Effects of seabed interaction on steel catenary risers

Steel catenary risers are an enabling technology for deepwater oil and gas production. A steel catenary riser consists of a steel pipeline suspended between the vessel and the seabed forming a catenary shape. Tools to analyse and design steel catenary risers show that the point where the steel catenary riser first touches the seabed, termed the touchdown point, has the highest stress and the greatest fatigue damage. Current understanding of pipe/soil interaction is limited and consequently there is concern within the industry regarding the conservatism of the analysis. In particular, the implications of pipe/soil interaction for maximum stress and fatigue damage at the touchdown point are significant. To address these concerns, research has been conducted into the following areas: Steel catenary riser trenches - using video survey data from installed steel catenary risers to determine the shape of seabed trenches. A steel catenary riser trench profile has been developed for use in finite element analysis. Pipe/soil suction force - i. e. the bond that forms between the riser pipe and a clay seabed. Experiments have been conducted and a pipe/soil suction model developed for use in steel catenary riser analysis. Pipe/soil stiffness - test data from the CARISIMA and STRIDE JIPs has been examined and a series of soil stiffness models for static penetration, small and large displacements, and cyclic loading have been developed for use in finite element analysis programs. Closed form and finite element models of steel catenary risers were constructed to determine the effect of the soil on stress and fatigue damage at the touchdown point. A finite element model of a representative steel catenary riser has been created and analysed using the seabed interaction models developed. The results show that the seabed trench, pipe/soil suction and soil stiffness have little effect on extreme stress in the steel catenary riser during normal operating conditions. However, pipe/soil suction is shown to have a large effect during slow drift motions where the stress in the riser at the touchdown point could double. The results from a closed form seabed model and finite element analysis show that the fatigue life of a steel catenary riser is sensitive to soil stiffness. If the soil stiffness used to model the seabed is too high the fatigue life may be underestimated; conversely, if the soil stiffness is too low the fatigue life may be over estimated.