**Q:**
Why do I get ERROR: RAO Computation Did Not Converge when
I am trying to compute RAOs?

REV 7.06

**A:**
This error is because one of the following:

Way to big of damping,

Way to big of stiffness, or

No mass is defined.

**Q:**
Why is there a
g*sin(theta) correction and not a g*cos(theta) term?

REV 7.05

**A:**
Think of the incline free body diagram from your college text book.
The incline is the roll or pitch in our case. In the free body diagram
it is just theta.

What we are trying to find is the change from one position to the next. First, we should look at the mean (position 0). Lets assume theta0 = 0. So, the equations are:

F0 = W*sin(theta0) = W*sin(0) = 0 N0 = W*cos(theta0) = W*cos(0) = W

At position 1, the equations are:

F1 = W*sin(theta) N1 = W*cos(theta)

Let's look at the change in friction force. So, dF = F1 - F0 = W*sin(theta) - 0 = W*sin(theta)

With the assumption of a small theta, sin(theta) -> theta. Therefore, dF = W * theta

or

dF = W * roll

Now, let's look at the change in the normal force. dN = N1 - N0 = W*cos(theta) - W = W*(cos(theta) - 1)

for a small theta, cos(theta) -> 1. therefore,

dN = W*(1 - 1) = 0

So, there is a g*sin(theta) correction, but the g*cos(theta) term cancels out.

The same works for pitch.

**Q:**
I have a piece that I want to be included for hydrodyanmic calculations
but I do not want it included for hydrostatic calculations. What
options do I used in the definition.

REV 7.04

**A:**
You need to give the part just a little bit of displacement.
You do this by defining it with a very small permeability.
A permeability of 0.001 should suffice. You can use the
-PERMEABILITY option when you define the piece.

**Q:**
Is the viscous damping linearized stochastically or harmonically?

REV 7.02

**A:**
By default MOSES linearizes harmonically via a "equivalent
linearization". If you tell it to, MOSES will perform
a spectral linearization. Remember that the viscous
forces depend on the relative velocity so that you
are not only linearizing the damping, but also
part of the force.
Click here (Section III.G) or
Here(Section XII)
for details.

**Q:**
How do I get only the dynamic response excluding the mean?

REV 7.01

**A:**
Define the envinment with the option

-USE_MEAN NOThis is an option on the &env command. If you are working with only the connector forces this is also an option on the ST_CFORCE command.

**Q:**
Do the damping matrices obtained using the v_matrices command
include both linear radiation damping and viscous damping coefficients?

REV 7.01

**A:**
No, what you get with
MATRICES
in FREQ_RESP contains both, what you get with
V_MATRICES
in HYDRODYNAMICS is only radiation. This menu does not know about the response.

**Q:**
How can I compute the Cargo Forces on many
points and how can I find the maxima for
different periods and headings?

REV 7.01

**A:**
(This is actually a sample.
Click Here for the test files.)
The quickest way to do this is with loops. This file
shows how to report forces for a set of points and how
to report forces for each point for a set of environments.
Basically, the sample is a sample of a loop within a loop.
Briefly, the points are first defined and then
a macro is used to loop on the point names. In the file
the points are defined within the MEDIT menu. This section
is labeled as "Define Report Pts". The poins can also be
defined as part of the body description in the data file.
A variable, which is the list of points, is made after the
points are generated.
In this file a set of environments is generated with
a loop. This section is labeled as "Devine env".
After the environments are generated the frequency response
menu is used for caluations and reporting. The rao's
are computed outside the loop. The rest of the computations
are done within the loops.
The outer loop (using p as the index) uses the point
names are first used to generate the response and force operators
at the location
of the point. This is done with the fr_point and fr_frcargo
commands.
The inner loop (using e as the index) uses the list
of environment names to calculate and report the forces
for each point-environment combination. Calculation of the
cargo forces is done with the st_fcargo command.
There is a section labeled "get max" where variables
are set so that the user will have the data available to
sort through. The command set_var is used to retrieve the
data from the database and the command &set is used to save
the maximum of the values calculated to that point.
Once all the point names and the environment names have
been exhausted the loops are closed. Be sure to close both loops.
The final section is labeles as "report". Here the maximum
of the values collected earlier are printed to the screen
and the log file.

