Environment Questions


Q: Can I use MOSES to plot the time history of a wave spectrum?
REV 7.04

A: There are two ways to plot this. The test file env_info shows how to plot the wave and wind time history. Alternativley, you can plot the time history of a wave specturm after you have performed a time domain simulation in the PRCPOST menu. The wave elevation "W. Elv" is part of the standard variable list produced with the POINTS command.

        PRCPOST
         points -events 0 9999 1
           vlist
           plot 1 2 -no
         end
        END
Remember you need to have associated the points you are interested in with the &DESCRIBE interest command. There is an example of this in the sample file mp_moor.cif


Q: Why is the wave elevation at a time where all terms of the Fourier series reinforce unreasonably large?
REV 7.04

A: Suppose you pick a set of frequencies so that each component has the same area. Then all Fourier coefficents are the same

 
     c(i) = sqrt( Hs**2/(16*N) )
 
where N is the number of coefficients. Now let us pick the phases so that at some time, Tb, cos( w(i)t + phi(i) ) = 1 for all coefficients. Then the amplitude at this time is
 
     e = sum c(i)
     e = N * sqrt( Hs**2/(16*N) )
     e = sqrt( N * Hs**2/16) )
 
which increases as the square root of N.


Q: What is reported when I use an environment containing more than one spectrum?
REV 7.04

A: The spectra being used will be included in the results of the &status env command. They will be listed starting with the spectrum defines with the -SEA option.


Q: Does MOSES perform multiple time domains when I use &ENV with the -E_PERIOD option for a time domain simulation?
REV 7.04

A: No. The -E_PERIOD option is completely ignored except in the frequency domain.


Q: Can I use MOSES to plot the sea/wave Spectrum?
REV 7.01

A: You can plot the sea spectrum from the MAIN menu:

        &status sea_spect -hard -plot
           vlist
           plot 1 3 -no
There is an example of this in the test files, please see env_info


Q: Is it possible for MOSES to generate 2-3 maximum waves together in a wave spectrum?
REV 7.01

A: Not easily. What you want is something that violates the properties of the wave spectrum; i.e. something that is a quite rare event. What you can do, however, is to cheat a bit - run a sumulation with the spectrum and then

  • Define a new environment with a regular sea and a height that represents the "maximum wave",
  • Pick a severe time in the original simulation, and
  • Restart the simulation at this time with the new environment.


Q: How does MOSES chose the phases doing a time domain simulation?
REV 7.00

A: If the sea was defined as a regular wave, then the phase is zero. If, however, the sea was specified via a spectrum another method is used. Here, the phases are chosen as

          phi(i) = -t_r / w(i)
Here, phi(i) is the phase for the ith component, w(i) is the frequency of the ith component, and t_r is a time. By using this scheme, you always get the same sea, and two time domain simulations will yield identical results. The time, t_r, has an interesting interpretation - it is a time at which all of the components reinforce, i.e. at this time the amplitude of the sea is the sum of all of the Fourier coefficients. Depending on the number of components you use, this sum is "unreasonably" large and this time is to be avoided during the simulation. Thus the default is set to be a large number. If you need to change it to perform Monte Carlo simulations, you can do so with:
          &DEFAULT -T_REINF T_R
Click here to read more about the &default command. This option can also be set in the &env command.


Q: Can I input a user defined wind spectrum?
REV 7.00

A: Yes. Please see the -W_SPECTRUM option with the &DEFUALT or the &ENV or the -W_HISTORY option with the &ENV command.


Q: Can a wind speed time series be modeled?
REV 7.00

A: You can put in a deviation of the wind from the mean. Please see &DATA command. You can also see the -W_HISTORY option with the &ENV command.


Q: What is the best way to specify a one minute wind speed to MOSES?
REV 7.00

A: Wind speed in MOSES is always with reference to the 1 Hour Wind Speed. Use the option

    -W_DESIGN API 60
or
    -W_DESIGN NPD 60
This will use either the API or NPD formulation to use the correct wind speed for 60 seconds based on the one hour speed. Please see the -W_DESIGN option with the &DEFUALT or the &ENV command.


