Silent running: How do you navalise a variable speed drive for a modern warship?
The Royal Navy's latest large procurement project, the Type 26 anti-submarine frigate, is proceeding well with many of the prime contractors being announced. One of the most fundamental is its propulsion system that has been awarded to General Electric's Power Conversion's naval business.
There are many challenges in providing propulsion on modern marine vessels, not least is one that fundamentally comes under the ambit of being a warship. The Type 26's primary role is searching for submarines, as well as a secondary role as a general support and humanitarian vessel. It means any propulsion system has to offer speed and efficiency, as well as near total silence – though not all at the same time.
The Type 26 is slightly larger than the Type 23 Frigate it is replacing, with a basic displacement of around 6500 tonnes. It is also slightly longer at 150m, and due to its varied multi-mission role the range of the Type 26 was one of the key drivers during its design, some 7000 nautical miles.
For this reason the ship uses a fairly well known 'hybrid' configuration in the marine industry known as combined diesel electric or gas (CODLOG). The reality for the Type 26 is that it will use a single large gas turbine that will directly drive both the ship's shaft lines via a splitting gearbox, in combination with four diesel generators. These will produce electricity and in turn power General Electric motors to drive the ships propellers at lower speeds and for near silent operation.
"When we are running in stealth mode the propellers do not run on batteries," said Paul English, marine business leader for GE Power Conversion. "The diesel generators keep running and produce the power to the propellers and the rest of the ship. So the noise of these engines is isolated by putting them on acoustic mounts and in an acoustic enclosure to reduce the airborne noise."
The gas turbines and the diesel generators will both use the same single onboard fuel – the NATO designated Dieso. While still broadly considered a distillate light fuel, it is slightly heavier than the diesel most of us are used to at service station pumps. However, it also has a much higher flash point, an obvious advantage for a ship likely to see combat during its service life.
For higher speeds, the ship uses the gas turbine. "In this mode the gas turbine drives through a splitting gearbox, and then into a second reduction gearbox, which then drives the shafts and propeller."
However, a gas turbine whirring away might well be an efficient and effective way of producing power and shifting the Type 26 to its top speed in excess of 28knots (32mph / 52kph), but when it needs to remain quiet and locate enemy submarines, its diesel generators kick in to enable near silent running. One of the key technologies enabling this propulsion system is its use of Variable Speed Drive (VSD) technology.
"When the ship is operating quietly the gas turbine and subsequent gearboxes shut down so to eliminate all the mechanical noise from those pieces of equipment," said English. "The propellers are then turned by the ultra-quiet GE propulsion motors, using electricity produced by up to four diesel generators. The motors receive their electricity from a combination of VSDs. A VSD is basically a frequency converter that controls the frequency it sends to the propulsion motor. We need to do this since the diesel generators run at a constant 'mains' frequency (60Hz), which is fine for the normal equipment on the ship - like the pumps, as their electric motors only need to work at one speed. But, to control the speed of the ship through the water we need to be able to vary the shaft speed, so we obviously need to be able to change the frequency we give to the propulsion motor.
"There are all sorts of ways of doing that, the Type 23 that this is replacing does it by using direct DC voltage to give us variable speed control, but the modern way of doing it is to modify the fixed AC supply waveform to one of a different frequency for the motor, via an initial conversion to a direct current. This is done by a technique called Pulse Width Modulation (PWM). We're already well in to the design phase for the motors and converters, with the motors being designed at the moment and the VSD technology being based on commercial equipment."
The VSDs are controlled by computer to create variable frequencies that enable the speed control of the propeller.
"The greater the frequency out, the faster the motor will go," said English. "Conversely the lower the frequency the slower the motor will go.
"It is based on our standard MV3000 range of marinised drives. We produce hundreds of these for use in commercial shipping and the core electronics are the same, but we have to navalise it."
Given the nature of the Type 26's primary role as an anti-submarine vessel, the drives have to ensure that the electrical waveform produced has very little noise and distortion, as any distorted waves going in to the motor will cause vibration and radiate noise in to the water.
This is achieved using a variety of technologies, including filtering techniques and the use of special PWM strategies to smooth the input to the motor ultimately turning the ships' propellers.
"The idea is it has to be very quiet as it is an anti-submarine frigate," said English. "So we put in a huge effort to reduce radiated noise from the ship to enable it to operate very effectively in that environment.
"And like much of the ship's components, generally, the VSDs need to be made more robust and shock hardened. If a ship suffers an explosion, for example, the VSDs need to be designed to survive. We do that by optimising their design using advanced dynamic computer modelling as well as simple techniques like putting it on specific mounts and surrounding it in a strong frame."
Fundamental to a low noise signature is the design of the motor. The motors, currently under design and development, must be carefully engineered to ensure that they generate a minimum of harmonics and to ensure the maximum of attenuation in the noise conduction paths. This work requires a great deal of computer modelling and the application of many years of data gathered from a large number of different noise quiet motor designs. There are very few companies in the world that have this capability and no other company has supplied more noise quiet, shock proof motors to the surface fleets of western Navies than GE.
By integrating gas turbines with an enhanced electric propulsion system, the Type 26 will be more efficient and have reduced fuel consumption compared to its predecessor as it is able to configure its electric propulsion system for a wider range of operational demands and over a wider speed range. The integration engineering to deliver a package that works, rather than just a collection of equipment, is key to this and integration is an area where GE Power Conversion has strength in depth; not just in the Naval arena but also in the wide range of commercial electric propulsion packages it produces.
And this sits well with General Electric's larger marine business. As the International Maritime Organization increasingly look to introduce guidelines around ship efficiency and in particular with CO2 legislation becoming an increasing possibility in many regions, the integration of both gas turbine and electric propulsion technology, in a hybrid arrangement is likely to become much more prevalent in the civil marine industry.
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