**Q:**
Is the static angle due to wind included in
the "Cargo Force Statistics"?

REV 7.01

**A:**
No, it is not. Neither is the static component
of gravity. (Notice that the heave force is
around .1 to .3 instead of being greater than 1)
The manual clearly states that these are *"DYNAMIC
FORCES"*. To get the total force you should
add the static force to the dynamic one. Notice
the cargo force results are maxima for each
degree of freedom. Thus you should take the
mean (static force) and add/subtract each
component which yields quite a few cases.

The "Cargo Force Statistics" output is generated with the ST_FCARGO command.

**Q:**
What coordinate system is used to report the
frequency domain results?

REV 7.00

**A:**
The body system is used for almost everything
is the frequency domain. The titles Surge, Sway,
Heave, Roll, Pitch, and Yaw are body system
terms. For forces we use Longitudinal, Transverse,
and Vertical which again are body system terms.
One thing that needs clarification is relative
motion which is reported is the Body system of the
first point.

**Q:**
Does MOSES consider hydrodynamic interaction among multiple bodies
when calculating the hydrodynamic coefficients and raos?

REV 7.00

**A:**
The precise answer is, not in general. There are two
alternatives, however. First, a body can interact
with an
"obstacle", or you can connect the multiple
bodies, compute the modes of the assembly, and
use the modes as generalized degrees of freedom.

**Q:**
I am looking at MOSES results from the output report
"CARGO G FORCE STATISTICS". Do these G forces include the
g*sin(theta) effect? If so, how can the Longitudinal G force
be reported as less than half of g times the maximum pitch angle?

REV 7.00

**A:**
Of course, the G*sin(theta) effect is included in the
CARGO G FORCE STATISTICS. If you examine your motion
response operators, you will notice that surge and pitch are
nearly 180 degrees out of phase. This means the surge component
of the G force is opposing the pitch component, resulting in the
lower G value. See related questions concerning phasing:
Click Here
for one and
here for
another.

**Q:**
I have cargo
placed symmetrically about the barge centerline.
Why do I get very different
vertical forces for the port and starboard cargo when I look at
the results for a 90 degree heading?

REV 7.00

**A:**
This is due to the heave/roll phase relationship. The vertical
component of roll will change signs depending on the direction of
roll. Sometimes this will be additive to heave, sometimes this
will subtract from heave. This depends on the position of the
cargo relative to the barge centerline, and the wave approach.

**Q:**
Why does to force I computed on some cargo
using the motions and the accelerations not agree with
the MOSES force.

REV 7.00

**A:**
The answer to your question is a word, **phasing**. MOSES
computes the forces correctly accounting for phasing: i.e.

fx = M INT [ { ax + g sin(pitch) }^2 s(w) dw ]What you are doing above is applying the triangle inequality to this expression as

fx <= M INT [ ax ^2 s(w) dw ] + M INT [ {g sin(pitch)}^2 s(w) dw ]This is a mathematical fact, the force statistics MOSES computes will always be less than or equal what you get with a check of the motion statistics. The reason you get the same results for heave is that there is no angle involved, in this case, equality holds.

**Q:**
If one does not specify the periods around the peak of the Roll
RAO curve, will MOSES statistics of Roll motion results be
inaccurate?

REV 6.02

**A:**
In general, your statement is correct, and it is always good
practice to provide a reasonable refinement of periods in
the range of the anticipated peak roll response. However,
this is more an issue of engineering judgment than one of
running MOSES. For most common barge transports, the default
encounter periods provided in MOSES will suffice. Normally,
there would not be much difference in results between a run
with the default periods, and a run with more periods in the
region of the peak roll response. Of course, if these results
are different, or your results are close to a pass/fail situation,
further study would be required.

