**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.

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 endThe 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 5These 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 < 0Here 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 ENDMore 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 -EVENTSso 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.