Submerged Floating Tube Bridges (SFTB) is a proposed non-conventional engineering solutions to cross fjords. The Norwegian Public Roads Administration have made a video showing the concept.

A SFTB, also known as an Archimedes Bridge, is essentially a buoyant tunnel that floats below sea surface. This ensures that surface ship traffic is not disrupted, while at the same time, protecting the structure from most of the environmental loads. The SFTB has to be supported by floating pontoons or moored to the seabed with cables.

The dynamic behaviour of these mooring systems needs special attention for safe design. Of particular concern are periodic changes in the cable tension, producing a parametrically excited system. These excitations might lead to parametric resonance or dynamic instabilities that cause excessive axial vibrations.

Sketch of mooring line model. Illustration by NTNU/Daniel Cantero Lauer.

NTNU has studied the mooring systems and their susceptibility to parametric excitation, both analytically approaches using simplified closed-form expressions, and numerically, allowing for more complex cable configurations, as well as the inclusion of hydrodynamic interaction and loading.

Cable displacement under parametric resonance for harmonic oscillation of support with amplitude of 0.01m including nonlinear and hydrodynamic effects. Figure by NTNU/Daniel Cantero Lauer.

Due to the uncertainty associated with many of the system parameters, particularly those related to the wave loading, the investigations are treated within a probabilistic framework. Extreme value responses can be estimated using enhanced simulation methods developed at NTNU. These results can be used to study the system reliability, leading to enhanced design procedures of mooring systems.