Submerged floating tunnels: Nonlinear dynamics of anchoring elements subject to hydrodynamic loads

Seabed-anchored Submerged Floating Tunnels (SFTs) are interesting modular structures which are deemed to be a valuable option for crossing deep and long waterways. Due to their inherent flexibility, SFTs are prone to the effect of dynamic loads. Among all of them, wave loads are deemed to be the most critical for both serviceability and fatigue life assessments of the SFT, because of its recurrent and cyclic nature. The local hydrodynamic response of the anchoring elements typically entails significant computational burden, making both numerical models and CFD simulations unfeasible solutions for the conceptual design phase of SFTs. In the present work, a reduced-order cross-sectional model of the SFT is formulated, by modeling the submerged tube as a two-dof rigid body and describing the planar motion of the anchoring elements according to the small-sag cable theory, accounting for both geometrical nonlinearities and supports motion. The coupled system of equations of motion is derived, and three different loading conditions are studied. Subsequently, the multiple timescales perturbation method is employed to obtain approximate closed-form solutions for the steady-state vibration amplitude of the tethers. Numerical validation of the sought expressions is then presented, and extensive parametric analyses are carried out, with the final aim of providing a quick and reliable tool to guide the conceptual design phase of SFTs.