New Generation Azipod Builds on Experience
The design of the coming ‘next generation’ Azipod units (Azipod XO) has drawn on operating experience gathered since ABB originated the concept.
• Storied roots: new applications
For a product still pushing the boundaries of what is possible in terms of ship manouevrability, and still finding new applications, it seems hard to believe that the prototype Azipod unit was initiated some 20 years ago.
This month, ABB announced that it will deliver Azipod C products through a new production line in Shanghai, China from the first half of 2011, while a completely new generation of Azipod units is heading towards formal launch in September 2009. In reaching these milestones ABB, which originated the concept with Masa Yards in 1989, believes it is worth taking stock of why the solution has come to be a first option for forward-looking shipowners, and how improving performance over time points to its inevitable adoption by an even wider pool of operators.
Today, Azipod units are available in power ranges of 1.5 MW-20MW and are found on offshore support vessels, icebreakers, ice-breaking cargo vessels, research vessels, drilling rigs, yachts, and on cruiseships, including the largest and most expensive of them all – Oasis of the Seas.
On paper, the Azipod concept was, from the outset, hard to resist. An AC motor driving a fixed-blade propeller located in a separate steering unit and able to turn 360 degrees around its vertical axis would immediately confer greater manouevrability and free up space inside the hull. Again, that the speed of the electric motor could be continuously adjusted allowed for the use fixed pitch propellers. The replacement of pushing propellers with pulling ones brought substantial hydrodynamic improvements; even in the concept’s early development
But ships do not sail on paper and, in the run up to the launch of its new generation, ABB is emphasizing how developments in electrical systems in general and experience at sea have contributed to refined thinking.
In its early incarnations, Azipod technology made a particular impression in the icebreaker sector. Indeed, the concept was derived from icebreaker operating challenges, where electric motors have been preferred to diesel engines because high propeller torque at low speeds of rotation is an important requirement.
However, it soon became clear that the benefits of Azipod technology extended beyond ice operations. The first Azipod rudder-propellers were installed on board passenger vessels in 1998 on the last two in a series of Fantasy-class ships for Carnival Corp (Elation and Paradise). Immediately, these ships achieved maximum speeds of 0.5 knots higher than their conventionally equipped sister ships, using the same amount of power.
Efficiency improved by 8 to 9%, which translated into approximately 40 tonnes in fuel savings per ship per week.
From here, ABB went on to develop a three Azipod system for what was then the largest cruise ship in the world - Voyager of the Seas – as well as the Compact Azipod unit, and the first Azipod unit to feature a contra-rotating propeller (CRP Azipod – 2002).
The Azipod concept also provided a solution to the seemingly incompatible needs of designing a ship exhibiting good properties for both open water use and ice operations. The Double Acting Tanker (DAT), developed at Masa-Yards’ Arctic Technology Centre (today Aker Arctic), operate on the principle of going aft first the ice to reduce the ice resistance of a ship, where the bow is optimized for open water operations.
As the number of merchant ships moving through ice infested water multiplies, it seems only a matter of time before Azipod units will be affixed to a large containership or a liquefied natural gas carrier. For the moment, however, ABB is focusing on the vibrant drill ship and semi submersible drill rig market.
Exemplary has been the launch of the Azipod CZ product, for drilling vessels with station keeping capabilities to cover the low power and high thrust applications According to ABB, due to far better internal and hydrodynamic efficiency, the performance of a 3.3MW Azipod CZ is comparable to a 3.8MW traditional mechanical thruster.
The unit features a unique underwater (de)mountable thruster that stands out from other thrusters due to its built-in high-efficiency electric motor. A fixed pitch propeller is mounted directly to the motor shaft, eliminating unnecessary mechanics such as gears, shaft lines and bearings, which makes the thruster unit resilient to mechanical stress and wear.
The Azipod C -type is available in two versions/applications; a main propulsion version for ships (Azipod CO) up to 4,5 MW and a thruster version with a nozzle for drilling rigs and vessels and other offshore applications requiring high thrust (Azipod CZ) up to 3,3 MW.
An early champion of the new unit has been Transocean, which installed the new series onboard its drilling rigs Developer Driller I and Developer Driller II. According to the company: “The Azipod CZ thrusters installed on Development Driller I and Development Driller II have today (April 16th, 2009) over 450.000 cumulative operation hours. They have provided Transocean with the reliability and efficiency necessary in DP operations.”
• Operating challenges
It is not hard to find other customers willing to endorse the merits of Azipod propulsion. Sinorail Bohai Train Ferry Co has Compact Azipod units on three train ferries operating between Yantai and Dalian. According to the company: “We adopted ABB Azipod for Zhongtie Bohai I, II, III. After several years operation and survey, these vessels achieve 25% fuel oil savings compared with similar ro-ro passenger vessels.”
