# best method for CRUNCH

to all, i am in the middle of finding the fatigue range for a turbine. I have used TurbSim top create 10 randomly seeded files per wind speed, for a range of wind speed 4-30m/s (plus a few others). My questioon is: how do other users suggest the rain flow files be created/used, i assume that all output files should be analysed in CRUNCH, giving 10 rcc files per wind speed, how can this data then be used? Should I have average each wind speed bin (although this to me seems to defy the logic in using 10 random seeds) or use the extreme values? Is there a way to instruct CRUNCH to use these files appropiately?

many thanks, Teddy Crockwell, Merseyside, UK

Teddy,

I’m going to assume that you want to compute the fatigue life of various parts of your turbine. I will also assume that you have a Raleigh distribution of wind speed in mind for the site you are designing for. It sounds like you have run a bunch of FAST simulations that used various wind speeds for a multiple of seeds.

If you have MatLab and its Statistics Toolbox, you can use my MCrunch code to compute the fatigue life. This calculation will tell you what fraction of the lifetime of the parts is consumed in a specified amount of time for a given wind-speed distribution. If you don’t have MatLab, let me know and I’ll try to come up with another option for you.

Marshall

Thankyou for your reply. Yes, i do want to calculate the fatigue life, mainly of the tower, therefore i need to find the forces acting on the turbine (rotor thrust, yaw bearing moment). Regarding my question, i am unsure of which wind speed seed to use for each wind speed bin, I am currently using the most extreme seed value, would you reccomend something different to this? Sadly I do not have MatLab, though i have written an excel spreadsheet to calcualte the DEL from the codes I am working to.
Many thanks, Teddy

Dear all,

Since this question will be about MCrunch it is probably of most interest to the author of the program (Marshall Buhl), however the answers could be helpful for more people. I am currently researching how one could use Individual Pitch Control (on the 1P and 2P harmonic) in order to reduce the cost-of-energy of the UpWind 5.0MW offshore wind turbine. For this I need to determine what the influence of the IPC is on the fatigue of several components. Right now, I’m approaching this with a rather crude method of applying rainflow counting on several moments and subsequently damage equivalent load calculations based on simple S-N curves. Since thus far I have only considered relative fatigue (how much reduction is obtainable compared to the baseline collective pitch control situation), however I would like to refine my fatigue calculations. As a first step, it would seem obvious to calculate the stresses in the material as opposed to using moments. MCrunch has the option to both calculate the damage equivalent load for a simulation run, as well as a fraction of the total life-time damage. How does MCrunch perform these calculations; there are many things to determine while the input to the MCrunch program is rather limited. Is the program able to calculate the total lifetime damage of a component by using the material and structural properties?
I hope my question is formulated clearly enough and I am looking forward to your thoughts. Let me also use this occasion to thank everyone at NWTC for making such a great range of software available to the public.

Kind regards,
Bob Prinsen

Dear Bob,

Marshall had pointed me to an initial draft for the Mcrunch Theory manual that he had started working on. It has some limited information. You can find it here

http://wind.nrel.gov/designcodes/postprocessors/mcrunch/MCrunchTheory_Draft.pdf

Hope it helps.

Best Regards,

Subin

Bob,

I’m really not an expert in material properties. I’m more into the dynamics and loads stuff. I generally consider that our simulators (such as FAST) are used to generate loads and then these can be applied to FEA models if you really want to understand strains.

Thanks for pointing to the draft theory manual, Subin!

Marshall