PRACE NAUKOWE POLITECHNIKI WARSZAWSKIEJ z. 95 Transport 2013
Lech Kobyli ski
Foundation for Safety of Navigation and Environment Protection
PROBLEMS OF HANDLING SHIPS EQUIPPED WITH AZIPOD PROPULSION SYSTEMS
The manuscript delivered, March 2013
Abstract: Large ships, mainly large cruise vessels, built during last two decades are quite often equipped with revolutionary propulsion devices known under the name AZIPODs. There are many reasons for choosing AZIPODs as main propulsion units, the main reason being excellent manoeuvring characteristics achieved. However in case of large propulsion units, having power of 15-25 MW, used for propulsion there are also some disadvantages and limitations, the last mainly related to operational factors. Handling of ships equipped with AZIPODS is different from handling conventional ships and in certain manoeuvring situations safety of the ship and of the propulsion units might be endangered.. Therefore some limitations imposed on handling procedures are necessary and it is essential that pilots and masters of ships fitted with AZIPODs must be specially trained. Keywords: ship manoeuvrability, podded propulsion, handling of ships with Azipods
1. INTRODUCTION
During recent times the new type of vessel has been introduced to the world shipping fleet in large numbers. This type is passenger cruise vessel. Statistics of ships in operation and on order reveals that more than 750 ships of different size that could be defined as cruise ships are recently in operation, more than about 50 of them are large ships of more than 250 metres in length and carrying sometimes as much as 6000 passengers. According to the definition cruise ship is usually a very large passenger ship that makes a roundtrip with several en route stops and takes passengers only at the port where trip begins and ends. This is not very strict definition but it makes clear that the passengers are holidaymakers and the main purpose of the ship is not to make fast trip but rather to assure pleasure spending time visiting several interesting places. This is fast developing business and the number of very large cruise ships in operation since the last decade of the last century is increasing rapidly. This is really type of ship of XXI century. Typical large cruise ship is shown in fig. 1.
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Fig. 1. Typical large modern cruise ship [Wikipedia, Cruise Ships]
It is essential that cruise ships must have excellent manoeuvring characteristics especially at low speed, because they are supposed to call quite often at small ports in remote places where there are no facilities such as deep berthing quays, powerful tugs, etc. Because of that, they are mostly fitted with azimuthing propellers that make possible to manoeuvre the ship effectively in a very confined area.
2. AZIMUTHING PROPELLERS
The idea of azimuthing propulsion units is that they are used at the same time as propulsion devices and as rudders. Azimuthing propulsion devices are known for quite a long time. The most popular azimuthing propulsion device is outboard motor used for many years for driving small boats and yachts. This idea was later followed by installing propulsion motor inside the hull with Z-form transmission where the propeller could be rotated around vertical axis. Later-on this type of propulsion device developed into Schottel propeller allowing to install higher power. In the early thirties of the last century cycloidal propellers were invented and manufactured, the main representative of which was Voith-Schneider propeller, rotating around vertical axis and with several vertical blades, the angle of attack of which is controlled by special mechanism. Azimuthing propulsion devices used for propulsion of large cruise ships are at present known under patented name AZIPODs, where the electric propulsion motor is contained in a rotatable housing or nacelle submerged under water. Opposite to the azimuthing propellers mentioned above, that usually were of rather small power not exceeding several hundreds kW, the AZPODs are of rather high power, of the order 14 to 25 MW and Problems of Handling Ships Equipped with AZIPOD Propulsion Systems 233 because of that, operation of ships fitted with such high powered propulsion units poses certain problems. The application of this type of propulsion devices is considered here. AZIPODs are characterized by two main qualities: Electric motor is located inside a hydrodynamic optimized submerged housing The total unit is rotated with the propeller(s) by 360 degree rotation providing large turning force in any direction without necessity of installation of rudder. Schematic lay-out of the AZIPOD is shown in fig. 2, whether fig. 3 shows standard installation of twin AZIOPIDs of the power of 17MW each onboard of a large cruise ship.
Slip rings Ventillation unit unit Air cooling Hydraulic steering unit
Ship hull Installation block
Bearings
Electrical Propeller Bearing, motor shaft seals
Fig. 2. Scheme of the AZIPOD unit [7]
Fig. 3. Typical AZIPOD propulsors with pulling propellers [10] 234 Lech Kobyli ski
Usually AZIPODs are installed in pairs, because such arrangement is able to provide excellent manoeuvrability at slow speed as well maintains good directional stability. However, there are known many variations of this propulsion systems using AZIPODs where propellers may be pushing or pulling, tandem or contra-rotating. In very large ships sometimes more than two AZIPODs are installed. In special types of ships, such as ships requiring dynamic positioning (off-shore supply ships, anchor handling off-shore ships etc) four or more AZIPODs are sometimes installed, however their power is much smaller than in cruise ships. Some different installations of AZIPODs are shown in figs. 4 and 5. Main suppliers are Rolls Royce Kamewa/Alstrom, Finland ABB Industry, Siemens- Schottel, and STN Atlas Marine/ John Crane-Lips.