Managing Fatigue in Deepwater Flexible Risers

摘要

Dynamic flexible risers are unique in the sense that they can accommodate substantial curvature variations combined with high tensile loads without compromising their fatigue capacity. However, as the fatigue endurance assessment becomes increasingly challenging with increasing water depth it is of utmost importance to know the fatigue driving mechanisms in flexible risers used for deepwater applications. Thus, key fatigue drivers are discussed in this paper including high inter-layer contact pressure due to severe tension and ambient hydrostatic pressure loads, corrosion fatigue in connection with high strength steel armour, riser structural properties (e.g. stiffness and damping), impact from riser VIV and second order springing effects of the floating unit. Furthermore, helpful fatigue analyses procedures are presented including a methodology based upon transfer functions that can determine the fatigue damage along the length of a riser thus facilitating an efficient irregular wave fatigue assessment. At present, dynamic flexible risers are used in connection with oil and gas floating production facilities in water depth down to approximately 2000m. However, research is ongoing to further expand the flexible riser capabilities thus meeting the requirements of the future offshore field developments at even more demanding water depth between 2000m and 3000m. Thus, it is demonstrated in the paper how a new flexible pipe concept, Flextreme~R, having excellent fatigue performance can be used for deep- and ultra-deepwater applications, and how failure modes normally associated with the conventional flexible pipe structure have been eliminated. Finally, ways to manage the risk of fatigue failure during operation of a deepwater flexible riser are addressed, including presentation of a condition monitoring technique using optical fiber technology. Also, a concept based upon active flushing of the riser annulus is described which can be used to extend the service life of in particular sour service risers.