Q: Can I do a code check of a rod connector?

A: Before REV 7.03 the answer is "partially". If you look at rod stresses there is a column with the maximum normal stress divided by the ultimate tensile strength. You can use this to get the stress check in RP2SK. With REV 7.03 columns are available to give the Von Mises stress divided by the Yield and for a real code check including hydrostatic pressure.

Q: What units do I use with the -emod option when defining a flexible connector?

A: You should use the same units as you would with the &default command. This will depend on the units you are working in. Click here to see the documentation.

Q: How are the allowable force AF(1), AF(2) used for the GSPR and LMU connector classes?

A: The same way they are for any connector - they are only used to convert a force into a utilization - the force divided by the allowable.

Q: Could you be a bit more precise on how the -FRICTION option works with GSPR connectors?

A: The forces in the GSPR without fiction are given by:

    Fx   = Kx * Dx
    Fy   = Ky * Dy
    Fz   = Kz * Da

    Fh   = sqrt ( Fy*Fy + Fz*Fz )
    Fm   = MU * abs ( Fx ) / Fh
    FACT = min ( Fm, 1)

A physical model for what we have just described is shown in the figure:

Q: What does the message "*** FATAL ERROR: Solution for Line Cannot Be Found" mean?

A: This is normally caused by a bad line definition. For instance, with too shallow a water depth and too much bottom slope, the fairlead can actually be below the ocean bottom.

Q: What does the message "*** WARNING: Tension Specified is Less Than Zero H Case" mean?

A: This message means the tension specified is less than tension caused by the weight of the line hanging straight down from the fairlead. When this is the case there are two solutions and MOSES has found one, but you may want the other.

Q: I am performing a launch analysis using a barge with double rocker arms. Why do the X and Z coordinates of the second rocker pin from the &STATUS G_LWAY report not match what I expect?

A: These coordinates are in barge coordinates after the first tiltbeam has rotated to the "maximum angle".

Q: Why do I get both tension and compression in a GSPR element when I say it is compression only?

A: It is because you defined the element incorrectly. The "business" direction of a GSPR is the element X direction; i.e. the special features such as the -CONEPY spring and tension/compression flags work only in the X direction. Your element has stiffness only in the Z direction so it will work in both tension and compression. To fix this you should change:

      ~COMP GSPR COMPRESSION Z 100
      CONNECTOR C ~COMP *NODE1 *NODE2

      ~COMP GSPR COMPRESSION X 100
      CONNECTOR C -EULER +Z ~COMP *NODE1 *NODE2

Q: How does MOSES treat the "contents" in a rod element?

A: If you define the contents with

• A TEMPRES command in the &DATA menu and then
• bind this to an environment with a -TEMPRES option on an &ENV command,

Q: I am having trouble defining a TLP tendon. What am I doing wrong?

A: This is not as simple a problem as one would imagine. You need to communicate to MOSES clearly that you have a taught rod element. Thus, you need to have:

• the two attachment points directly above one another and
• the total length of the tendon needs to be less than the distance between the points.

Consider

   ~tendon ROD 558.8 88.9 -refine  5 -len   6.10
   ~tendon ROD 711.2 25.4 -refine 30 -len 365.76
   ~tendon ROD 711.2 28.6 -refine 60 -len 604.48
   ~tendon ROD 558.8 88.9 -refine  5 -len   3.66 -depan 1000
   &loop p ( 11 12 13 21 22 23 31 32 33 41 42 43 )
      &set anc = &point(*t%p -g )
      &set anc = &token(1:2 %anc) -1002
      CONNECTOR tendon%p ~tendon -a_locat %anc *t%p
   &endloop

You now have a properly defined tendon but it does not have the tension you wish. You can accomplish this with the command:

    &connector tendon@ -l_ten 500

Q: Can we get mooring line fatigue results at different locations along the line, such as the interfaces for a chain/wire/chain line makeup?

