Can structural stress be calculated in FAST

[Xiexianping99]

hello，
I want to use fast to calculate the stress of the tower structure, what can I do ? How to modify the source code, this method is feasible, hope to get your help

Best regard！

[jjonkman]

Dear @Xiexianping99,

OpenFAST does not directly calculate stress; instead, OpenFAST calculates loads. For the blades and tower, OpenFAST will calculate the reaction loads at various cross sections along the beam e.g. axial force, transverse shear forces, transverse bending moments, and torsion moment. These loads can be used to calculate local stresses using cross-sectional analysis. For simple circular cross sections such as often used in a tower, the cross-sectional analysis could be analytical. This topic has been discussed several times on our forum, e.g.:
https://forums.nrel.gov/t/mlife-user-defined-distribution/1970
https://forums.nrel.gov/t/difference-between-fast-and-openfast-results/4665/58

Best regards,

[Xiexianping99]

hello，
Thank you for your answer. Yes, I also saw the discussion in the forum. I am now using the fast7 seismic version to study the dynamic characteristics of single pile wind turbines. Considering that the tower may yield locally under large load conditions, I want to carry out plastic analysis of the supporting structure.
First of all, we want to modify the fast7 code to make it realize the output of the normal stress time history, and calculate the normal stress of the ring section according to the formula in the forum. Do you know that this method is feasible and difficult ? Secondly, a yield condition is set to judge the yield degree of the tower, and then the yield part is iterated to solve the stiffness matrix of the tower.
This is a simple idea,I do not know in the fast7 version can be achieved ?
Best regards,

[jjonkman]

Dear @Xiexianping99,

I'm sure you could implement the equations on the forum to compute the normal stress within the FAST v7 source code, but normally I would expect the sectional analysis would be done in a post-processing step.

It sounds like you want to use the stress to update the tower stiffness matrix at run time. This would likely be difficult, as discussed in a similar question on our forum about the modeling of blade stiffness changes as a result of delamination: https://forums.nrel.gov/t/making-fault/1260.

Best regards,

[Xiexianping99]

Dear Sir
Thank you very much for your answer. I saw the post you pushed me. I learned that to modify and update the stiffness matrix and mass matrix is to add a scale factor in front of the corresponding code. This way is feasible. I now want to add an if statement. For example, when the load condition reaches a certain value, such as the stress reaches the yield strength of the tower, the tower will fail. I consider this problem by modifying the stiffness and mass to affect the motion equation. How do I add the if statement as the beginning of judging the tower failure.

Based on v7, I learned that the stiffness, mass and modal shape of the tower are defined in the subroutine Coeff, and the calculated motion equation is defined in RTHS ( ). I want to add a conditional judgment statement again. Where to modify ? Could you give me some advice ? Give some reference to modify the code ?

The second problem is that I want to consider the initial defects of the tower, such as the initial displacement defects, how can I modify it ? I see the .fst file has two parameters TTDspFA, TTDspSS, I consider the words only need to change the two parameter values or also need to modify the source code ?
Best regards,

[jjonkman]

Dear @Xiexianping99,

It sounds like you have a good understanding of what needs to be changed. The big challenge mentioned in the forum post is that, while you can change the tower stiffness, it is not straightforward to change the tower mode shape during the simulation. And changing the stiffness without the corresponding mode shape is not physically consistent, so, the solution may not be fully correct (small changes to stiffness are probably OK, but not large stiffness changes).

The IF condition you are proposing would be best implemented where the tower stiffness (KTFA, KTSS) is used within SUBROUTINE RtHS() of FAST v7.

