Connector Questions


Q: How can include damping in a cable?
REV 7.04

A: Use the -DAMPING option on the class definition command.


Q: Can I add a drag force to a spring buoy?
REV 7.04

A: No. If drag is important, then the buoy should be modeled as a body.


Q: Is the ~roller class only for use with a pipe assembly ?
REV 7.04

A: Yes, it is only useful for a pipe assembly. This is one of several types of flexible connectors.


Q: Why does my mooring line go below the sea floor?
REV 7.04

A: This happens because you have a line with a spring buoy. The manual says:

     The -CLUMP options adds a "clump weight" of weight CW
     (bforce) at the end of the current segment. If CW is less
     than zero for the first segment, the program will assume
     that there is a spring buoy connected between segments 1
     and 2. In this case, CW is the negative of the maximum buoy
     displacement and CLEN is the length of the pendant line to
     the buoy. Thus, the connection will be constrained to lie on
     CLEN feet or meters below the water surface until the load
     that the lines exert on the buoy is equal to the maximum
     displacement. The buoy will then sink, so that the connection
     is in equilibrium. The solution for the catenary is exact
     except that the water depth is ignored for the first segment;
     i.e. MOSES does not consider the possibility of grounding
     between the spring buoy and the fairlead.
The last sentence says it all. MOSES is not correct when any segement of the line between a spring buoy and the fairlead goes below the ground.


Q: Why is my spring buoy floating in air?
REV 7.04

A: Your problem is that you are using a buoy with a H_CAT. The manual says

      "A H_CAT ignores both the bottom and the water line, so a
      spring buoy cannot be used here.By default, a H_CAT ignores
      the weight of the element so that it is really a tension only
      SL_ELEM. A value of EXACT for FLAG considers the weight."
You can simulate what you want by using a B_CAT with a sloping bottom.


Q: How can I model a flexible boom for crane barge?
REV 7.04

A: Two ways. Since most of the flexibility comes from the flexibility of the backstays. You can make the boom a body and connect it to the barge. This is not easy and is numerically difficult. Instead, you can include this flexibility as a "spring at the end" of the lifting line.


Q: Can I do a code check of a rod connector?
REV 7.02

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. Click here to read more about the post-process of rods.


Q: What units do I use with the -emod option when defining a flexible connector?
REV 7.02

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?
REV 7.00

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. Click here to read more about flexible connectors.


Q: Could you be a bit more precise on how the -FRICTION option works with GSPR connectors?
REV 7.00

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

    Fx   = Kx * Dx
    Fy   = Ky * Dy
    Fz   = Kz * Da
Here, Ki are the spring constants in each direction and Di are the corresponding relative deformations. The -FRICTION option can be used to limit the force in the element y and z directions based on the element x force. After the force components Fx and Fz have been computed, they are checked against the product of Fx and MU (the friction coefficient); i.e. we compute
    Fh   = sqrt ( Fy*Fy + Fz*Fz )
    Fm   = MU * abs ( Fx ) / Fh
    FACT = min ( Fm, 1)
and then actually apply FACT * Fy and FACT * Fz.

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

Click here to read more about flexible connectors.


Q: What does the message "*** FATAL ERROR: Solution for Line Cannot Be Found" mean?
REV 7.00

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?
REV 7.00

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: Why do I get both tension and compression in a GSPR element when I say it is compression only?
REV 7.00

A: It is because you defined the element incorrectly. You have defined it with:

$
$****************************************
$
&instate -loc 0 0 2 0 0 0
medit
   ~cccc gspr compression z 100
   connector e ~cccc   *1
end
The &instate command sets the initial distance between the nodes to 2 units. The "business" direction of a GSPR is the element X direction; i.e. the special features such as the -X_PY 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 the "business" direction to be in the body -Z direction:
   ~cccc gspr compression x 100
   connector e ~cccc -euler -z  *1
Click here to read more about -euler option.


Q: How does MOSES treat the "contents" in a rod element?
REV 7.00

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,
the mass of the contents will be handled correctly. In particular, they will increase the hoop stress as the depth increases and they will add mass to the system perpendicular the rod but not along its centerline. Click here for an example.


Q: I am having trouble defining a TLP tendon. What am I doing wrong?
REV 7.00

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.
If either of these conditions is not satisfied you will have difficulties.

Consider

&set ne = &point(coordinate *ne -g)
&set nw = &point(coordinate *nw -g)
&set se = &point(coordinate *se -g)
&set sw = &point(coordinate *sw -g)
medit
   ~tendon ROD 55.88 8.9 -refine  60 -len  960   -depan 1000
   &loop p (ne nw se sw)
      &set anc = &token(1:2 %(%(p))) -1000
      *anc%p  %anc
      &type anc%p  %anc
      CONNECTOR tend%p ~tendon *%p *anc%(p)
      &type CONNECTOR tend%p ~tendon *%p *anc%(p)
   &endloop
end
Seveal things are on interest here. To understand this you should know that the points *ne *nw *se and *sw are at 39 units below mean water level. First is that I used &point(coordinate *XX -g) to get the x and y coordinate of the top of the tendon. Then I defined a bottom node *anc%p of the tendon to be the same x and y coordinate. I am sure of this because the &point string function gives me the location of the top. Also, I placed the z coordinate of the anchor a distance greater than the sum of the length of the segments.

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
where 500 is the desired tension. Click here to read more about defining a ROD element. Click here to read more the &point command. Click here to read more the CONNECTOR command. We have two samples that have tendon TLPs TLP_TEND and TLP_SL.


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?
REV 7.00

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?
REV 7.00

A: First, you need to obtain the force-deflection properties of the fender, normally provided by the fender supplier. Then, use the -X_PY 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. For an example of how to define fenders here.


Q: How do we perform a mooring analysis for the transient, one line failed case, as per API RP2SK?
REV 7.00

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?
REV 6.01

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. To read more on the &STATUS F_CONN command click here. To read more on the ST_CFORCE command click here.


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

      ~CMP COMPELM 100 -LEN 3.00 -X_PY 0     0 1e-10 1.80 \
                                        1000 1.90 5000 2.00
   
What is the free length (length when the connector has zero load) of this connector?
REV 5.05

A: The free length is 3, the length specified with the -LEN option. The -X_PY option specifies a spring of zero length By viewing it this way, you can use the -X_PY 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?
REV 5.05

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 the connector cannot be negative! Now, consider the following:

      ~cmp gspr compression x 100 1000 \
       -len 2.00 -x_y 0     0 1e-10 1.80 \
       1000 1.90 5000 2.00
MOSES will attempt to build a table of force vs. deflection for forces up to 5000 (it uses the largest force for the table limit when you specify a -x_py). The deformation due to the -x_py is 2 and there is an additional deformation of 5000 * 2 / AE due to the "line part" of the connector. When you add these two deformations together, you find that the compression in the element is more than the original length. Thus, the deformed length of the element is negative. This leads to all manner of numerical difficulty! The fix here is to either make the free length longer, or stop the table sooner, i.e. use -len 2.00+something, or make the 2.00 in the -x_py 2.00-something.


Q: Can I use a spring between two nodes of the same part?
REV 5.05

A: Yes, this is allowed.


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?
REV 5.03

A: These coordinates are in barge coordinates after the first tiltbeam has rotated to the "maximum angle". More on &STATUS G_LWAY can be found here.