A: No, you get fatigue at the most highly stressed location in the line.

Q: How can I model a fender attached to a quay?

A: First, you need to obtain the force-deflection properties of the fender, normally provided by the fender supplier. Then, use the -CONEPY option to define these properties to MOSES. You want to specify a generalized spring here (GSPR), so you do not worry about the weight per length, diameter or Young's Modulus. Since you are defining springs in series, select a relatively stiff spring constant for the GSPR. Also, use a sense of COMPRESSION for the GSPR, and be sure to define this in the global system. Click here for the relevant section in the Reference Manual, and here for an example showing how to define a GSPR.

Q: How do we perform a mooring analysis for the transient, one line failed case, as per API RP2SK in MOSES?

A: The following advice illustrates how I would perform this analysis, but you need to make sure the requirements of API RP2SK are fulfilled. First, run the time domain simulation for the intact mooring condition. A sensible observation time here is 20 minutes. From this intact run, determine the most heavily loaded line, and the time this occurs. Inactivate this line, and rerun the time domain simulation using TDOM -RESET xxx, where xxx is the time of maximum load. This new portion of the time domain simulation should also be run for at least 20 minutes.

You can set TTRA_SET to FIND on &ENV -TIME, which instructs MOSES to find a maximum during the simulation that corresponds to a simulation with NCYCLES. You can then examine the time domain statistics, and see if the difference between the maximum value and the predicted maximum is acceptable to you. If there is substantial difference between these values, try increasing your observation time. Click here for reference:

Q: Why is it that when I check the static forces in the lines either side of the pulley using &STATUS F_CONN the force is the same, but when I perform a similar check, but using dynamic loads from the frequency domain with ST_CFORCE the force either side of the pulley is not the same?

A: Pulleys really are nonlinear in nature. The force in the lines either side of the pulley are the same, but there is also a force at the center of the pulley, acting normal to the body where it is attached. Better answers for those systems with pulleys are provided in the time domain, where the nonlinear effects are included.

Q: I am confused by the -CONEPY and -SEND options when defining connector classes. Suppose that I have

      ~CMP COMPELM 100 -LEN 3.00 -CONEPY 0     0 1e-10 1.80 \
                                        1000 1.90 5000 2.00
   

A: The free length is 3, the length specified with the -LEN option. The -CONEPY option specifies a spring of zero length By viewing it this way, you can use the -CONEPY option to alter the material behavior of the "basic" connector.

Q: Why do I sometime get the message "*** WARNING: Table Contains Negative Element Lengths" when I model a compression only connector?

A: This is relatively complicated, we first need to talk a bit about how MOSES handles "taut line" connectors. One of these connectors has a free length, a length so that when the two ends are separated by the length there is zero load in the connector. MOSES builds a table of length vs. force based on the data you supplied, and will interpolate a force from this table based on the current distance between the ends. For tension only elements, there are no physical restrictions. For compression elements, there are - the length of of the connector cannot be negative! Now, consider the following:

      ~cmp compelm 100 -len 2.00 -conepy 0     0 1e-10 1.80 \
                                        1000 1.90 5000 2.00

Q: I have a compression only connector, and I am having trouble finding equilibrium and I also get a message about capsizing when I try to do a time domain. How can I get around these difficulties?

A: Your basic problem is that your connector is too stiff. You defined the connector with the class

      ~cmp compelm 100 -len 2.10 -conepy 0     0 1e-10 1.90 \
                                        200000 2.00         \
                                 -emod 1e10

There are two things here. First, the stiffness is probably not correct in the first place. Often one looks only at the connector and forgets that it is also part of a larger whole which also deflects. Second, this is a classic case where one needs to use a "gap". A gap is different numerically than a compression only element. Instead of using the difference in location as a compelm uses to compute force, a gap imposes a constraint - there is no relative deflection between the two ends. This problem is much better behaved numerically.

Q: can I use a spring between two nodes of the same part?

A: Yes, this is allowed.