Q: How can I get trajectory information written to a file?

A: Use the STORE command in the Disposition Menu. For details, Click Here.

Q: How can I change the precision of the results MOSES reports?

A: There is no way to change the nicely formatted reports. You can, however, get a report with as much precision as you want by scaling the column and then either viewing or storing it. For example if you want to look at the trajectory with more precision, you could specify:

     TRAJECTORY -CG
       VLIST
       C_SCALE XXX 5:9
       VIEW 1 5:9 -HARD
       REPORT LOCATION
     end

Q: Why does a floatover analysis sometimes stop with "instabilities"?

A: Instabilities occur when you have large changes in applied forces between time steps. You probably need a smaller time step. The size depends on the stiffness of your connections, but we have used as small as .05 seconds.

Q: How does Moses calculate the righting moment in a time domain simulation?

A: In reality, the concept of a "righting arm" is not used in MOSES at all (except for hydrostatic stability). Instead, MOSES computes the forces (and moments) of all force acting on a body at each time step.

Q: How would you connect a model in the time domain so that the connections will not affect the motions?

A: I have had good results using four lines at 45, 135, -135, and -45 degrees. Make these line 25mm in diameter and 1000 m long. Finally pretension them to get equilibrium at the nominal position. To be more complete, I use:

   &dimension -dim meters tonnes
   medit
      cl_del ~mline
      ~mline h_cat 25.4 -len 1010 -buoy 0 -wtpl 0 -dep 0
      connector portbow -anc 225 1000 ~mline *prtbowd
      connector stbdbow -anc 135 1000 ~mline *stbbowd
      connector portaft -anc 315 1000 ~mline *prtaftd
      connector stbdaft -anc  45 1000 ~mline *stbaftd
   end
   &connector @ -l_tens 5

Q: Is it necessary to generate RAO after the 'g_pressure' command if I am doing a mooring in time domain?

A: No, the RAOs have no influence on the time domain. More on the G_PRESSURE command can be found here.

Q: What does the message "*** ERROR: Launch Leg Has Offset" mean?

A: When performing a launch analysis, the elements that make up the launch legs must be continuous. This message indicates they are not. The usual cause of this is bad offset information for an element in the launch legs.

Q: How can I get zero peaks from the statistics of a time domain sample?

A: Finding peaks is not quite as simple as it may seem since we must worry about noise. Let

    max  = max ( val(1) - mean , .....)
    fact = 1./max
    a    = fact * ( val(i+1) - val(i) )
    b    = fact * ( val(i)   - val(i-1) )

    abs(a) >  tol  and
    abs(b) >  tol  and
    a*b    < 0

One should not be too distressed by having zero peaks since the peaks are only used in computing the statistics of them and in predicting the maximum response.

Q: How can I plot jacket buoyancy vs time?

A: Use the Disposition Menu to do this. The result of VLIST is a list of available variables. Jacket displacement will typically be labeled something like "Displ:JACKET", but this depends on the body names in your particular problem. You need to find the number associated with "DISP:JACKET". Now you can issue the PLOT command with the appropriate variable numbers. For instance, "PLOT 1 24 -NO" will work, if variable number 24 corresponds to your jacket displacement. Your commands should look similar to the following.

   PRCPOST
     TRAJECTORY
        VLIST
        PLOT 1 24 -no
      END
   END

Q: I am performing a time domain vessel motions analysis, and I am having problems like "Simulation Terminated Due To Instability" and coprocessor faults. What am I doing wrong?

A: You need some soft mooring lines to hold the barge in proximity of the waves for the time domain simulation. You need 4 lines, one at each corner, at 45 degrees off the longitudinal, similar to how a model test tank would hold the barge in the middle of the tank during a model test. Mooring lines need to be soft enough so as not to effect the motions, but stiff enough to keep the barge from drifting. This method is refered to in an earlier question, click here.

Q: If the vessel heading changes during a time domain analysis, how does MOSES compute wave forces on the vessel for the changed directions?

A: MOSES will interpolate between the headings used for computing hydrodynamic pressures on the hull. Thus, it is important to use a sufficient number of headings on the G_PRESSURE command.

Q: How can I perform a time domain simulation in a new process which starts at the last event in an existing process?

A: Use

     &DESCRIBE PROCESS NEW -EVENTS

Q: What causes the roll and yaw angles to change radically when a body nears a pitch angle of -90 degrees?

