Abstract
The long-term distribution of stress cycles is a crucial part of the fatigue life assessment of offshore structures. This research studies the wave and current induced motion of a top tensioned riser connected to a semi-submersible platform. To account for the directionality, the platform’s directional response amplitude operator (RAO) is applied.
Since this research aims to calibrate Weibull distribution parameters, a long-term analysis of stress cycle distribution for all sea states is run. A finite element model is developed to solve riser dynamics under the effect of waves, currents, and top vessel motions. The code is solved for 87 sea states in the time domain, and then the stress cycles are derived by the Rainflow counting method. Finally, the long-term stress ranges distribution is fitted to a Weibull distribution. Comparison of the results in cases with and without directionality, shows that directional assumption leads to a broader distribution for stress ranges.
