Free-Hanging Riser Fatigue Improvement Using High Strength Steel

Abstract High-strength steel, particularly X80 grade, has gained significant attention in recent years for its potential applications in offshore risers due to its superior strength and durability characteristics. Focusing in a new free-hanging riser system called the “Residual Curvature Riser Ballast Distributed” (RCR-BD), this work investigates the application of high-strength steel in a large envelope of RCR-BD configurations and compares the results with those from X65 grade. Numerical global analysis consists of the evaluation of in-place and fatigue analyses, taking into account a comprehensive matrix of envelope load cases for X65 and X80. Further optimisations were done mixing X65 and X80 to improve fatigue performance. Results show that the reduction in the wall thickness by using of high-strength steel for RCR-BD applications can decrease hangoff tension, overall utilisation factor and can be beneficial for fatigue. However, it has been detected that it may depend on the ratio: outer diameter over submerged weight. Improvements up to 30% is observed in this study on fatigue life, suggesting that X80 may play an enabler role of RCR-BD concept.