Critical speeds

Measurements in all 3 directions confirm that vibrations are predominant in the longitudinal direction...

 

Vibex,

Can you post the analysis? I'd be curious of the magnitudes and frequency patterning.

Are you in the great white north?

-Tom

 

Like this one? http://www.marinfo.gc.ca/en/flotte/DesGroseilliers.asp#Specs

Sorry if I sound dumb, just want to be sure we are using the same terminology... Are you saying that X-axis acceleration is the predominant one?

 

Hard to tell, but something like the one in the link above.

Yes, vibration levels are predominant in the X-axis/axial/longitudinal direction.

 

That is really interesting. It is a wave and not a usual vibrating membrane or panel.

 

Yes, it is not a most common vibrating mode of a hull, and is usually not a predominant one. But the superstructure can easily vibrate in X-axis dimension - especially if the deckhouse is high enough, like this one: http://marineinsight.com/wp-content/uploads/2010/10/superstructure.jpg . And that can be a driving force for the X-axis vibration of the hull too.

I've made and attached a drawing (see the pdf below) which shows one possible mechanism which can generate the vibrations in the way you have described it:

The superstructure, oscillating in the X direction, transmits inertial forces to its base, which transmits it to the hull beneath. A hull girder would not normally vibrate in X direction, but it can do so when coupled with the superstructure, which acts as the exiting force. This vibration would very probably be localized, because frictional damping of structural elements along the hull length would impede its longitudinal propagation.

Hopefully, the picture is clear enough but if you have questions about it, just ask. Of course, only a calculation of natural frequencies involved can tell if this hypotesis is correct. At the preliminary level, I can tell that the natural frequencies of a superstructure can be several times higher than the natural frequencies of the hull.

At a preliminary level I can tell this:
- a 10.000 t vessel can have a natural frequency of the hull typically ranging between 1.5-2.5 Hz. Let's say it's around 2.0 Hz, but you can get a more precise value by applying Kumai's formula to your ship.
- it's superstructure can have anything between 5.0 Hz and 15.0 Hz (to be verified with FEM analysis or with the analytical formula for cantilevered box-beam - if applicable).

The possible exciting forces for the superstructure's longitudinal vibration could come from:
- a 4-bladed prop revving at 1.7 Hz will give the first two exciting frequencies of around 6.8 Hz and 13.6 Hz, hence falling in the range of natural frequencies of the superstructure.
- a wrongly placed engine room, with respect to modal nodes, can also be a culprit.
- the engine firing frequency can be resonant with the superstructure too, so that's another thing to be investigated.
- finally, another possible source of local X-axis vibrations could be the thrust bearing transmitting a high-frequency oscillatory forces (cavitation? turbulence? CPP malfunctioning?) to the hull.
Please take note - all the above numerical values are generic (statistical). A numerical analysis of this specific ship has to be performed in order to individuate the most probable causes of vibrations.

Now, what can you do about it? There are coslty solutions - like changing the propeller type and number of blades, changing the shaft RPM's, or reinforcing the structure (but the outcome of the latter is always an unknown). Or you can go the cheap way - by trying to avoid the RPM range where the resonance occurs. Which is what the shipbuilder did...

My 2 cents worth.

 

Forgot to mention - a crane, a mast, or whatever other installation above the deck can behave in a way described above...

 

You could join a very very long que. Some ships, for whatever reason have minor vibration problems. These vibrations can be minor, or some drift into being major. The degree of severity depends upon ones frame of reference. If you have a vibration problem going up through the revs, but at a rpm that the vessel does not run at under normal operation, then it is an inconvenience, that is all. Hence the wording in the operating manual.

You can spend vast amounts of money and time trying to pin point the source/cause of the vibration. Bottom line is, is it worth it? These problems are often only solved (if at all) by being on-board and/or taking measurements all over, ones you ahve yet to do. We could only offer our thoughts. But being on-board focuses the thoughts much more to possible causes....which cannot be described on a forum, since no "answers" have popped out!

Lets say for argument sake, that the vibration is caused by the resonant frequencies of that very eloquently described above by Daiquiri. Is it worth the expense of changing the prop, simply because of an annoying level of vibration going through the rev range??

I used to travel by car ferry from the island i used to live on to the mainland several times each month. As the ship slowly accelerated out of the port and going through its rev range to full RPM, there were exciting frequencies which made my table, with my cup of tea and cake, vibrate in an unhealthy manner. But this last about 5 seconds, then gone. Was it annoying, yes....is it a real problem...no. Would the vibration cause fatigue, of course, but how much?. Is the vibration going to shorten the structures life, probably not, since it is transitory and only at the start and end of a journey....so not much in reality!

Thus you need to put the problem into context.

See above.

 

A ship is an elastic structure, it will vibrate. It's nearly always excited by the propelling machinery either directly or through hydrodynamic flow effects.

It's possible that you have not noticed the fundamental and you are looking at a harmonic. Critical questions :
What does your FFT look like
What was the frequency range of the vibration sampling and the sampling rate?
Were the prop vibration and beat frequencies also sampled?
Do the vibrations intensify with varying hull pressure from passing waves?

And I presume those props will be in kort nozzles? And are the aft sections deep V or shallow?

Flow instability can cause some serious vibration issues too. Is there a critical speed or does the resonance occur at the shat RPM regardless ? Although in twin screw ships the wake distribution is less likely to cause vibration problems.

Sometimes a coupled vibration is present in a greater amplitude than the excitation at some multiple often when the primary is well damped. Bulkheads particularly need de-tuning at times usually by stiffening or adding mass or even a damped mass. At 121 Hz I'd suggest it's a 2 node vertical standing wave.

And oh yes! This still occurs in new vessels despite FEM modeling. It's a very complex phenomena . Sometimes physical models have to be constructed because FEM just doesn’t cut the mustard.

 

When the Japanese get really annoyed they sell the ferries to countries like this, where they still stay in service for many years. There is one here where the whole car deck seems to ripple at a certain rpm.

You could look at it positively and say it is a sign of stiffness: a wet rag doesn't vibrate at any frequency.