Compartment Questions


Q: What are the units for "Max DHead" and "Pres Head" generated by the command "Tank_bal"?
REV 7.05

A: Anything labeled "head" has unit of blength; i.e. either feet or meters.


Q: I have a tank with open valves and it is totally submerged. Why is it not 100% full?
REV 7.05

A: You have air traped in the tank. Once all valves submerge it is likely that you will have air trapped.


Q: What does the message

   *** ERROR: No Solution to Ballasting Problem Found"
mean?
REV 7.00

A: This means the program could not find a solution to the ballasting problem as posed. First, check if your initial condition is reasonable. Is your draft and trim such that the vessel displacement is larger than the applied weight? Then, check to make sure you have provided MOSES sufficient tanks to ballast, and there is sufficient size to these tanks to affect the desired change. You will normally need at least 4 tanks, a set of 2 each side of the centerline (port and stbd), and a set of 2 each side of the LCF (fore and aft).

This error is generated if the &CMP_BAL has problems.


Q: What does the message *** ERROR: In Computing Tank Depth" mean?
REV 7.00

A: This message usually occurs with very strange mean tank shapes. If you can not detect any obvious errors, send us your input files.


Q: Can I stop flooding a tank and have it continue with the current percent full?
REV 7.00

A: There is no direct way to do this, but the commands:

   &set amt = &compartment( tank  -amount)
   &compartment -correct -amount %amt
will accomplish what you want for the compartment "tank". The manual page on the string funcion &compartment( ) is here. The manual page on the command to fill compartments &compartment is here.


Q: When I flood the TUBTANK representing a jacket leg member, will MOSES assume the member is flooded for hydrostatic collapse?
REV 7.00

A: No, MOSES does not map the pressures for a TUBTANK compartment to the structural element, so hydrostatic collapse is not considered properly in this case. The manual page on the TUBTANK can be found here


Q: I am doing a dynamic upending of a spar. After a compartment gets "almost full" (99.99%) the flow rates and the heads change erratically. What is going on here?
REV 7.00

A: What you have here is a numerical problem. While the upending may be gentile, the internal pressure computation is not. In fact,

     P = C/V
where V is the volume of air. When a compartment get 100% full, V is zero and the pressure is large. You need a smaller time step to get good results here.


Q: What causes message "*** ERROR: TANK1 adds negative weight either permeability or definition wrong"?
REV 6.02

A: What does the message Either Permeability or Definition Wrong, mean? The most likely cause is you have defined your compartment so that the normals for the panels you created point in the wrong direction. In particular, if you have used the -BOTH option to define both sides of a compartment cross section, be sure to adhere to the MOSES Reference Manual:

"In this case, one first defines the positive portion and proceeds around the contour to define the negative portion. In all cases, one starts at the bottom of the station (minimum z and zero y) and proceeds around counter clockwise (point 2 is has non decreasing z from point 1)"

Also, be sure the permeability for interior compartments is negative.


Q: Can I define TUBTANK between the same joints where I have a flooded member?
REV 6.00

A: No, this is totally incorrect. If you flood the compartment, you have twice the amount of flooding in your model. MOSES has no way to tell if a TUBTANK is coincident with a flooded member, where the flood flag is set using BEAM -FLOOD yes.


Q: Why do I have problems finding hydrostatic equilibrium after issuing the "CMP_BAL" command with the "-CORRECT" option but I have no problem if I omit "-CORRECT" option?
REV 5.07

A: You have the wrong permeability sign on your compartment description. A wrong sign in the permeability using the "-CORRECT" option produces forces in the wrong direction, so, equilibrium cannot be found. The permeability should be positive for a buoyant piece (exterior compartment) and negative for a hole or an interior compartment. The manual page on the -CORRECT option can be found here .


Q: How can I model a TUBTANK where the inside diameter changes along the length?
REV 5.07

A: What you need is more than one tubtank per compartment:

     &describe compartment lega1
     tubtank 1000 *1 *2
     tubtank 1050 *2 *3
     tubtank 1000 *3 *4
etc. MOSES will combine these so that water properly flows to the proper location (provided that you have defined the total compartment). In other words, what you define for a compartment is the total enclosed volume between two bulkheads, and you can combine any number of pieces to accomplish this task. The manual page on TUBTANK can be found here.


Q: Why do I get a strange vertical center of gravity when I fill a non rectangular compartment half full?
REV 5.03

A: You did not use &COMPARTMENT -CORRECT. Without the -CORRECT option, the program uses an average depth of the tank to find the vertical center of gravity. The manual page on the -CORRECT option can be found here .


Q: Why does the &CMP_BAL command allow compartments to fill below the minimum specified with the -LIMITS option?
REV 5.03

A: The -LIMITS option applies to compartments specified after the option. Therefore, the correct syntax is &CMP_BAL BNAM -LIMIT 40 100 :TNAM to specify a minimum of 40% and a maximum of 100%. The order of the options on a command is important, and many options act on the data following the -OPTION on a command line. Then manual page on the -LIMITS option can be found here.


Q: I ballasted compartments of a barge using &COMPART -CORRECT -PER :TSEL 100. Why when I use &COMPART -FLOOD TNAME1 to evaluate damage, and then &COMPARTMENT-NO_FLOOD TNAME1, is the compartment not flooded "correctly".
REV 5.01

A: The command you used to "-NO_FLOOD" the compartment used the default flooding method. To be sure of getting the flooding method you want you can specify it on the command; i.e.

     &COMPARTMENT -CORRECT -NO_FLOOD TNAME1.
The manual page on the option -NO_FLOOD can be found here.