Learning Forum

[d1227078662]

Dear all:

I would like to know why after adding a mooring system to a ship, there are multiple damping values (some much higher than the standard damping values and some much lower than the standard damping values) in the direction of the ship's bow rock and transverse swing when using AQWA to calculate the anchor chain tension. The following figure shows the material properties and arrangement of two anchor chains.

                 

                   

             

[Usman]

Hello,

I am trying to create a new post. After writing it, when I submit the post, it dissappears. Can you please let me know how did you create a post in this learning forum?

[d1227078662]

When you're done creating a post first save it, then go to it from the saved draft and edit it to publish it

[Yu Bi]

Hi, there, it looks like there are some unstable natural mode results, and for unstable cases, the critical damping percentage is negative. See Aqwa theory manual section 11.4 equation 11.19 for more information. 

There could be a lot of reasons for an unstable natural mode. From the cable settings you shared, what could be improved is the maximum expected tension, which may be too large compared to the initial tension. The maximum expected tension is not a material property like ultimate tensile strength but defines the furthest point in the 2D quasi-static cable database. Talking about the cable database, the negative/positive dZ range settings are relative distances from the initial position, so 50 meters/60 meters seem to be a bit too large if this is not the relative distance you expect your structure to move in the vertical direction. However, if you use cable dynamics, these parameters won't matter as they won't be used.

I would suggest you adjust those settings and solve them again or try cable dynamics.

 

[d1227078662]

Dear Yu

Thanks for your reply.

When I use the cable dynamics and adjust the pre-tension of the anchor chain, I still get instability with negative critical damping percentage. The results of tension calculations during cable time domain calculations as well as the results of Yaw and Roll rocking of the ship don't seem to be reasonable, should I increase the critical damping in these two directions.

     

[Usman]

Dear,

It is a humble request. Can you please let me know how the IVC mixture in tutorial 2 (Dual fuel combustion modelling using a 60% sector mesh) of Forte was calculated? I followed the reference paper (Refernced at the end of the tutorial) and used the given values in the IVC calculator:

 

mass of gasoline = 104.96mg

mass of diesel = 23.04mg

Phi = 0.77

EGR = 0.455

However, I did not get the same values which are provided in the dual fuel tutorial. Can you please let me know how we can use the IVC calculator to find the mass fractions of species in a homogenous mixture of gases at IVC when gaseous fuel is premixed with the air and liquid fuel is injected directly into the cylinder?

 

Many thanks!

[d1227078662]

I'm sorry, but I don't know much about that.

[Yu Bi]

Okay, therefore we can exclude the possible causes of the mooring line settings. It then could be caused by the small stiffness in the sway and yaw directions. You can find the mooring stiffness matrix in the EQUILIBRIUM.LIS or EQUILIBRIUM.MFK file. Given the mooring line position in this model, I expect the diagonal stiffness for sway-sway and yaw-yaw to be small while the non-diagonal stiffness for sway-yaw coupling probably could be negative. You can also have a check of the non-diagonal sway-yaw value of added mass and radiation damping. As these two mooring lines don't prove much stiffness in the sway and yaw direction, the negative stiffness/damping/added mass of the sway-yaw coupling term could result in an unstable natural mode in the sway-yaw coupled direction. To prove stiffness for yaw and sway motion, you could add two more cables on the other side of the structure and ensure the cables are not slack at the structure equilibrium position, and then the system could be more stable.