Azipull everywhere

16 October 2006

Azipull azimuth thrusters

Since its first application in ferry propulsion in 2004, the Azipull azimuth thruster from Rolls-Royce has been welcomed into other markets. More than 100 units are in service or on order as the main propulsion for offshore service vessels. Now several product tankers are in operation, driven by Azipull thrusters, and units are on order for a gas carrier and a passenger vessel.

The Azipull thruster takes the mechanical elements of a gear-driven azimuth thruster and puts them into a quite different hydrodynamic package. A pulling propeller – CP or FP – is mounted ahead of the leg, which is a hydrodynamically optimised streamlined unit incorporating the gear house and a lower fin. The leg/housing/fin combination recovers swirl energy from the propeller slipstream which would normally be wasted, converting it into additional forward thrust. At the same time the underwater unit has more rudder effect than a conventional azimuth thruster, improving the steering ability of many hullforms. This can also save energy since less thrust vectoring is needed to keep a steady course.

A double ended catamaran ferry was the first application for Azipull propulsors. Four AZP085 units, each rated at 1,340kW, placed one at each corner of the symmetrical ferry helped the vessel reach its required service speed of 22 knots. This ferry, named Stavanger, operating on a Norwegian fjord route, has now built up about 20,000 running hours on each of its thrusters. Other vessels of the same Fjellstrand type with AZP085 units are also in service in Turkish waters.

The Azipull size range was soon extended to create thrusters suitable for other markets, and now comprises four sizes AZP085, 100, 120 and 150, covering unit powers up to 5,000kW.
These thrusters have caught the imagination of the offshore industry, and have proved themselves in supply vessel propulsion. Comparative trials on Bourbon Tampen against conventional azimuth thrusters showed a reduction in fuel consumption of at least 16 per cent in normal service, with much lower vibration levels. To date more than 50 shipsets of Azipull thrusters have been sold for offshore vessel propulsion.

Smaller merchant vessels are also a relevant application for Azipull thrusters, and the first installations in product tankers are now at sea. Traditionally, product tankers have been conventional single screw vessels, but increased focus on manoeuvrability in confined waters and propulsion redundancy for safety’s sake is causing owners to embrace new thinking.

Azipull thrusters are attractive not only for their efficiency but because their rudder effect can provide effective steering for hullforms which are in themselves not particularly stable. Bro Deliverer is the first of a series of four 14,500dwt Brostrøm D-class product tankers and is the first tanker to go into service with Azipull propulsion. Rolls-Royce is supplying each vessel with two Azipull AZP120 thrusters rated at 2,380kW. They provide propulsion and steering, and are supplemented by a Kamewa TT1850 tunnel bow thruster of 1,000kW. Loaded service speed is about 13 knots. By using two azimuth thrusters for propulsion, each with its own medium speed main engine, independent steering and separate fuel systems for each engine, a high level of propulsion redundancy has been achieved, and Bro Deliverer has RP class notation from DNV and Clean Design. Broström’s D-class tankers have a deadweight of about 14,500dwt on a draught of 8.0m, an overall length of 146.8m, a beam of 22m and 14 tanks with a capacity of 18,556m3 when 98% full. They are being built at Jinling shipyard in China.

Bergen Star, a product tanker built in Turkey for Bergen Tankers, features Azipull thrusters in an innovative NVC 604PT ship design developed by Rolls-Royce. Here the thrusters are coupled to the main engines mechanically with shaft generators in the same short shaftline, giving propulsion redundancy and the ability to supply power for cargo pumps and ship services from one engine in port. Because the machinery is so compact, about 5 per cent more tank volume can be provided in a given overall length of vessel.

Recently, the application of Azipull thrusters as the main propulsion system for merchant vessels extended further, with the ordering of a pair of thrusters for a 7,500m3 gas carrier to be built in Poland for the Netherlands shipowner Anthony Veder. This vessel is unusual in being designed to carry liquefied natural gas (LNG), liquefied petroleum gases (LPG) or petrochemical gases. Two AZP120 size thrusters with fixed pitch propellers will be driven by frequency controlled motors in a diesel electric system.

The latest application field for Azipull thrusters is in passenger vessel propulsion. Two AZP120 units, each taking 2,310kW, will propel a new ship for the Hurtigruten Norwegian coastal route which is under construction at Fincantieri in Italy. The vessel will be named Fram, and is to operate the normal Bergen/Kirkenes route in winter, but in summer will cruise to Greenland. For this reason Ice-class has been specified for the Azipull thrusters.

A market that is starting to show interest in the Azipull concept is the superyacht industry. Prosperous times have produced owners who want yachts from 50m and up, for their own pleasure or for charter. There has been a tendency to concentrate on visual appearance, comfort and style, quite naturally, while below the waterline solutions remain very traditional.

Particularly for faster displacement yachts with service speeds up to 24 knots, Azipull-based propulsion systems can be effective. The thrusters can be used in a variety of mechanical and diesel electric transmissions, giving freedom in positioning the machinery and opening the way for different accommodation layouts. Where a yacht makes long voyages and then requires good manoeuvring capabilities, Azipull thrusters can provide an ideal solution. The cost aspect of reduced fuel consumption may not be as important as in some other markets, but lower emissions because of better propulsion efficiency are increasingly a factor. Generally reduced noise and vibrations from a well-engineered Azipull installation can be attractive in this discerning market.