Q: I have a semi-submersible platform with mooring lines and risers. Can I simulate flooding of one of the columns in static process?

A: The STATIC PROCESS and HYDROSTATIC menus are magic in the sense that they were designed to do very specific traditional things. Thus they do not have the generality of most of MOSES. In particular, theses menus consider only a single body and connectors (other than a sling in Static Process) are ignored.

For your problem you will need to construct your own process:

• Do your flooding,
• Find equilirium, and then issue the command
• &event_store xx

Repeat this until you have stored events for for all situations of interesti. You can then use the PRCPOST MENU to report the results.

Q: Is there a problem with the &APPLY command? It appears that it is using the square of the fraction I tell it to apply.

A: In fact, what your file contains is:

    &apply -fraction -load_group hullwl %prop%
    &apply -fraction -force #jyforce %prop%

Q: Can MOSES calculate the natural period of a ship?

A: The mass in the equations of motion for a ship in the frequency domain depends on frequency. Thus, there is no classical natural frequency. Almost everybody talks about a natural period and when they do, they really mean where the response peaks.

Q: Is it possible to get the hydrostatic stiffness matrix from MOSES?

A: Not directly. If you do the following

     &status force
     &instat -move bod 0 0 1 0 0 0
     &status force

Q: Why are my plot titles not centered?

A: You have specified a proportional font for graphics (Times Roman, Helvetica, etc.). Choose a fix space font (Courier) and they will be properly aligned.

Q: How can I change the number of lines per page in the out file?

A: You need to change the "style" of the output page. In particular, use the command

   &STYLE OUTPUT -LPI   NUM

Q: Is there a summary that contains beam K Factors?

A: Yes, look at

   &SUMMARY
       BEAM PROPERTIES

Q: Is the added mass coefficient reported by MOSES the total added mass? In other words, will the total added mass be the displacement x the added mass coefficient reported or displacement x (1 + added mass coefficient reported).

A: This is a normal confusion. what we report is the added mass. What many others report is the hydrodynamic mass. The differ by 1; i.e. the hydrodynamic mass is the basic mass plus the added mass.

Q: can I get MOSES to output a table of data?

A: Yes, you can. The STORE command in the DISPOSITION MENU is designed to perform this task. By default it emits a "CSV" file but you can change this to emit an HTML file. If you want maxima, over several situation you can keep track of a set of data and then use &BUILDG to put the data into a matrix and the STORE to emit the table.

Q: Can you explain how to use &FILE?

A: The &FILE command is designed to be a general interface between the MOSES user and the computer's file system.

It is not designed to replace &DEVICE -USE_FILE which, in days gone past, was the way to change the file associated with a channel; i.e. the old command

    &DEVICE -USE_FILE G_DEVICE COW.EPS

    &CHANNEL GRA_DEV  -FILE COW.EPS

Q: What does the message "*** FATAL ERROR: Write Failed IOSTAT = xxx Disk Is Probably Full !" mean?

A: OK, this one really is obvious. If your disk is full, MOSES will not run.

Q: What does the message "*** FATAL ERROR: Unexpected END OF FILE on Type" mean?

A: During an INMODEL command, you asked MOSES to read a file that does not exist, or does not have the permission to be read. Check your use of &INSERT and &DEVICE -AUXIN, and make sure the referenced files can be opened.

Q: What does the message "*** OS ERROR: Refused to Allocate Space Probably Out Of Swap Space" mean?

A: This is an allocation request to the operating system that was not satisfied, see your System Administrator.

Q: What does the message "*** SECURITY ERROR: Update Is Incompatible With Installation" mean?

A: This means you downloaded the wrong update to MOSES from our website. For instance, you downloaded an update for Rev6.02, yet your present MOSES installation is Rev6.01. The download page warns against this.

Q: What does the message "*** SECURITY ERROR: XXXXX Is Not Authorized For This Version" mean?

A: This means you did not purchase MOSES Complete, and you are running a file that requires a command What does the message you do not have. This typically happens when running some of the test cases.

Q: What does the message "*** WARNING: No Data For A PICTURE" mean?

A: This usually means an invalid selection criteria is active. Use &REP_SEL -SELALL, or &PICTURE -RESET to check. The message can also mean there really is no data, caused by a problem while reading the model.

Q: Why do I get the message "*** WARNING: EQUILIBRIUM NOT FOUND WITHIN TOLERANCE"?

A: This means the program could not find equilibrium within the given tolerance using the given number of iterations. Normally, the user needs to determine the cause for this: either the program is chattering about a solution, or more iterations are required to find a solution. Typical solutions for this include specifying a better initial condition, or changing the default -OMEGA value. Click Here for further discussion.

Q: We are considering to use MOSES for lowering analysis. A typical case is for instance to install a cover or a template at the seabed. We typically want to analyze 4 different phases:

1. Lifting in air before the cover meets the surface
2. Lowering through the surface
3. Cover fully submerged, typically 5m below surface
4. Cover close to seabed, a few meters above the sea bed.

However; I am not sure of what the best approach would be for phase 2. I am thinking about carry out many time domain analyses of this phase, where I use different waves for each run. Then report the maximum and minimum (to check for uplift) loads in the slings for each run. After carrying out maybe 30-40 runs, I could collect the data and establish confidence intervals for the mean loads in the slings and use these results to establish limiting environmental criteria for the lifting operation. A Monte Carlo approach so to speak. Do you have any experience with these kind of analyses in Moses? Is it possible to define a "lowering velocity" for the cover when doing time domain analyses of the lifting process? As the cover is partly submerged, will Moses take buoyancy variations as the wave passes by into account? In that case, I guess I must use very small time steps in the analyses...

