Considering the stress cycles in joints and members due to wave induced forces on offshore platforms, fatigue analysis is therefore one of the most important analyses in offshore platforms design. Most of steel jacket type platforms located in areas with relatively high ratios of operational sea-states to maximum design environmental events; fall in acceptable safety margin in inplace and seismic analyses. But in fatigue analyses they will face critical condition. Therefore it seems that utilizing control mechanisms with the aim of increasing fatigue life in such platforms will be more preferable to merely deck displacement control. Investigation of tuned mass dampers adjustable parameters optimality for vibration control of wave excited structures implies that optimum tuning and damping ratios are strongly dependent to sea-state in addition to system parameters. The efficiency of optimally designed tuned mass damper for fatigue damage mitigation in realistic steel jacket platforms has been evaluated in this study. Full stochastic spectral analysis method has been employed to estimate the maximum fatigue damage as objective function to be minimized. This leads to a min-max problem which has been solved with Micro Genetic Algorithm. Furthermore the efficiency of a variable TMD with different optimum mass for each sea-state has been evaluated in comparison with a TMD with constant parameters.

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