Building a DTU 10MW Turbine numerical model using Orcaflex

Dear NREL Forum Users,

Hello. I am looking to compare results between OpenFAST and other analysis tools (Orcaflex) for a FOWT using the DTU 10MW Turbine. While I have completed the numerical model construction using OpenFAST, I am facing difficulties in building a numerical analysis model using Orcaflex.

The detailed properties of the blade pose a challenge as I find it difficult to construct the numerical model in other analysis tools solely based on the input data from OpenFAST.

Therefore, if anyone has information regarding the detailed specifications of the DTU 10MW turbine blade, I kindly request you to share it.

I would greatly appreciate any assistance and will not forget your kindness.

Thanks!

Dear @Jisu.Lim,

Can you clarify what information you need on the DTU 10-MW wind turbine blade that is not available in the OpenFAST model?

Best regards,

Dear Jason.Jonkman

Thank you for the prompt response!

As mentioned in your response, based on my judgment, I have arranged a list of parameters that cannot be obtained from OpenFAST.

Parameters (Unit) :

  1. Blade element thickness (%)
  2. Aerodynamic centre x ,y (%)
  3. Blade element Rz (%)
  4. Ry/Rx
  5. COM offset (%)
  6. Inertia twist (deg)
  7. EI (kN.m^2)
  8. Pre-bend curvature (rad/m)
  9. Neutral axis offset (%)
  10. Shear centre offset (%)
  11. Structural twist (deg)

There may be aspects within the arranged parameters above that can be obtained from OpenFAST, but I may not have recognized them.

Your understanding on this matter is kindly requested.

Additional inquiries:
In the DTU 10MW OpenFAST version I am using, the counts for NBlInpSt (Elastodyn_Blades) and BldNodes (AeroDyn) differ, with 51 and 37, respectively.

In the data of IEA 15MW RWT, blade information is defined with the same count at Elastodyn and AeroDyn.

Are there any different issues that might arise when the count is different, as in the case of DTU 10MW?

I appreciate your ongoing efforts to contribute to the activation of offshore wind energy system research.

Best Regards,

Dear @Jisu.Lim,

Are you trying to set your OrcaFlex model to match the OpenFAST model as close as possible, or are you trying to match the DTU 10-MW definition as close as possible? Also, are you trying to model the DTU 10-MW directly, or the IEA Wind 10-MW, which is based on the DTU 10-MW? Documentation of the IEA Wind 10-MW wind turbine is available here, which answers some of your questions: GitHub - IEAWindTask37/IEA-10.0-198-RWT.

I’m not sure why the OpenFAST model of the IEA Wind 10-MW includes a simplified representation of the blade, but certainly OpenFAST itself is capable of including all of the effects you mention (by modeling the blade structural dynamics in BeamDyn instead of ElastoDyn). If you are trying to match the existing OpenFAST model (with the blade structural dynamics modeled in ElastoDyn), the following equivalencies between your parameters and OpenFAST parameters exist:

Blade element thickness (%) - Not used by OpenFAST, but can be inferred from the airfoil shapes
Aerodynamic centre x ,y (%) - AeroDyn blade input file inputs BlCrvAC and BlSwpAC
COM offset (%) - The COM lies along the pitch axis in an ElastoDyn blade model
Inertia twist (deg) - The flapwise and edgewise inertias are neglected in an ElastoDyn blade model
EI (kN.m^2) - ElastoDyn blade input file inputs FlpStff and EdgStff
Pre-bend curvature (rad/m) - Aerodynamically, this is represented via AeroDyn blade input file input BlCrvAng; structurally, the pre-bend curvature is zero in an ElastoDyn blade model
Neutral axis offset (%) - The neutral axis lies along the pitch axis in an ElastoDyn blade model
Shear centre offset (%) - The shear centre lies along the pitch axis in an ElastoDyn blade model
Structural twist (deg) - ElastoDyn blade input file input StrcTwst.

I’m not sure I know what Rz or Ry/Rx is; please clarify.

In OpenFAST, the aerodynamic and structural nodes can be independently specified and spatial mesh-to-mesh mapping is used to transfer data between the aerodynamic and structural modules. That said, ElastoDyn input NBlInpSt does not locate the structural analysis nodes; rather, NBlInpSt specifies the locations of distributed blade structural data, which is then interpolated to the structural analysis nodes defined via ElastoDyn input BldNodes.

Best regards,

Dear @Jason.Jonkman

Thank you for your prompt and insightful response!
Let me clarify my current research goals. I am working on building a DTU 10MW RWT in Orcaflex, and I am utilizing DTU 10MW OpenFAST data for the construction process.

In other words, I’m not building an IEA 10MW model; instead, I’m focused on constructing the DTU 10MW Orcaflex model.

  1. Modeling object: DTU 10MW Orcaflex model

  2. Reference data: DTU 10MW OpenFAST model

Additionally, I am attaching the DTU 10MW OpenFAST file for your reference.
(Extracted from the NREL Forum DTU 10-MW Reference Wind Turbine Topic).

I have compiled my questions regarding your response on the parameters as follows:

1) Aerodynamic centre x ,y (%) - AeroDyn blade input file inputs BlCrvAC and BlSwpAC

→ The AeroDyn data I have attached does not include 'BlCrvAC' and 'BlSwpAC'

->I also aim to represent the aerodynamic center coordinates (x, y) with the leading edge as the
reference axis and express the position as a percentage of the chord length. However, it appears
challenging to confirm this information in the OpenFAST data.

