**Q:**
What is the reason of the warning

*** WARNING: Releases Created Element Singularity,
Problems May Result Due to Fixup

and how to do I solve this problem?

REV 7.06

**A:**
This message means that you have too many releases on a beam.

Most common reason is releasing torsion on both ends. Likewise releasing Axial, y shear, or z shear on both ends could cause the problem.

I personally think only in very rare cases should there be any releases.

**Q:**
Why is the hydrostatic bending moment different from
the structural one computed from RAOs?

REV 7.05

**A:**
The two results deserve to be different:

For the hydrostatic bending moment, MOSES assumes that the wave changes the shape of the water surface into a sine wave, finds equilibrium with this "lumpy" sea and then computes the moment.

For the hydrodynamic results, linear hydrodynamics is assumed; i.e. the sea remains flat but there is a pressure field and the sum of the forces is no longer zero, but is Ma.

Thus, on the one hand you have a nonlinear sea (but ignoring radiation and diffraction) that is equilibrated with buoyancy and the other you have a linear sea that includes all of the effects and that is equilibrated with inertia. The differences get greater the more that the mass distribution differs from the buoyancy distribution.

**Q:**
What is the local axis system of a vertical member in a
topside which is sitting on a barge?

REV 7.05

**A:**
The answer depends on your defaults (if you have what we shipped,
MOSES will have the same system as SACS), the -CA, -REFN, and
-DIR_LOC options on the element definition. This is described
here
Probably the best way to really see that the beam is
properly positioned is to look at a GL rendered picture
of the system.
Finally, a BEAM PROPERTIES summary will give you the
cosines of the angles between the element Y axis and the
part X, Y, and Z axes.

**Q:**
What is the difference between LCASE -TIME and
CASES -TIME?

REV 7.03

**A:**
Basically, CASES -TIME combines the results of LCASE -RAO. LCASE
-TIME combines the input loads which would be used for LCASE -RAO
before the stiffness analysis; i.e. you can get good stuff with
a nonlinear solution with LCASE -TIME, but there are no nonlineraties
with CASES -TIME.

**Q:**
How can I choose the times for load cases after
separation during a launch?

REV 7.01

**A:**
You can do them every second or two from separation
until both the top and bottom have there greatest
submergence. This will make sure that you have
properly checked hydrostatic collapse.

**Q:**
How does MOSES compute the membrane stress for plate elements?

REV 7.01

**A:**
The plate is composed of either three or four constant strain
triangles. The stress in each triangle is computed from
the displacements at the nodes and then averaged.

**Q:**
What does the message
*** ERROR: Singularity In Stiffness At *X.XXX"
mean?

REV 7.00

**A:**
This means there is a problem with the model you used
for the structural analysis. In particular, it is
reporting a singularity in the stiffness matrix at
the referenced node. In general, there are two classes
of singularity: local and global. The local ones are
caused by stiffness mismatches between elements and
the global ones are caused by not completely restraining
the system; i.e. here the structural system can move
without causing a change in load. This often happens
when one has a simple "stick" model of a vessel.
MOSES adds water springs to restrain the vessel, but
since all the nodes are in a line you still get
a rigid singularity in RX.

**Q:**
What does the message
"*** WARNING: xxx Sig. Figures Lost At *X.XXX"
mean?

REV 7.00

**A:**
This is an indication of problems with the model, related
to relative stiffness at the referenced node. By
looking at all the elements that frame into this node, it
should be apparent which element is causing the problem.
Significant figures lost up to 4 or 5 are typically
ignored, as long the overall structural results look
good. Somewhere beyond 6 significant figures lost, the
stiffness matrix is declared singular.

**Q:**
What does the message
"*** WARNING: Fixing Zero On Diagonal Of Stiffness At *X.XXX"
mean?

REV 7.00

**A:**
There is a zero on the diagonal of the stiffness matrix
at the specified node, and MOSES is adding a large
spring at that degree of freedom.

**Q:**
What does the message
*** FATAL ERROR: Nodes Are Not Connected to Rest
mean?

REV 7.00

**A:**
It means that you are trying at least two, independent structures.
In MOSES the subset of the model you are trying to solve must
be connected. This error normally occurs when one is trying to
transport more than one piece of cargo on a single barge.
To make this problem "go away" you can either make each piece
of cargo a separate piece and solve them one at a time, or
connect them with weak beams. Many of these details are
taken care of automatically with the Automated Installation
Tools.
To see which elements are not connected,
take the list of nodes that comes
with this error message, create a selection
criterion. Use the criterion to show a picture of the
structure: &PICT ISO -ENDS :LIST -COLOR SELECTED.

