Moving Anchor Point

Hello everyone,

I am trying to model a mooring system for a barge. There are two sets of mooring lines. Primary mooring line (1 line) run parallel the length of the barge and the ends are connected to ground anchors. Secondary mooring lines (10 lines) are running perpendicular to the primary mooring line with one end connected to the primary line at equally spaced interval and the other end connected to the barge. 

I would request if someone can help me to understand how to model the 10 connection points between the primary line and the 10 secondary lines. As these points will be not be fixed and will change their position with the motion/ movement of the barge.

Thanks in advance!

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  • Hi Sid,

    From the description in the message I believe you want to use the "-clump" option when you define the mooring lines.

    I am not clear on the set up.  Can you provide a sketch?

    Georgina Maldonado

  • Dear Gerogina,

    I thank you much for such a quick response. Please see the sketch attached. Hope this will help you understand my problem better.

    Sid

  • Dear Georgina,

    The file you attached really helped me to model the connection points and I have been able to do so in my model. One more thing, I am not able to achieve an equilibrium for the barge. I feel it might be due to the small bodies (bodies at the connection points). Can you please shed some light on this?

    Thank you. 

  • Hi Sid,

    You are correct.  Equilibrium difficulties are probably due to the 10 extremely small bodies.

    When you use the "&equi" command MOSES sets a tolerance for the forces.  When the bodies are in a position where the residual forces are less than the tolerance it reports equilibrium is found.  The tolerance is set with the "&equi -tolerance" command.  The values after "-tolerance" tell MOSES was percentage of gravitational force of the bodies is the tolerance.  For a multi-body problem MOSES picks the smallest of the values. 

    Since your analysis has small bodies the tolerance for the large body is set unreasonably small.  You can try asking for equilibrium in portions.  You can use "&equi -ignore bodyname DOF".  Try alternating between the large body and the small bodies in the list of "bodyname".

    This tells MOSES what bodies DOF to ignore when picking tolerance and reviewing the residuals.  Due to the large difference in size of your bodies you will probably always get the message "WARNING Equilibrium not found within tolerance."  You will have to make a visual check of the forces and make an engineering judgement.

    I hope this helps.

    Georgina Maldonado

  • Dear Georgina, 

    I do understand what you explained. What if I didn't input any weight for those small bodies?

    While describing the small bodies in the .dat file you had used #weight command to define a small weight with each body. I deleted the #weight command line and re-ran the analysis. The equilibrium converged in 1 iteration. Is this the right thing to do or does this affect the mooring analysis on a whole?

    Thanks

  • Hi Georgina, 

    After running the static wind load analysis using &env, I checked the forces on the barge using &status force. I found the apart from the wind force there is a huge inertia force as shown in the excerpt below from the output file. 

    1. I believe this is again due to the small bodies. Please note that for above analysis I had deleted the weight on all the small bodies.

    2. Even with the weights on the small bodies I am getting such inertia forces however in that case, flex connector loads on the body is very small.

    3. Also by removing the small bodies and just simply using the secondary mooring lines (not using the primary mooring line) the inertia forces disappear.

    Please help me resolve this issue. 

  • Hello Sid,

    There are two ways to approach this problem.

    Method A.

    Pre-calculate the stiffness of the two-line mooring arrangement in the three degrees of freedom.  Then input those values as GSPR elements.

    Method B

    Model the connector points as bodies.

    We are looking at method B.

    This is a multi-body analysis.  The equations of motion use mass, damping, and stiffness.  For the small bodies the stiffness in the matrix will be dominated by the lines.  Damping will probably be very, very small.  When you look at the equations of motion mass is in the denominator and is multiplied by wave frequency.  When you use a value of 0 for mass you are putting 0 in the denominator.  You need to put a value, even if it is very small for the bodies connecting the lines.

    You sent the table resulting from the &status force command.  Without knowing the context of the commands before I cannot comment.  Were you in equilibrium before the &env command?  Did you try to find equilibrium after the &env command?   It appears you are not in equilibrium.

    I hope this helps.

    Georgina Maldonado

Reply
  • Hello Sid,

    There are two ways to approach this problem.

    Method A.

    Pre-calculate the stiffness of the two-line mooring arrangement in the three degrees of freedom.  Then input those values as GSPR elements.

    Method B

    Model the connector points as bodies.

    We are looking at method B.

    This is a multi-body analysis.  The equations of motion use mass, damping, and stiffness.  For the small bodies the stiffness in the matrix will be dominated by the lines.  Damping will probably be very, very small.  When you look at the equations of motion mass is in the denominator and is multiplied by wave frequency.  When you use a value of 0 for mass you are putting 0 in the denominator.  You need to put a value, even if it is very small for the bodies connecting the lines.

    You sent the table resulting from the &status force command.  Without knowing the context of the commands before I cannot comment.  Were you in equilibrium before the &env command?  Did you try to find equilibrium after the &env command?   It appears you are not in equilibrium.

    I hope this helps.

    Georgina Maldonado

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