**Q:**
I have computed pressures
on the hull for encounter periods ranging from 4 seconds to 30
seconds. Can I uses these to look at a
sea state with a peak period of 4 seconds?

REV 6.02

**A:**
No, this is not good practice. You should always make sure you
have computed hull pressures for several periods either side of
the peak period of the spectra you expect to use. MOSES will
perform an extrapolation where no data exists, but you will always
get better answers using computed data.

**Q:**
If I want to obtain the maximum roll response, do I compare the
period of peak roll response with the environment peak period or the
environment mean period?

REV 6.01

**A:**
Peak period.

**Q:**
Why is it that when I
use strip theory I get a roll angle of 33 degrees and yet in
diffraction I get a roll angle of 22 degrees?

REV 6.01

**A:**
You do not have enough panels for a good diffraction solution.
As a general rule of thumb, you want to have approximately 1000 panels
for most monohull vessels when using 3D Diffraction. It is not
surprising your 273 panel diffraction answer is different from your
strip theory answer.

A final word is in order here. Strip theory is quite good for monohull vessels, particularly in roll. It is widely accepted for most deck barge transports. It is not so good for surge, and is simply not correct for multi-hulled vessels, such as semisubmersibles. The moral of this story is do not be a man with two watches, you will never know the correct time. Stick with strip theory for most common barge transports, go to 3D Diffraction when surge is important in a mooring problem, or when analyzing mufti-hulled vessels.

**Q:**
When I lower the position of my cargo on the vessel
I find that the roll angle is now increased. Why is this
happening?

REV 6.01

**A:**
Changing KG influences not just stability but also the period of peak
roll response for the system. If decreasing KG moved the period
of peak roll response closer to the peak period of the wave spectrum
used in the analysis than that would increase the roll response.
Check the roll RAO curve or table for beam seas.

**Q:**
How can I calculate the quadratic transfer function of a vessel
in MOSES?

REV 6.01

**A:**
As I understand it, a quadratic transfer function is used
to evaluate second order effects on a floating body exposed
to waves. These second order effects are really slowly varying
wave drift. You can view wave drift data by using the
v_mdrift
command.
If you are expecting to take wave drift output from MOSES,
and use this as input to another program, you will likely
have to perform some kind of conversion for units or format.
MOSES was designed to solve all aspects of floating body
problems, not to directly output quadratic transfer functions.

**Q:**
Can I get MOSES to give me a correct wave force on a
tank setting on the sea bottom?

REV 6.01

**A:**
Actually, you need to do nothing. MOSES automatically eliminates
all panels below the mud line. Just make sure that the tank
extends below that bottom and everything will be fine.

**Q:**
What are the dimensions for radiation damping coefficients in
the MOSES output?

REV 5.10

**A:**
The output says "Values Normalized by Mass with Weight = xxx".
What MOSES is reporting is the weight of the body in question,
and needs to be converted to mass to compute a damping coefficient
to be used outside of MOSES. The translational values have units
of 1/sec, the rotational values have units of ft^2/sec or
meters^2/sec.

**Q:**
We have found that the solutions of the RAOs differ depending on the
number of headings which were run for the
G_PRESSURE command.
Shouldn't they be the same?

REV 5.10

**A:**
MOSES uses the solution for the previous heading as an initial
estimate for the current heading. Thus, there will be small
differences (within the convergence tolerance) in the solution
depending on the number of headings. There is nothing you can do
to change this fact.

**Q:**
I am comparing motion forces obtained by using
FR_FCARGO and ST_FCARGO,
and defining an environment with -SPREAD 2.11.
Why is there a substantial difference between these two results?

