Fatigue from platform motion

[Crusader911]

Haven't done this in a while. I have 10-year and 100-year accelerations and wind loads, but I'm not sure how to translate that into cycles or account for cumulative damage from cycles at lower accelerations. In other words, should I be using some sort of statistical distribution (I have a dim memory of having done something like that for a client like 10 years ago). I know how to build a FAT text file and pull it into Caesar, I just need a little direction on what to use as a basis for the cycle distribution.

 

[DSB123]

If you are doing a fatigue analysis in accordance with ASME VIII Div 2 (as probably you should be) then Caesar does not perform the calculations strictly in accordance with ASME VIII Div 2 even though the documentation says otherwise. On the statistical point, sometimes a weibul distribution is used but I would have thought that the number of platform motions(i.e wave height distribution) would have been specified in the Clients documentation.

 

[bxtsafe]

Hi Crusader911,

I'm actually don't use the ASME standards for doing my job, so I'm going to answer you how I would deal with this issue without using a standard, although I find important to use it.

I imagine you're doing a fatigue analysis. Basically, there are 2 ways to do it: A) fatigue analysis in time domain; and (B) fatigue analysis in frequency domain.

Marine loading are random by nature. So, the most time-saving way to do it is to do a fatigue analysis in frequency domain. First, you have to do a FFT on each wind turbine loading and apply this loading in your component/system. After that, you should do FEA using this loading. The results will be the stresses measured in the frequency domain. After that, you should perform a frequency-based fatigue analysis. The results of it is the life of the structure, measured in each surface node.

frequency-based FEA and Fatigue analyses are complex subjects. So, I recommend to read some texts. For fatigue, I recommend chapter 10 of the book Fatigue Testing and Analysis: Theory and Practice of Yung-Li Lee.

If you have any other question, please let me know.

Regards,