Corrosion Fatigue of Steel Catenary Risers in Sweet Production

Steel catenary risers (SCRs) are commonly used for deepwater oil and gas developments and the most economic material of construction is generally carbon manganese (C-Mn) steel. These risers suffer cyclic loading principally due to vessel movements, and vortex induced vibration (VIV) from passage of marine currents. For this reason, close attention is paid to fatigue design and girth weld quality, and fatigue testing is commonly carried out on procedure test welds. A further advantage of C-Mn steel is that good quality welds can readily be made, and more importantly, freedom from defects can be assured by reliable ultrasonic inspection. In sweet corrosive environments, when significant hydrogen effects would not be anticipated, a range of environmental effects on fatigue could be envisaged under different conditions, and at different stages of fatigue crack growth. For example, in early stages of growth, corrosion could blunt crack tips, and therefore slow the growth rate, whereas under other circumstances, or later in life, corrosion could provide additional crack extension, and accelerate growth. It has been demonstrated in this programme of fatigue crack growth rate and endurance testing that the most aggressive conditions in terms of corrosivity may not give shortest fatigue lives in testing. The results of tests comparing behaviour in air and in a very highly corrosive aqueous environment at 60°C saturated with CO₂ (conditions which could not be sustained in production) have been explained by reference to competing effects of fatigue and corrosion. Comparison has been made with other published data. Important safety implications surrounding conditions for project-specific corrosion fatigue testing for riser design are considered.