Spectral Fatigue Analysis

Hello sir/madam,

I am Nalinii here and I am a final year student from UTP.I need to run spectral fatigue analysis for jacket platform.I would like to know the step to conduct spectral fatigue analysis in SACS.I would be really glad if sir or madam will be able to help me to show me  the procedure  especially after the eigenvalue analysis because I am having problem starting from that part.

  • Nalinii,

    The typical steps to perform a spectral fatigue analysis in SACS are as follows:

    1. Create pilehead superelement (optional)
    2. Extract mode shapes
    3. Transfer function generation
    4. Spectral fatigue

    It appears that you have already performed step 2 so you should have your mass matrix (dynmas) and mode shape (dynmod) files. You then need to generate a transfer function which is used to convert the wave spectra into stress ranges for the fatigue analysis. This wave response analysis utilizes the dynmod and dynmas files to generate a SACS common solution file (saccsf) which contains the responses which define the transfer function. You can then run a spectral fatigue analysis on this saccsf file to generate your fatigue damage.

    I would recommend looking at Sample08 located in your SACS installation directory which contains a working example of what I just described. You may also want to refer to Section 4.3 of the fatigue manual for a more in depth discussion of spectral fatigue as well as Sample Problems 1 and 2 in Section 6.0 of the manual for other examples of this type of analysis.

    Regards,

    Geoff

  • Hello Mr.Geoff,

    Thank You so much for the answer.I have one more question.Can we use general wave response module instead of deterministic wave/transfer function generation. How does it affects the analysis ?Where we can get the manual to know the differences between these two modules ? Thank you in advance, sir.

     

    regards,

    Nalinii

  • Nalinii,

    General Wave Response and Deterministic/Transfer Function Generation are both part of the Wave Response module so they are very similar in execution. In fact, the only difference between the two is the input that you provide in you wave response input file (wvrinp). The Wave Response module is designed to compute the dynamic response of a structure due to wave action, but you as the user have control over how the waves are applied (e.g. random wave or deterministic) and what output is generated (e.g. max base shear, max overturning moment, etc.). However for a spectral fatigue analysis, a transfer function is required. Without a transfer function you cannot convert the wave spectra into a structural response and will not be able to generate stress ranges for your fatigue analysis.

    I would recommend reading the Wave Response manual for more information about transfer function generation.

    Regards,

    Geoff

  • Hello Mr.Geoff,

    What does this error means ?

    Thank You in in advance and have a nice day :).

  • Nalinii,

    When you generate a transfer function using the wave response module, SACS uses fourier series to represent the generalized forces for each mode due to the wave loading specified. And since a fourier series is just a series of sine waves used to represent a more complex function, there will always be a slight difference (error) between the two curves. More fourier series terms should reduce the error between the two curves. You can set the fourier series error tolerance in columns 31-40 on the WROPT line in your wave response input file (wvrinp). You may refer to section 3.2 of the Wave Response manual for more information about this procedure.

    Regards,

    Geoff