Centrifuge modeling of steel catenary risers in the touchdown zone

Field surveys of installed catenary risers have shown the creation of deep trenches cut into the seabed around the touchdown point. This section is a critical to the fatigue life of Steel Catenary Risers (SCRs) as it consists of a repetitive complex fluid-riser-soil interaction and often contains the maximum deflection and bending moment. Current practice and design programs typically assume a flat, rigid, "non-interacting" seabed and do not account for the complex nonlinear processes associated with the repetitive interaction of the riser with the seabed and trench formation. An important step forward in the state-of-the-art would therefore be to improve the fluid-riser-soil interaction models that are currently used in the analysis and design procedures for steel catenary risers. -- The thesis presented herein focuses on the development of a novel centrifuge experimental tool for modeling a 0.5 m outside diameter, 108 m long steel catenary riser at the touchdown zone and to investigate the trench formation mechanism arising from the fluid-riser-soil interaction and its influence on the fatigue stresses. This thesis provides the details of a SCR model developed for the geotechnical centrifuge at C-CORE, the associated scaling laws, and the results of two series of physical tests along with the confirmatory finite element analyses. Further, it discusses the current status of the state-of-the-art on steel catenary riser modeling at the touchdown zone, identifies the knowledge gaps, and provides recommendations for future research based on physical and numerical models. -- The results of the physical tests indicated that the SCR model was consistent with engineering theory and centrifuge scaling laws, verified using finite element analysis. It was also demonstrated that there was a considerable reduction in bending and tensile fatigue stresses due to the change of trench geometry and the increase in trench depth from its original mudline state, potentially increasing the fatigue life of the SCR.