Remarkable New Marine Propulsion Systems

[anorlunda]

TL;DR

Some modern ships use a new propulsion system called Azipod. These remarkably simple and effective devices are made possible by modern power electronic cycloconverters.

I recently came back from a cruise on MS Eurodam. The ship was equipped with two 17.6 MW Azipod propulsion systems. The captain explained a bit about them in a Q&A session, and I researched a bit more because I'm curious as an engineer. I'm sharing it here on PF for the benefit of other engineers. The innovations are more than just electrical, hence General Engineering.

The Captain said that the ship has 6 diesel generators that produce 64 MW AC power at 11 KV (50 Hz? 60 Hz? I didn't ask.). Cycloconverters connect these fixed frequency AC sources to provide variable frequency AC power to the Azipods. Slip rings allow the Azipod to rotate for steering purposes without twisting cables.

In a recent thread, I said that the most powerful motors were synchronous AC. I was not thinking about permanent magnet synchronous. The Azipod uses permanent magnet synchronous motors, fed by a variable frequency supply, in sizes up to 20MW. Apparently, they can operate through the entire speed range from zero to maximum RPM with no special starting windings or mechanisms.

We normally think of BLDC motors as very different things than AC synchronous motors, but with a cycloconverter in the middle to convert the frequency, the difference between BLDC and AC synchronous reduces to the difference between nearly square and nearly sinusoidal waveforms. Therefore, the BLDC concept stretches from the smallest hobby drone helicopter, to motorcycles, to EVs, to the largest 20 MW marine propulsion. That's impressive.

On the mechanical side, the Azipod system eliminates long propeller shafts, through-hull penetrations with moving parts, rudders, stern thrusters, controllable-pitch propellers and reduction gears. The motor and bearings are directly cooled by sea water.

All of that leads to efficiency gains, but I don't know how much. I also imagine that 6 diesel generators plus 2 Azipods make for great reliability via simplicity and redundancy.

Tug boats are not needed, even in ice.

Disclosure: Azipods are a product of ABB. I am an ex-ABB employee, but in a different field. This post is not a promotion for ABB.

 

[Dr.D]

This is not new; it has been around quite a few years and is well known in the marine engineering world.

 

[Greg Bernhardt]

Whether new or less new, I find this very interesting, thanks @anorlunda!

 

[256bits]

I suppose the pod propulsion system has had some birthing problems in the beginning and has matured over time. Sealing, heat production and cooling, motor design ( permanent magnet ), bearings - thrust and support, and ease of maintenance. In the event of a failure within the pod or slip ring area, the ship would have to go into dry dock for repairs which is still problematic if the ship is not near such a repair facility, whereas the standard shaft through the hull has a lot of equipment readily accessible within the floating ship.

I do wonder what the check up and preventative maintenance schedule looks like.

 

[anorlunda]

I do wonder what the check up and preventative maintenance schedule looks like.

One of the articles referenced in the wiki article says 5 years design interval between dry dock maintenance of the azipod.

Conventional propulsion has its own vulnerabilities. Rudder damage and cutlass bearings come to mind. (Do they use that word cutlass bearing on large ships?)

You can remove the azipod without letting water in. That suggests that you might be able to swap a failed azipod with a spare while in the water. But I have not seen any claims that that plan to try that.

 

[Nik_2213]

IIRC, Azipods and their 'fixed' kin had a bumpy start, with several high-profile failures. Don't hear much going wrong now, so well up the learning curve...

Speaking of diesels, didn't a ship recently have trouble off Scandinavian coast when 'severe' sea-state sloshed air into their oil lubrication system, and the engine management system sensed this, shut down ?
On a lee shore, in a gale...
Brrr...