Steel catenary risers (SCR) connect seabed pipelines and flow lines to floating structures used for oil and gas production in deep waters. Waves and currents induce motions of the structure and the risers. The repeated motions of the risers at the touchdown zone in turn induce loads on the seabed soil and might eventually cause fatigue damage to the risers. The analysis of riser 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 riser fatigue damage, which is currently not appropriately quantified in existing design methods.

This paper presents centrifuge model tests simulating the repeated vertical movement of a length of riser on clay seabed. During the tests, the pipe was subjected to cyclic motion of constant vertical displacement amplitude. Particle Image Velocimetry (PIV) analyses are performed to investigate the soil failure mechanism during the penetration and uplift of the length of riser, including cavity formation, transition from shallow and deep failure mechanism, and evolution of failure mechanism as the load cycle increases.

The test results show a significant progressive degradation of penetration and uplift resistances upon the onset of riser cycling which is due to the remoulding of the soil. Furthermore, formation of weakened zone confines the development of failure mechanism as the number of load cycles increases. These results could provide further insight into the touchdown zone modelling as SCR present as an economical solution as oil exploration and production carried out in greater water depth.

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