Forum
Mooring Lines
Hello all,
I am trying to design a mooring system having the mooring lines divided into several mooring elements. That is I would like to separate my lines into three sections, and assign different mechanical properties to each section. In addition, I wish to add a clampweight at the end of the first section of each lines, which I managed to do with the addmass keyword.
My problem concern the connection points of the different section of the lines. I have created different subjoints in space where i come to attach the lines by creating them using the moormembers table (i.e., first section connected between the floating structure and joint_in_space_1, second section between joint_in_space_1 and joint_in_space_2, and third section between joint_in_space_2 and GRD). No subconstraints nor submembers are added for the connection.
The model is well created, but when running the simulation nothing happens. The system behaves like it is fully constrained somewhere. Normally when in the ramp up time the lines becomes flexible and come to their equilibrium position, but it is not the case here, they remain straight and do not move. Nothing happens neither after the ramp up time.
I was wondering if my model was correctly created, and if Qblade was able to run such a configuration? If not, is there another way to connect the different sections of the lines?
Thanks in advance,
Steven
Hi Steven,
what you are trying to do should work – however, I tried it myself and found that it doesnt – so you actually found a bug…
I could identify the issue: the “lone joint” that is created and not constrained to anything via the constraint table leads to a singular mass matrix – thats why the simulation cant find a solution and nothing is happening on the screen. I have already fixed this for the next release.
Now, what you can do as a workaround until the fix is public is to add some mass to the “problematic” joint that should connect the mooring lines with the “ADDMASS” keyword. Adding a very small mass (such as 0.001kg) should already fix the issue for you.
Let me know if that helped.
BR,
David
Hello David,
Thank you very much for your answer. I have tried your solution and the model seems to work properly. The mooring lines are well attached and behave more realistically.
However, I think that I have come to another problem. While running several simulations, in particular an extrem case with strong current, the structure translates in the direction of the wave, current and wind, and come in contact with the “lone joint”. This “lone joint” then feels to be ”stuck” to its position, inducing a false dynamic response of the floater. Further, the conserned mooring line do not behaves as expected.
In addition, when running this extrem case, it feels like the floater has a lack of buoyancy. I was wondering if the tower of the turbine was considering hydro forces, such as the buoyancy force. If not, is there a way to include such buoyancy force in the tower mast?
Just to let you know, I am working on the Hexafloat, where i have redesigned a 10kW model.
Thanks in advance for your time,
Steven
Hi Steven,
this particulat joint should not behave any different the other parts of the structure – so I dont really understand what you mean by “stuck”. If it is not constrained to anything but the mooring lines then it should act as a connecting joint between those. Maybe you can attach a screenshot that explains your observation, or even better a project that contains this situation?
Regarding the tower: Towers cannot have hydrodynamic properties associated to them, however you can simply model a part of the (or the complete) tower as the substructure – this will then contribute to the buoyancy. The substructure flexible element definition is mostly equivalent to the tower structural data table.
BR,
David
Hi David,
Thanks for your answer.
Please find attached a screenshot to explain the situation. I would be happy to share the project file with you, but privately only ( via email for instance). Note that in normal condition the floater do not sink like on the picture (it has an expected draft of about 4m).
When applying a smaller wave and no current, the free surface still goes a little above the tower mast. I am afraid that modelling part of the mast as a substructure would remove some aerodynamic propertries, such as the vibrations, tower shadow, himmelsakam effect ect.. Do you think it would be feasible to keep the mast as it is, but extend a fictional part of the substructure into the tower mast with no mass just to get the hydro forces?
Thanks again for your time,
Steven
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Hi Steven,
I think in general there is nothing wrong with the model behavior. The joint that is interconnecting the mooring lines is acting only a positional, but not as a rotational constraint – so the behavior that you are observing is most likely a result of the cable bending stiffness properties.
This doesnt really explain why your floater is “sinking” in some situations and not in others. Mooring line dynamics could play a role in this however – you can checkout the forces that are acting by looking at the mooring line sensors in the structural time graph.
Your idea of using a “massless” substructure cylinder to generate buoyancy at the position of the tower is quite good and should work!
BR,
David
Hello David,
Thank you for your answer. I have reduced the second moment of inertia of the mooring lines to reduce the bending stifness, it behaves now as expected! As for the sinking, the ”massless” substructure cylinder in the tower mast works to provide additional buyancy.
I would have another question, a bit out of topic here, but how do you specify the aerodynamic coefficients of the the hub/nacelle of the turbine? We are able to see the thrust applied on it in the graphics, but i cannot find the way to specify the exact dimension of the hub nor its drag coefficient.
Thank you,
Steven
Hi Steven,
the nacelle thrust that you see in the graphs is the combined thrust of the rotor in the nacelle coordinate system. The nacelle itself, or any drag acting on it, is not modeled.
BR,
David
Quote from David on 27. March 2023, 21:10Hi Steven,
what you are trying to do should work – however, I tried it myself and found that it doesnt – so you actually found a bug…
I could identify the issue: the “lone joint” that is created and not constrained to anything via the constraint table leads to a singular mass matrix – thats why the simulation cant find a solution and nothing is happening on the screen. I have already fixed this for the next release.
Now, what you can do as a workaround until the fix is public is to add some mass to the “problematic” joint that should connect the mooring lines with the “ADDMASS” keyword. Adding a very small mass (such as 0.001kg) should already fix the issue for you.
Let me know if that helped.
BR,
David
Hello david,
I have also encountered the same problem, may I ask where is the setting of “ADDMASS”
Thank you，
Camelia
Hi Camelia,
check the documentation on substructure joints:
https://docs.qblade.org/src/user/turbine/substructure.html#substructure-joints
The ADDMASS keyword, that can be used to assign mass to a joint, is explained there.
BR,
David
