MoorDyn Line segment minimum length

Wind & Water Computer-Aided Engineering Software Tools
1

Hi,

I am trying to run a simulation in OpenFAST with active tension control of mooring lines in a floating wind turbine platform. Each line has 5 segments. I am getting the following error message depending on the amplitude of the actuation:
FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption1: Active tension command will make a segment shorter than the limit of half its original length.

Is there a way to change or disable the constraint on the minimum segment length?

If my understanding is correct, any commanded variation in cable length is added/subtracted from the length of the last line segment (fairlead connection). Therefore, the discretization of the cable (i.e., number of segments) limits how much the cable can be shortened.

2

Hi - your understanding is correct. Unfortunately, at this stage there isn’t a good alternative. You are welcome to modify the code to disable the constraint, but the model is at risk of stability problems if the segment length becomes too small. The more robust solution I have in mind is a method of dynamically adjusting all segment lengths (and node positions) mid-simulation. However, this isn’t something we are planning to implement in any current projects so I’m not sure of the outlook for it happening.
Matt

1 Like
3

Hi, it seems the format for the MoorDyn input file has changed from OpenFAST v3.1 to 3.4.1, in particular the cable control column seems to be missing. I am wondering how to set the cable control channels in the newer version of OpenFAST for simulink commands.

Thanks

4

Hi Jacob, length controls are now in their own optional section of the input file. The format is as follows, where the first column is the channel and the second line is which line end(s) the command will be applied to.

---------------------- CONTROL ----------------------
ChannelID Line(s)
() (,)
1 1,2,3,4
2 5

5

Hi Matt, thank you for the quick response.