# Using Mlife for a lifetime fatigue estimation

Hi everyone,

I want to use MLife to determine the fatigue lifetime for the 5MW NREL
turbine.
So far, I have run test 18 without problems. After reading up in the forum,
I’ve to download MLife to do fatigue estimations, but I can’t find the
Can somebody help me?

Thanks and best wishes,
Julius

Dear Julius,

A similar question was asked and answered in my post dated Jun 17, 2020 in the following forum topic: viewtopic.php?f=4&t=805&start=30.

Best regards,

Best wishes,
Julius

Hi Jason,

I managed to run the mlife_certification_test file provided by mlife.
Now I want to adjust the input files in such a manner that I can determine the lifetime of the 5MW NREL turbine for all three cases specified in the IEC-61400 standard.

1. To do so, do I have to modify the DLC data files, or do I have to import the data files from FAST?
2. I’m wondering how I can plot the corresponding Wöhler curve?

Best wishes,
Julius Scherf

Hi Julius,

I’m not sure what you mean by “DLC data files”. Are you referring to the FAST time-series output files?

MLife will not plot the S-N curve for you, but you can plot it yourself using the values of L_Ult, L_MF, and m using equation (2) from the MLife Theory Manual: drive.google.com/file/d/1d7uWya … sp=sharing.

Best regards,

Hi Jason,

The Problem concerning the DLC files solved itself.
But regarding the S-N curve, I have a question. I understand that I can calculate the number of cycles using equation (2) with L_Ult and L_MF provided by MLife, but I’m not sure about L_i^RF. Can I understand it as such that it represents the standard deviation from each load case?

Best wishes,
Julius Scherf

Dear Julius,

L^RF_i is the cycles load range about a fixed mean value. The S-N curve is a plot of L^RF_i (S) versus N_i (N).

Best regards,

Dear Jason,

thank you. I appreciate your effort.

Best wishes,
Julius

Dear Jason,

I set the Weibull-Shape factor to 2, so I can assume a Rayleigh wind distribution. Further, I want to determine the lifetime damage for three different mean wind speed values (7.5; 8.5; 10 m/s). The strange thing that I observe is that the ultimate load decreases from 8.5 to 10 m/s. Is there a fundamental error I don’t see. Do you have any ideas?

Best wishes,
Julius

Dear Jason,

I’m sorry, I forget to tell you that the ultimate load for the lifetime damage was calculated for RootMMxy1, which was determined as described in the MLife manual.

Best wishes,
Julius

Dear Julius,

I’m sorry, but I’m not sure I understand. How are you arriving at the ultimate load for RootMMxy1?

Best regards,

Hi Jason,

I wasn’t sure where exactly to post this but it is regarding L_Ult and the use of MLife.

I am wondering what values I should insert for L_Ult in the .mlif file for MLife? The turbine I am using is the 15MW offshore wind turbine. Please let me know what you think.

Thanks and best regards,
Matthew

Dear Matthew,

LUlt has been discussed many times on our forum, e.g. see:

Best regards,

Dear Jason,

Best regards,
Matthew

Dear Jason,

I am wanting to perform a mooring fatigue life analysis using my tests from FAST V8, I have used Map++ in my simulations and wanting to know how to use mlife to process these results and work out the fatigue life.

Best regards,
Andriana.

Dear Andriana,

MLife can be used to post-process the fatigue of mooring line tensions (in terms of DELs, lifetime damage, etc.) just like it can with any other turbine or support structure load.

That said, MAP++, being a quasi-static model, does well at predicting the mean and low-frequency mooring line loads, but not the dynamic / high frequency mooring line loads. So, I would expect the mooring line tension ultimate and fatigue loads to be underpredicted by MAP++. MoorDyn is better when mooring line loads are to be analyzed.

Best regards,