Centrifuge Model Study of SCR Motion in Touchdown Zone

Steel catenary risers (SCRs) connect seabed pipelines and flow lines to floating structures used for oil and gas production in deep water. Waves and currents induce motions of the structure and the risers. Repeated horizontal and vertical interactions of SCR at the touchdown zone would induce loads on the seabed soil, and would eventually cause fatigue damage to the risers as the number of load cycle increases. The analysis of SCR fatigue damage is heavily dependent on the soil model. Soil behaviour at touchdown zone such as soil remolding, stiffness degradation and deformation of the seabed at the touchdown zone further complicate the accurate assessment of SCR fatigue damage, which is not appropriately quantified in existing design methods. This paper presents model tests simulating the repeated vertical movement of a length of riser on a geotechnical beam centrifuge at the National University of Singapore. During the tests, the pipe was subject to cyclic motion over fixed vertical displacement amplitude from an invert embedment of 0.5 pipe diameter to 3 pipe diameters into the soil. The results from centrifuge model tests show a significant progressive degradation of soil strength and diminution of excess pore water pressure as the number of load cycle increases. The increased soil strength degradation and diminishing excess negative pore water pressure during cyclic motion of the SCR reduce fatigue damage. In view of different types of environment loadings experienced by the floating platform, test results obtained from parametric studies on the effect of cyclic displacement amplitude on soil stiffness degradation will also be presented in this paper. Lastly, a simple upper bound plasticity solution will be presented and compared with the experimental result.