The mode shapes of a monopile wind turbine tower

I am currently trying to calculate the first and second mode shapes of a monopile wind turbine tower. I’m using OpenFAST v2.6 right now. Can you help me verify if my approach is correct?

1.Modify the discretized structural parameters of the tower in the BMode file.
2.Utilize the independent subDyn program to calculate the equivalent mass and stiffness matrices of the single-pile structure at the TP (transition piece) location, taking into account pile-soil interaction. Specify the SSIfile to consider. Which degrees of freedom should be enabled for the connection joint? How should fluid damping be applied? Is it necessary to couple with HydroDyn, and if so, how should the coupling be done?
3.Next, modify the equivalent mass and stiffness matrices in the BMode file, then run the program to calculate the tower’s mode shapes.

Best regards,

Dear @HaiYan.Liao,

I generally agree with the approach you outlined.

Pile-soil interaction can be included within SubDyn, but you can also model the pile as cantilevered to the seabed (without pile-soil interaction).

Damping is not needed to calculate the tower mode shapes via BModes.

I’m not sure what you mean when you ask, “Which degrees of freedom should be enabled for the connection joint?”

Best regards

Dear Jason,
Thank you for your response. I apologize for my previous vague statement.
1.The joints are base reaction joints and interface joints in the subdyn file. I am not certain which degrees of freedom need to be enabled for calculating the equivalent mass and stiffness matrices of the pile structure at the TP location, such as RctTDXss and RctTDYss?
2.I did not seem to find the effect of water on underwater single piles in the subdyn file. Is it added through joint additional concentrated masses?
3.Are the equivalent mass and stiffness matrices obtained through the subdyn module inclusive of pile-soil interaction and the effect of water?
4. Can the mode shape parameters obtained through BMode be directly used to modify the tower fore-aft mode shapes and tower side-to-side mode shapes in the Elastodyn_Tower file, and then proceed with openfast simulation?
Best regards

Dear @HaiYan.Liao,

Here are my responses:

  1. You should enable all 6 DOFs of the interface joint (ItfTDXssItfRDZss = 1). If you want to model the pile-soil interaction in SubDyn, you should enable the DOFs of the reaction joint were stiffness is nonzero (RctTDXssRctRDZss = 1); you can disable the other reaction joint DOFs (RctTDXssRctRDZss = 0).
  2. SubDyn only considers the structural dynamics of the substructure. Within OpenFAST, the effect of water, including hydrostatic and hydrodynamic loads, is accounted for via the HydroDyn module.
  3. The equivalent stiffness matrix would include the effect of pile-soil stiffness specified within SubDyn, but the equivalent mass matrix does not include the effect of water per (2). For monopiles, the only water-related term that could influence the mode shapes is the hydrodynamic added mass along the pile. You could try to approximate this effect, but often, this added mass is assumed to be small and, thus, neglected when calculating the tower modes.
  4. Yes, except that you must fit a polynomial through the tower mode shapes calculated by BModes for use in OpenFAST (because ElastoDyn requires that you specify the tower mode shapes in terms of coefficients of the polynomial fit), e.g., via the spreadsheet ModeShapePolyFitting.xls.

Best regards,