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The first part of this page is largely derived from information provided by DCN in relation to machinery options for aircraft carriers displacing 40,000 to 60,000 tonnes - in recent years DCN has studied engineering configurations for French, Indian and Argentinian aircraft carriers.
Mechanical SolutionsMechanical engineering solutions consist of a system with two or three independent units, each made up of typically one of two prime movers driving a propeller via a gear reduction system and propeller shaft. Possible combinations range from a homogeneous collection of gas turbines or diesels, to a heterogeneous mix of gas turbines and diesel engines. Common architectures are:
The following diagram shows an example mechanical propulsion architecture with a homogeneous set of gas turbine prime movers driving two shafts via gearbox's.
The ships electricity generation system can consist of diesel alternators or gas turbines alternators.
Electric solutionsElectric propulsion solutions with two or more shafts usually incorporate electric motors which drive a propeller. If an electric motor and a propeller are closely combined in one unit, this is called POD which can be mounted externally to the main hull. The electric motors are supplied by electricity generators which may consist entirely of gas turbines alternators, or be or a combination of diesel alternators and gas turbines alternators. The diagram below shows an example of a "full electric" example of architecture. The "prime movers" are two Rolls Royce WR-21 gas turbine alternator sets rated at 21MW, and four diesels alternators of 5-7MW, divided in to two units each powering a 28MW electric motor which drives a propeller shaft.
Hybrid solutionsHybrid solutions are combine elements of mechanical and electric solutions. An electric motor system drives a propeller via a reducing gear box and shaft. The electric motor is used for low speed. For high speed, a gas turbine is also coupled directly to the gear box. With hybrid solutions, the generating plant is usually made up of gas turbine and diesel alternators. Other hybrid architectures include pods and one or more conventional shafts (BAE Systems has proposed this for CVF at one stage). The diagram below gives an example of a hybrid architecture.
Steam SolutionsThese solutions consist of propeller shafts driven by steam turbines. The necessary steam is produced by either oil-fired boilers or by nuclear reactors. The ships generating plant is usually a combination of steam turbo-alternators and diesel alternators. Sizing the ships propulsion systemA CTOL aircraft carriers propulsion system must be powerful enough that it is possible for the ship to reach the speed necessary to catapult the aircraft embarked under the most constraining environmental conditions (no wind, high air temperature). This speed depends upon both the performances of the plane and that of the the catapults. A second sizing criterion is the speed attainable in a damaged condition. If damage occurs to a propeller shaft, it is essential that the aircraft carrier is still able to reach sufficient speed to allow the aircraft to land back on. Other operational criteria that affect the choice and power of an aircraft carriers propulsion plant include endurance, the range achievable at given speeds without refuelling or replenishing, the ability to sail at low speeds for significant periods of time, and stopping distance (capacity to stop the ship within a given distance). Selection Criteria Clearly several very different solutions exist for powering a medium size aircraft carrier.. The ultimate choice selected depends upon the importance the user gives to one or other of the criteria. The criteria to be considered can be gathered in to several families: operational criteria, impact upon the design of the aircraft carrier and its dimensions, economic criteria, industrial criteria. The operational criteria are numerous. In addition to the obvious ones related to speed (maximum, maximum sustainable, cruising, ...) and and power, there is the flexibility. This is characterized by the ability to vary the speed of the aircraft carrier without requiring important changes in the configuration of the propulsion machinery or generating plant. (Example: in a "full electric" architecture, a large variation in speed may result starting or in stopping one or more additional sources of power). This ability to support large variations of speed is an asset for an aircraft carrier whose mission profile typically includes frequent variations of speed during aircraft operations. Military related requirements such as the effect of shock, , acoustic noise, and survivability, etc must also be analyzed. Common sense dictates that high value unit of the surface flotilla, with a payload of expensive aircraft, which may be carrying tactical nuclear weapon, and which will have a crew and airgroup of at least 1200 people, must have robust engineering and propulsion, which is resistant to a significant degrees of combat damage. The type of propulsion system preferred will have a direct impact on the design of the aircraft carrier. For example, exhaust uptakes and funnels may dictate position of the island, or even require the presence of two separate islands above widely separated engine rooms. Fuel consumption will require more or less internal volume being taken up by fuel bunkerage for a given endurance and range. Lastly, and it is a major consideration for CTOL carriers, the choice of a non-steam propulsion system requires the development and fitting of boilers specifically for the steam supply to the catapults, although the emergence of electro-magnetic catapults may remove this requirement in the next 10-15 years.. Economic criteria take into account the total cost of ownership cost i.e. not only the cost of acquisition but also the costs of operation and maintenance. Industrial criteria should take into account the maturity of the
