Hydrodyn NBody maximum limit

Wind & Water Structural Analysis
1

Hello,

I am working on modeling floater felxibility in OpenFAST, and I am using the multi-body option in HydroDyn to distribute the potential flow hydrodynamic loads obtained from WAMIT. I was wondering if there is a maximum limit for the number of bodies (NBody), and if so, how to go about increasing it?

I am running OpenFAST v3.5.3 and whenever I increase NBody beyond 11 I get this error:

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Thanks in advance for your help.

Best,
Serag

2

Dear @Serag.Abdelmoteleb,

I’m not aware of any hard limit to NBody in HydroDyn. Are you saying that simulations run as expected with NBody = 10, but then produce a Program Exception error when you increase NBody to 11? Can you share your HydroDyn input file(s)?

Best regards,

3

Hi @Jason.Jonkman,

Thanks for the quick reply!

Yes, that is what I meant. When NBody is more than 11, I get the aforementioned error. I have put together 3 examples of the model with which I am working. The difference between the examples is the number of bodies that represent the floater, as given below:

  • RigidLPT : NBody = 1
  • RigidC1P1: NBody = 11
  • RigidC1P2: NBody = 14

The first two run without issues, while the last one gives the error. Please find the input files attached here.

Best regards,
Serag

4

Dear @Serag.Abdelmoteleb,

Can you enable Echo = TRUE in HydroDyn and make sure that the input file is being read in properly for all cases by reviewing the .HD.sum files?

Best regards,

5

I have enabled the ECHO mode and ran all three models. The WAMIT body information (volume, offset, and buoyancy loads) seem to be read correctly. The radiation impulse response functions also seem to be read correctly with the following information:

  • 1 matrix with entries for 36 values for the case with NBody=1 and NBodyMod=1
  • 11 and 14 matrices with entries for 21 values for NBody=11 and 14, respectively, with NBodyMod=3 (I presume a symmetry assumption is implemented here).

(I have edited this reply, so please disregard the previous comment.)

6

Dear @Serag.Abdelmoteleb,

@Lu.Wang of NREL has tracked this issue down to a simple bug in HydroDyn, now fixed via the following pull request in the OpenFAST repository: HD bug fix: prevent array index out-of-bound error in HDOut_MapOutputs when more than 9 potential-flow bodies are present by luwang00 · Pull Request #2397 · OpenFAST/openfast · GitHub.

Best regards,

1 Like
7

Dear @Jason.Jonkman,

Thank you and @Lu.Wang very much for your help. I will try the modified version.

Best,
Serag