MoorDyn parameters

Wind & Water Computer-Aided Engineering Software Tools
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Dear @Matthew.Hall ,

Sorry for being so demanding ! (I just wanna understand what is done behind MoorDyn dll).
I was reading MoorDyn User’s Guide - December15, 2015. So, if i understand well:

  • dtC is the coupling time step. Is it user defined ? In fact, it is written in page 4 that MoorDyn adjusts its internal time step size dtM. For me, dtM is specified in the input file in solver options. Could you please clarify this point ?

  • I will say that dtC is user defined. Therefore, it is an input argument for the function LinesCalc (MoorDyn in C++). If i fix dtC to 0.01 sec and dtM is defined in the input file and it equals 0.001 sec, that means MoorDyn will do calculation for N =dtC/dtM=10 steps ?? Could i choose these two parameters such that N=1 in order to let MoorDyn computes fast ?

  • Finally, in order to be sure that i get it: In the page 13 in the same document, you provide an example how to couple MoorDyn with MATLAB. In this example the variable dt is dtC right ?

Best Regards

Riad

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Also, i have another question if you dont mind.

in this paper: Validation of a lumped-mass mooring line model with DeepCwind semisubmersible model test data - ScienceDirect.
you said that MoorDyn neglects wave kinematics and you justify that in section 3.4
This neglection is valid in the standalone version of MoorDyn and not in OpenFast right ?

Best regards

Riad

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Dear @Jason.Jonkman

In OpenFast, when MoorDyn wants to compute tension forces, it is given the position and the velocity of the fairleads right ? the units of the angular position in the position vector is degree or radians ?

Best Regards,

Riad

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Hi Riad,
Sorry, I don’t check this forum enough. Here are a few responses:

  • Angular units for MoorDyn function calls are radians.
  • The validity of neglecting wave kinematics depends entirely on the scenario being simulated. Currently in OpenFAST, MoorDyn does not include wave kinematics by default. There is a special manual way of doing it, but it is not documented and requires running twice as many simulations. In future work we will make a direct coupling between SeaState wave kinematics data and MoorDyn.
  • The coupling time step, dtC, is determined by whatever code calls MoorDyn. For example, it is usually equal to the global time step in OpenFAST, or in a custom code it can be set to whatever you like.
  • Your dtC/dtM math is correct. Setting dtC=dtM is usually a bad idea because MoorDyn often needs a small dtM for stability.
  • In the original Matlab coupling example, what’s written as dt is the dtC value, yes.

Best,
Matt

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