A Case Study on Optimising Flexible Riser Fatigue Design

Abstract The fatigue life of flexible risers is a critical factor in offshore system design due to inherent safety implications. Breaches in the outer sheath or condensed water can lead to annulus flooding, which needs to be accounted for during design to prevent integrity concerns during the operating life of the asset. The annulus environment alters the applicable SN curves used for tensile armor fatigue calculations and can significantly affect the riser’s predicted fatigue life. Because the integrity of the outer sheath cannot be guaranteed, industry practice requires accounting for a flooded annulus from day one, which can lead to conservative and costly riser designs. This paper presents the case study of a pliant wave flexible riser connected to a Semi-Submersible. The study examines the impact of key parameters on fatigue life predictions, including operating pressure and temperature, inter-layer friction, annulus condition, and current loading. The results provide actionable insights into unbonded flexible riser fatigue life optimization and management. It is shown that the predicted fatigue life can significantly increase by accounting for reduced operating pressure over the asset life at the design phase, or by revisiting the accumulated fatigue damage to account for actual annulus conditions during the operational phase. Global riser and local fatigue analyses were conducted using industry-recognized software, OrcaFlex and BFLEX, respectively. Automation tools enabled concurrent operation of both software packages, improving efficiency, and facilitating a comprehensive sensitivity analysis across various operational conditions.