Load Group Questions


Q: What does the message "*** ERROR: LGROUP *C1A Spans Parts" mean"?
REV 7.00

A: The error message says that an attribute of the load group has a point associated with it that is in a part which is at different than the one to associated with the load group. For example:

&DESCRIBE PART COW
*C1A 10 30 20
&DESCRIBE PART DOG
&DESCRIBE LOAD GROUP *C1A
#WEIGHT *C1A 10 1 1 1
will produce this message because the point *C1A belongs to part COW and the load group *C1A belongs to part DOG.


Q: I have input the wind and current loads as data tables. For some reason I get no current loads. Why is this?
REV 6.01

A: MOSES uses the global location of the point defined on the #TABLE command to decide whether or not to compute wind/or current. In your case you have this at +8.6 and you have a draft of 3.6, so it is out of the water.


Q: I am using #PLATE commands to model wind areas in my model. When I look at the summary from LOADG_SUM ATTRIB, some of the areas are negative. Have I done something wrong?
REV 5.07

A: No, the areas are correct. For this summary report, the normal derived by the order of coordinates on the #PLATE command is used, occasionally resulting in a negative area. For the actual computation of wind force, the absolute value of the area is used.


Q: When defining #DRAG should I specify the displacement in metric tonnes * g?
REV 5.06

A: This confusion between mass and force units has more to do with the way SI units are typically used than with our software. Outside the US, 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. One never specifies mass to MOSES, always force.


Q: What is #DRAG?
REV 5.03

A: This is a linear drag matrix applied at some specified point on a body. This is normally found in a barge definition in our library of barges, along with the #AMASS command. Without these commands, drag and added mass of the barge would not be included in a jacket launch.


Q: When using #TANKER why do I sometimes get current and wind forces in a direction opposite of what I expect?
REV 5.03

A: This is a function of the OCIMF data. #TANKER uses the curves proposed by the Oil Companies International Marine Forum (OCIMF) which were based on extensive wind tunnel and tank tests on typical tankers and published in the book, "Prediction of Wind and Current Loads on VLCCs" (now in its second edition, 1994). We have incorporated digitized versions of these curves into MOSES; therefore, what is derived is essentially what was measured. The question now becomes why are these forces in a direction which surprise you - the answer is probably lift, but this depends on your expectations. The hull will behave like an aerofoil where the flow does not separate immediately at the bow (and particularly if it is cylindrical as far as wind loads are concerned). As a consequence, the longitudinal force components may be "negative"; i.e. up current or upwind, for some directions. Rest assured, however, the resultant force is always downweather - there is a net drag!


Q: How can I get beam wind force when I specified head wind only?
REV 5.03

A: You specified X, Y and Z values for area on the #AREA command. The #AREA command defines a plane on which the environment acts using these values. The force normal to this plane is then calculated by the program. To truly separate the different planes when describing cargo, use one #AREA command for each plane.


Q: How does #LSET work?
REV 5.03

A: #LSET is a user defined force. These forces can be -FORCE is used to apply the load set, the forces are immediately applied to any beams. To get the forces on a load group, the load group must also be applied. Conversely, the force will not be applied to either beams or load groups unless &APPLY -FORCE is used.