Azimuthing AC propulsion system to drive new generation vessels

Azipod fitted to an offshore vessel

The Azipod propulsion system developed jointly by ABB Industry and Kv?rner Masa-Yards has made an important breakthrough into the offshore sector. Two 5-MW Azipod systems are to be fitted to the multipurpose icebreaker/offshore support vessel which Finnyards is building for the Finnish National Board of Navigation. This will be the first use of the system in an offshore support vessel, though it has been installed in a number of tankers, according to Mikko Niini, Kv?rner Masa-Yards vice president for marketing and sales.

Azipod, which stands for azimuthing podded drive, represents a revolutionary approach to propulsion. The system consists of an AC electric motor installed in a pod which directly drives a fixed pitch propeller. The pod itself is installed outside the hull.

The AC motor is controlled by a frequency converter, giving full and smooth torque in either direction and at low speeds. The short shaft line comprises propeller, seals, bearings and shaft.

The unit can azimuth through 360° and is used for steering the vessel, thus dispensing with the rudder. The electric motor provides full and smooth torque in either direction and at low speeds, thus giving excellent maneuverability.

The system eliminates the need for much heavy and bulky equipment required by conventional propulsion systems, including long shaftlines, reduction gears, controlled-pitch propellers and transversal stern thrusters. Construction is made much easier, the maintenance load considerably lightened, and mechanical losses eliminated.

In DP mode, the Azipod system with its fixed pitch propellers offers much greater fuel economy than the controlled pitch propellers installed in vessels with conventional diesel-electric propulsion - an important factor for the offshore sector which is so dependent on DP functioning, according to Thomas Hackman, marketing manager for ABB Industry´s marine division in Helsinki.

Maneuverability

Azipod was originally developed in the late 1980s for icebreakers and ice transiting vessels, and the Finnish National Board of Navigation is fully aware of its capabilities in this respect.

But the system also offers excellent maneuverability and full dynamic positioning capability, features which will be vital to the vessel´s offshore role during summertime, when it will be engaged on subsea construction and well workover in the North Sea.

In this respect, offshore vessel operators will be watching with keen interest to see if the Azipod system lives up to its promise. As far as tanker operators are concerned, the experience so far has been very encouraging, Niini says.

After Azipod was retrofitted to the Uikki and Lunni 16,000dwt Arctic products tankers, sea trials showed that vessel maneuverability in narrow passages and harbors was greatly improved, and performance in the open seas was also better than expected.

Long-term monitoring of fuel consumption showed that in the most common speed range the Azipod vessel consumed about 10% less fuel than sister vessels operating with conventional propulsion and controlled pitch propellers.

The crash-stop distance was also found to be substantially reduced. Reversal can be performed either by reversing the propeller speed or by turning the Azipod unit through 180°.

Double acting tanker

To take full advantage of the benefits of the Azipod system in both ice and open water, Kv?rner Masa-Yards has now developed the double acting tanker (DAT) concept, in which the tanker proceeds ahead in open waters and astern through ice - the bow is designed for open water functioning and the stern for icebreaking.

For Hackman, Azipod´s breakthrough into the offshore sector is also a vindication of ABB´s promotion of AC electric propulsion for the marine sector.

It has developed two types of drive technology for the marine market: the Cyclo, which consists of a cycloconverter controlling a synchronous motor, and the Sami PWM frequency converter, which controls a cage induction motor. The Cyclo is for powers of 3-20 MW/drive, and the Sami PWM for 0.5-8 MW/drive.

For the offshore market the Sami PWM (pulse width modulated) is most relevant, not least for vessels with DP thruster drives.

Since it supplied AC electric main and thruster propulsion systems to Allseas´ Lorelay pipelay vessel in 1986, ABB has made regular deliveries to the offshore market. Complete electric power systems have been delivered to Statoil/Bergeson´s multipurpose shuttle tanker Berge Hugin, and to the three dedicated shuttle tankers on the Heidrun Field.

Other orders and deliveries for AC propulsion and power plants include Falcon´s Peregrine IV DP drillship, the Finnish National Board

of Navigation´s Fennica and Nordica icebreaker/offshore vessels, Esso Norge´s Balder FPU, the Reading & Bates/Conoco deepwater drillship, Saga´s Varg FPSO, Bluewater´s Glas Dowr FPSO, Statoil´s Norne FPSO, the Rasmussen/Smedvig FPSO, and Stolt Comex Seaway´s Seaway Falcon pipelay vessel.

"It´s well accepted by most of the operators we talk to that PWM technology is by far the most advantageous for offshore vessels, or any vessel using DP capability," Hackman says.

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