Dynamic responses of the unbonded composite riser with multi-spacing coupling interference

The interference among risers has always been a research focus within marine engineering. By riser, we mean the equipment that transports oil, gas, and other fluid media in marine environments, connecting the offshore platform and subsea wellhead. The “unbonded composite riser-smooth riser” serves as the research object of this paper, which experimentally investigates the riser group vibration with multi-spacings. This study reveals the interference mechanism of risers with interlayer slip by reviewing the dynamic response of unbonded composite risers in various arrangements. The results indicate that increasing displacement of a side-by-side unbonded composite riser is subjected to the interference and internal cable interlayer slip at high flow velocities. The displacement interference rate peaks, particularly at S/D = 4 (S/D is the spacing ratio, where S represents the distance between the axis of the two risers and D represents the outer diameter of a riser). By contrast, changes in the spacing ratio have little influence on the dominant frequency. The strong energy dissipation capacity and high mass-damping ratio of side-by-side unbonded composite risers efficiently decrease vibration caused by fluid forces. Side-by-side unbonded composite risers occur in a delayed “lock-in” region as opposed to smooth risers under the same conditions. Furthermore, the wake and clearance flow interference on the unbonded composite riser results in a maximum difference of up to 5 times greater in the average value of displacement interference rate.