With respect to phase 4, I wonder whether Moses takes into account the increase of added mass as the cover gets close to the seabed.

A: I normally look at the problem a bit differently. Basically, you have one body connected to another; one floating, the other being supported. The two important factors are that the length of the support line (and hence its stiffness) is being changed and the second body changes it proximity to the water and the bottom.

We recently did a study of lowering a jacket and we found that the maximum load occurred before the jacket entered the water but the maximum DAF occurred when the jacket was completely submerged. Also if the water depths are large then a resonance will occur for some length of line.

That having been said the answers to your specific questions depend on how you model the cover.

• Morrisons equation is treated "correctly"; i.e. the current position is used in computing the added mass and the excitation. The bottom effect is ignored.
• Diffraction is "correct" at the position used to compute the pressures. As the body is lowered a true diffraction solution does not change. There is a option, -WAVE_RUN, on &PARAMETER which will compute the Froude Kryloff pressure "correctly", but the change in added mass is still ignored.
• A frequency domain approach is ok before the cover can contact the water and perhaps after it is totally submerged. I would, however, use the time domain for locations from the cover being at +- the maximum wave amplitude to the water plane.
• You can define a line change speed and simulate the lowering as a true time domain process.
• I would probably look at three locations: cover in the air, cover just touching the still water, and cover just submerged. Now, run time domains at these positions to find which one is critical and use it to establish an environmental limit.
• Actually, the above may not be what I would do, depending on the water depth. For deep water, you need to consider the mass and weight of the line and need to look at the response for all depths since you can have a resonance for some intermediate depth. Here, I would probably use frequency domain to find the critical depth, do a time domain at that depth, and then use the critical position to find the environmental limit.
• The simple connectors do not include the effect of their mass. You must either add 1/3 of this mass to the cover (a standard vibration approximation) or use a rod element.

Q: Why doesn't the conversion between metric tons and kilo-newtons change when using SI units and changing the value for the acceleration of gravity in the program?

A: This confusion between mass and force units has more to do with the way SI units are typically used than with our software. In Europe, it seems common to use mass units to define a weight. As a convenience, our software allows this by accepting metric tons as a force measure. This is why changing the value of "g" has no effect on the difference between metric tons and kilo-newtons.

Q: I issued the command &DEFAULT -DEPTH to change the water depth after INMODEL and MOSES does not take the command. It seems that I can change the water depth only by issuing the &DEFAULT command before the INMODEL. Am I missing something here?

A: Yes, you are missing something. Water depth is a property of the environment and is set on the &ENV command. The default water depth &ENV uses is set by &DEFAULT. Thus to actually change the water depth, you need to issue &ENV -DEPTH. The reason it worked if you put the &DEFAULT before the INMODEL is that after the INMODEL, a new environment is created which uses the defaults currently in force.

Q: Why does the log file show a negative bottom clearance?

A: You have forgotten to specify the water depth. To do this, use &DEFAULT -DEPTH DEPTH, followed by &ENV. Since the water depth is treated as part of the environment, the &ENV command is required for the water depth to be recognized by the program.

Q: What formula is used in MOSES for determining radius of gyration?

A: The recepies MOSES uses are those in any undergraduate dynamics book. Local plate inertia is ignored, and beams are assumed to be slender; i.e. the size perpendicular to the centerline is small in comparison to the length. Other than this, the computation is exact.

Q: Why does MOSES sometime suddenly abort with the message "*** FATAL ERROR: Bad Time In NUMEVE"?

A: You are plotting pictures of the events to the screen as you proceed. One should never produce pictures to the screen while using input from a file. This really confuses MOSES. The error you reported is a symptom of the confusion.

Q: Is the bottom clearance (or the maximum submergence) measured from the lowest joint or does MOSES really compute the lowest point?

A: MOSES really computes the lowest point (based on the data he is given); i.e. the size of any beam is used to compute the lowest point.

Q: We get the message that we cannot get more than 8 points out of the &INTEREST REPORT. Since 8 points is really not enough, Is there any way to increase the number of points?

A: The 8 point limitation stems from the amount of real estate we have available on a page for the report. If you need more, loop over the commands providing the information you want, changing the interest points in each step.

Q: I have a system composed of several "things". Should I model them as a single body with several parts, or as separate bodies?

A: Unfortunately, the answer here depends on what you are interested in investigating. First, Bodies are considered to be rigid (except when using generalized coordinates). Thus, if there is to be relative motion between two "things", then they must be separate bodies. Even it there is no relative motion, there are times when you may want to use separate bodies. Suppose that you are interested in the forces which hold two things together. If these two things are parts of a single body, there is no way to "look" at the connecting forces except by doing a structural analysis. If, however, these things are modeled as two bodies connected by some system of connectors, then the connection forces are readily available. Remember, however, that these connection forces are computed assuming the bodies to be rigid, so a stress analysis will yield somewhat different results, depending on the flexibility of the bodies.

Q: How can I arrange multiple bodies in space?

A: This is perhaps one of the more difficult tasks which one has to perform in MOSES. The simple answer is that you use the &INSTATE command. (For details, click here.) You need to specify the location and orientation of each body. There are several different ways of doing this, but perhaps the easiest to see is:

&INSTATE -LOC BOD1 X1, Y1, Z1, RX1, RY1, RZ1 \
-LOC BOD2 X2, Y2, Z2, RX2, RY2, RZ2

Q: Is the stiffness matrix (hydrostatic and connector) used in MOSES a 3 X 3 matrix with coupling terms, using the assumption of small motions?

A: No, this stiffness matrix is a N X N matrix where N is 6 times the number of bodies. In the frequency domain, it is assumed to be constant. In the time domain, it is computed correctly at each time step.