How to perform spectral fatigue analysis using SACS

We are trying to perform a fatigue spectral analysis for our structure, but we are unsure how we do this in SACS. 
The information we have been given for the spectral data is as follows:
- Velocity RAOs
- Motion RAOs
- Acceleration (with G) RAOs
- Acceleration (without G) RAOs
- Jonswap spectrum input (3 cases)
We have two questions:
1) What is the methodology for inputting the RAOs and spectral data into SACS in order to calculate fatigue damage at individual joint details?
2) What is the method for combining the 4 different sets of RAO inputs?
Thanks in advance.
  • Stephen,

    You need to do a Tow Fatigue analysis. Tow Fatigue will analyze fatigue damage of a structure by generating transfer functions using motion RAO's (Velocity, Motion or Acceleration RAO's) and combining those transfer functions with the wave spectra (Jonswap). I don't think you actually need to combine the 4 sets of RAO's as they should simply be 4 different ways to represent the same set of data. However if you do indeed have 4 sets of RAO's they would most likely not be for the same fatigue environment and could be combined linearly using SACS fatigue damage output.

    Tow Fatigue Workflow:

    1. Tow Analysis
      1. SACS Model File - SACS input file
      2. Tow Input File - Tow Input File with RAO's and wave definitions (WAVDEF)
      3. Tow Runfile - Select Solve Option to generate a common solution file
    2. Fatigue Analysis
    1. Fatigue Input File - Fatigue Input File with SEAS option and wave spectra definitions (WSPEC)
    2. First Common Solution File - Common solution file generated from tow analysis (saccsf)
    3. First Seastate Input File - Tow input file with wave definitions (towinp)

    You can select the option on the FTOPT2 line to output a fatigue damage file which can then be used as input for a second fatigue analysis. It will always be valid to linearly combine tow fatigue damages from separate fatigue environments (e.g. tow fatigue and in-place wave fatigue since the responses are not coupled in any way). Refer to Section 2.8 of the Tow Manual for more information about specifying RAO's and wave definitions and Section 3.4.1 of the Fatigue Manual for more information on specifying the wave spectra.

    Regards,

    Geoff