Session: 04-01-02 Flexible Pipes and Umbilicals II

Paper Number: 80863

80863 - The Long-Term Stress Range Distribution of Marine Risers Considering the Effect of Wave Directionality 

The long-term distribution of stress cycles is a crucial part of the fatigue life assessment of offshore structures. The stress cycles are usually assumed to follow a Weibull distribution. The proper selection of these parameters affects the fatigue prediction process. In the first steps of the design process, the shape and scale parameters of the distribution are selected based on the extreme value of the stress cycle that the structure is supposed to endure during its desired lifetime. So, the fatigue life prediction is very dependent on the extreme event prediction. This research calculates the extreme value of stress cycles of a top-tensioned riser connected to a semisubmersible platform. To account for the directionality, the platform's directional response amplitude operator (RAO) is applied. The and the extreme events are predicted by a directional form of environmental contours method. 

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. On the other hand, extreme events are predicted by the directional environmental contours method. Finally, the distributions ate compared to find their relationship. The study shows that the parameters proposed by the standards are far from being conservative and may lack enough safety. 

Presenting Author: Zohreh Sadat Haghayeghi Amirkabir University of Technology

Authors:

Zohreh Sadat Haghayeghi Amirkabir University of Technology
Mohammad Javad Ketabdari Amirkabir University of Technology