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Systems Articles (2) |
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From time to time, this page will be liven up with general description articles regarding various systems, functioning, technic or design we can find on modern cruise ships. Since these articles will be available in the forum too (but without images), you're free to react, to add your own commentaries, precision, or to add your experience and knowledge. If you would like a specific system to be treated here, simply ask it directly through the forum or e-mail me, I'll try to do it, with the help of other readers if they want.
Articles yet available:
- Gas-turbine propulsion system
Gas-turbine propulsion system:
MILLENNIUM is the first large cruise ship to use a new propulsion's system: Electric motors feed by alternators driven by combined gas turbine and steam turbine. This system could interest few ones since it will be more and more used in the next buildings. This is an innovative technical feature and, besides marketing, economy of scale, new activities available, etc..., propulsion's technic represents an important element of the product cruise and of course cruise ship design, the main purpose of this site.
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Here,s a picture of one of the two turbines onboard the ship (source: Chantiers de l'Atlantique). |
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The principal advantages of this system are: - lower costs of exploitation (lower and easier maintenance) - reduction of nocive emissions (partially because gas oil instead of fuel; -80% oxide of azote and -90% oxide of sulphur). - gain of volume and weight considerable (especially combined with Azipods; about 900 tons and 50 pax cabins + 20 crew's cabins have been added). - lower noise and vibrations level, so better comfort and lower probability of failure for several equipments). Here's a brief working of the system: Electric power, for propulsion and other needs of the board, is produced by combined cycle (COGES type): gas turbines and steam turbines. The two main alternators (25 MW, 3,600 tr/min) are driven by two gas turbines type LM2500+ built by General Electric. They are stem from large aircraft ones. Each gas turbine is equipped with a recuperative boiler (recuperation of the heat issuing of the combustion of gas in the turbine) which produces the necessary steam to drive a steam turbine (one for the 2 gas turbines) used to drive 9MW alternator. The thermic output is then 43% instead of 39% with gas turbine only. The previous version of this gas turbine model, the General Electric LM2500 is available for a long time onboard US Navy ships and other Navies (the LM2500+ has a higher power of 25%). They are very reliable turbines, only one serious breakdown for 48,800 hours functioning according statistics. For Celebrity, these 48,800 hours would represent 10 years of commercial exploitation of the MILLENNIUM. More, in case of failure, a gas turbine could be able to be replaced in 8 hours only. (a spare one is stored onboard). To obtain the same final power, lot of heavier and bigger Diesel Engines would be necessary, with higher maintenance costs and higher noise and vibrations. Here, it's interesting to notice, gas turbines are (at this time) only interesting in high speed vessels (like warships or quite fast passenger ships - max speed of 25kts and service speed of 24 kts for MILLENNIUM). This is due to higher price of gasoil instead of fuel for diesel engines and better output of diesels in lower speed too. gas turbine systems, since they use gasoil, has the advantage to avoid pre-heating systems necessary for fuel in classic installation (which must be at 130°C before going within diesels), so a risk of fire. A quantity 1000 less important of lubrication's oil is necessary too, (so its treatment and elimination of residues too). Four "small" classic Diesel/alternators are available too for alternative/spare electric power production and sometimes for very low speed/mooring periods.
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You can see the location of one of the gas turbine onboard MILLENNIUM shown by the red arrow I added on the following picture. (source: Chantiers de l'Atlantique). |
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Besides the 3 other MILLENNIUM class ships ordered in Chantiers de l'Atlantique, 6 "VANTAGE" class ships, (stem from MILLENNIUM ones) using similar gas turbine system are planned at Meyer Werft for RCI/Celebrity group. (the RADIANCE series) That is quite a turn in the search of "always lower costs for companies and lower prices for customers" cruise product. And as this system presents several other advantages such ecology or comfort...
To complete this article, here are other types of systems using gas turbines available in ship's industry: I'll try to keep a as succinct description as above and not add invading details. - A COGAS (or COSAG) system uses gas turbines (combustion of the gasoil using the same basic principle than combustion of the kerosen for aircrafts) and geared steam turbines ( using a steam produced by a classical boiler system using gasoil or fuel). Each sort of turbines (gas or steam) work separately and they drive shaft(s) through a gears. Alternator(s) are usually mated to the shaft(s) or gear(s) to produce the necessary electric power for the board (but not for the propulsion). COSAG systems were only used on British military ships "TRIBAL", "COUNTRY" class and BRISTOL entering in service between 1961 and 1973. - A COGAD (or CODAG) system, uses classical diesel engines instead of steam turbines in the example above (COGAS) and diesel engines or gas turbines can work separately too (and sometimes drive separated shafts too). Diesel engines are used at low speed or at mooring and gas turbines for high speeds. CODAG systems were used for the first time for large vessels by Soviets (and they were the pioneers for gas turbine systems application in large vessels) from 1961 for their "PETYA", "POTI", and other class later. The first US Navy ship to use this system was the "RELIANCE" class (Coast Guard) launched on 03-25-1963. - A COGAG system is a combination of several gas turbines only. The first large vessels to use it was the Soviet "KASHIN" class in 1964. Derived combinations of different powers gas turbines (COGOG) or with Diesel engines (COGOD or CODOG) are available too. The first large passenger vessel with COGAG system was, the car-ferry FINNJET of May 1977 which can link the 660 nautical miles between Finland and Germany in 22h30min at the average speed of 30.5 kts. (24,065GRT, 212.8m X 25.4m; 2 Pratt & Whitney gas turbines of 75,000 HP, stemed from DC10 aircraft). By the way, she's no longer a COGAG ship since the 1981's refit because, due to economical recession in Europe, two 15,500 HP Wartsila Diesel engines were added (areas on the main car deck were sacrificed) to reach 18-20 kts and do the same trip in 38 hours during off-peak saison. From these explanations and the ones above, we can see MILLENNIUM doesn't use a classical COSAG system for two reasons. - First, she does not have primary boiler(s)/steam turbine(s) system but only primary gas turbine system and THEN, the heat issued of the combustion of gasoil in the two gas turbines is recuperated to heat a boiler which produces steam for a third smaller steam turbine. This steam turbine is only here to improve the output (43% instead of 39%) of the whole system. This is in fact a sort of COGAG system which is improved and not a COSAG one. - Secondly, compared to a traditional COSAG system, turbines don't drive shafts (through gears) and then propellers directly but alternators to feed the two electric Mairmaid pods. MILLENNIUM is the pioneer with that system, even if other gas turbine solutions (CODAG for example) will be used by other companies. The 40-year long use of gas turbine systems onboard warships in many navies worldwide shows us the reliability of this system despite the facts problems can always happen, but this is the case with "traditional" systems too. |
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