**Q:**
Can I find out the reactions between the bottom
of a jacket and the ground during a deck mating?

REV 7.00

**A:**
If you believe that the jacket is a rigid body
then the reactions are part of the analysis.
To include the stiffness of the jacket you can
do a "multi body" stress analysis:

STRUCT &SELEC :B -SEL JACKET DECK LCASE -PROCESS T1, T2, ..... BODSOLVE -BODY :B ENDThis will consider the stiffness of the jacket but not any dynamics of the elements in the deck and jacket.

**Q:**
How can I analyze a situation where extra braces are
added after cargo has been placed on a barge?

REV 7.00

**A:**
I suppose you are asking how you can get a
solution in the time domain which has the
effect of the extra braces on dynamics, but
not the incorrect effect on statics.
This can be accomplished as follows:

STRUCT LCASE -PROCESS BEFORE 0 SSOLVE END MEDIT ~BRACE TUBE 36 1 BEAM ~BRACE *B1 *C1 END STRUCT LCASE -PROCESS AFTER 0 SSOLVE END TDOM STRUCT LCASE -PROCESS T010 10 SSOLVE END STRPOST CASES -COMBINE A010 T010 1. BEFORE 1. AFTER -1 BEAM CODE -LOAD A010Here we make three trips through the structural solver:

- The first captures the correct static situation before the braces are added,
- The second captures the incorrect static situation after the braces are added (the braces have static load here), and
- The last captures the incorrect dynamics.

**Q:**
I have a rectangular barge modeled as a line of beams with
a uniform load distribution. Why are the structural bending
moments and shears not what I get from statics?

REV 7.00

**A:**
The difference is the way the loads are applied. The weight is
divided equally among the nodes. The buoyancy forces, however,
are "lumped" to the nodes at the ends of the panels. Now, suppose
that you have N+1 nodes and a weight (or buoyancy) of L.
We have

-L/(N+1) force applied to each node due the weight, andfor each interior nodes

L/N force due to buoyancy.At the ends, however, you do not get to add the contribution due to a panel on each side. Here you have

L/(2N). Thus you have a net force on each node of L [ 1/N - 1/(N+1) ] in the interior, and L [ 1/(2N) - 1/(N+1) ] at the ends. Using your data, thisis

12500 [ 1/100 - 1/51 ] = 120

**Q:**
How does MOSES do modal analysis?

REV 6.02

**A:**
MOSES actually does not perform modal analysis. MOSES does
extract modes and their corresponding natural frequencies. To
perform this task, the method of subspace iteration is used as
outlined in Bathe's book, *Finite Element Procedures in
Engineering Analysis* (ISBN 0-13-317305-4). This is a robust
method which deals with the complete stiffness and mass matrices and
does not require a "Guyan Reduction" for efficiency. With most
other programs, the modes are the solution; in MOSES, they are the
beginning. Since the mass and stiffness matrices will normally
change as a simulation progresses, MOSES uses the modes as
additional degrees of freedom during a simulation. In this way,
the natural frequencies for the current situation are used
instead of those at some nominal position.

**Q:**
How does MOSES "inertia relieve" structural load cases?

REV 6.01

**A:**
For all load cases except for those created using LCASE -STATIC,
the loads will sum to zero. For this to happen,
MOSES will compute an extra acceleration so that the
total force, including this extra acceleration, sums to
zero. This results in a change in force due to inertia
for every little piece of the model.

**Q:**
I have a hull hydrostatic model and a hull structural model , whereby I
compare the deflections of the two models. The structural model is composed
of two longitudinal beams and some transverse link beams, both made up of
prismatic sections.
The deflection results do not compare nicely. Why is it so ?

REV 5.10

**A:**
Without having the data it is impossible to say exactly what is going on, but:

- Never model the structure of a vessel as two beam (unless,of course, you are modeling a catamaran). It is impossible to get the shear and torsion correctly this way. Use either a single beam or a complete beam/plate model.
- Is the difference a rigid motion? Longitudinal strength uses a beam pinned at the bow and stern, did you restrain your structural model the same way?
- The answers will never be exactly the same because of the way the load is applied for a structural analysis. For more information, click here.

**Q:**
What are the properties of the water springs that MOSES automatically
generates during an upending analysis.?

REV 5.08

**A:**
I am assuming here that you are performing a structural analysis of a
jacket upend. The water springs are computed on a node by node basis,
and are an approximation of the waterplane area associated with each
node. Specifically, the spring constant at a node is the total
waterplane area divided by the total number of nodes under water.
If you do not want the water springs, add your own restraint and the
water springs will not be provided by the software.

**Q:**
I want to export the applied loads, so I issued the commands

STRUCTURAL EXP_ALOADWhy do I get an error message?

REV 5.07

**A:**
You can get this error message for quite a few reasons. In fact,
what MOSES is telling you with this message is that he is missing
some information. In your case, it is the load information. The
EXP_ALOAD command says to export the loads for each defined load
case, but you have defined none! Hence the message. You need

STRUCTURAL LCASE ... EXP_ALOADwhere what follows on the LCASE command defines the load case to be exported.