Fatigue Reliability Assessment for Flexible Riser Armour Wires

Oil and gas exploitation in regions of ultra-deep water has already been started and there is a great expectation of growth in the coming years, mainly in the region known as Pre-Salt, at Brazilian’s coast. Operators, which have the concession to exploit this region, have adopted different solutions with different configurations using rigid and flexible pipes for this initial phase. However, the last structural type mentioned shows to be much more versatile, providing attractive cost/benefit solutions to the operators, in which, only in Brazil, it is forecasted a demand about 4.000 km of flexible pipes in the next years [1]. With new challenges coming, one may ask the following question: What is the structural reliability of each failure mode of these flexible pipes? Motivated by this question, this paper has the objective to do a preliminary analysis, under the structural reliability view, of the critical failure mode for the design of flexible pipes faced in Brazil, represented by the fatigue phenomenon on metallic layers. The work assesses the fatigue reliability of the armour wires, located in the bend stiffener region, for two 6″ flexible risers configurations, free-hanging and lazy-wave. They are considered to be installed in ultra-deep water (2140m depth) and are submitted to the same external environmental loads. The fatigue reliability methodology used is based on that presented in Ref. [2] including new random variables. Sensitivity analyses are also performed for the variables that most contributes to the quantified probability of failure. Calibrated safety factors (SF) are obtained for different target failure probabilities.