Available output channels from MoorDyn module

Wind & Water General
1

Hi,

I am trying to get out of a FAST simulation the (x,y,z) components of the mooring line tension for a floating wind turbine (semisub).

I have seen in the MoorDyn user guide (p.8 ) that e.g. the label “Con4pX” in the “OUTPUTS” section of the MoorDyn input file would give me the X position of one of the fairlead points. But when I try to get out the force by using “Con4fX”, FAST crashes.

Is there another way to output the mooring forces? Thanks

2

Hi,

What version of FAST and MoorDyn are you using? I didn’t have all the output channels set up in the first MoorDyn versions, but the “Con4pX”-type outputs should be working in the latest version.

If updating doesn’t solve the problem or you’re already using the latest version, let me know what error message you see and I’ll try to help.

Cheers,
Matt

3

Hi Matt,

I have a similar problem as Antonio. I’m trying to output the vertical force at node 20 of line 1. According to the MoorDyn user guide I thus defined an output L1N20fZ in the MoorDyn input file. However with this setting FAST crashes.

I have tried this with FAST v8.16 and OpenFAST v1.0.0., but neither works. To be more precise, I have used the executables which are available in the downloadable archives from the NREL websites, nwtc.nrel.gov/OpenFAST, respectively.
I always get the following error message:
[b]CalcOutputs_And_SolveForInputs:SolveOption1: MD_CalcOutput:In MDIO_WriteOutputs:
Unsupported output quantity 13 requested from Line 1.

FAST encountered an error during simulation initialization.
Simulation error level: WARNING[/b]

When I try to output the position L1N20pZ, that works without problems, but I would like to have the single components of the force so that doesn’t help.
Do you have an idea what could be the reason for this problem?

Best regards,
Paul

4

Hi Paul,

MoorDyn in FAST isn’t currently set up to output the net force components along the line - mostly because I wasn’t sure this sort of output would be useful and have a clear physical meaning. The net forces on a node internal to the line would include tensions pulling in opposite directions and would be equivalent to the product of mass and acceleration at the node. The net forces at nodes at the END of the line, however, are more clear and useful; they could correspond to the tension components at an anchor or fairlead. Are these line-end forces what you’re looking for? If so, you can access them by asking for the net force on the Connection that corresponds to the end of the line (e.g. “Con1fZ”).

Let me know if that solves what you’re looking for, or if you do need the net force component at a node along the line.

Best,
Matt

5

Hi Matt,

Thanks for your fast answer.

I think I’m indeed looking for the line-end forces, because I’m interested in the vertical force component of the mooring that acts on a floating platform to check some hydrostatic calculations. So, to my understanding, the sum of the fZ forces of all fairleads, i.e. the fZ forces on the connections to the end of the lines at the fairleads, is exactly what I’m looking for. Please correct me, if I’m wrong.

Thanks for pointing me to this solution.

Best regards,
Paul

6

Hi Paul,

Yeah, that sounds like the right approach. Summing the Con#fZ outputs (where # is connection number) for all the fairlead (“vessel”) connections should give the total vertical force you’re looking for. Glad there’s a solution available for what you’re looking for.

Best,
Matt