C-CORE successfully tested in our geotechnical centrifuge, the equivalent of a submerged 108m, 0.5m outside diameter Steel Catenary Riser (SCR) over a soft clay seabed. The test was part of a larger research program led by BP America, to investigate the fatigue stresses associated with extreme storms and vessel movements on SCRs in the touchdown zone. C-CORE’s research involved a series of physical centrifuge tests, followed by numerical finite element analyses, drawing heavily on the geotechnical, structural, mechanical and hydrodynamics principles.
An SCR is a long steel pipe used in medium and particularly deep (> 300m) water to connect a floating vessel to facilities on the seafloor. SCRs offer advantages over other riser types, including ease of installation and lower cost. Despite these advantages, estimating the fatigue life of an SCR is hampered by incomplete understanding of riser–soil interaction at the touchdown zone, where cyclic interaction of the riser with the seabed occurs. Developing a model for seabed stiffness requires characterization of a number of complex nonlinear processes including trench formation, nonlinear soil properties, and soil suction associated with breakaway of the riser from the seafloor. Therefore, SCR interaction at the touchdown zone remains an active research topic for offshore engineers.
C-CORE’s centrifuge tests modelled a continuous submerged SCR, starting from about 5m above the seabed and extending through the touchdown zone and into the surface zone. During each test, the model riser underwent a series of dual frequency heave and surge motions, simulating a vessel under various storm and hurricane conditions. The model riser was 2.7 m in length and was fully instrumented with axial and bending moment strain gauges placed at close intervals. Further instrumentation included monitoring the riser embedment and deformations at various locations along its length. Riser testing like this in a geotechnical centrifuge is unprecedented, and the results have provided invaluable insight into the riser–soil interaction mechanism.