S-N curves for FOWT reference

Discuss theory and modeling of wind-turbine structures.

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Benjamin.Marechal
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Organization: D-ICE engineering
Location: France

S-N curves for FOWT reference

Postby Benjamin.Marechal » Fri Dec 21, 2018 10:08 am

Hello,


I'm in a team working on a controller for the OC3 hywind (spar) floating turbine and we want to perform the load analysis with MLIFE to obtain the damage equivalent loads and, most important, the lifetime of the main components of the turbine.

To do this, we need for each component the Wohler exponents and the ultimate loads (which are related to the S-N curves).
I read in the FAQ (https://nwtc.nrel.gov/FAQ#TypeLMF) that a way to obtain an ultimate load is to multiply the maximum value of the load by a factor like 20.
I do not think this method is a good way to obtain realistic lifetime of the components because ultimate loads obtained by this method are correlated to the stress components endure during the simulations. For me, the ultimate loads should only take into account the material of the components.

Could you share ultimate loads values and Wohler exponents (or S-N curves) for the following components of the OC3 hywind FOWT:
- Tower base loads (TwrBsMxt, TwrBsMyt)
- Hub and rotor loads (LSShftMxs, LSSTipMys, LSSTipMzs)
- Yaw Bearing loads (YawBrMxn, YawBrMyn, YawBrMzn)
- Blade root loads (RootMxb*, RootMyb*, RootMzb*, and aggregate loads on x and y)

Also, do you considere that the choice we have made for the load components are relevant?


Best regards,
Benjamin Maréchal

Jason.Jonkman
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Re: S-N curves for FOWT reference

Postby Jason.Jonkman » Fri Dec 21, 2018 12:31 pm

Dear Benjamin,

I read in the FAQ (https://nwtc.nrel.gov/FAQ#TypeLMF) that a way to obtain an ultimate load is to multiply the maximum value of the load by a factor like 20.
I do not think this method is a good way to obtain realistic lifetime of the components because ultimate loads obtained by this method are correlated to the stress components endure during the simulations. For me, the ultimate loads should only take into account the material of the components.

Could you share ultimate loads values and Wohler exponents (or S-N curves) for the following components of the OC3 hywind FOWT:

I agree, which is why the FAQ explains that, "[ideally], LUlt would be based on a strength (e.g., FEA) analysis of the cross section of the component in question."

However, the OC3-Hywind system is a conceptual design and I'm not aware of a strength analysis that's been. So, I don't have ultimate strength loads to share with you. For the Wohler exponent, we typically use 3, 4, or 5 for steel components and 8, 10, or 12 for composite components in the absence of more specific information.

I agree with your selection of outputs, except that I would normally look at the shaft moments in the rotating frame and at the main bearing (i.e., LSSGagMxa, LSSGagMya, LSSGagMza). Likewise, I would look at the yaw-bearing loads in a coordinate system that does not yaw with the nacelle (i.e., YawBrMxp, YawBrMyp, YawBrMzp).

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Benjamin.Marechal
Posts: 4
Joined: Fri Feb 09, 2018 6:43 am
Organization: D-ICE engineering
Location: France

Re: S-N curves for FOWT reference

Postby Benjamin.Marechal » Fri Dec 28, 2018 10:53 am

Hello Jason,


Thank you for answering quickly.
For the controller we are developing, it is very important to be able to evaluate the lifetime of the turbine.

I understand that no strength analysis on OC3-Hywind is available.
Are you aware of a wind turbine for which the ultimate strenght loads (or the S-N curves) are available?
The best case would be a floating turbine of at least 5 MW.


We also have another question regarding the total length of the simulations for fatigue assessment.
In the article Simulation-Length Requirements in the Loads Analysis of Offshore Floating Wind Turbines, it is stated that "approximately ten 10-min simulations for each wind speed bin should be used to obtain converged statistics".
For the fatigue assessment we perform, we use a full distribution of environnemental conditions, which represents a very large number of cases. We don't think we need to do 100 minutes of simulations for each case.
Which cumulative length of simulations would you advice to perform a proper fatigue assessment?


Thank you for your time and Happy New Year,
Benjamin Maréchal

Jason.Jonkman
Posts: 4104
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
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Re: S-N curves for FOWT reference

Postby Jason.Jonkman » Sat Dec 29, 2018 6:52 am

Dear Benjamin,

I'm not aware of any floating wind turbine model (FOWT) available in the public domain, which has the ultimate strength loads or S-N curves available for the tower, shaft, and blade root (that doesn't mean there aren't any). You may need to make your own assumptions for your own purposes.

I can't recommend a specific cumulative length of simulation to perform a proper fatigue assessment for FOWT. I think more work is required on this topic before general statements can be made. I suggest you do a convergence study based on your own FOWT and metocean conditions. Certainly, though, for a given condition, fatigue loads converge more quickly than ultimate loads (the paper you reference covers both ultimate and fatigue loads).

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov


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