solution suggested, its expected lifespan (a modern aircraft carrier is
expected to last 30-50 years!), and emerging solutions -
CVF PropulsionSince the CVF project was formally initiated in 1999, Integrated Full Electric Propulsion (IFEP) has been a front runner. IFEP theoretically offers considerable advantages over current generation propulsion systems including: no mechanical gearing, flexibility in component location, potential economies in fuel consumption, manning and maintenance. A BAE spokesman noted in March 2002: "Because of the demands for power in a carrier this size, full electric propulsion provides the maximum flexibility and survivability. There is no need for a large engine room - you can site the turbines around the ship". IFEP also helps with damage control by eliminating vulnerable drive shafts and gearboxes. Disadvantages include: higher initial costs, some very heavy and bulky equipment, increased space requirements, greater complexity. Overall its considered that adopting IFEP for the CVF will offer the Royal Navy massive benefits, most particularly in terms of through life costs and range / endurance - both big issues for the RN, but only if the MOD is willing to accept the prerequisite increase in hull size and the higher initial procurement costs, which is perhaps becoming increasingly doubtful. Since the late 1990's it has been expected that the core of any IFEP system for the CVF's was likely to be the 25MW inter-cooled, recuperated Northrop Grumman/Rolls Royce WR21 gas turbine. (In 2000 the Royal Navy selected a de-rated 21.5MW variant of the WR-21 ICR for its new Type 45 destroyers). This recycles hot exhaust gases both to reduce the IR signature and to provide fairly uniform fuel consumption at high or low power. However alternatives have always been considered, the 25MW General Electric LM2500+ gas turbine (preferred by the French) and more recently the 36MW Rolls Royce Marine Trent. Early studies showed that four WR-21's in an IFEP configuration could propel a 30-40,000 tonnes CVF at a maximum speed of 30 knots. However as CVF grew in size, speed inevitably dropped. During 2002, with the BAE and Thales teams now considering CVF concepts of 60,000 tonnes or more, the new Rolls-Royce Marine Trent MT30 became increasingly favoured over the WR21. Because of it's significantly greater power output (36MW compared with 25MW), fewer GTA packages are required for achieving the required generating power.
Indeed despite being 45% more powerful than the WR21,
the MT30 unit actually requires far less volume and weighs much less
than the WR21 when the latter's complex intercooled and recuperating
system is included. Also, although no figures are published, it is
likely that the MT30 costs less to procure, although over a 20+ year life cycle the higher fuel efficiency of the WR21 starts to
tell in terms of saving on fuel cost and overall through life costs.
The Thales consortium quickly committed to IFEP for CVF and they brought Alstom on board as part of their team to provide expertise in electric propulsion and power distribution, bringing to bear their experience on Type 45 destroyers and elsewhere. Thales' noted that the adoption of an IFEP system would allow the ship's electrical generators to be dispersed throughout the vessel giving improved survivability. IFEP also opened the way to using podded drive propulsors ("pods"), a technology now common in modern cruise ships such as the 82,000 tonnes displacement (150,000 GRT) Queen Mary 2. In conventional systems, electric motors are located inside the ship's hull. With the new system, the motors are installed in pods fastened to the hull, which eliminates long shaft-lines. Each pod includes a propeller. The pods can rotate a full 360°, so they do not only propel the ship - they handle manoeuvring as well. A ship can have two or more pods. One reason why pods were considered by both teams for the future carrier is to reduce the risk of delays and problems during build. In a 'shafted' solution, the shaft itself, the bearings, thrust blocks and other components have to be put into the ship early on. But instead, podded propulsion units can be added in just days at a later phase in the shipbuilding programme. Each pod can be fitted in five days, and this can be done at the end of the build. Ship maintenance is easier, since the pods can be mounted and removed without moving the ship to a dry dock. Using propulsion pods free up a large amount of space inside the hull, and the pods are quieter and generate less vibration. Furthermore, podded propulsion also improves hydrodynamic efficiency by up to 10% (thus reducing fuel consumption) and manoeuvrability in confined waters and berthing.