REV 5.10

**A:**
No, you are not doing anything wrong, this is related to your
use of the -spread option, and the mathematics used to synthesize
the sea. The problem is that at some point in space the waves will
always reinforce, sometimes they will cancel.
Just imagine two regular plane waves,
with the exact same period, coming from different
directions. It is easy to see that there will be maximums
and minimums in the ocean, spreading evenly in space,
and more importantly since they have
the same period they will stay stationary in space.
When we transform the waves to the time domain we still get
the same waves, also stationary in space with maximums and
minimums.

If the characteristic periods of your sea are large enough, your vessel will have different responses depending on where it is located in space. It may be on a maximum spot, it may be on a minimum spot. It is a problem of representing the randomness of the sea with the spreading technique, which is nothing but using exactly the same sea spectrum for different headings with a scaling factor.

**Q:**
Can I use a hull model composed of various
PGEN
definitions that share surfaces for hydrodynamics?

REV 5.10

**A:**
Absolutely not! You must have a single enclosed surface for
hydrodynamics computations. You should use
COMPART_SUM
panels to make sure your panel areas sum to zero. If you have appendages
such as skegs to include for hydrostatics, use "PGEN -DIFTYPE none"
to exclude these for hydrodynamics. If you want to include the
effects of a moonpool on the motions, you really should use the
&surface menu to create a proper 3D Diffraction mesh.

**Q:**
What does the message
"*** WARNING: Negative On Diagonal Of Damping"
mean?

REV 5.10

**A:**
It means you have negative values in either your damping values and/or
your added mass values resulting from the hydrodynamic pressure
computation. We discovered users were not checking their added
mass and damping output, so we added this warning message. This can
be caused by a bad or strange hull definition, close proximity to the
bottom, or irregular frequencies.

What we mean by "bad" hull definitions are those that do not have enough detail in areas of the vessel with a change of shape, such as the bow or stern. Often, adding more planes or panels in these areas can make the problem go away. Another good check would be to square off the ends of the vessel, and see if the warning is still reported. With 3D Diffraction, adding panels is automatic by using &PARAMETER -M_DIS. Depending on the size of the vessel, a good panel size is 3 to 4 meters, and 500 to 1000 panels is a reasonable number. Some vessel shapes may require up to 3000 panels, but much more than this takes too long to compute. The optimal aspect ratio for diffraction panels is 1 to 1, meaning a square, but rectangular panels are acceptable. The program will create triangular panels when needed to avoid warped panels. The user can control the resulting mesh by careful selection of the points used to define the planes.

Strange hull shapes can include floating jetty type structures that look like an upside down "T" in cross section. Here you have a horizontal vessel surface in close proximity to the water surface, and this can cause negative added mass. In this case, the negative values and resulting motions are probably real, but thorough checking is advised. If you get the warning with both Strip Theory and 3D Diffraction, this increases the chances the answers are real.

Negative damping can also occur when the keel of the vessel is close to the seafloor. In this case, the water under the keel gets "pushed" against the bottom, and this "hydraulic spring" effect is captured in MOSES as negative damping.

Occasionally, negative damping is caused by irregular frequencies, a numerical phenomenon where the computational algorithm breaks down at particular frequencies. In essence, these are the natural frequencies at which water would slosh inside the vessel. Numerics being what it is, the problem being solved becomes "ill conditioned" in the neighborhood of these frequencies.

The onset of irregular frequencies depends on draft, length, and beam, but for a 400 X 100 foot barge it is around 3.5 seconds. For H851, is it about 6.5 seconds. More panels makes the ill conditioning better. Therefore, before you can declare you are subjected to irregular frequencies, you need to fine tune and check your model carefully. If the negative damping occurs at only one or two periods where you know there is little or no spectral energy, these periods can be safely deleted from the computed periods list. Another option is to remove the offending periods from the g_press command, but use them on the rao command. MOSES will interpolate or extrapolate results for the removed periods.

