Reporting Questions


Q: Is the -RECORD opion in the DISPOSITION Menu broken?
REV 7.06

A: No, it is not broken. You are making a common mistake in confusing "RECORD NUMBER" with TIME. The record number is just the row number of the matrix of data. It is easier to use the -EVENT option on the command that put you in the Disposition Menu to cull data than to use -RECORD.


Q: How does MOSES check vortex shedding?
REV 7.05

A: This is a four step process:

  • Calculate natural periods of each element From Structural Dynamics, by Mario Pas, it lists the formula for a simply supported beam as
              mode = sin [ n * pi* x / L]
          
    Instead of this, MOSES uses
              mode = sin [ n * pi* x / kL]
          
    i.e. we use the effective length kL instead of the length.
  • Calculate velocity which "matches" the natural period
             Vel = Str.Num * Diam / fn
          
    Here Str.Num is the Strouhal number which we take to be 0.2 and is essentially constant for cylinders, and diam is average diameter of element and fn is natural frequency in Hz.
  • Region II is described in the paper "Synopsis of lift, drag, and vortex frequency data for rigid circular cylinders" John H. Lienhard (1966) is the region with Reynolds Number between Re = 3e5 and 3.5e6. There is no vortex shedding within this region, In the &SUMMARY report,
             Vbeg/Vend = Re * nu / D
          
    Where nu is kinematic viscosity, D is the diameter, and Vbeg and Vend are the vlocities at the beginning and end of Region II.
  • The comment "check" is used when velocity due to the natural frequency (Vcr) is below Vbeg or above Vend.


Q: How is the EVE_INC parameter for the -EVENT option for reporting commands in the PRCPOST Menu related to the time step specified on the &ENV command?
REV 7.05

A: If EVE_INC is omitted (or 0) the events computed will be reported. Suppose you specify:

     -EVENTS 100 200 1
Then you will get results reported at 100, 101, 102, ..... 200. Now, suppose you originally had computed these with a time step of 2. MOSES will compute the reported event at t by a linear interpolation of the computed events at t1 and t2. Here t1 is the maximum computed event less than t and t2 is the minimum computed event greater than t. The -EVENTS option is available on most of the commands in the Post-Processing of a Process Menu.


Q: Can MOSES save the values in the DISPOSITION menu to a CSV file?
REV 7.05

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. The LMU test creates an html table using stored data. Click here to see the commands in the LMU test.


Q: Why do I sometimes get the message "*** INFORMATION: Too Many Selected Maximum is 8 - Excess is Ignored" when I am reporting locations of points of interest?
REV 7.04

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: What does "WET RADII OF GYRATION" mean?
REV 7.04

A: It means the radii of gyration considering the effect of contents in the compartments. The radii of gyration is reported as part of the buoyancy and weight report. Please see the &status command.


Q: Does MOSES report single or double amplitude motions/accel results?
REV 7.02

A: MOSES never uses "double amplitude". The RAOs are titles as motion amplitude/wave amplitude; so if you multiply an RAO by a wave amplitude, you get the motion amplitude. When MOSES does statistics, again this a motion, not a "double amplitude". It the output says 20 degrees, you will get 20 degrees one way and -20 degrees the other. The same is done for structural integrity. The results are checked for the mean + the amplitude and mean - amplitude.


Q: Is it possible to get the hydrostatic stiffness matrix from MOSES?
REV 7.01

A: Not directly. If you do the following

     &status force
     &instat -move bod 0 0 1 0 0 0
     &status force
The difference in the bouyancy force between the two the force status reports will give you the heave row of the stiffenss. If you do this for each degree of freedom, you will get the complete matrix. Click here to read more about &status. Click here to read more about &instate.


Q: Why are my plot titles not centered?
REV 7.01

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?
REV 7.01

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

   &STYLE OUTPUT -LPI   NUM
Click Here to look at the details.


