Q: How can I choose the times for load cases after separation during a launch?

A: You can do them ever second or two from separation until both the top and bottom have there greatest submergence. This will make sure that you have properly checked hydrostatic collapse.

Q: How does MOSES compute the membrane stress for plate elements?

A: The plate is composed of either three or four constant strain triangles. The stress in each triangle is computed from the displacements at the nodes and then averaged.

Q: What does the message *** ERROR: Singularity In Stiffness At *XXXXX" mean?

A: This means there is a problem with the model you used for the structural analysis. In particular, it is reporting a singularity in the stiffness matrix at the referenced node. In general, there are two classes of singularity: local and global. The local ones are caused by stiffness mismatches between elements and the global ones are caused by not completely restraining the system; i.e. here the structural system can move without causing a change in load. This often happens when one has a simple "stick" model of a vessel. MOSES adds water springs to restrain the vessel, but since all the nodes are in a line you still get a rigid singularity in RX.

Q: What does the message "*** WARNING: xxx Sig. Figures Lost At *XXXXX" mean?

A: This is an indication of problems with the model, related to relative stiffness at the referenced node. By looking at all the elements that frame into this node, it should be apparent which element is causing the problem. Significant figures lost up to 4 or 5 are typically ignored, as long the overall structural results look good. Somewhere beyond 6 significant figures lost, the stiffness matrix is declared singular.

Q: What does the message "*** WARNING: Fixing Zero On Diagonal Of Stiffness At *XXXXX" mean?

A: There is a zero on the diagonal of the stiffness matrix at the specified node, and MOSES is adding a large spring at that degree of freedom.

Q: What does the message *** FATAL ERROR: Nodes Are Not Connected to Rest mean?

A: It means that you are trying at least two, independent structures. In MOSES the subset of the model you are trying to solve must be connected. This error normally occurs when one is trying to transport more than one piece of cargo on a single barge. To make this problem "go away" you can either make each piece of cargo a separate piece and solve them one at a time, or connect them with weak beams. Many of these details are taken care of automatically with the Automated Installation Tools. To see which elements are not connected, take the list of nodes that comes with this error message, create a selection criterion. Use the criterion to plot the structure: &PICT ISO -ENDS :LIST -COLOR SELECTED.

Q: can I find out the reactions between the bottom of a jacket and the ground during a deck mating?

A: If you believe that the jacket is a rigid body then the reactions are part of the analysis. To include the stiffness of the jacket you can do a "multi body" stress analysis:

    STRUCT
        &SELEC :B -SEL JACKET DECK
        LCASE -PROCESS T1, T2, .....
        BODSOLVE -BODY :B
    END

Q: How can I analyze a situation where extra braces are added after cargo has been placed on a barge?

A: I suppose you are asking how you can get a solution in the time domain which has the effect of the extra braces on dynamics, but not the incorrect effect on statics. This can be accomplished as follows:

    STRUCT
        LCASE -PROCESS BEFORE 0
        SSOLVE
    END
    MEDIT
       ~BRACE TUBE 36 1
       BEAM ~BRACE *B1 *C1
    END
    STRUCT
        LCASE -PROCESS AFTER  0
        SSOLVE
    END
    TDOM
    STRUCT
        LCASE -PROCESS T010  10
        SSOLVE
    END
    STRPOST
       CASES -COMBINE A010 T010 1. BEFORE 1. AFTER -1
       BEAM CODE -LOAD A010

• The first captures the correct static situation before the braces are added,
• The second captures the incorrect static situation after the braces are added (the braces have static load here), and
• The last captures the incorrect dynamics.

Q: Why if I use S_BODY and S_PART to select two of my 8 bodies does BODSOLVE still and try to solve all 8?

A: Until December 2004, the manual was misleading. S_BODY, S_PART, and S_REST did not work with BODSOLVE. Click here to read the new documentation.

Q: How does MOSES "inertia relieve" structural load cases?

A: For all load cases except for those created using LCASE -STATIC, the loads will sum to zero. For this to happen, MOSES will compute an extra acceleration so that the total force, including this extra acceleration, sums to zero. This results in a change in force due to inertia for every little piece of the model.

Q: I have a rectangular barge modeled as a line of beams with a uniform load distribution. Why are the structural bending moments and shears not what I get from statics?

A: The difference is the way the loads are applied. The weight is divided equally among the nodes. The buoyancy forces, however, are "lumped" to the nodes at the ends of the panels. Now, suppose that you have N+1 nodes and a weight (or buoyancy) of L. We have

-L/(N+1) force applied to each node due the weight, and

L/N force due to buoyancy.

 
L/(2N). Thus you have a net force on each node of
 
L [ 1/N - 1/(N+1) ]  in the interior, and
 
L [ 1/(2N) - 1/(N+1) ] at the ends. Using your data, this
 

12500 [ 1/100 - 1/51 ] = 120

Q: What are the properties of the water springs that MOSES automatically generates during an upending analysis.?

A: I am assuming here that you are performing a structural analysis of a jacket upend. The water springs are computed on a node by node basis, and are an approximation of the waterplane area associated with each node. Specifically, the spring constant at a node is the total waterplane area divided by the total number of nodes under water. If you do not want the water springs, add your own restraint and the water springs will not be provided by the software.

Q: I want to export the applied loads, so I issued the commands

    STRUCTURAL
        EXP_ALOAD

A: You can get this error message for quite a few reasons. In fact, what MOSES is telling you with this message is that he is missing some information. In your case, it is the load information. The EXP_ALOAD command says to export the loads for each defined load case, but you have defined none! Hence the message. You need

STRUCTURAL
LCASE ...
EXP_ALOAD

Q: How does MOSES do modal analysis?

A: MOSES actually does not perform modal analysis. MOSES does extract modes and their corresponding natural frequencies. To perform this task, the method of subspace iteration is used as outlined in Bathe's book, Finite Element Procedures in Engineering Analysis (ISBN 0-13-317305-4). This is a robust method which deals with the complete stiffness and mass matrices and does not require a "Guyan Reduction" for efficiency. With most other programs, the modes are the solution; in MOSES, they are the beginning. Since the mass and stiffness matrices will normally change as a simulation progresses, MOSES uses the modes as additional degrees of freedom during a simulation. In this way, the natural frequencies for the current situation are used instead of those at some nominal position.