I am reproducing the force-displacement relationship of OC4 DeepCwind Model just as what Dr. Jonkman did in the reference paper: Definition of the Semisubmersible Floating System for Phase II of OC4_5Dec2012. I get the same results as Dr. Jonkman but I have a question about the Pitch restoring moment when the surge varied independently with all other displacements zero-valued. see Figure 5-2 in the reference paper.
According to the result, when the platform surge displacement is positive, which usually because of the wind and wave in the x direction, the resulting pitch moment is also positive. If this is true, the pitch moment will not act as a restoring force which decrease the pitch angle, but instead of increasing the pitch angle.
This problem can be solved by means of increasing the net weight of the mooring lines in water, which is the heave load acting on the platform. But I am not sure what I am understanding is right.
Thank you in advance if someone can help me with this problem.
I agree that the mooring system arrangement for the OC4-DeepCwind semi has the effect that a positive surge will induce a positive pitching moment. This results from the surge motion tugging on the upstream mooring line and making the downstream lines more slack; I’m not sure changing the weight of the mooring lines would change that. However, I’m not aware that this mooring arrangement causes any problems either.
Base on the discussion between you and Jinsong, I would like to ask, how can we reproduce the force-displacement relationship of the OC4 DeepCwind Model?
Yes, you can run separate simulations with different platform displacements to derive force-displacement relationships. You can do this in FAST (e.g. by running separate simulations with different initial platform displacements in ElastoDyn) or using one of the standalone drivers for the various mooring modules. I’m not too familiar with the Python wrapper for MAP++, but this may be a good approach–see the online MAP++ documentation for more information: map-plus-plus.readthedocs.io/en/ … index.html.
The factor of -1000 in the forum post you referenced was because the matrix analyzed was in kN instead of N and had the opposite sign.
I tested the python wrapper for MAP++ that you have mentioned for the original FAST Test25 DeepC floating wind model. It seems possible to define the variation of surge displacement in the provided python driver (map-plus-plus.readthedocs.io/en/ … figuration):
The vertical pretension V = 631915.26 [N] indicates that this result coincides with that of FAST simulation.
What I wonder is: which value(s) in the above outputs (there is a linearized 6X6 stiffness matrix for the whole mooring system together with pretension and some other outputs for Line 1) should I use to generate the force-displacement relationship plot?
The figure uploaded by Jinsong Liu above shows the total mooring reaction load about the platform reference point. If this value is not directly generated by the python wrapper for MAP++ (again I have not used this feature myself, so, I’m not familiar with the details), you could derive it by summing up the fairlead tensions from all lines (Fx, Fy, Fz and the associated moments using the moment arms to the platform reference point).
I calculated the force-displacement relationship based on the python wrapper. I generated exactly the same graphs for displacements in surge, sway and heave as given in Figure 5-2, Page 30 of the document “Definitions of the DeepCwind semi…”(nrel.gov/docs/fy14osti/60601.pdf). For example, the force-displacement relationship for displacement in surge generated by me is:
which is exactly the same with that shown by Jingsong Liu (also the DeepCwind document).
However, the graphs I generated for rotational displacements are different from those provided by the DeepCwind document, e.g., for roll displacement(similar for pitch), it looks like
If I amplify the restoring moments by a factor of 17~20, then my graphs for rotational displacements look same as those in the official document. For example, if I amplify the roll restoring moment for displacement in roll by a factor of 17, the second graph in the above last post will be
which are almost the same with those in the official document.
I cannot understand why those discrepancies occur in the rotational displacements. If possible, could you please confirm the correctness of the results in roll, pitch and yaw displacements in Figure 5-2 of the “Definition of DeepCwind semisub…” document? Because I’m a little doubtful for those discrepancies in the magnitude.
The linearized mooring restoring (stiffness matrix) you calculated by the MAP++ wrapper in your post dated Feb 01, 2018 seems to match well with that reported in the OC4-DeepCwind specifications report in Eq. (5-15). If you estimate the slope of the force-displacement curves from the plots, the slope derived from the published plots is consistent with this stiffness matrix. I’m not sure why your force-displacement curve is showing less roll-roll and less yaw-yaw stiffness, but this seems inconsistent with the linearized stiffness matrix you’ve confirmed.
Traceback (most recent call last):
File “D:\FastV8\Mooring\mmasciola-map-plus-plus-f30012b568d7\mmasciola-map-plus-plus-f30012b568d7\python_driver\main.py”, line 35, in
from mapsys import *
File “D:\FastV8\Mooring\mmasciola-map-plus-plus-f30012b568d7\mmasciola-map-plus-plus-f30012b568d7\python_driver\mapsys.py”, line 28, in
class Map(object):
File “D:\FastV8\Mooring\mmasciola-map-plus-plus-f30012b568d7\mmasciola-map-plus-plus-f30012b568d7\python_driver\mapsys.py”, line 626, in Map
lib.map_offset_fairlead.argtypes = [MapInput_Type, c_int, c_double, c_double, c_double, c_char_p, POINTER(c_int)]
File “C:\Python27\lib\ctypes_init_.py”, line 375, in getattr
func = self.getitem(name)
File “C:\Python27\lib\ctypes_init_.py”, line 380, in getitem
func = self._FuncPtr((name_or_ordinal, self))
AttributeError: function ‘map_offset_fairlead’ not found
I’m new to python. Could you tell me how to do with this errro? Or could you tell me how to modify the two files when download from the website ?
For now, you can comment lines 626 - 632 in mapsys.py. The most recent changes on git should have been pushed to the ‘next’ branch, but I was mistakenly working in master.
I’m sorry, I just see your message to me.
Glad to hear that your problem has already been solved.
If you have further questions, please ask me again.
You can also send to my email. My email is: liuyingyi@riam.kyushu-u.ac.jp
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