Many research papers have been written concerning this topic. One of the more useful ones is "On the Significance of Negative Added Mass", by T. Vinje, presented at the Eighth International Conference on Offshore Mechanics and Arctic Engineering, The Hague, March 19-23, 1989.

**Q:**
How are the phases defined for RAOs?

REV 5.06

**A:**
See the
MOSES Sign Convention document.

**Q:**
Why are the heave and rotational "unit-forces" computed
from
ST_POINT
not the same as when computed from
ST_FCARGO?

REV 5.06

**A:**
What you are seeing here is numerical differences. In MOSES,
we "work hard" to compute the moments of a spectrum. In
particular, we assume that the response operator varies
linearly between the points at which they are computed, and
then carefully integrate this with the wave spectrum
values. This is all fine and good except for your problem.
You see that the acceleration is the second moment of the
motion so it is computed according to the above procedure.
The heave force, however, is the zeroth moment of its spectrum.
Thus, according to the above rule, it is computed assuming
a linear variation. Any differences should be minor. If
they are not, you need to compute the results at more periods.

**Q:**
I have a body, composed entirely of tubular members. When
I compute frequency domain results for this body, I get all
zeros for the added mass, damping, and exciting forces.
How can this be?

REV 5.05

**A:**
There are three reasons that you can receive a report of all zeros:

- The body is not in the water.
- You are looking at the wrong report. There are two
reports which give the information you described. One
of these is is obtained in the HYDRODYNAMICS Menu,
the other in the FREQUENCY RESPONSE Menu. The one
from HYDRODYNAMICS reports results
*only*for panels. You have no panels, hence these should be zero. The report you get in the FREQUENCY RESPONSE menu will contain results for both panels*and*Morrisons elements. - You asked for the results to be reported before they were computed. Hydrodynamic forces due to Morrisons equation are computed when they are needed. Hence, there are no hydrodynamic forces on your body until after the first RAO or SRESPONSE command is issued.

**Q:**
If I use the command
FR_POINT &BODY(CG DECK)
will I get the RAOs of the motion of the DECK cg in its current position?

REV 5.05

**A:**
Maybe. The FR_POINT does yield the RAOs of a point about
its mean position. These RAOs are represented in the body
coordinate system. If, however, you have more than one body
the syntax you described may give the results for the
"wrong" body. The documentation of FR_POINT
(For details, click here)
warns you that if you use the syntax above, MOSES will use the
"current body". This may not be the body you want unless you
explicitly use the command &DESCRIBE BODY DECK immediately before
FR_POINT. It is easier to use FR_POINT *DECKCG where the point
*DECKCG has been defined to have the location of the CG. Also,
I am not quite sure what you mean by "in the correct position".
The way moses views life, points are "particles" attached to
bodies, not locations. Thus, the RAOs you get are the raos of
a fixed particle of a body - regardless of the location of
the particle. I am afraid that you are confusing the
representation of the RAOs with the location of the point.
As stated above, the RAOs are represented in the body system,
i.e. a heave is perpendicular to the "deck" not the waterplane.

**Q:**
How does MOSES treat nonlinear connectors in the
frequency domain?

REV 5.05

**A:**
MOSES uses the tangent stiffness matrix at the mean
position in the frequency domain. This statement applies
equally well to all stiffnesses, hydrostatic, weight, etc.
What is applied in the frequency domain is the derivative
of the force evaluated at the current configuration when
the RAO or SRESPONSE command was issued. Of course if
a tension only (compression only) connector has zero
load at this position, it will not contribute to the stiffness.

**Q:**
What coordinate system is used when reporting the results
of the
FR_POINT
command?

REV 5.05

**A:**
The body system of the body containing the point. Normally,
one should specify a point name on the
FR_POINT
command to
define the point of interest. Thus, MOSES will get the
coordinates in the system it wants, and it will know the
body being considered. The results are reported in the
body system. If you are interested in global motions,
you can use the
PMOTION command.

