Coupled motion characteristics of a large-scale tunnel element suspended by immersion rigs in the vicinity of seabed

Abstract

During the near-seabed immersion stage, the submerged tunnel element suspended by immersion rigs and anchored to the seabed is at risk of possible bottom grounding and collision with the installed elements. Specifically, the wall effect would have an important role on the motion characteristics of the tunnel element when it approaches the seabed (The wall effect is the influencing behavior of one large planar surface approaching another large planar surface underwater). To investigate this effect, this research conducts 3D experiments using a 1:60 scale model of a combined tunnel element and immersion rig system. The six-DOF motions of the tunnel and immersion rig are measured simultaneously for various regular wave conditions. The collected measurements for different relative clearances are compared in both time and frequency domain. Nonlinear effects of the near-seabed interference are found to be particularly significant when the relative clearance ratio (clearance beneath tunnel / height of tunnel element) is smaller than 0.26, especially when the incident wave period is near the resonant roll period of the tunnel/rigs. To further explore the contributing factors of the seabed interference and to determine suitable operating conditions in practice, a fully coupled tunnel-rig numerical model is built based on the potential flow theory. Additional damping is also included in the coupled motion equations for the resonant roll scenarios and good agreement with experimental data is achieved for relative clearance ratio of δ/d ≥ 0.42. Overall, the experimental and numerical results demonstrate that besides the relative clearance, the near-wall effect of the seabed is also closely associated with the roll of the tunnel element and thereby has a strong dependence on the incident wave height, wave period and wave obliquity. The study is expected to be helpful for understanding the near-seabed effect on the responses of large-scale rectangular floater so that the installation and positioning can be performed accurately and effectively.