Time Domain Questions


Q: We are currently determining maximum mooring loads for a single point mooring and are having problems with what looks like snap loading in the system. I have made the following variations to determine why this is happening:

  • time steps,
  • Newmark coefficients,
  • mdist properties,
  • adjusted periods hydrodynamics calculated, and
  • checked catenary excursion vs tension properties.
How to I solve this?
REV 7.06

A: In situations like this you need to check the effect of the time step. If you have a seriously stiff system (you will if you have snap forces), you will need a small time step. How small is small? My first try would be .1. If this gives same results as what you already have then this is probably the truth. If not cut the time step in half and repeat.


Q: I have generated a spectrum a time trace. Why is it spikey instead of being nice and smooth?
REV 7.06

A:

You get the peaks at the periods specitified on the -S_PERIOD, TW(1), TW(2), ......., TW(n) and -MD_PERIOD, TD(1), TD(2), ......., TD(n) options of &ENV and nothing for all other periods.


Q: How can I get trajectory information written to a file?
REV 7.04

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?
REV 7.04

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
The report you get with VIEW will have the trajectory multiplied by XXX, but the one you get with REPORT will not. For more information on C_SCALE, Click Here, and for more information on VIEW, Click Here.


Q: Why does a floatover analysis sometimes stop with "instabilities"?
REV 7.04

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?
REV 7.04

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?
REV 7.03

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
These lines are so soft that they will have negligable influence on the motions.


Q: Is it necessary to generate RAO after the 'g_pressure' command if I am doing a mooring in time domain?
REV 7.01

A: It depends on what parts of the drift force you want included. You need to compute the RAOs before you perform a time domain analysis if you are going to include radiation and Coriolis terms.


Q: What does the message "*** ERROR: Launch Leg Has Offset" mean?
REV 7.00

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?
REV 7.00

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) )
Then MOSES computes a peak when
    abs(a) >  tol  and
    abs(b) >  tol  and
    a*b    < 0
Here tol = 1e-5. So it is quite possible to have more "up crossings" of the mean than you have peaks.

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?
REV 7.00

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
More on the Disposition Menu can be found here.


Q: How can I perform a time domain simulation in a new process which starts at the last event in an existing process?
REV 7.00

A: Use


     &DESCRIBE PROCESS NEW -EVENTS
so that the events of the last process are stored in the new one. You can now -RESTART the old time domain.


Q: I am performing a time domain vessel motions analysis, and I am having problems like "Simulation Terminated Due To Instability". What am I doing wrong?
REV 6.02

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?
REV 6.02

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: What causes the roll and yaw angles to change radically when a body nears a pitch angle of -90 degrees?
REV 5.06

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?
REV 5.06

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?
REV 5.05

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