By

Helge Christian Tesch

Odense Steel Shipyard Ltd.

Convenor for ISO/TC 108 SC2/WG 2

　

Why do we need a new Standard one could ask?

　

I will try to give some reasons and examples on what the new standard will mean when we are talking about vibration figures.

　

In the old standard (ISO 6954-1984E) see attached figure- Guidelines for evaluation of vertical and horizontal vibration in merchant ships (peak values). We use the term "single amplitude maximum repetitive peak value."

　

・This term is very difficult to measure because you need a time signal on a trace, and if you then have several frequency components in the same time signal, it will be difficult to distinguish between the different "single amplitude maximum repetitive peak value" with respect to distinct frequency.

　

・Measurement equipment with appropriate filters could directly give figures called peak values, but they would always either be average or rms figures, but never "single amplitude maximum repetitive peak value".

　

・The old standard does not take into account if two or more single order responses were dominating at the same time. You only need to look at them individually.

　

・The old standard have the term "crest factor" which you have to multiplied with measured single order RMS value to get to "single order maximum repetitive peak value".

　

・All these factors made it very difficult and time consuming to reach the real "single amplitude maximum repetitive peak value".

　

・To overcome some of above difficulties all kind of unofficial interpretation has been introduced such as broad band measurement "sum of all orders RMS value" without weighting filter.

　

With new development in measurement equipment it became obverse that the solution, on vibration criteria for habitability, was to have a broad band "sum of orders RMS value" with weighting filter (similar to noise criteria).

　

The vibration in ships accommodation area are typical dominated by frequencies in the range from 0.5 - less than 100 Hz. As seen in graph from the old standard the human sensitivity for velocity vibration decreases below 5 Hz. you can accept higher vibration values. So if we want to have criteria for "sum of all orders". We need to have a weighting filter and we have chosen the combined curve filter in ISO 2631 part 2. Which in substance reflect the human sensitivity for velocity vibration similar to the graph in the old standard, see attached figure B.1 and figure A.1.

　

So what does this means in real terms?

　

I have here an example from a ship that I visit some time ago.

　

・First graph shows velocity vibration responses of individual dominating orders in mm/s rms.

　In the upper speed range two individual orders are dominating at the same time.

　

・Second graph shows the sum of all orders without and with combined filter accordingly to the new standard.

With the combine filter the values reduces:

　

It is the intension of this new standard that the acceptance criteria in different areas for habitability shall be agreed between builder and owner in the ship specification.

　

In the new standard you can find some guidance on acceptance criteria. See attached page 3 from the new standard.

　

But it must be made clear that a ship specification stating "Vibration in accommodation shall fulfil ISO 6954-2000", makes no sense.

　

It must be followed up with a list of areas with agreed acceptable vibration figures in velocity or acceleration terms.

　

Thank you for your attention