"Pods are a proven commercial solution and they offer us a lot of advantages, notably manoeuvrability and the flexibility they bring to the shipbuilding programme," said Thales' Robertson in 2002. "But there are some outstanding shock and signature issues." BAE Systems took a similar view. "We are looking closely at podded propulsion because it holds a number of attractions," explains Chief Engineer Scott Whiteford. "But it is a technology not yet proven for the military environment. One option we are considering is a hybrid arrangement with a conventional centre shaftline and two podded drives." In its final AP2 proposals submitted in November 2002, Thales baselined a power train of 4 x 36MW Rolls-Royce MT30 gas turbine alternators (31MW electrical output) plus 5MW service diesel generators. The ships generators were capable of 150MW in total, and were connected to the ship's systems by 2,000km of cable! In the proposal, the CVF was fitted with a military version of the Mermaid pod unit similar to that installed in the Queen Mary 2. The design had four 21.5MW pods totalling 85MW, each in a hydrodynamically optimised azimuthing body housing an electric motor in a "push" configuration. The pod structure and prop-blade would be built by Rolls-Royce, and Alstom would supply the electrics. Use of IFEP allowed the generator sets to be placed where ever the designers chose. "The reason we went for [pods] is they save us space within the ships, and for an aircraft carrier you can use them to manoeuvre more easily. Each pod steers in 180 degrees and with the bow thrusters the vessel can self-berth." said Simon Knight of BMT Defence Services after the CVF design he had helped develop was selected in early 2003. "The principal worry with pod engines is the shock loading." Pod engines have not shown a great resistance to shock. The effect has been to move the shaft out of alignment within the pod so that it comes into contact with other parts of the engine, causing the bearings to fail. In the commercial market pod engines have used conventional roller bearings. To improve the shock resistance the Future Carrier Alliance has gone for sleeve bearings that offer greater and more constant contact within the bearing itself. It is hard to translate the 85MW aggregate rating of the pods in to a traditional "shp" rating or an estimated speed based only upon the published data because this is their power consumption not output, combined with the hydrodynamic efficiency associated with pods (a reduction of 5-7% in drag is common), and also the slightly less than optimal hull form planned for CVF. The final Thales proposal had a designed maximum speed of 28 knots on the equivalent to roughly 100,000 shp, by way of comparison, the old 50,000 tons (not tonnes) HMS Ark Royal [IV] had 113.4MW (152,500shp) on 4 shafts, for a maximum speed of 30.5kts Meanwhile the BAE Systems led CVF design had slightly different ideas. Rolls-Royce, who was an active part of BAE team and so were probably not giving Thales all its ideas, suggested 2 x 25MW WR21-ICR GTA's (21MW output), plus 2 or 3 MT30 GTA packages for high speed boost, powering a centreline shaft with wing pods. With this configuration, the new carriers could normally cruise on the two very fuel efficient WR21's, but kick in powerful MT30's as "boost" when higher speeds were required. The partial commonality with the WR-21 powered T45's for spares and logistical support was also considered to be a significant advantage. Although the Thales CVF design was selected over the BAE
design in January 2003, the Royal Navy liked the laters propulsion
arrangements and in February 2003 asked if the Thales/BMT
CVF design could be similarly modified by the Future Carrier Alliance,
working with Rolls-Royce and Alstom. The optimum location for the position of the main propulsion system was carefully examined in early CVF studies, with the need to maximize the hangar space below decks a major consideration. The gas turbine generator units could be mounted in the superstructure, this would require a large island and reduce the flight deck area, but by avoiding volumous air intake/venting trunking to low machinery spaces will enable a larger and wider hanger. The comparative advantages of the two layouts was extensively debated within the DPA and the two competing industrial teams, but operational analysis and aviation generation studies demonstrated that the extra flight deck space associated with a small island(s) would be more valuable than the extra hanger space, so traditional main hull located engine rooms were selected. In the chosen Thales design, the two engine room units are widely separated, each one directly below an island to minimise the length