Q: How does MOSES combine the response to the two sea states defined with "-A_SEA" option?
REV 6.02

A: The way MOSES treats the sea is to build a Fourier Series for each heading specified with the -HEADING option and with period specified with the -PERIOD option. Now, with a normal environment there is a single series. With two spectra or with spreading there are more than one.

These series are used directly in the time domain to compute the wave height and this is used to compute the force one the body. In the frequency domain, these series are used to compute response Fourier Coefficients if you ask for SRESPONSE.

If you simply compute RAOs and then ask for statical results the coefficients are combined with the RAOs to produce the response spectrum and the statistics are deduced from it. The view here is that the integration is over heading and periods. You could view it as computing a response spectrum for each and then adding them because the results are the same. The treatment is the same regardless of what you are computing statics: motions, cargo forces, connector forces, etc.

To look at a test for this, Click Here: and look at the test named "a_sea".


Q: How can I display report data like the spectra and SN Curves?
REV 6.01

A: You can get information on the energy ordinates that make up a spectra by using the sp_point command. For SN curves, there is the &status sn command.


Q: How can I input the Phillips Constant of the JONSWAP spectrum in MOSES?
REV 6.01

A: You don't. There are many formulations of the JONSWAP spectrum which are mathematically equivalent, but they all contain different parameters. Our formulation uses:

  • a period which is the peak divided by 1.2958,
  • the significant height, and
  • the peakedness parameter gamma.
Notice that the Phillips constant does not appear. You need to find out the particular formulation of the equation being used and see how this constant is defined. Now, compare the formulation with the one MOSES uses to see how this particular Phillips constant is related to the significant height. Then input the corresponding significant height.


Q: How can I change wave spreading in MOSES?
REV 6.01

A: The short answer is you should not do this. In general, the default provides a conservative answer and there are problems with spreading in the time domain. Check Here to see a discussion on why spreading gives "strange" results in the time domain.

If you must change it, use the -SPREAD option on the &ENV command. Here one specifies an exponent, EXP which is used in the wave spreading function, F:

            F(THET) = COS(H - THET)**EXP  for -pi/2 < THET < pi/2
 
            F(THET) = 0                   Otherwise
Here H is the mean wave heading. Now, for a heading of 65 degrees away from the mean heading the spreading would be as follows:
      COS(65)**2   =   0.1786    for EXP = 2
      COS(65)**4   =   0.0319    for EXP = 4
      COS(65)**200 =   1.55e-75  for EXP = 200

The default in MOSES is to assume unidirectional waves, using the EXP value of 200. I believe this is commonly referred to as "long crested waves". Any values for EXP lower than 200 will result in less conservative results, so this practice should be considered carefully. As I understand it, "short crested waves" refers to waves where wave spreading is considered, meaning some value less than 200 for EXP.


Q: I have been given several periods for a JONSWAP spectrum, which should I input to MOSES?
REV 6.01

A: You need to use the mean period, this is peak period divided by 1.2948, Tm = Tp/1.2958.


Q: How can I represent a Pierson-Moskowitz or Bretschneider spectrum?
REV 5.08

A: All the various spectral types can be described with one general equation. For instance, a JONSWAP spectrum with a gamma of 1.0 is the same as an ISSC spectrum. To represent a Pierson-Moskowitz spectrum, use an ISSC spectrum with a mean period that gives a wave steepness of 23.27. This steepness is what the Pierson-Moskowitz spectrum assumes. Therefore, one needs to choose the proper mean period:

       Hs/(gT^2/2pi) = 1/23.27

so

       T = sqrt(23.27*Hs*2pi/g).


Q: Why do we get a transient phase in the wave profile of a regular wave?
REV 5.06

A: The transient you noticed for a regular wave is the result of the default setting &DEFAULT -RAMP 20 in your moses.cus file. This controls the time interval over which the sea will be linearly ramped from zero to its full value, which in this case is 20 seconds. This can be set at the top of the command file with either the &DEFAULT, or the &ENV, command.


Q: Could you possibly reply with reference(s) that describe the practical aspects of the "Pierson-Moskowitz and "Bretschneider" spectra?
REV 5.03

A: Two good sources of information on Sea Spectra are papers by Michel: "Sea Spectra Simplified", January 1968, Marine Technology and "Sea Spectra Revisited", Winter 1999, Marine Technology.