Q: Is there a summary that contains beam K Factors?
REV 7.01

A: Yes, look at

   &SUMMARY
       BEAM PROPERTIES
This provides the Buckling Length times the K Factor divided by the element length. Click here for details.


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).
REV 7.01

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: What is the meaning of the "wave force" I get when I issue the &STATUS FORCE command?
REV 7.00

A: The result of all &STATUS command are applicable to the current, static, condition. As a result, the "wave force" is the mean drift force and the "drag" force is that due to current alone. There is no way to get the time variation of the wave force.


Q: Please explain why MOSES categorizes a 2083mm OD tube with a thickness of 50.8 mm as a "non-tube" in the JOINT PROPERTIES summary?
REV 7.00

A: MOSES finds the largest "thru member" and proclaims it the chord. In your case the diameter of the chord is 600 mm. Now, the diameter of the chord must be greater than any other tubular element or it cannot be built. Thus, MOSES assumes that the "big" tube is really not a tube at all.


Q: Why is the information about GMT and GML sometimes missing when I issue &STATUS B_W?
REV 7.00

A: This is because the traditional view of GM is only correct if the weight equals the buoyancy. If this is not true then the stability of the system gets much more complex. Thus, to avoid confusion, GMT and GML are only reported when the weight minus the buoyancy is less than some small number. Click here to see the manual page.


Q: Is the bottom clearance (or the maximum submergence) measured from the lowest joint or does MOSES really compute the lowest point?
REV 6.01

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: When I issue the command &STATUS FORCE, I get a several different types of forces listed, what are these?
REV 5.08

A:

  • WEIGHT : This is the weight of a portion of the system.
  • CONTENTS: The weight of ballast in compartments or fluid in an element.
  • BUOYANCY: The buoyancy of an a portion of the system.
  • WIND : The wind force acting on a portion of the system,
  • V_DRAG : The viscous drag acting on a portion of the system. This is the velocity squared term in Morison's equation, the viscous roll damping, or the vicsous drag on a piece with the -CS_CURR option. This can be either an excition due to wave or current, or a damping in still water.
  • WAVE : This is the linear exciting force on a portion of the system.
  • R_DRAG : The radiation damping due to hydroddynamics or that due to a #DRAG load attribute.
  • SLAM : This is the force due to mass transfer into the system; i.e. it is the velocity time the mass flow rate term in the equations of motion.
  • CORIOLIS: This is the force due to Coriolis accerleration. It also produces a "slowly varying" force.
  • W_DRIFT : This is the nonlinear part of the wave force, or the slowly varying "wave drift force". In MOSES, it does not contain an approxation of the Coriolis force. This is different than most other programs.
  • DEFORMATION: This is the force on a body due to deformation of the body. This only occurs when a body has generalized degrees of freedom.
  • EXTRA : This is a "extra" force that can be added to produce equilibrium in a given configuration. This is useful to "cover up" modeling errors or errors in the environment.
  • APPLIED : This is a true force applied to a portion of the body.
  • INERTIA : This is the mass of the body times the acceleration.
  • A_INERTIA: This is the added inertia times the acceleration.
  • C_INERTIA: This is the inertia of the contents times the acceleration.
  • FLEX_CONNECTORS: This is the force due to flexible connectors.
  • RIGID_CONNECTORS: This is the force due to rigid connectors.
  • TOTAL : This is the sum of all of the other contributions.


Q: Why, do I get a different value for KMT from the &STATUS report as compared to CFORM results for conditions that have trim?
REV 5.03

A: If you have zero trim, the program will provide the same result from both methods. This difference also occurs if the vessel origin is placed at midships. The reason for this is the axis transformation that takes place from the global to the vessel coordinate system. The GMT value we report in &STATUS is the derivative of the roll restoring moment (with respect to roll angle) divided by the displacement in vessel coordinates. The value we report for KMT with the CFORM command is the traditional naval architecture type value, in global coordinates.


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?
REV 5.03

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.