Dynamic substructuring technique for submerged floating tunnels

Seabed-anchored Submerged Floating Tunnels (SFTs) are structures made of watertight tubular segments, assembled, and kept floating under the water table by a spread system of anchoring elements connected to the seabed. Due to their inherent flexibility, SFTs are prone to the effect of dynamic loads such as seismic and hydrodynamic loads. These peculiar features have since long lead to the assessment of the SFTs’ dynamic response by resorting to the simplified model of a beam on an elastic foundation. This amounts to totally neglect the mass of the anchoring elements and, therefore, their local vibration modes. In this work, the mooring system is modeled as a discrete series of taut-strings possessing axial extensibility, while the tunnel is considered as a continuous Euler-Bernoulli beam. Within this modeling choices, a dynamic substructuring technique is presented and used to obtain both the natural circular frequencies and the mode shapes of the coupled system.