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Q&A forum: Vessel Motions
(displacements, velocities and accelerations)

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Doing a further analysis of a 55m 600ton vessel, cog 25 from ap, I wanted to see how much a point 10 behind ap would go up/down for a gangway landing.
What I saw is that the surge (forward movement) in the middle of the vessel is about 1m backwards, while the P point behind the vessel goes 3.6m forward – the vessel must be an accordion?
... The discrepancy between the CofG and point 'p' is due to the value entered for surge phase angle in your client's calculation, which must be entered manually – frictional phase angles are not calculated in the RAO calculator.
The calculation result at point 'p' is correct in all calculations. The surge phase angle must be altered to obtain the correct value at CofG ...
... A fuller explanation will be added to the appropriate web-page for the Vessac calculator ...

Trying to use Vessacc and RAO we found that applying the DVN rules was giving lower results than the harmonic for small waves.

We bought another version through one of my young engineers (Vasilis Papadopoulos) and then we found that there was a problem in the previous editions of Vessacc and RAO which I got.

We check also the waves but we found no problem between the 2 editions.

I am sending you, for your consideration, the slides with the comparison, together with the excel file of the data.
The results are for a specific angle μ (angle of wave direction in respect to vessel direction), and a specific angle θ (angle through the wave). For different angles you get bigger or smaller x,y,z components for acceleration.

Your customer has indeed recognised an error in the earlier version (1.0) of Vessac
It is a coding error that was corrected in the later version (1.2)
... this was the only correction made to the calculator
The latest version is correct and the earlier version is not correct.
The issue raised by your customer only applied to the Classical Harmonic calculation option of version 1.0, it did not apply to the DnV Rules for Ships calculation option.

Any customer running version 1.0 of this calculator may contact CalQlata direct for a free copy of version 1.2 in which the above error has been corrected.

...
Having read the technical help, I would like to ask you the following:
Input zᴿᴬ is defined as heave RA in m. In the technical help doc you say that you must enter the amplified response itself, and the amplified response operator (RAO).
Output zᴳ is defined as heave in m @ CoG

I would expect output zᴳ to have the same value with input zᴿᴬ, which is not the case when I run the analysis.

Is it possible to give me some insight?
...
zᴿᴬ is the RA (in metres) at the vessel's centre of gravity
zᴳ is the actual movement of the vessel at the CofG taking acceleration and velocity and phase angles into account

The maximum heave at the vessel's centre of gravity will not necessarily occur at the peak of the wave and accelerations and velocities will cause the vessel to move independently of the water in which it sits.
Some months ago I purchased the above program to calculate ship motions. This program based on DNV AS (Det Norske Veritas) - Hull Design Ships with Lengths 100m and above - Part 3 Chapter 1.
I believe I may have found some anomalies in your calculation of the solution of the final accelerations in sections 602, B700 and 701 and B800 and 801.
My focus is particularly directed on the use of the offset term Rp and RR used in the base equations ar and ap defining the oscillatory acceleration terms given in section 403 and 503. These equations enable the final combined transverse (at), combined vertical (av) and combined longitudinal (al) accelerations to be evaluated.
The particular terms in these equations use the vertical and longitudinal offset distances or moment arms - a want of a better term, which are used in the aforementioned base equations ar and ap to define the specific directional equations ary, apx, arz and apz.
I believe the incorrect offset distances have been used, as I have evaluated these equations myself using the original DNV documentation source and compared these with results of the Calqlata V1 program i purchased.
I would be grateful therefore if you would check these equations and confirm whether my anomaly findings are indeed valid.

Whilst vertical acceleration usually induces the highest loads on-board a ship, unnecessary conservatism can result in unnecessary costs.

Para; 602 of the code uses a formula for 'av' that is based upon all three contributing RAOs (heave, roll and pitch) being in-phase at the same time and also co-incident with extreme design conditions.

I have always applied the following slightly less conservative approach to this analysis:
The highest of √[az² + avr²] and √[az² + avp²] (roll or pitch) and have included this approach in the Vessel Motions calculator.

In order to apply the more conservative formula, users should perform the following calculation for av:
av = √[az² + avr² + avp²]

I checked an example I did some time ago, using the 2012 DNV Rules, and you have an error in the “Vessel velocity” units.
Units on the software show m/s.
Converting ship speed in knots to m/s gives the wrong answer for common acceleration parameter a0.
The units for V on the bottom should show “vessel velocity (knots)” and not “vessel velocity (m/s)”

This observation is of course correct:
When using the 'DnV Rules for ships' calculation option, you should ignore the stated input units (m/s) and enter your value in 'knots'.

Any customer still using this calculator that wishes to receive a corrected version of the calculator - free of charge - please contact us and we'll send you a revised version.

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