Centrifuge modelling of steel catenary riser–seabed interactions under three-dimensional cyclic motions

Steel catenary risers (SCRs) are prone to fatigue failure due to the cyclic motion of the host floating platform. While previous studies have primarily focused on one-dimensional (1D) and two-dimensional (2D) motions of SCRs, real-world scenarios often involve complex three-dimensional (3D) motions that can significantly affect SCR–seabed interactions and SCR fatigue. To fill this gap, centrifuge model tests are conducted to enhance understanding of cyclic SCR–seabed interactions under episodic 3D motions. The results indicate that 3D motions produce the highest cyclic stress in the SCR, approximately 20% higher than that from 2D motions at the end of the fifth motion packet. This highlights a critical design concern regarding SCR fatigue under episodic 3D motions. In addition, the steady cyclic stress increases with the number of motion packets and stabilises by the seventh motion packet in the 3D motion scenario, suggesting that the cyclic stress at the seventh episodic motion packet can serve as a reference for evaluating the SCR’s fatigue. Contrary to conventional understanding that lower seabed stiffness reduces cyclic stress, this study reveals that increased SCR embedment depth in a softer seabed can lead to higher cyclic stress.