# Wamit-Wave height input??

Discussion in 'Software' started by sud, Feb 28, 2009.

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### sudJunior Member

Anyone who has worked on Wamit, please clarify me that how do we input wave height parameters to calculate wave forces?

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### jehardimanSenior Member

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### sudJunior Member

I didn't get you. Please tell me in detail. You can send me some guidance on my email id sud.suman@gmail.com because i have started learning it for my B.tech project ad i m facing problems in it

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### sudJunior Member

I didn't get you. Please tell me in detail. You can send me some guidance on my email id sud.suman@gmail.com because i have started learning it for my B.tech project ad i m facing problems in it

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### jehardimanSenior Member

WAMIT works in the frequency domain. I takes a unit wave height (1ft or 1m or defined by ULEN) at a given frequency and then determines potential flow field around the body to generate a force and/or response amplitude operator (RAO) for the freely moving body for that frequency by integrating the pressure over the body. The given flow fields, forces, and/or RAO are only at that frequency and relative to a unit wave height. Because this is a unit response (i.e. the response to a wave of 1 unit height) for that specific frequency, it scales directly with wave height.

To get maximum forces for a given seaway, we have to use spectral superposition. Note and understand that WAMIT does not give actual time domain forces, but rather the maximum forces can be infered by stoichastics. How this is accomplished is subject to various interpertations.

If this doesn't make sense, you need to go back and understand unit response, RAO's, and spectral analysis before diving into WAMIT.

Last edited: Mar 1, 2009
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### sudJunior Member

yeah thnx for the guidance. I got it.

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### sudJunior Member

I have one more query. RAO for sway motion is coming out to be of the oreder of 10^-6 for wave periods 8-12 s.
Although for surge motion, its coming out to be around 0.8, which i have found is more or less fine. What can be the reason for so less value of Sway RAO?? Plz reply ASAP.

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### jehardimanSenior Member

What is your wave direction? WAMIT will give no sway for waves dead in the head or stern.

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I haven't personally used WAMIT. However i worked on a project a few years ago where the client provided me with their WAMIT output, for guidance. The values were in general, twice that given by class such as DNV, which was the reference and classification society used for design/construction. Suffice to say I didn't use them, I also used my own database of load cases too.

So you may be over cooking the structure!

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### sudJunior Member

Obviously for wave angle 90 deg.
anyways i had sorted out the problem, thnx for the reply bdw.

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### sudJunior Member

@jehardiman
For a definite, suppose 0 deg heading, i am getting forces at different frequencies. At 8 second, the non dimensional mean drift forces is coming out to be around 50. So, for calculating mean drift forces should i take the maximum force at 8 second time period only. In this case it will be= (density)*g*A^2*L (L=1).. (taking significant wave height as 1.8m)...so it will come around 1587KN.. Does it seem correct for a 180m long and 40m wide rectangular cuboid structure with a draft of 4.8m??

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### jehardimanSenior Member

So, a Mean Drift Force of 356Kips on a 34,500 ton barge in 6 foot, 8 second waves (325 foot, 106m wave length)?

Well by bounding the problem (something you should have done before any computer simulation), significant wave orbital acceleration at a depth of ~2m is 4.08 ft/sec^2 so orbital mass force should be about 9,853Kips. So a Mean Drift Force of 3-4% seems reasonable, espically if you consider that the wavelength is ~0.5L and ~2B.

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### sudJunior Member

Yea, i checked using the pressure integration method (expression given in Faltinsen), the value coming out to be same...
but tell me about the reference of the statement u have made
""significant wave orbital acceleration at a depth of ~2m is 4.08 ft/sec^2 so orbital mass force should be about 9,853Kips. So a Mean Drift Force of 3-4% seems reasonable, espically if you consider that the wavelength is ~0.5L and ~2B.""

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### sudJunior Member

means 3-4% of......this one..

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### jehardimanSenior Member

Mean Drift Force is the average force in the direction of wave travel caused by energy losses to the body's presence. Think of it this way, in a perfect wave orbital in the x direction I get a sinusodial force on the body going from 0 to +9853kips to 0 to -9853kips back around to 0. However there are losses due to flow dynamics and phase lag. The intergration of these losses over the cycle generates the Mean Drift Force....i.e. the average force needed to hold the body in location. Note that this occurs with the water also, a mass flow is generated in the direction of celerity (see Wiegel, Oceanographical Engineering). Given a moment of consideration, it is obvious that for a normal "ship shaped" body that the larger the peak mass forces, the larger the Mean Drift.

A quick inspection will show that peak mass forces are larger when the body's principle dimension in the direction of celerity is not a "whole" wave length. Conversely there are smaller peak mass forces when the body's principle dimension is much, much, smaller than the wave length, i.e. we expect peaks at L/lamda of 1/2, 1.5, 2.5, etc and minimums at < 1/10, 1, 2, 3, 10, etc. This is why it is very important for stationary ocean structures to be designed for the conditions at the site.

In you your case, the L/lamda is ~ 1.7 so we don't expect a large ratio of peak force to Mean Force. 3-4% seems reasonable. 150 ST mooring load for 34,500 LT ship is not that out of place compared to the emperical data from mooring designs and other things I have done.

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