Experimental and numerical investigation of transient responses and mooring tensions of SFT induced by sudden mooring failure

This paper examines the transient responses and line tensions of a submerged floating tunnel (SFT) subjected to sudden mooring-failure incidents through experiments and numerical simulations. A 1/100-scale SFT-section model, constructed from acrylic material with 8 mooring lines, was tested in a 2D wave tank based on Froude similarity to evaluate the transient effects of mooring failures. The same system was numerically modeled using lumped masses connected by linear and torsional springs for structural elasticity and the Morison equation for hydrodynamic load calculations. The numerical simulations for various line-failure scenarios were systematically compared with experimental results, demonstrating good agreement. Both approaches indicate that transient tension overshoots immediately after a line failure can be substantial, especially at high buoyancy-to-weight ratios (BWRs). After verifying the transient behaviors, the numerical model was extended to investigate cases with lower BWRs and a more realistic, longer SFT configuration with additional mooring lines. In these cases, transient effects are appreciably reduced, as expected. In the case of longer SFT, the present Morison-force-based result is compared with a much more rigorous discrete module beam (DMB) approach in solving hydroelasticity, including wave diffraction/radiation effects. Furthermore, mooring-line loss results in a nonsymmetric mooring system, leading to more nonlinear SFT dynamics under regular wave conditions.