Best regards,

[Xiexianping99]

Dear Sir
Thank you for your answer. I want to consider the initial defects of the tower, such as the initial displacement defects, how can I modify it ? I see the .fst file has two parameters TTDspFA, TTDspSS, I consider the words only need to change the two parameter values or also need to modify the source code ?
Besides,I am building a 10MW offshore monopile wind turbine with a water depth of 30 m and a pile length of 45 m below the mud line, considering the SSI effect. To build a linear py spring, I encountered difficulties at this time. I do not know how to modify the normalization coefficient of the tower. Seeing how the spring stiffness in the OC3 project is for 5MW, how should I select the spring stiffness of 10MW ?
Best regards,

[jjonkman]

Dear @Xiexianping99,

I'm not sure what you mean by "initial displacement defects". TTDspFA and TTDspSS in FAST v7 are the initial conditions of the tower-top displacements, which can be set nonzero to reduce the simulation start-up transients.

You can read how the soil-pile model used in OC3 Phase II was derived in the corresponding documentation, available from: https://drive.google.com/drive/folders/0B0KGNSHvXXgCazJDZmRuWVQzaDA?resourcekey=0-0SIeHrGx-WiOnSKsNsikRA&usp=drive_link.

I'm not sure if you are trying to match specific soil-pile conditions for a specific 10-MW turbine or not, but OC6 Phase II considered soil-pile interaction for the DTU 10-MW turbine atop monopile. You can find the corresponding model specifications for this model here: https://sharea2e.atlassian.net/wiki/spaces/OC6/pages/532283453/Model+Information+2.

Best regards,

[Xiexianping99]

Dear Sir
Thank you for your answer.
First of all, the first question I mean is to add a boundary condition in the dynamic equation, similar to the initial tower displacement conditions. Furthermore, in the process of solving the motion equation, considering this initial condition, similar to the traditional structural dynamics, a y is added to the term k.x. It is not known whether this can be considered in multi-body dynamics ?
You sent me OC6 second link I can not open, no access. But I can open the net. Can you send it to me again ?

[jjonkman]

Dear @Xiexianping99,

I'm still not sure I really understand what you mean by "initial displacement defects", e.g., I don't know what you mean when you say, "a y is added to the term k.x".

Regarding the Confluence site for OC6 Phase II, you must create an account to access that data. The corresponding NREL technical report is available in the NREL publication database (https://www.nrel.gov/docs/fy21osti/79938.pdf), but if you want to access the full model rather than just the publication, you'll need to create an account on the Confluence site.

Best regards,

[Xiexianping99]

Dear Sir
I am very sorry I didn 't express my thoughts clearly.The following attachments are my preliminary ideas. Do you see if it is feasible to do so in FAST ? For FASTV7 I change, which part of the FAST code should I modify ?

Thank you for the link, I registered the account is still no access. But I look at NREL 's report on the OC6 project. I am also using FAST 7 to build a 10 MW single pile wind turbine with a water depth of 30 m. When building the DS model, I modeled the linear DS and nonlinear DS models mentioned in the OC3 project. It just happens that the file you sent above provides a series of nonlinear py curves. Can I use this directly ? Can you send me a linear DS model and a nonlinear DS model of FAST7 version of the case for my reference ?

Best regards,
xie.docx

[jjonkman]

Dear @Xiexianping99,

Regarding your proposal for "initial displacement defects", this sounds equivalent to applying a constant force on the right-hand equal to -K * u_0. You could add this force in the FAST v7 source code at the same location where K * x is added to the equations of motion.

I don't have a FAST v7 example that implements nonlinear p-y curves, but I'm sure that would be possible to add. We do have an example of FAST v7 SUBROUTINE UserTwrLd() that implements linear distributed springs, which is available in the following topic on our forum: https://forums.nrel.gov/t/turbine-soil-interaction-influence-on-the-mode-shapes/481/13. I'm sure this example could be modified to include nonlinear p-y curves.

Best regards,

[Xiexianping99]

Dear Sir
I read your advice on the first question and I think it ' s quite right. I will try to move in that direction next.
But I now look at the linear py curve :https://forums.nrel.gov/t/turbine-soil-interaction-influence-on-the-mode-shapes/481/13, in the forum to understand that he is based on the API specification, I do not understand how its stiffness is calculated ?As the link shows.
Best regards,
API Rules.docx