Structural response of the Bjornefjord Submerged Floating Tunnel in current flow
G. Papadopoulos (TU Delft - Mechanical Engineering)
A Metrikin – Mentor (TU Delft - Offshore Engineering)
KN van Dalen – Graduation committee member (TU Delft - Dynamics of Structures)
João M. de Oliveira Barbosa – Graduation committee member (TU Delft - Dynamics of Structures)
RHM Ogink – Graduation committee member (TU Delft - Dynamics of Structures)
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Abstract
The Submerged Floating Tunnel (SFT) is an innovative type of transport structure, with significant advantages in crossing long, deep and wide water areas, compared to more conventional types of bridges. Recognizing its potential, the Norwegian Public Roads Administration is planning to use such technology, in creating a ferry-free highway that will connect the west coast of Norway. The objective of the present Master thesis report is to develop a model on a tether-stabilized SFT and investigate the static and dynamic load effects acting on such a structure in order to estimate how safe can such a structure be. The modeling of the SFT has been based on prototype designs, specifically developed for Bjornefjord in Norway. Focus has been put on capturing the Vortex-Induced Vibrations generated on the structure, when interacting with a current flow. Matlab has been used to describe the SFT and solve the dynamic problem. Facchinetti's wake oscillator model is applied to couple the tunnel motions with the vortex shedding. In the present report, the tether motions are not taken into account and, conservatively, tethers are modeled as springs. The 100-year current speed was used to excite the structure and it was found that the SFT is not influenced significantly. An SFT configuration with a free-span length of 210m, between two consecutive tethers along the total length, is capable enough to dissolve any VIV effects. Only when a free-span length of 700m is used, vibrations in the cross-flow direction make their presence felt. A simple estimation of the 100-year swell wave force was also performed, showing similar forcing magnitudes as the current force with an excitation frequency much closer to the 1st natural frequency of the structure. This renders the structure a lot more sensitive. Furthermore, the effect of an unexpected tether failure was investigated and showed that the system can safely reach to a new equilibrium position, without any progressive damage. Overall, despite all the hesitation around this concept, it has proven to be quite promising.