of air downtakes and exhaust uptakes while offering good damage control. This arrangement is possible thanks to the flexibility of IFEP and propulsion pods. Battery's and several large diesel generators will provide emergency power if the prime movers fail for any reason. In June 2003, the DPA asked the Aircraft Carrier Team to look at smaller CVF design in order to reduce costs, and changes to the engineering were also considered. One option was reverting to WR21 gas turbine's, but it was decided that dropping one of the MT30's was more cost-effective. However it was recognised that with just two powerful gas turbines as the only prime movers, the CVF would be very restricted as to its one G-T cruising speed "window" for achieving maximum endurance, and the optimum cruising speed would not necessarily suit accompanying ships. Also, aircraft carriers frequently change speed while operating aircraft and having to constantly double or half the power available is very uneconomic, another problems is that the loss of just one gas-turbine for any reason might prevent the carrier operating heavily loaded aircraft in low wind conditions. It was also found that considerable benefits were accrued by adding diesels generators to the remaining gas turbines in an arrangement known as COmbined Diesel-Electric and Gas Turbine - CODLAG. The specific fuel oil consumption (SFOC) of a typical large gas turbine is much higher than that of a medium-speed diesel engine, which is very fuel efficient through a wide range of power output. Gas Turbines have reasonably good specific fuel consumption (i.e. fuel efficiency) only when working near their maximum power output, consumption is particularly high at part load and typically exceeds 400 g/kWh as the load drops below 20%. At maximum load, the consumption is still 207 g/kWh even for the modern and very efficient MT30. By comparison with gas-turbines, the consumption of a large medium-speed diesel engine is extraordinarily frugal, below 175 g/kWh at high load. Nor is the consumption of a diesel engine as load-dependent as that of the gas turbine; it increases only by about 30 g/kWh when the load is dropped to 20 %. This large difference in SFOC between the gas turbine and the diesel engine gives rise to a typical characteristic of a CODLAG machinery - the fuel consumption is very low when the ship is operating on diesel engines alone, but increases rapidly when the gas turbine(s) are started. Lower fuel consumption was not the only cost factor favouring the addition of diesel engines. Gas turbines also demand high-quality fuel such as marine gas oil (MGO), which is significantly more expensive than heavy fuel oil (HFO), normally used in diesel engines. Indeed, the price of MGO can be twice as high as HFO. Adding diesels to the machinery mix for CVF offered a compromise between low weight and space demand for high power that are best met by gas-turbines, and the good economy obtainable from a diesel-electric plant. There are various ways to operate a CODLAG ship, but the most economical way is to use the diesel engines as much as possible and only use the gas turbine as a booster unit when the power demand exceeds the available diesel output. Installing diesels in the CVF's mainly for cruising, boosted by powerful gas turbines for higher speeds (e.g. when operating aircraft), gives the command and far more flexibility when determining the carriers speed compared with having just two large gas turbine alternators as the prime movers. It also enables the engines to run closer to their optimum load, which in turn improves the fuel economy. The solution is also more resilient due to having more prime movers, and its also thus far easier for engineering staff to perform maintenance on one engine without affecting the available power too much. But unfortunately this new hybrid CODLAG arrangement, plus the use of pods and full or integrated electric propulsion, was relatively expensive - a major concern for a project projected to be far over budget.. Thus during the Autumn of 2003 the Future Carrier Alliance, in conjunction with Rolls-Royce, carefully considered against the baseline the cost-benefits of several more conventional options that offered immediate savings in money, and possibly reduced weights and volume requirements:
By October 2003 thinking favoured adopting option (A) for the
smaller CVF design concepts being considered at the time. A decisive
factor was the
intense pressure to cut project costs, (A)
was the cheapest of the electric propulsion options considered in
terms of initial build cost. T B
Brochures From Rolls Royce:
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© 2004-8 Richard Beedall unless otherwise indicated. |
