Force Computation Questions

Q: Do I need to calculate the RAOS before a time domain analysis if I want to include radiation and (or) Coriolis forces?
REV 7.05

A: The drift force has three components.

1. mean drift which is always present (we will call this static drift)
2. dynamic drift which is due to the motions of the body (we will
call this radiation), and
3. a force due to any rotational motion (Coriolis).
The results of TDOM are dependent on the RAO calculations. You need to compute the RAOs before you perform a time domain analysis if you are going to be including radiation and Coriolis terms.

Q: If I specify that a tube is -FLOODED YES and the tube accelerates, is the inertia of the entrapped fluid included or omitted?
REV 7.05

A: MOSES assumes that the tube is open at the ends. Thus, the inertia included for acceleration perpendicular the centerline and excluded for acceleration parallel the centerline.

Q: How can I find the drag and inertia coefficients that Moses uses for tubular members?
REV 7.05

A: These are set with an &PARAMETER command. All values set with this command are defined in a moses.cus file somewhere. MOSES reads this file from /ultra/data/progm, /ultra/data/local, %HOME%/moses.cus and finally in the local directory. You can find out what they are with the command &STATUS PARAMETER.

Q: Why do my "Wave Dirft" forces look strange?
REV 7.05

A: You do not have a good panel model for diffraction. Remember, the drift force depends on the gradient of the velocity potential while the first order forces depend only on the potential. Thus, the drift forces are more sensitive to numerics than the first order ones.

Q: How can I specify different added mass and damping coefficents for different plates?
REV 7.05

A: You can use either #PLATEs or #AREAs, and specify the added mass multiplier with -AMASS and the drag multiplier with -DRAG.

Q: How does MOSES compute the added mass on a plate?
REV 7.02

A: The added mass for plates and #PLATES is computed as defined in DNV Classification Notes 30.5; i.e. the added mass depends on the aspect ratio of the submerged portion of the plate. For #AREAs, it is computed as outlined in the DNV notes except that the coefficient is the one for an aspect ratio of 1.

Q: Is there a way to scale the drag loads on a beam to check for slamming?
REV 7.01

A: The during a time domain, the slam loads are automatically included in the loads on a beam. If you do anything, they will be accounted for twice.

Q: Can I input directionally dependent Cd / Cm coefficients for a beam?
REV 7.01

A: Not directly. MOSES treats all Morrisons equation members as equivalent tubes. If you feel that this matters then you should not model them as beams but as "pieces" or as a set of #PLATES.

Q: How is the current/wave drag force on an area computed?
REV 7.01

A: See Chapter XIII of:

Q: How is the wind force on an area computed?
REV 7.01

A: The basic equation is:

    F = A * Ch * Cs * Rho * v **2 /2
  • A is an area,
  • Ch is the height coefficient
  • Cs is the shape coefficient
  • Rho is the density of air, and
  • v is the wind speed.
The meaning of A and the direction of the force depend on the current setting of a MOSES parameter. If you use -AF_ENVIRONMENT YES then the force is in the opposite direction of the wind and A is the area projected to the wind. If -AF_ENVIRONMENT NO was used, the the direction of the force is in the direction of the normal to the area and A is the area.

Q: How can I find out the drag force on a tendon?
REV 7.00

A: There is no direct way to get this result because it is not really useful except for checking. Thus, you can:

  • Turn off the environment,
  • Issue the command &STATUS F_ROD,
  • Turn on the environment with the current, and
  • Again issue the command &STATUS F_ROD.
the difference in the internal forces between the two &status results is the effect of the drag on the tendon. In particular, the first and last values can be used to compute the values of the force and where it is located.

Q: Can you define your own spectral force in MOSES?
REV 7.00

A: I am not sure what you mean by spectral force, but you can define a force and define a time variation to the force and bind them together. Click here for the Reference Manual and here for a test.

Q: Does MOSES use shielding effects when computing the drag force on a tubular structure?
REV 7.00

A: Drag force is computed for all members, no shielding is assumed.

Q: Why does the ST_EXFORCE command report drift forces which do not look correct?
REV 5.08

A: What you are getting with ST_EXFORCE is not wave drift, but first order wave excitation. You can look at wave drift raos in the Hydrodynamics Menu with V_MDRIFT. To see the results in an irregular sea, do:

       &ENV -SEA .....
The "wave force" printed here will be the mean wave drift corresponding to the sea you just defined.