Pod mechanics move forward

Like a child growing up, the path to technical maturity isn?t always trouble-free. So in spite of its problems, it should always be remembered that podded propulsion technology remains in its infancy.

Computer design and models can take theory so far. But there comes a stage with all new technology where it has to be applied in full-scale. Often it is only at this stage that some shortcomings in the design emerge. These shortcomings and the work needed to rectify them cost money, but this should not be completely unexpected.

If there is a clear benefit to the technology, customers will stick by it provided the shortcomings aren?t seen as insurmountable. For this latter reason it is in the manufacturers? best interest to keep the market fully informed as to the extent of the shortcomings and what developments are taking place to overcome them.

Mermaid modifications

Rolls-Royce says the recent problems with Mermaid pods, most notably on Celebrity Cruises? Millennium-class ships, are related to bearing and seal failures. It adds that it has now made agreed modifications to its pods in consort with ship owning customers and shipyards. But beyond this, the company offers no possible explanations as to why it thinks the bearings and seals have failed nor on the nature of the modifications, despite repeated

questioning.

What is known is that the bearings are supplied by SKF and the seals are Wärtsilä Propulsion?s (formerly John Crane-Lips) Deep Sea Seals brand. The seal failures are understood to have resulted from bearings overheating due to

premature wear. The cause of

this premature wear remains

undetermined.

A spokesman for SKF says: “In those limited instances where early failure has been reported we are utilising our competencies and those of our customers to come to a clear understanding of the underlying causes. This will lead to jointly developing a solution.”

Demetrios Kaparis, senior vice president at Celebrity Cruises, recently raised the issue of personnel training in relation to the Millennium-class ships and their new technologies. This may suggest that a causal element of the pod problems lies in the way the pod suppliers, shipping companies and shipyards equip onboard personnel with the information and experience that allows them to best handle new systems immediately after vessels are delivered.

But this and numerous other suggestions floating about (see previous article) are mere conjecture. The sooner Rolls-Royce and others involved in Mermaid pods can forge opinion based on the release of firm engineering fact is better for all concerned.

Fin addition

ABB is anxious to distance itself from any negative publicity affecting pods. The company acknowledges it had problems with its first cruise ship installations a couple of years back, but says Azipod ships have since operated without any unscheduled drydockings. To date, Azipods have accumulated over 500,000 operating hours on the 20 vessels on which they have been installed, says the company.

ABB has, since the problems with Carnival Cruise Line?s Paradise and Elation, changed the supplier of the roller bearings next to the Azipod?s propeller. It feels the new bearings, combined with development to the lubrication system, are more suited to the job they have to do. By switching to SKF, it is, interestingly, using the same configuration here as Rolls-Royce is on its Mermaid pods. It has retained the original FAG thrust bearing configuration.

The company has also modified its pods for open-water use with the addition of a fin on the underside of the pod housing. The company explains that this has two positive effects: It helps to recover part of the rotational energy that the pulling propellers put into the water and it gives the pods a better balanced steering effect.

ABB believes that because it was first to market with the Azipod ? the prototype delivery was 12 years ago ? and these initial installations were small scale, its designers have been able to develop and refine the technology as installations have grown in size to ensure maximum reliability from the product it supplies today. The bearing switch and fin are examples of this.

Two prop balance

The Siemens Schottel consortium may be playing catch up on ABB with its SSP propulsor, but it has found success by selling four 11MW SSP 10 propulsors to German shipyard SSW. These were installed on two recently completed ropax ferries ? Nils Holgersson and Peter Pan ? for German ferry operator TT-Line.

Heinz Naujoks, chief superintendent at TT-Line, says the two ships have, to date, traded without experiencing any problems with their pod systems. Comparing the new vessels to other diesel electric vessels within the fleet operating the same Travemünde, Germany to Trelleborg, Sweden service he says: “We find the ones with podded propulsion are more reliable.”

Naujoks says the ships have demonstrated greater manoeuvrability allowing them to enter port, particularly in Trelleborg, in higher wind forces. He adds that the fuel efficiency is also “much improved” compared to other vessels operating the service.

Concerns over the water integrity of the pod are addressed through having additional seals in the shaft end of the pod. One seal is permanently monitored, meaning if water should pass through it an alarm is raised. The oil system for the bearings is also monitored, although not with metal detection, says Naujoks.

He says simulator training of seafarers in the operation of podded ships combined with joystick control restricts power to a maximum 2MW for close-in manoeuvring, helping to minimise potential for pod bearing overload in these circumstances. He believes the overall balance of the system is enhanced because each pod has two propellers.

First Dolphins

Later this month (August), the first Dolphin pods ? a product of an alliance between STN Atlas and Wärtsilä Propulsion (formerly Lips) ? are scheduled for installation on Radisson Seven Seas Cruises? Seven Seas Voyager. The ship, being built by the T.Mariotti shipyard in Genoa, Italy has two Dolphin pods.

STN is responsible for complete system integration of the pods. Wärtsilä Propulsion developed the pod shape.

The two complete pods for Seven Seas Voyager were assembled by VEM, which has a build contract with STN Marine, at its factory in Dresden, Germany. VEM also built the air-cooled six-phase synchronous motors installed in the pods. These produce 7,000kW each at 170 rev/min and supply drive to the pods? fixed pitch propellers. The motors receive powered through synchroconverters supplied by Alstom?s German factory.

The pods contain bearings supplied by SKF. Reinhard Swoboda, senior manager of drive systems at STN Atlas Marine Electronics says the bearing configuration is similar to that used in the Siemens-Schottel Propulsor and quite different to that used in Mermaid pods. Face-type seals supplied by Wärtsilä Propulsion are used to prevent the ingress of seawater into the pod.

“It was really hard with the bearings and the sealings and so on, but I think we have the right solution now,” says Swoboda.

Prior to delivery VEM tested all components, including the electric motors, to ensure the integrity and reliability of the pods, says Swoboda. He adds that further tests will be carried out during installation and during the vessel?s sea trials, which are scheduled to commence at the end of the year.

The Seven Seas Voyager is seen as a pilot installation. Swoboda says STN and Wärtsilä Propulsion are keen to get some meaningful data together before putting the Dolphin on full sales release.

Pods in Service

The Dolphin is the one pod system not to have any form of participation in the European Union backed

Pods in Service

program. ABB is represented with data from Radiance of the Seas; Rolls-Royce with Millennium; and Schottel with Nils Holgersson.

Henk van den Boom of Dutch research institute Marin and the manager for the program, explains that the project is seeking to gauge the loads on the pods and the ship structure by measuring stresses, vibration and cavitation etc. He says the program is mainly looking at safety aspects, but also embraces reliability.

Data produced from each system is potentially commercially sensitive, so van den Boom is not in a position to release any results. The extensive analysis and evaluation of the feedback from the ships in service could though shed considerable light on some of the current problems pods are having. So the completion of the project next year and the results released will be greeted with much interest.