MHK RM1 Question

Hello everyone,

I have a question regarding the MHK RM1 r-test : Does OpenFAST provide an option to replace the fixed support with springs at the bottom of the tower ? Additionally, I would like to know if it’s possible to measure the deflection at each node along the tower. If so, which output list parameter should I use to track this data?

Dear @Tareq.AbuAgolah,

I’m assuming you are referring to the fixed-bottom MHK RM1 r-test: r-test/glue-codes/openfast/MHK_RM1_Fixed at main · OpenFAST/r-test · GitHub; is that correct?

You can model a flexible substructure or a spring between the tower and ground in OpenFAST through the SubDyn and/or External Platform MCKF sub-structure modules (CompSub = 1 or 2). If you simply want a spring/stiffness matrix between the tower and ground, the later would be appropriate.

You can certainly output the tower deflection, using ElastoDyn outputs TwHt1TDxt or TwHt1TPxi, etc. for each tower output node. For an explanation of the difference between these two outputs, see my post dated Jun 13, 2017 in the following forum topic: SubDyn problem with larger substructures.

Best regards,

Hello @Jason.Jonkman

Thank you for your answer. Could you please elaborate on how I can add these modules to the fixed-bottom MHK RM1 r-test, as they are not currently included in the files?;r-test/glue-codes/openfast/MHK_RM1_Fixed at main · OpenFAST/r-test · GitHub

Regarding the tower deflections, I have tried using these outputs, and they all yielded zero as shown below.

The only output parameter that provided deflections was Tower-Top / Yaw Bearing Motions: YawBrTDxt, YawBrTDyt, YawBrTDzt.

Dear @Tareq.AbuAgolah,

Documentation on SubDyn is provided here: 4.2.5. SubDyn User Guide and Theory Manual — OpenFAST v3.5.3 documentation and documentation on External Platform MCKF is provided here: 4.2.6. ExtPtfm — OpenFAST v3.5.3 documentation.

If ElastoDyn output TwHt3TDxt is marked as INVALID (which will return zeros), this implies that you have not identified 3 tower strain gage outputs. You should set NTwGages >= 3 and select which tower analysis nodes you want for output via TwrGagNd.

Best regards,

Dear @Jason.Jonkman ,
I have a question regarding modeling a spring between the tower and ground in OpenFAST through the SubDyn module. I have been trying to implement the following stiffness matrix as shown in the picture below,but I keep getting the following error:

It’s worth mentioning that the code runs fine when I use the example stiffness matrix provided in the documentation( SubDyn’s SSI File (OC4 Jacket SSI File).

Any insights or advice on what might be causing this error would be greatly appreciated.

Thank you,
Tareq Abu Agolah

Dear @Tareq.AbuAgolah,

I would guess you are getting an error about rigid-body modes because you have no stiffness in the heave and yaw directions because K_33 and K_66 are zero.

Best regards,