Avoiding large initial hydrodynamic forces right after start

[MarianAlbers]

Hi everyone,

I'm currently conducting simulations of a bottom-fixed monopile with OpenFAST 4.1.2, using SubDyn for the MP and ElastoDyn for the tower. Mostly everything runs smoothly but I found that for specific wave seeds the wave height time series is such that the hydrodynamic forces at the very start (t=0) are quite high, which in combination with a zero-initialized platform (the MP) leads to a simulation crash. Lowering the time step did not help in those cases, such that I simply changed the wave seed. I was wondering if there is any better option to avoid this behavior, like an initial ramp to avoid this force "shock" at the very startup?

Thanks and best wishes
Marian

[jjonkman]

Dear @MarianAlbers,

Based on the limited information you've provided, it has hard to tell what is happening.

As discussed in other posts, we generally recommend including a correction step (NumCrctn > 0) when SubDyn is enabled, based on the current form of the implicit loose coupling in OpenFAST v4.X. Do you have that, and if not, does it resolve your issue?

Can you clarify what structural damping you've enabled in your SubDyn model (Guyan and/or Craig-Bampton), and how many Craig-Bampton modes you are considering? Does reducing Nmodes, increasing JDampings, or including Guyan damping (e.g., GuyanDampMod = 1 with RayleighDamp(2) > 0) help?

Best regards

[MarianAlbers]

Dear @jjonkman,

Thank you for your answer.

Until now I was using NumCrctn = 0 and switching it to 1 resolves the issue. In my current model I'm using both Guyan and CB damping with 2 CB modes, i.e. that section of my SD file looks like:

             2   Nmodes      - Number of internal modes to retain. If Nmodes=0 --> Guyan Reduction.
           0.7   JDampings   - Damping Ratios for each retained mode (% of critical)
             1   GuyanDampMod - Guyan damping {0=none, 1=Rayleigh Damping, 2=user specified 6x6 matrix}
    0.0, 0.01   RayleighDamp - Mass and stiffness proportional damping coefficients (Rayleigh Damping) [only if GuyanDampMod=1]
             6   GuyanDampSize - Guyan damping matrix (6x6) [only if GuyanDampMod=2]

Previously I had not used Rayleigh damping and then using NumCrctn = 1 lead to high frequency oscillations in the substructure loads, therefore I refrained from using it. But now that I'm also using Guyan damping your suggestion to use a correction step actually solves the issue without introducing those oscillations. The only drawback is that the computational effort is higher now. When I run a another simulation with and without correction step the results are nearly identical.

Thank you for your help
Marian

[jjonkman]

Dear @MarianAlbers,

I'm glad adding a correction step with Rayleigh damping of the Guyan modes helped.

FYI: When adding a correction step, you may be able to increase the time step a bit to reduce the impact of the correction step on the computational expense.

Best regards,