Fatigue is one of the key design considerations for moorings, risers, and umbilicals in offshore floating systems. Due to differences in materials, components, configurations, and functionality, these slender structures have distinct drivers for fatigue performance. On the other hand, they also share a number of the same challenges. Typical slender structure design configurations include taut and catenary mooring systems; steel catenary riser, steel lazy wave riser; and catenary, lazy wave, and pliant wave configurations for umbilicals. This paper introduces the fatigue performance drivers for mooring lines, risers, and umbilicals. The paper also compares the fatigue performance of moorings, risers, and umbilicals, and provide cross-disciplinary guidance to the integrated design of floating systems, thereby helping improve the overall system reliability.
Moorings, risers, and umbilicals demonstrate quite distinct fatigue characteristics but share some similar challenges. Mooring fatigue performance is influenced by tension and corrosion. Chains in fairlead locations and on the bottom are the most critical fatigue locations for mooring fatigue. Recent fatigue test data show that mean tension significantly impacts chain fatigue and thus whole mooring system fatigue life. Fatigue is a significant challenge for riser design due to tension and bending. The hangoff and touchdown zones are usually the most fatigue critical areas for risers as well. Similar to the mooring design, corrosion contributes to riser fatigue. Riser configuration, internal fluid slugging, as well as seabed stiffness significantly affect riser fatigue performance. Besides tension and bending, one of the specific features for umbilical design is the interaction between the different elements within an umbilical cross-section, which significantly impacts its life. Since this effect is more significant in regions with higher tension and curvature, the hang off region is usually the critical area for umbilical fatigue.
The offshore industry is moving towards integrated design and systematic approaches. Although there is abundant literature discussing the fatigue of mooring, risers, and umbilicals separately, there is very limited published literature comparing the performance of these three types of slender structures. This paper can serve as a useful reference for a floating system integrated design. Cross disciplines can also benefit by leveraging the design philosophy and mitigation strategy of different types of slender structures.