However, ABB does not shy away from the challenges in persuading owners to adopt the solution. One factor may be the shipping industry’s notorious conservatism, but another is the questions over the reliability of podded propulsion in general, after a series of high profile cruise ship installations suffered leaks in seals and bearing wear, leading to unscheduled drydockings.
In the short term, podded propulsor suppliers including ABB replaced materials used in their bearings and initiated elaborate maintenance support, but deeper thinking was required.
In taking Azipod units through from prototype to full scale industrialization, ABB’s Azipod has been established as more or less the only podded propulsor in continuous demand. While this may not have been desirable in the sense of concept acceptance, it bears testament to the fact that the supplier has systematically met the technical and operational challenges arising.
To meet the challenges of total life cycle costs and penetrating new shipsegments with Azipod propulsion, ABB said some years ago that it had embarked on a development program for next generation Azipods. The resulting Azipod X, which will be launched this autumn, features many new and innovative solutions.
It is no surprise to learn, for example, that this product features an entirely revised bearing and sealing arrangement, involving the complete separation of the oil and water seals, and a void space factored into the hull design to accommodate possible seal leakages or ingress from the sea. A 200litre capacity drainage space featuring integral pumps connects either to the ship’s bilge system, or to its oily water separator. Separation will also confer a longer lifetime, because lubrication can be optimised.
Again, the hull has been optimized, to allow greater access for internal sealing and bearing maintenance at sea, where the former design required drydocking.
CFD analysis and tank tests at Finnish research institution VTT yielded a modified, slimmer hull shape. Depending on the frame size of the unit, the torpedo diameter is 100mm - 200mm slimmer, for example.
Antti Lehtela, sales and marketing manager, Azipod Propulsion, ABB Marine, said: “Hydrodynamic efficiency was improved during ‘normal’ project design work over time, as the propeller design was improved and adjustments were made to Azipod geometry from project to project. Improvements made in this way have achieved 6%-8 % gains in efficiency compare to the first cruise liner application. In the new generation, hydrodynamic efficiency has been further improved by reducing the propeller hub diameter, reducing the diameter of the torpedo and by improving the geometry of the strut, and slightly reducing its thickness. This has achieved a further 2 % improvement.”
Another aspect of the new generation has already been witnessed on board the much lauded and recently delivered Celebrity Solstice. The Azipod units on this ship feature a higher supply voltage (3000V), meaning that cabling can be smaller and easier to install.
ABB has also made efforts to improve communication with the operator of the system and his/her understanding of its overall status, so that the risk of human error is minimized, and reliability can be enhanced because mechanical stress and wear are reduced and identified early. According to Mr Lehtela: “The intelligent control system gives the operator advice, for example, ‘not recommended operational mode’, so that the ship’s master can see beforehand how much power he can get using different Azipod steering angles.”
Other key refinements include the relocation of the vertical turning axis, a modification reckoned to save 20% in steering torque.
In line with wider industry trends, and drawing on its experience in developing the Compact Azipod, the entire next generation of Azipod units will also feature electric, rather than hydraulic steering gear. “Electrically-steered Azipod units are controlled by variable speed drives that offer several advantages,” said Mr Lehtela. “Efficiency is higher, installation is easier and maintenance needs are fewer due to the lack of hydraulics, and space is saved. In addition, less oil is needed, there are no leakages, the unit is more environmentally friendly, and there is less noise.”
ABB’s Azipod units will henceforward be subject to new product nomenclature. Rather than being denoted by size, or propeller type, Azipod units will be coded by product and usage. Large units will be denoted by ‘V’, while compact units will be denoted ‘C’. Next generation units will be denoted by ‘X’. Open water units will be further described ‘O’, while ice application units will be described as ‘I’, units with a nozzle are described with ‘Z’ and units for Contra Rotating applications as C. For example, a large Azipod for ice applications will be designated as VI, while new generation open water application will be denoted Azipod XO. and Compact Azipod open water use as Azipod CO.
If lessons learned during operating experience have been applied to design, the same can be said of ABB’s commitments to service support. “Our emphasis in service is in preventive maintenance,” said Mr Lehtela, “thus reducing considerably the operator's total Life Cycle Costs. Azipod service has been reorganized in order to better serve our customers.” He cited the development of a new Propulsion Condition Monitoring System, which would “bring with it totally new service products”.
Again, last year, ABB opened a new, dedicated Azipod service workshop in Houston USA, targeting to both drilling and cruise market. One satisfied customer has proved to be Transocean, which commented: “ABB Azipod service/maintenance support, which is provided from ABB’s Houston Workshop is meeting Transocean’s expectations.”