine,” Riddle explained.
Ultimately engines become greener by becoming more efficient and using less fuel. Much depends on the speed of the engine – the stroke – but the fuel injection system and turbo-charging methods are also important factors. “When looking at efficiency we are trying to reduce the losses,” Riddle said. “The things we looked at on the Wärtsilä 31 are: combustion shaping, combustion with injection and the use of high maximum cylinder pressure – so we are constantly looking at improving fuel injection technology.”
Wärtsilä is also supplying the CSS with a selective catalytic reduction (SCR) unit, that will reduce the NOx [nitric oxide] emissions of the ship to adhere to International Maritime Organisation (IMO) Tier III regulations. The company provides its own SCR units without using a third-party provider which reduces the need to complete onboard certification whilst the ship is in construction. Marine engine development is a specialist capability and with few providers of both gas turbines and diesel engines competition is tough. Driven by improvements in the commercial sector companies are able to offer more efficient and capable prime movers to their naval customers and give warships the right power and propulsion outfit to suit the mission profile. The Rolls-Royce MT30 has been challenging the GE LM2500 over the past decade in the GT market with its power density and competitive through life maintenance cost reductions. Improved maintenance is also a key driver for Wärtsilä as it improves efficiency with its latest 31 engine offerings.
by Tim Fish