posted 07-24-2000 06:33 AM
Hello everyone,In another topic regarding the firm order of the QM2 postponed until mid-august, a talk about azipod propulsion's system has been started with questions from Cambodge, interesting "provocation" :-) from Malcolm (quickly depicting this system as still a problematic one) and an answer from Terry. However, as it is still a not very well known system, I would like to add few precision, explanations and give an additional detailed answer with the wee documentation I have here.
Here are few datas about ELATION / PARADISE and MILLENNIUM pods:
- ELATION: 2 azipods, developed by ABB Marine, adjustable on 360°of 14,000 kw each, running at variable speed (that's why azipod systems and diesel-electric ones in general, needn't variable pitch blades propellers but fix ones, a substantial economy and simplification regarding propellers) between 0 to 146 turns/min.. Of course, to go astern during manoeuvres or to brake the ship, the rotation is reversible, electrically by inverting polarities and 20 secondes are
necessary to go from full ahead to full astern.
The electric motor inside the pod is fed with a tension of 6660 V. The whole pod is 170 tonnes heavy and can operate a 180° turn in 22.5 secondes.
Compared to the traditional inboard elecric engines / shafts of the other ships of the same class (FANTASY...), the general output is improved by 8%. More, they are lot of more manoeuvrable and the gyration's diameter is 35% reduced.
The electric engine and working parts are cooled with air.
We have to notice too, these pods have no fire extinguishing's system. Indeed, in case of fire in the pod, a smoke's detector
stop immediately the ventilation and the resulting absence of oxygen in conjugation with the natural cooling because of the
exterior sea water's circulation would stop flames.
- MILLENNIUM: 2 azipods, Mermaid, design resulting of a collaboration between Alstom and KaMeWa, 19,500 kw
each and a general functioning comparable to the ABB Marine ones above.
To sum up, the main advantages are:
- far best manoeuvre ability, (so economies too. y the way FANTASYs, without pods and with their 3 stern thrusters of 1,5 Mw each, are already able to manoeuvre near a quay without tug's assistance until 35 kts winds and this despite their 7,600 mē of longitudinal sail's surface, so you can imagine to use the whole power - 2 X 14 Mw - as a lateral thrust in case of short emergency with the pod's ships).
- best output so economies of exploitation
- absence of stern thrusters, shafts, less bearings.
- better comfort because of lower level of vibrations.
- better flexibility during the construction since the propulsion's engines can be set after, avoiding few delay's problems and
permitting the continuation of the other rooms (or blocs setting) in the hull.
- gain of volume for the addition of more lucrative spaces for passengers,
- faster and easier maintenance (especially in the case of very serious failure of electric propulsion engine which needs its
replacement in shipyards), so economy of exploitation.
To answer to Cambodge too, the diameter of propellers is indeed about the same than for traditional systems one (but pod system is now the traditional one!), so about 6 meters. As Terry judiciously said too, accidents and risks of damages with pods in case of the ship runs aground are about the same than with any ship. I don't think; in case of very serious damages, it would be more difficult and slow to change a pod than a whole shaft/bearings/supports/watertight
supports/propellers/rudders line, it seems it would rather be the contrary. In the ELATION and PARADISE, the very aft
structure of the hull has been reinforced to support the additional weight and thrust's strength due to the azipods. These pods and their propellers, (as you can see in the MILLENNIUM's picture below), are not under the keel level and so are not more exposed than a shaft system. We can even say, because of the better manoeuvrability and reaction in case of emergency depicted above, risks to occur impacts are theoretically smaller.
To finish, advantages and reliability of pods have been widely demonstrated (a 11,4 Mw azipod had been tested first by ABB with a tanker in -30° C air conditions in very cold waters and that was absolutely conclusive). Recent troubles with PARADISE are not due to pod's principle but because of a defective watertight seal so the sea water entered in the pod. Now, the delay (and so the cancelation of the few next cruises) to repair it is mostly due to delay of livraison of spares (as Terry said) from European firms than a big problem about the principle or the realization and reliability of the pod system itself.
Even better, the MILLENNIUM's problems during her maiden cruise was not due to pods at all and even not because of it
gas-turbine system! The cause was "simply" a break box in the electrical circuit between alternators driven by gas-turbines and the electric engines of the pods. This break box, after the problem was localized, was first bypassed to allow electrical feed of the propulsion engines, and then replaced (or repaired) at Rostock in Germany during an afternoon and an evening "only". The type of breakdown which can arrive with any classical diesel-electric ship.
Of course, other breakdowns with pods (or gas-turbines) can and will probably arrive, but this is the case with any mechanical or electrical device. In about two years of exploitation, ELATION and PARADISE had not occurred particular serious problems with their pods. Same thing with VOTS.
Of course, as I'm interested (and probably other readers) in informations about that, all additional ones are greatly welcomed.
Bye.
[This message has been edited by Vaccaro (edited 07-24-2000).]
posted 07-24-2000 06:33 AM
