Heave motion as a function of global time in Seakeeper

[Lisa86]

Hello everybody!

My Name is Lisa and I'm a naval architect student from Hamburg in Germany. This is my first post in this forum, and maybe someone can help me.
My Problem is, that I want to compare the ship motions by waves, for two different ships.
Because I’m interested in mooring these two vessels, I used Seakeeper to simulate the heave and pitch motion.
Now i require this movements as a function of time, to compare the difference in height directly.
Does someone know how to do this in Formsys's Seakeeper or another PC-program?

Thank you!
Lisa

[jehardiman]

Just to clear this up, do you have the output motion spectras, the motion RAO's, or a time series? To determine "average" and "most probable maximum" motions between two different platforms, you need to do stoichastic analysis in the frequency domain, not direct comparison of motions in time series.

If you have time-displacement data you can use A Fast Fourier transform (FFT) to turn it into the frequency domain. From this spectra you can calculate the "average" , "significant" and "most probable maximum". If you have the RMS response or RAO's (both output from Seekeeper then you can use spectral analysis directly. See Dynamics of Marine Vehicles by Bhattacharyya or Mechanics of Wave Forces on Offshore Structures by Sarpkaya & Isaacson.

Edit: I just re-read the OP,

Because I’m interested in mooring these two vessels, I used Seakeeper to simulate the heave and pitch motion.
Now i require this movements as a function of time, to compare the difference in height directly.

Is it that you have the Response Spectra and now want to make a time series? That is a whole other ball of wax, and generally not advised or used in technical discussions because of the problems of either "cherry picking" the answer you want, or missing the answer you need.

[Lisa86]

Hello Jehardiman.

It seems like I was on the wrong way to solve my problem. To answer your question, yes, I have the RAO’s of both vessels, and originally I really wanted to compare the motions in time series. Obliged to your post, I think I have to chose another way for an informative result . I don’t know yet how to

calculate the "average" , "significant" and "most probable maximum"

like you said. I ordered the book from Bhattacharyya (thanks for your recommendation) in my library, hope this helps. Can you say some words about the direct spectral analysis I can make out of the Seakeepers RAO’s?
I am truly grateful for your expert advise.

[jehardiman]

Lisa86.

Have you got your answer or do you need more help? Bhattacharyya does a better job detailing the problem than I could do in a froum, but if you have a specific point you are stuck on, I could push you towards the proper answer.

[Lisa86]

Hi!
At the moment I actually not make any progress. My problem is to compare the RAO’s scientifically and to make a proposition about the relative motions. I have never done an inverse Fourier-Transformation before, but I think that would be one way.
In Bhattacharyya’s book I can’t find an example according to my problem, maybe You can give me a hint?

[BMcF]

We have used a program called WAMIT to predict the relative motions in heave between different sized vessels in very close proximity to each other. The program allows for any orientaion of the two vessels or floating structures relative to each other and to the wave field. Your Seakeeper results should be compataible..or easily made compatible with the WAMIT input data requirements.

I've seen sufficient correlation study done betwen WAMIT results and verification model tests to say the results are quite usefull for preliminary design and motion studies.


I cannot provide you with more info than that (what is carried in my head) because I am currently on travel and away from myu office. However, a Google search on WAMIT will surely turn up something.

Otherwise, your frequency-domain results would certainly be adequate to develop the statistical description of relative heave motions between two vessels that are 'somewhere' in the same sea spectrum (exceedance of some limit criteria per hour, for example, and the RMS differential heave and heave rate, etc). What the FD results will never do well, however, is include the effects of sheltering and interference between two vessels close to each other in a seaway and those effects are very significant.