**Q:**
Are the heave motions from the frequency
domain coupled with rotations?

REV 5.05

**A:**
Yes.

**Q:**
How does MOSES predict the "maximum" response of quantities in an
irregular sea?

REV 5.03

**A:**
As stated in the MOSES Reference Manual,
&DEFAULT -PROBABILITY
STAT PDATA controls the statistics which will be
defined when computing the statistics of quantities in an irregular
sea. The default is for STAT to refer to the MAXIMUM, and PDATA
is 3.72 which provides the 1/1000 highest response, based on a
Rayleigh distribution. This is the statistical multiplier the root
mean square will be multiplied by to obtain a maximum. When
DURATION is used for STAT, the time specified is used to determine
the statistical multiplier. See equation 11.22 of the Time
Series, Spectra and Extreme section in
How MOSES Deals With: (PDF).

**Q:**
Where is the assumed center of rotation
for vessel motions?

REV 5.03

**A:**
Our software does not assume a center of rotation for
vessel motions, since there really is no such thing.
This question normally arises from the old pendulum
motion techniques, assuming 20 degrees roll in 10
seconds for instance. Sometimes this question also
stems from supplying accelerations for use in a
structural software package. Some of these packages
require not only a center of rotation, but also
angles, to calculate the g*sin(theta) effect. When
using the Statistics of Forces report from our
software, this effect is already included.

**Q:**
Surely your answer to the above question is not
correct! The moment of inertia depends on the
axis system and the roll depends on the moment of
inertia. How can the roll not change with a change
in axes?

REV 5.03

**A:**
The mistake in your argument is that you are thinking
in only one degree of freedom! If I may for a moment
change your argument slightly, if I fix a point on the
barge, and watch the roll, it will change when the
fixed point changes. This is a true statement. The
fallacy lies in the fact that normally a barge does
not have a point which is fixed! When the axis system
is changed, more must happen than a change in the
roll moment of inertia - the entire inertia matrix
must change. Changing the inertia matrix changes
the coupling between the degrees of freedom
(in this case roll and sway) and it also changes the
applied roll moment. If this change is done correctly,
the roll RAO will not change, but the sway one will!
The change in sway is not a real change in the sway
at all, it is simply a manifestation of the fact that
by changing the origin of the axis system we are now
computing sway at a different physical point. If you
take the RAOs computed with the new axis system and
use them to compute RAOs at the old origin, you will
find that the results are the same.

**Q:**
Why do my connectors have no load in the
frequency domain, yet indicate load in
static equilibrium?

REV 5.03

**A:**
The RAO command was executed prior to the
&EQUI command.

**Q:**
Why do I get a substantial roll response in head seas?

REV 5.03

**A:**
This can happen for a variety of reasons, including an
asymmetric hull description. The most likely cause, however,
is gyroscopic coupling. We solve the problem correctly using
a full NxN mass and stiffness matrices. In most cases, the
distribution of mass produces an inertia matrix which has
roll/pitch coupling. Thus, the pitch excites the roll
through mass coupling.

**Q:**
How can I find the maxima of a value defined with RAOs?

REV 5.03

**A:**
There are several way of *estimating* the maximum.
Normally, one does this statistically by assuming that
the peaks are Raleigh distributed. If one makes this
assumption, then he only needs to define what he means
by maximum. In MOSES, you define what you mean. This is
done by giving a duration to the seastate, or defining
a multiplier. If you give a duration, MOSES will use
the moments of the response spectrum and the duration
to produce the probable maximum value. Alternatively,
you can define a multiplier for the RMS of the spectrum.
The default is to use a multiple of 3.72 which defines
the maximum to be the average of the 1/1000th highest
values. Other values for this multiplier are:

2.00 - Significant value 2.54 - average of 1/10th highest values 3.03 - average of 1/100th highest values