Dear Dr. Jonkman,
Thank you for your reply.
I’m sorry for my late response. I spent time to extract mooring stiffness matrix from pymap and I can extract the mooring stiffness matrix now. the difference of the mooring stiffness matrix as shown below is within the margin of error when I set the reference point at (0,0,0).
Mooring stiffness matrix from pymap
41181 0 0 0 -2815428 0
0 41181 0 2815430 0 0
0 0 11941 0 0 0
0 2815431 0 310785098 0 -49
-2815429 0 0 0 310784963 0
0 0 0 -81 0 11566656
Mooring stiffness matrix from OpenFAST
41180 0 0 0 -2821000 0
0 41180 0 2821000 0 0
0 0 11940 0 0 0
0 2821000 0 311100000 0 0
-2821000 0 0 0 311100000 0
0 0 0 0 0 11560000
1 Then I obtain the natural frequencies in my linearization model. Here comes the question. The frequency of roll DOF will vary when I change the origin of global coordinate such as a point at still water level (SWL) or a point below SWL as well as pitch DOF. I list the frequencies of floated foundation . What has to be noted is that the frequencies masked as Morison is extracted from OpenFAST linearization module, and I recalculate mass matrix and stiffness matrix when the origin coordinate changes in both models. Morison formula is used to calculate the hydrodynamic loads with PotMod=0 and PropPot = false in Hydro.dat file. Little differences can be found only when the origin of coordinate is at SWL which confuses me. Does the coupling of surge and pitch modes cause this difference(OC3 Spar natural frequency - #6 by Jason.Jonkman)?However there are differences of frequencies when the origin of coordinates is attached at the farilead. It’s hard but could you give me some suggestions?
Origin of global coordinate: at SWL:
Morison 0.008076962 0.008076965 0.031520257 0.031536235 0.032642968 0.039389928
My model 0.00806388 0.00806388 0.067199128 0.067214905 0.03295249 0.039381796
Origin of global coordinate: 70m below SWL:
Morison 0.008076128 0.008076131 0.031553544 0.031569608 0.032642928 0.039389973
My model 0.008063163 0.008063164 0.023876158 0.023882588 0.032952562 0.039394495
Origin of global coordinate: 78.01m below SWL:
Morsion 0.008076128 0.008076131 0.031553544 0.031569608 0.032642928 0.039389973
My model 0.008063644 0.008063646 0.033586929 0.033597137 0.032952548 0.039395842
2 Extra damping and stiffness are provided when potential-flow solution is applied to calculate hydrodynamic loads with ExctnMod=1 and RdtnMod =1. And there aren’t any extra stiffness and damping when Morison formula is used in OpenFAST. Is it necessary to add extra stiffness and damping in my own model in the late simulation when I use Morison formula since it seems that it’s difficult to take extra damping and stiffness into consideration in OpenFAST when Morison formual is used?
Best regards and thank you in advance,
Xing Tan