2) COM offset (%) - The COM lies along the pitch axis in an ElastoDyn blade model
Neutral axis offset (%) - The neutral axis lies along the pitch axis in an ElastoDyn blade model
Shear centre offset (%) - The shear centre lies along the pitch axis in an ElastoDyn blade model

→ Are you indicating that the CoM (Center of Mass), Neutral Axis Offset, and Shear Center Offset are all equal to "PitchAxis" value in "ElastoDyn_Blades" input?

->Similarly, I would like to represent this input with the leading edge as the reference axis and express the position as a percentage of the chord length. Is it possible?

3) In OpenFAST, the aerodynamic and structural nodes can be independently specified and spatial mesh-to-mesh mapping is used to transfer data between the aerodynamic and structural modules. That said, ElastoDyn input NBlInpSt does not locate the structural analysis nodes; rather, NBlInpSt specifies the locations of distributed blade structural data, which is then interpolated to the structural analysis nodes defined via ElastoDyn input BldNodes .

→ Isn’t it correct that the ElastoDyn input 'BldNodes' in the last part of your response should be AeroDyn input 'BldNodes'
→ If I understand correctly, are you saying that the structural data for ElastoDyn_Blades is interpolated and applied to the nodes defined in AeroDyn?

Please point out the parts of my question that you find unclear, and I’ll do my best to explain.
Also, I appreciate your continued enthusiastic responses.

Best regards,

Dear @Jisu.Lim,

Here are my responses:

  1. AeroDyn does not currently have a specification of the leading edge location. AeroDyn inputs BlCrvAC and BlSwpAC determine the location of the aerodynamic center. When BlCrvAC =BlSwpAC = 0, this means that the aerodynamic center lies along the pitch axis. If OrcaFlex needs to know the leading edge location in addition to the aerodynamic center, you could assume a location of the leading edge (e.g., 25% of chord along the chord axis upstream of the pitch axis), in which case locating the pitch axis / aerodynamic center is easy (i.e., 25% of chord).

  2. The PitchAxis input in the ElastoDyn blade input file is not used when AeroDyn v15 is enabled in OpenFAST. (PitchAxis is only used by ElastoDyn when AeroDyn v14 is enabled.) Again, you can follow my suggestion in (1) to locate the centers along the pitch axis by assuming a location of the leading edge.

  3. No, I mean ElastoDyn input BldNodes, which defines the number of structural analysis nodes in ElastoDyn and can be independent of AeroDyn input NumBlNds, which defines the number of aerodynamic nodes in AeroDyn.

Best regards,

Dear @Jason.Jonkman

Your kind response has been very helpful.

I am currently constructing the Orcaflex 10MW DTU model, referencing your response. Moreover, I find the topic “Definition of parameters” on the NREL Forum, which covers Blade parameters, to be very helpful.

If I encounter any further uncertainties during the process of constructing the Orcaflex DTU 10MW model, I will reach out with additional questions.

Thank you for the kind and thoughtful responses thus far.

Best regards,

1 Like

Dear @Jason.Jonkman

Thanks to your detailed response, I was able to build the DTU 10MW Turbine with Orcaflex. I appreciate it

After the construction, I intended to perform an Equilibrium test using Orcaflex and OpenFAST, and I have some questions that arose during this process that I would like to ask.

When setting all Platform initial conditions in ElastoDyn to zero and conducting the analysis in OpenFAST with an output start time of 0, I can observe that all 6DOF output start from zero in the results(Y axis), as shown in the figure below.

Q1) According to this interpretation, it is possible to consider that when the analysis starts, it begins not from a Static Equilibrium state but rather from a state where all 6DOF are fixed at zero. Is it correct that the analysis starts directly from the position where 6 degrees of freedom (6DOF) are set to zero, rather than from a static equilibrium state, as described?

Q2) As the analysis begins, tensions for each line are displayed, as shown in the figure below. Is this tension associated with the Platform’s Static Equilibrium state, or is it from the state where 6 degrees of freedom (6DOF) are set to zero?

If you have any doubts or need additional explanation regarding the questions I’ve asked, please feel free to let me know.

Thank you.

Best regards,

Dear @Jisu.Lim,

Here are my responses:

  1. OpenFAST currently starts the simulation based on the initial conditions defined by the user; it does not start from a static-equilibrium condition unless those are the initial conditions the user set. It looks like you’ve set the initial conditions of all platform displacements equal to zero, but you could change that via the settings in the ElastoDyn input file.

  2. MoorDyn does perform an internal static-equilibrium solve for the internal mooring analysis nodes before the start of the simulation (e.g., to define the catenary shape of the lines). This equilibrium solve is fully internal to MoorDyn and based on the initial platform displacements defined within ElastoDyn (which may differ from zero).

FYI 1: We are working on the development of a full-system static-equilibrium solve for OpenFAST, but this is not ready for release yet.

FYI 2: I do see in your results that the floater heave settles out slightly less than zero; in general, we recommend that OpenFAST models of floating wind turbines be defined such that the mean heave is zero; i.e., that the full-system weight plus vertical mooring pretension balances with the vertical buoyancy in the undisplaced condition. So, your model could be “tweaked” a bit by reducing weight or pretension or increasing buoyancy.

Best regards,

Dear @Jason.Jonkman

Thank you for always providing thoughtful responses :grinning:

I fully understand the content you provided in your response.
It will be very helpful in comparing with Orcaflex and its results.

I will ask again when I have another question. Always grateful.

Best regard,