A: What you are seeing is correct and there is no fix for it! This is a problem which has plagued all for centuries: any set of Euler angles has a "singularity". For our set the singularity is at a pitch of 90 degrees.

At a pitch of 90 degrees, there is no unique way to compute roll and yaw. As a result, when you pass through this angle, "strange things appear to happen", the roll and yaw angles may change by 180 degrees. Nothing, however, is really happening - as pictures of the model will show. This is simply numbers changing to protect their integrity.

The only advise I can give here is to change your coordinate system so that you do not go through a pitch of 90; i.e. change the coordinate system so that during the process the structure rolls instead of pitches (use a &DESCRIBE PART XXX -MOVE).

This works so long as the pitch does not change by over 90 degrees during the simulation.

Q: When I issue, the command POINTS from the PRCPOST Menu, I get things labeled "X:*NODE", ..., M-X:*NODE". I thought that the M- columns would be the angles of rotation for the body, but they appear to be translations. How can I get the angular motion of the body?

A: A quick answer is that you use TRAJECTORY. This gives you the location (including Euler angles), velocity and acceleration of either the origin or the CG of the body and some other general data.

The columns that you thought were angular motions are really the motion (displacement vector) of the point from its reference position. For more details, Click Here.

Q: How can I find out the velocity of a point during a time domain simulation?

A: You need to use the POINTS command in the Process Post-Processing Menu.

Q: I am going to do a time domain simulation of a vessel in an irregular sea. What are your recommendations as to periods used and time step?

A: In the absence of any more concrete information, I would say that the defaults should suffice. There are three sets of periods that are involved here: those specified by -PERIOD on the G_PRESS command, those specified by -PERIOD, -MD_PERIOD, and -W_PERIOD on the &ENV command. The defaults for those specified by -PERIOD are those specified on an &DEFAULT command. command (in the last moses.cus file). Unless you changed them, the defaults are:

     -PERIOD     25.  20.  19.  18.  17. 16.  15.  14.5    \
                 14. 13.5  13.  12.5 12. 11.5 11.  10.5    \
                 10.  9.5   9.   8.5  8.  7.5  7.   6.5    \
                  6.  5.5   5.   4.5  4.  3.               \
     -P_DRIFT   60 70 80 90 100 120 140 160 180 200        \

The periods specified via -MD_PERIOD are a more difficult question. These define the periods at which second order wave forces will be generated. The important thing here is to have some periods in the neighborhood of the "natural frequency" of the mooring system. You may need to play with these using SRESPONSE to see where to place them in the time domain.

While you need to have a time step suitably small so that the wave forces can be accurately integrated, the stiffness of your system is normally the governing factor. For many problems, a time step of .75 - 1. works quite nicely. This is for a free vessel, or one with a "soft" mooring. As the stiffness of the connection increases, the time step must decrease! There is no general rule, but normally .05 is small enough. One thing to watch out for here is that MOSES is solving integro-differential equations and keeps a limited history. As you make the time step smaller, portions of the history are neglected. Thus, with MOSES, you can have too small a time step, i.e. too small a time step may have more numerical error than a larger one.

Q: What does the message "*** WARNING: CONVOLUTION KERNEL HAS BAD PROPERTIES" mean?

A: In general, MOSES uses an inverse Fourier transform to account for the frequency dependence of the added mass and damping in the time domain. The result of this procedure is that the equations of motion are no longer differential equations, but integro-differential ones. The numerics of this inverse transform are quite sensitive to the shape of the added mass and damping. We do our best to compute the inverse, but we must deal with the data at the discrete points you used in computing the hydrodynamic pressures. Since the form of the kernel can have important consequences on the motion of the system, we check our computation. In particular, we compute the Fourier transform of the kernel and compare it with the original damping matrix. If the comparison is not good, we give this warning. Normally, the warning can be safely ignored. If, however, you get "bad results" (instabilities, large motions, etc.) you better heed the warning. The "fix" here is to not use the convolution, but in its stead, use an added mass and damping matrix at a single frequency. Normally, one chooses the frequency of the mean period of the sea being applied. Also, using a smaller time step has been known to remove the warning message.

Q: Can I change the time step in the middle of a time domain solution?

A: No. The convolution used for the hydrodynamic coefficients does not allow for this. However, any time domain simulation can be stopped and then restarted with a different time step.

Q: Is there a way to obtain the wave velocity for a wave slamming problem?

A: No, not at this time. On our list of things to do is to develop a reasonable method for dealing with wave slam.