3.0 fatigue analysis-jacket (using sacs)

1. Make a copy of the inplace sacinp file. Rename the file as sacinp.fatigue.
2. Make following changes to this file:
   1. In the LDOPT card change the Water Depth as per bid to account for absence of Storm Surge
   2. Change the Cd and Cm values
   3. Delete all loads except wave loads
   4. Change the Kinematic factor to 1.0(see bid) in the WAVE card
   5. In the Wave Height field, change the wave height to Average wave height of the specific wave
   6. Enter MS for obtaining maximum base shear value
   7. Delete all Load combinations from this file.
   8. Advance the wave in steps of 4degrees
   9. The number of static steps will therefore be 360/4 = 90
3. Run this file in Static Linear static analysis Select sacinp.fatigue*
4. In the output listing a saclst file is obtained from which the crest positions for minimum and maximum shear can be found for different wave heights and different wave directions.
5. Obtain three angles between angle1(X) and angle 2(Y) as (Y-X)/4 and remaining three angles as ((360-Y)+X)/4
6. Enter the 6+2 angles in the sacinp file and change the number of steps to1 in increments of 1degree.
7. Run the sacinp file in Linear Static Analysis.
8. A saccsf file is obtained which will be used for the fatigue analysis.
9. Create a ftginp.* file.
10. In the FTOPT card enter design life(see bid) and factor(see bid, usually 2.0). Also enter the fatigue time period as the Wave Period (generally 1 year).
11. Initially apply API X Prime for source of S-N curve i.e. APP.
12. For SCF’s use Kuang & Wordsworth for all joints initially.
13. In the FTOPT2 card give PT, Export Fatigue data, Tubular Inline Check, Inline Tubular SCF-AWS, and Effective Thickness Ratio-2WAL
14. Specify the SCF limits in the SCFLM card as a maximum of 6.0 and a minimum of 1.6
15. For Grouted Joints enter Joint Override with SCF Option as Marshalls Method.
16. In the GRPSEL RM card specify dummy structures, appurtenances, Risers and conductors which are not part of the main structural Member but attract Wave Load.
17. In the EXTRAC HEAD AE card enter a cut off of 0.5 to extract joints which have a fatigue life of less than 50 years.
18. Now, in the Runfile Wizard go to Post - Fatigue Damage and Run the ftginp file with the saccsf file and note down the joints which have service life of less than 50 years.
19. Now, Extract these joints individually by using the EXTRAC card in the ftginp file.
20. On running the file again as in step no.18, a ftgext file is created automatically.
21. In the ftgext file we can make changes in the Chord Thickness, or S-N curve to be used etc. to find the optimum change that has to be made to a chord/brace. Note down the specific changes that need to be made for a specific joint.
22. Now, in the ftginp file enter the JNTOVR cards for the joint overrides that need to be specified as noted from the ftgext file.
    NOTE: In the case of grouted joints, if the S-N curve needs to be changed, use the API X curve with effective thickness option i.e. AXP option.
23. Run the Final ftginp file with the saccsf file till no joint have a service life
    less than 50years.
24. The output files that are obtained from the run are the ftglst and the ftgext files.

LARSEN & TOUBRO LIMITED
STEP BY STEP GUIDE FOR JACKET DESIGN USING SACS SOFTWARE

RESEARCH & DEVELOPMENT
ENGINEERING & CONSTRUCTION DIVISION
LARSEN & TOUBRO LIMITED